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<!-- doc/src/sgml/libpq.sgml -->
<chapter id="libpq">
<title><application>libpq</application> &mdash; C Library</title>
<indexterm zone="libpq">
<primary>libpq</primary>
</indexterm>
<indexterm zone="libpq">
<primary>C</primary>
</indexterm>
<para>
<application>libpq</application> is the <acronym>C</acronym>
application programmer's interface to <productname>PostgreSQL</productname>.
<application>libpq</application> is a set of library functions that allow
client programs to pass queries to the <productname>PostgreSQL</productname>
backend server and to receive the results of these queries.
</para>
<para>
<application>libpq</application> is also the underlying engine for several
other <productname>PostgreSQL</productname> application interfaces, including
those written for C++, Perl, Python, Tcl and <application>ECPG</application>.
So some aspects of <application>libpq</application>'s behavior will be
important to you if you use one of those packages. In particular,
<xref linkend="libpq-envars"/>,
<xref linkend="libpq-pgpass"/> and
<xref linkend="libpq-ssl"/>
describe behavior that is visible to the user of any application
that uses <application>libpq</application>.
</para>
<para>
Some short programs are included at the end of this chapter (<xref linkend="libpq-example"/>) to show how
to write programs that use <application>libpq</application>. There are also several
complete examples of <application>libpq</application> applications in the
directory <filename>src/test/examples</filename> in the source code distribution.
</para>
<para>
Client programs that use <application>libpq</application> must
include the header file
<filename>libpq-fe.h</filename><indexterm><primary>libpq-fe.h</primary></indexterm>
and must link with the <application>libpq</application> library.
</para>
<sect1 id="libpq-connect">
<title>Database Connection Control Functions</title>
<para>
The following functions deal with making a connection to a
<productname>PostgreSQL</productname> backend server. An
application program can have several backend connections open at
one time. (One reason to do that is to access more than one
database.) Each connection is represented by a
<structname>PGconn</structname><indexterm><primary>PGconn</primary></indexterm> object, which
is obtained from the function <xref linkend="libpq-PQconnectdb"/>,
<xref linkend="libpq-PQconnectdbParams"/>, or
<xref linkend="libpq-PQsetdbLogin"/>. Note that these functions will always
return a non-null object pointer, unless perhaps there is too
little memory even to allocate the <structname>PGconn</structname> object.
The <xref linkend="libpq-PQstatus"/> function should be called to check
the return value for a successful connection before queries are sent
via the connection object.
<warning>
<para>
If untrusted users have access to a database that has not adopted a
<link linkend="ddl-schemas-patterns">secure schema usage pattern</link>,
begin each session by removing publicly-writable schemas from
<varname>search_path</varname>. One can set parameter key
word <literal>options</literal> to
value <literal>-csearch_path=</literal>. Alternately, one can
issue <literal>PQexec(<replaceable>conn</replaceable>, "SELECT
pg_catalog.set_config('search_path', '', false)")</literal> after
connecting. This consideration is not specific
to <application>libpq</application>; it applies to every interface for
executing arbitrary SQL commands.
</para>
</warning>
<warning>
<para>
On Unix, forking a process with open libpq connections can lead to
unpredictable results because the parent and child processes share
the same sockets and operating system resources. For this reason,
such usage is not recommended, though doing an <function>exec</function> from
the child process to load a new executable is safe.
</para>
</warning>
<variablelist>
<varlistentry id="libpq-PQconnectdbParams">
<term><function>PQconnectdbParams</function><indexterm><primary>PQconnectdbParams</primary></indexterm></term>
<listitem>
<para>
Makes a new connection to the database server.
<synopsis>
PGconn *PQconnectdbParams(const char * const *keywords,
const char * const *values,
int expand_dbname);
</synopsis>
</para>
<para>
This function opens a new database connection using the parameters taken
from two <symbol>NULL</symbol>-terminated arrays. The first,
<literal>keywords</literal>, is defined as an array of strings, each one
being a key word. The second, <literal>values</literal>, gives the value
for each key word. Unlike <xref linkend="libpq-PQsetdbLogin"/> below, the parameter
set can be extended without changing the function signature, so use of
this function (or its nonblocking analogs <xref linkend="libpq-PQconnectStartParams"/>
and <function>PQconnectPoll</function>) is preferred for new application
programming.
</para>
<para>
The currently recognized parameter key words are listed in
<xref linkend="libpq-paramkeywords"/>.
</para>
<para>
The passed arrays can be empty to use all default parameters, or can
contain one or more parameter settings. They must be matched in length.
Processing will stop at the first <symbol>NULL</symbol> entry
in the <literal>keywords</literal> array.
Also, if the <literal>values</literal> entry associated with a
non-<symbol>NULL</symbol> <literal>keywords</literal> entry is
<symbol>NULL</symbol> or an empty string, that entry is ignored and
processing continues with the next pair of array entries.
</para>
<para>
When <literal>expand_dbname</literal> is non-zero, the value for
the first <parameter>dbname</parameter> key word is checked to see
if it is a <firstterm>connection string</firstterm>. If so, it
is <quote>expanded</quote> into the individual connection
parameters extracted from the string. The value is considered to
be a connection string, rather than just a database name, if it
contains an equal sign (<literal>=</literal>) or it begins with a
URI scheme designator. (More details on connection string formats
appear in <xref linkend="libpq-connstring"/>.) Only the first
occurrence of <parameter>dbname</parameter> is treated in this way;
any subsequent <parameter>dbname</parameter> parameter is processed
as a plain database name.
</para>
<para>
In general the parameter arrays are processed from start to end.
If any key word is repeated, the last value (that is
not <symbol>NULL</symbol> or empty) is used. This rule applies in
particular when a key word found in a connection string conflicts
with one appearing in the <literal>keywords</literal> array. Thus,
the programmer may determine whether array entries can override or
be overridden by values taken from a connection string. Array
entries appearing before an expanded <parameter>dbname</parameter>
entry can be overridden by fields of the connection string, and in
turn those fields are overridden by array entries appearing
after <parameter>dbname</parameter> (but, again, only if those
entries supply non-empty values).
</para>
<para>
After processing all the array entries and any expanded connection
string, any connection parameters that remain unset are filled with
default values. If an unset parameter's corresponding environment
variable (see <xref linkend="libpq-envars"/>) is set, its value is
used. If the environment variable is not set either, then the
parameter's built-in default value is used.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQconnectdb">
<term><function>PQconnectdb</function><indexterm><primary>PQconnectdb</primary></indexterm></term>
<listitem>
<para>
Makes a new connection to the database server.
<synopsis>
PGconn *PQconnectdb(const char *conninfo);
</synopsis>
</para>
<para>
This function opens a new database connection using the parameters taken
from the string <literal>conninfo</literal>.
</para>
<para>
The passed string can be empty to use all default parameters, or it can
contain one or more parameter settings separated by whitespace,
or it can contain a <acronym>URI</acronym>.
See <xref linkend="libpq-connstring"/> for details.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQsetdbLogin">
<term><function>PQsetdbLogin</function><indexterm><primary>PQsetdbLogin</primary></indexterm></term>
<listitem>
<para>
Makes a new connection to the database server.
<synopsis>
PGconn *PQsetdbLogin(const char *pghost,
const char *pgport,
const char *pgoptions,
const char *pgtty,
const char *dbName,
const char *login,
const char *pwd);
</synopsis>
</para>
<para>
This is the predecessor of <xref linkend="libpq-PQconnectdb"/> with a fixed
set of parameters. It has the same functionality except that the
missing parameters will always take on default values. Write <symbol>NULL</symbol> or an
empty string for any one of the fixed parameters that is to be defaulted.
</para>
<para>
If the <parameter>dbName</parameter> contains
an <symbol>=</symbol> sign or has a valid connection <acronym>URI</acronym> prefix, it
is taken as a <parameter>conninfo</parameter> string in exactly the same way as
if it had been passed to <xref linkend="libpq-PQconnectdb"/>, and the remaining
parameters are then applied as specified for <xref linkend="libpq-PQconnectdbParams"/>.
</para>
<para>
<literal>pgtty</literal> is no longer used and any value passed will
be ignored.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQsetdb">
<term><function>PQsetdb</function><indexterm><primary>PQsetdb</primary></indexterm></term>
<listitem>
<para>
Makes a new connection to the database server.
<synopsis>
PGconn *PQsetdb(char *pghost,
char *pgport,
char *pgoptions,
char *pgtty,
char *dbName);
</synopsis>
</para>
<para>
This is a macro that calls <xref linkend="libpq-PQsetdbLogin"/> with null pointers
for the <parameter>login</parameter> and <parameter>pwd</parameter> parameters. It is provided
for backward compatibility with very old programs.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQsocketPoll">
<term><function>PQsocketPoll</function><indexterm><primary>PQsocketPoll</primary></indexterm></term>
<listitem>
<para>
<indexterm><primary>nonblocking connection</primary></indexterm>
Poll a connection&apos;s underlying socket descriptor retrieved with <xref linkend="libpq-PQsocket"/>.
<synopsis>
int PQsocketPoll(int sock, int forRead, int forWrite, time_t end_time);
</synopsis>
</para>
<para>
This function sets up polling of a file descriptor. The underlying function is either
<function>poll(2)</function> or <function>select(2)</function>, depending on platform
support. The primary use of this function is iterating through the connection sequence
described in the documentation of <xref linkend="libpq-PQconnectStartParams"/>. If
<parameter>forRead</parameter> is specified, the function waits for the socket to be ready
for reading. If <parameter>forWrite</parameter> is specified, the function waits for the
socket to be ready for write. See <literal>POLLIN</literal> and <literal>POLLOUT</literal>
from <function>poll(2)</function>, or <parameter>readfds</parameter> and
<parameter>writefds</parameter> from <function>select(2)</function> for more information. If
<parameter>end_time</parameter> is not <literal>-1</literal>, it specifies the time at which
this function should stop waiting for the condition to be met.
</para>
<para>
The function returns a value greater than <literal>0</literal> if the specified condition
is met, <literal>0</literal> if a timeout occurred, or <literal>-1</literal> if an error
occurred. The error can be retrieved by checking the <literal>errno(3)</literal> value. In
the event <literal>forRead</literal> and <literal>forWrite</literal> are not set, the
function immediately returns a timeout condition.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQconnectStartParams">
<term><function>PQconnectStartParams</function><indexterm><primary>PQconnectStartParams</primary></indexterm></term>
<term><function>PQconnectStart</function><indexterm><primary>PQconnectStart</primary></indexterm></term>
<term id="libpq-PQconnectPoll"><function>PQconnectPoll</function><indexterm><primary>PQconnectPoll</primary></indexterm></term>
<listitem>
<para>
<indexterm><primary>nonblocking connection</primary></indexterm>
Make a connection to the database server in a nonblocking manner.
<synopsis>
PGconn *PQconnectStartParams(const char * const *keywords,
const char * const *values,
int expand_dbname);
PGconn *PQconnectStart(const char *conninfo);
PostgresPollingStatusType PQconnectPoll(PGconn *conn);
</synopsis>
</para>
<para>
These three functions are used to open a connection to a database server such
that your application's thread of execution is not blocked on remote I/O
whilst doing so. The point of this approach is that the waits for I/O to
complete can occur in the application's main loop, rather than down inside
<xref linkend="libpq-PQconnectdbParams"/> or <xref linkend="libpq-PQconnectdb"/>, and so the
application can manage this operation in parallel with other activities.
</para>
<para>
With <xref linkend="libpq-PQconnectStartParams"/>, the database connection is made
using the parameters taken from the <literal>keywords</literal> and
<literal>values</literal> arrays, and controlled by <literal>expand_dbname</literal>,
as described above for <xref linkend="libpq-PQconnectdbParams"/>.
</para>
<para>
With <function>PQconnectStart</function>, the database connection is made
using the parameters taken from the string <literal>conninfo</literal> as
described above for <xref linkend="libpq-PQconnectdb"/>.
</para>
<para>
Neither <xref linkend="libpq-PQconnectStartParams"/> nor <function>PQconnectStart</function>
nor <function>PQconnectPoll</function> will block, so long as a number of
restrictions are met:
<itemizedlist>
<listitem>
<para>
The <literal>hostaddr</literal> parameter must be used appropriately
to prevent DNS queries from being made. See the documentation of
this parameter in <xref linkend="libpq-paramkeywords"/> for details.
</para>
</listitem>
<listitem>
<para>
If you call <xref linkend="libpq-PQtrace"/>, ensure that the stream object
into which you trace will not block.
</para>
</listitem>
<listitem>
<para>
You must ensure that the socket is in the appropriate state
before calling <function>PQconnectPoll</function>, as described below.
</para>
</listitem>
</itemizedlist>
</para>
<para>
To begin a nonblocking connection request,
call <function>PQconnectStart</function>
or <xref linkend="libpq-PQconnectStartParams"/>. If the result is null,
then <application>libpq</application> has been unable to allocate a
new <structname>PGconn</structname> structure. Otherwise, a
valid <structname>PGconn</structname> pointer is returned (though not
yet representing a valid connection to the database). Next
call <literal>PQstatus(conn)</literal>. If the result
is <symbol>CONNECTION_BAD</symbol>, the connection attempt has already
failed, typically because of invalid connection parameters.
</para>
<para>
If <function>PQconnectStart</function>
or <xref linkend="libpq-PQconnectStartParams"/> succeeds, the next stage
is to poll <application>libpq</application> so that it can proceed with
the connection sequence.
Use <function>PQsocket(conn)</function> to obtain the descriptor of the
socket underlying the database connection.
(Caution: do not assume that the socket remains the same
across <function>PQconnectPoll</function> calls.)
Loop thus: If <function>PQconnectPoll(conn)</function> last returned
<symbol>PGRES_POLLING_READING</symbol>, wait until the socket is ready to
read (as indicated by <function>select()</function>, <function>poll()</function>, or
similar system function). Note that <function>PQsocketPoll</function>
can help reduce boilerplate by abstracting the setup of
<function>select(2)</function> or <function>poll(2)</function> if it is
available on your system.
Then call <function>PQconnectPoll(conn)</function> again.
Conversely, if <function>PQconnectPoll(conn)</function> last returned
<symbol>PGRES_POLLING_WRITING</symbol>, wait until the socket is ready
to write, then call <function>PQconnectPoll(conn)</function> again.
On the first iteration, i.e., if you have yet to call
<function>PQconnectPoll</function>, behave as if it last returned
<symbol>PGRES_POLLING_WRITING</symbol>. Continue this loop until
<function>PQconnectPoll(conn)</function> returns
<symbol>PGRES_POLLING_FAILED</symbol>, indicating the connection procedure
has failed, or <symbol>PGRES_POLLING_OK</symbol>, indicating the connection
has been successfully made.
</para>
<para>
At any time during connection, the status of the connection can be
checked by calling <xref linkend="libpq-PQstatus"/>. If this call returns <symbol>CONNECTION_BAD</symbol>, then the
connection procedure has failed; if the call returns <function>CONNECTION_OK</function>, then the
connection is ready. Both of these states are equally detectable
from the return value of <function>PQconnectPoll</function>, described above. Other states might also occur
during (and only during) an asynchronous connection procedure. These
indicate the current stage of the connection procedure and might be useful
to provide feedback to the user for example. These statuses are:
<variablelist>
<varlistentry id="libpq-connection-started">
<term><symbol>CONNECTION_STARTED</symbol></term>
<listitem>
<para>
Waiting for connection to be made.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-connection-made">
<term><symbol>CONNECTION_MADE</symbol></term>
<listitem>
<para>
Connection OK; waiting to send.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-connection-awaiting-response">
<term><symbol>CONNECTION_AWAITING_RESPONSE</symbol></term>
<listitem>
<para>
Waiting for a response from the server.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-connection-auth-ok">
<term><symbol>CONNECTION_AUTH_OK</symbol></term>
<listitem>
<para>
Received authentication; waiting for backend start-up to finish.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-connection-ssl-startup">
<term><symbol>CONNECTION_SSL_STARTUP</symbol></term>
<listitem>
<para>
Negotiating SSL encryption.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-connection-gss-startup">
<term><symbol>CONNECTION_GSS_STARTUP</symbol></term>
<listitem>
<para>
Negotiating GSS encryption.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-connection-check-writable">
<term><symbol>CONNECTION_CHECK_WRITABLE</symbol></term>
<listitem>
<para>
Checking if connection is able to handle write transactions.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-connection-check-standby">
<term><symbol>CONNECTION_CHECK_STANDBY</symbol></term>
<listitem>
<para>
Checking if connection is to a server in standby mode.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-connection-consume">
<term><symbol>CONNECTION_CONSUME</symbol></term>
<listitem>
<para>
Consuming any remaining response messages on connection.
</para>
</listitem>
</varlistentry>
</variablelist>
Note that, although these constants will remain (in order to maintain
compatibility), an application should never rely upon these occurring in a
particular order, or at all, or on the status always being one of these
documented values. An application might do something like this:
<programlisting>
switch(PQstatus(conn))
{
case CONNECTION_STARTED:
feedback = "Connecting...";
break;
case CONNECTION_MADE:
feedback = "Connected to server...";
break;
.
.
.
default:
feedback = "Connecting...";
}
</programlisting>
</para>
<para>
The <literal>connect_timeout</literal> connection parameter is ignored
when using <function>PQconnectPoll</function>; it is the application's
responsibility to decide whether an excessive amount of time has elapsed.
Otherwise, <function>PQconnectStart</function> followed by a
<function>PQconnectPoll</function> loop is equivalent to
<xref linkend="libpq-PQconnectdb"/>.
</para>
<para>
Note that when <function>PQconnectStart</function>
or <xref linkend="libpq-PQconnectStartParams"/> returns a non-null
pointer, you must call <xref linkend="libpq-PQfinish"/> when you are
finished with it, in order to dispose of the structure and any
associated memory blocks. This must be done even if the connection
attempt fails or is abandoned.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQconndefaults">
<term><function>PQconndefaults</function><indexterm><primary>PQconndefaults</primary></indexterm></term>
<listitem>
<para>
Returns the default connection options.
<synopsis>
PQconninfoOption *PQconndefaults(void);
typedef struct
{
char *keyword; /* The keyword of the option */
char *envvar; /* Fallback environment variable name */
char *compiled; /* Fallback compiled in default value */
char *val; /* Option's current value, or NULL */
char *label; /* Label for field in connect dialog */
char *dispchar; /* Indicates how to display this field
in a connect dialog. Values are:
"" Display entered value as is
"*" Password field - hide value
"D" Debug option - don't show by default */
int dispsize; /* Field size in characters for dialog */
} PQconninfoOption;
</synopsis>
</para>
<para>
Returns a connection options array. This can be used to determine
all possible <xref linkend="libpq-PQconnectdb"/> options and their
current default values. The return value points to an array of
<structname>PQconninfoOption</structname> structures, which ends
with an entry having a null <structfield>keyword</structfield> pointer. The
null pointer is returned if memory could not be allocated. Note that
the current default values (<structfield>val</structfield> fields)
will depend on environment variables and other context. A
missing or invalid service file will be silently ignored. Callers
must treat the connection options data as read-only.
</para>
<para>
After processing the options array, free it by passing it to
<xref linkend="libpq-PQconninfoFree"/>. If this is not done, a small amount of memory
is leaked for each call to <xref linkend="libpq-PQconndefaults"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQconninfo">
<term><function>PQconninfo</function><indexterm><primary>PQconninfo</primary></indexterm></term>
<listitem>
<para>
Returns the connection options used by a live connection.
<synopsis>
PQconninfoOption *PQconninfo(PGconn *conn);
</synopsis>
</para>
<para>
Returns a connection options array. This can be used to determine
all possible <xref linkend="libpq-PQconnectdb"/> options and the
values that were used to connect to the server. The return
value points to an array of <structname>PQconninfoOption</structname>
structures, which ends with an entry having a null <structfield>keyword</structfield>
pointer. All notes above for <xref linkend="libpq-PQconndefaults"/> also
apply to the result of <xref linkend="libpq-PQconninfo"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQconninfoParse">
<term><function>PQconninfoParse</function><indexterm><primary>PQconninfoParse</primary></indexterm></term>
<listitem>
<para>
Returns parsed connection options from the provided connection string.
<synopsis>
PQconninfoOption *PQconninfoParse(const char *conninfo, char **errmsg);
</synopsis>
</para>
<para>
Parses a connection string and returns the resulting options as an
array; or returns <symbol>NULL</symbol> if there is a problem with the connection
string. This function can be used to extract
the <xref linkend="libpq-PQconnectdb"/> options in the provided
connection string. The return value points to an array of
<structname>PQconninfoOption</structname> structures, which ends
with an entry having a null <structfield>keyword</structfield> pointer.
</para>
<para>
All legal options will be present in the result array, but the
<literal>PQconninfoOption</literal> for any option not present
in the connection string will have <literal>val</literal> set to
<literal>NULL</literal>; default values are not inserted.
</para>
<para>
If <literal>errmsg</literal> is not <symbol>NULL</symbol>, then <literal>*errmsg</literal> is set
to <symbol>NULL</symbol> on success, else to a <function>malloc</function>'d error string explaining
the problem. (It is also possible for <literal>*errmsg</literal> to be
set to <symbol>NULL</symbol> and the function to return <symbol>NULL</symbol>;
this indicates an out-of-memory condition.)
</para>
<para>
After processing the options array, free it by passing it to
<xref linkend="libpq-PQconninfoFree"/>. If this is not done, some memory
is leaked for each call to <xref linkend="libpq-PQconninfoParse"/>.
Conversely, if an error occurs and <literal>errmsg</literal> is not <symbol>NULL</symbol>,
be sure to free the error string using <xref linkend="libpq-PQfreemem"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQfinish">
<term><function>PQfinish</function><indexterm><primary>PQfinish</primary></indexterm></term>
<listitem>
<para>
Closes the connection to the server. Also frees
memory used by the <structname>PGconn</structname> object.
<synopsis>
void PQfinish(PGconn *conn);
</synopsis>
</para>
<para>
Note that even if the server connection attempt fails (as
indicated by <xref linkend="libpq-PQstatus"/>), the application should call <xref linkend="libpq-PQfinish"/>
to free the memory used by the <structname>PGconn</structname> object.
The <structname>PGconn</structname> pointer must not be used again after
<xref linkend="libpq-PQfinish"/> has been called.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQreset">
<term><function>PQreset</function><indexterm><primary>PQreset</primary></indexterm></term>
<listitem>
<para>
Resets the communication channel to the server.
<synopsis>
void PQreset(PGconn *conn);
</synopsis>
</para>
<para>
This function will close the connection
to the server and attempt to establish a new
connection, using all the same
parameters previously used. This might be useful for
error recovery if a working connection is lost.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQresetStart">
<term><function>PQresetStart</function><indexterm><primary>PQresetStart</primary></indexterm></term>
<term><function>PQresetPoll</function><indexterm><primary>PQresetPoll</primary></indexterm></term>
<listitem>
<para>
Reset the communication channel to the server, in a nonblocking manner.
<synopsis>
int PQresetStart(PGconn *conn);
PostgresPollingStatusType PQresetPoll(PGconn *conn);
</synopsis>
</para>
<para>
These functions will close the connection to the server and attempt to
establish a new connection, using all the same
parameters previously used. This can be useful for error recovery if a
working connection is lost. They differ from <xref linkend="libpq-PQreset"/> (above) in that they
act in a nonblocking manner. These functions suffer from the same
restrictions as <xref linkend="libpq-PQconnectStartParams"/>, <function>PQconnectStart</function>
and <function>PQconnectPoll</function>.
</para>
<para>
To initiate a connection reset, call
<xref linkend="libpq-PQresetStart"/>. If it returns 0, the reset has
failed. If it returns 1, poll the reset using
<function>PQresetPoll</function> in exactly the same way as you
would create the connection using <function>PQconnectPoll</function>.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQpingParams">
<term><function>PQpingParams</function><indexterm><primary>PQpingParams</primary></indexterm></term>
<listitem>
<para>
<xref linkend="libpq-PQpingParams"/> reports the status of the
server. It accepts connection parameters identical to those of
<xref linkend="libpq-PQconnectdbParams"/>, described above. It is not
necessary to supply correct user name, password, or database name
values to obtain the server status; however, if incorrect values
are provided, the server will log a failed connection attempt.
<synopsis>
PGPing PQpingParams(const char * const *keywords,
const char * const *values,
int expand_dbname);
</synopsis>
The function returns one of the following values:
<variablelist>
<varlistentry id="libpq-PQpingParams-PQPING_OK">
<term><literal>PQPING_OK</literal></term>
<listitem>
<para>
The server is running and appears to be accepting connections.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQpingParams-PQPING_REJECT">
<term><literal>PQPING_REJECT</literal></term>
<listitem>
<para>
The server is running but is in a state that disallows connections
(startup, shutdown, or crash recovery).
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQpingParams-PQPING_NO_RESPONSE">
<term><literal>PQPING_NO_RESPONSE</literal></term>
<listitem>
<para>
The server could not be contacted. This might indicate that the
server is not running, or that there is something wrong with the
given connection parameters (for example, wrong port number), or
that there is a network connectivity problem (for example, a
firewall blocking the connection request).
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQpingParams-PQPING_NO_ATTEMPT">
<term><literal>PQPING_NO_ATTEMPT</literal></term>
<listitem>
<para>
No attempt was made to contact the server, because the supplied
parameters were obviously incorrect or there was some client-side
problem (for example, out of memory).
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQping">
<term><function>PQping</function><indexterm><primary>PQping</primary></indexterm></term>
<listitem>
<para>
<xref linkend="libpq-PQping"/> reports the status of the
server. It accepts connection parameters identical to those of
<xref linkend="libpq-PQconnectdb"/>, described above. It is not
necessary to supply correct user name, password, or database name
values to obtain the server status; however, if incorrect values
are provided, the server will log a failed connection attempt.
<synopsis>
PGPing PQping(const char *conninfo);
</synopsis>
</para>
<para>
The return values are the same as for <xref linkend="libpq-PQpingParams"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-pqsetsslkeypasshook-openssl">
<term><function>PQsetSSLKeyPassHook_OpenSSL</function><indexterm><primary>PQsetSSLKeyPassHook_OpenSSL</primary></indexterm></term>
<listitem>
<para>
<function>PQsetSSLKeyPassHook_OpenSSL</function> lets an application override
<application>libpq</application>'s <link linkend="libpq-ssl-clientcert">default
handling of encrypted client certificate key files</link> using
<xref linkend="libpq-connect-sslpassword"/> or interactive prompting.
<synopsis>
void PQsetSSLKeyPassHook_OpenSSL(PQsslKeyPassHook_OpenSSL_type hook);
</synopsis>
The application passes a pointer to a callback function with signature:
<programlisting>
int callback_fn(char *buf, int size, PGconn *conn);
</programlisting>
which <application>libpq</application> will then call
<emphasis>instead of</emphasis> its default
<function>PQdefaultSSLKeyPassHook_OpenSSL</function> handler. The
callback should determine the password for the key and copy it to
result-buffer <parameter>buf</parameter> of size
<parameter>size</parameter>. The string in <parameter>buf</parameter>
must be null-terminated. The callback must return the length of the
password stored in <parameter>buf</parameter> excluding the null
terminator. On failure, the callback should set
<literal>buf[0] = '\0'</literal> and return 0. See
<function>PQdefaultSSLKeyPassHook_OpenSSL</function> in
<application>libpq</application>'s source code for an example.
</para>
<para>
If the user specified an explicit key location,
its path will be in <literal>conn->sslkey</literal> when the callback
is invoked. This will be empty if the default key path is being used.
For keys that are engine specifiers, it is up to engine implementations
whether they use the <productname>OpenSSL</productname> password
callback or define their own handling.
</para>
<para>
The app callback may choose to delegate unhandled cases to
<function>PQdefaultSSLKeyPassHook_OpenSSL</function>,
or call it first and try something else if it returns 0, or completely override it.
</para>
<para>
The callback <emphasis>must not</emphasis> escape normal flow control with exceptions,
<function>longjmp(...)</function>, etc. It must return normally.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-pqgetsslkeypasshook-openssl">
<term><function>PQgetSSLKeyPassHook_OpenSSL</function><indexterm><primary>PQgetSSLKeyPassHook_OpenSSL</primary></indexterm></term>
<listitem>
<para>
<function>PQgetSSLKeyPassHook_OpenSSL</function> returns the current
client certificate key password hook, or <literal>NULL</literal>
if none has been set.
<synopsis>
PQsslKeyPassHook_OpenSSL_type PQgetSSLKeyPassHook_OpenSSL(void);
</synopsis>
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
<sect2 id="libpq-connstring">
<title>Connection Strings</title>
<indexterm zone="libpq-connstring">
<primary><literal>conninfo</literal></primary>
</indexterm>
<indexterm zone="libpq-connstring">
<primary><literal>URI</literal></primary>
</indexterm>
<para>
Several <application>libpq</application> functions parse a user-specified string to obtain
connection parameters. There are two accepted formats for these strings:
plain keyword/value strings
and URIs. URIs generally follow
<ulink url="https://datatracker.ietf.org/doc/html/rfc3986">RFC
3986</ulink>, except that multi-host connection strings are allowed
as further described below.
</para>
<sect3 id="libpq-connstring-keyword-value">
<title>Keyword/Value Connection Strings</title>
<para>
In the keyword/value format, each parameter setting is in the form
<replaceable>keyword</replaceable> <literal>=</literal>
<replaceable>value</replaceable>, with space(s) between settings.
Spaces around a setting's equal sign are
optional. To write an empty value, or a value containing spaces, surround it
with single quotes, for example <literal>keyword = 'a value'</literal>.
Single quotes and backslashes within
a value must be escaped with a backslash, i.e., <literal>\'</literal> and
<literal>\\</literal>.
</para>
<para>
Example:
<programlisting>
host=localhost port=5432 dbname=mydb connect_timeout=10
</programlisting>
</para>
<para>
The recognized parameter key words are listed in <xref
linkend="libpq-paramkeywords"/>.
</para>
</sect3>
<sect3 id="libpq-connstring-uris">
<title>Connection URIs</title>
<para>
The general form for a connection <acronym>URI</acronym> is:
<synopsis>
postgresql://<optional><replaceable>userspec</replaceable>@</optional><optional><replaceable>hostspec</replaceable></optional><optional>/<replaceable>dbname</replaceable></optional><optional>?<replaceable>paramspec</replaceable></optional>
<phrase>where <replaceable>userspec</replaceable> is:</phrase>
<replaceable>user</replaceable><optional>:<replaceable>password</replaceable></optional>
<phrase>and <replaceable>hostspec</replaceable> is:</phrase>
<optional><replaceable>host</replaceable></optional><optional>:<replaceable>port</replaceable></optional><optional>,...</optional>
<phrase>and <replaceable>paramspec</replaceable> is:</phrase>
<replaceable>name</replaceable>=<replaceable>value</replaceable><optional>&amp;...</optional>
</synopsis>
</para>
<para>
The <acronym>URI</acronym> scheme designator can be either
<literal>postgresql://</literal> or <literal>postgres://</literal>. Each
of the remaining <acronym>URI</acronym> parts is optional. The
following examples illustrate valid <acronym>URI</acronym> syntax:
<programlisting>
postgresql://
postgresql://localhost
postgresql://localhost:5433
postgresql://localhost/mydb
postgresql://user@localhost
postgresql://user:secret@localhost
postgresql://other@localhost/otherdb?connect_timeout=10&amp;application_name=myapp
postgresql://host1:123,host2:456/somedb?target_session_attrs=any&amp;application_name=myapp
</programlisting>
Values that would normally appear in the hierarchical part of
the <acronym>URI</acronym> can alternatively be given as named
parameters. For example:
<programlisting>
postgresql:///mydb?host=localhost&amp;port=5433
</programlisting>
All named parameters must match key words listed in
<xref linkend="libpq-paramkeywords"/>, except that for compatibility
with JDBC connection <acronym>URI</acronym>s, instances
of <literal>ssl=true</literal> are translated into
<literal>sslmode=require</literal>.
</para>
<para>
The connection <acronym>URI</acronym> needs to be encoded with <ulink
url="https://datatracker.ietf.org/doc/html/rfc3986#section-2.1">percent-encoding</ulink>
if it includes symbols with special meaning in any of its parts. Here is
an example where the equal sign (<literal>=</literal>) is replaced with
<literal>%3D</literal> and the space character with
<literal>%20</literal>:
<programlisting>
postgresql://user@localhost:5433/mydb?options=-c%20synchronous_commit%3Doff
</programlisting>
</para>
<para>
The host part may be either a host name or an IP address. To specify an
IPv6 address, enclose it in square brackets:
<synopsis>
postgresql://[2001:db8::1234]/database
</synopsis>
</para>
<para>
The host part is interpreted as described for the parameter <xref
linkend="libpq-connect-host"/>. In particular, a Unix-domain socket
connection is chosen if the host part is either empty or looks like an
absolute path name,
otherwise a TCP/IP connection is initiated. Note, however, that the
slash is a reserved character in the hierarchical part of the URI. So, to
specify a non-standard Unix-domain socket directory, either omit the host
part of the URI and specify the host as a named parameter, or
percent-encode the path in the host part of the URI:
<programlisting>
postgresql:///dbname?host=/var/lib/postgresql
postgresql://%2Fvar%2Flib%2Fpostgresql/dbname
</programlisting>
</para>
<para>
It is possible to specify multiple host components, each with an optional
port component, in a single URI. A URI of the form
<literal>postgresql://host1:port1,host2:port2,host3:port3/</literal>
is equivalent to a connection string of the form
<literal>host=host1,host2,host3 port=port1,port2,port3</literal>.
As further described below, each
host will be tried in turn until a connection is successfully established.
</para>
</sect3>
<sect3 id="libpq-multiple-hosts">
<title>Specifying Multiple Hosts</title>
<para>
It is possible to specify multiple hosts to connect to, so that they are
tried in the given order. In the Keyword/Value format, the <literal>host</literal>,
<literal>hostaddr</literal>, and <literal>port</literal> options accept comma-separated
lists of values. The same number of elements must be given in each
option that is specified, such
that e.g., the first <literal>hostaddr</literal> corresponds to the first host name,
the second <literal>hostaddr</literal> corresponds to the second host name, and so
forth. As an exception, if only one <literal>port</literal> is specified, it
applies to all the hosts.
</para>
<para>
In the connection URI format, you can list multiple <literal>host:port</literal> pairs
separated by commas in the <literal>host</literal> component of the URI.
</para>
<para>
In either format, a single host name can translate to multiple network
addresses. A common example of this is a host that has both an IPv4 and
an IPv6 address.
</para>
<para>
When multiple hosts are specified, or when a single host name is
translated to multiple addresses, all the hosts and addresses will be
tried in order, until one succeeds. If none of the hosts can be reached,
the connection fails. If a connection is established successfully, but
authentication fails, the remaining hosts in the list are not tried.
</para>
<para>
If a password file is used, you can have different passwords for
different hosts. All the other connection options are the same for every
host in the list; it is not possible to e.g., specify different
usernames for different hosts.
</para>
</sect3>
</sect2>
<sect2 id="libpq-paramkeywords">
<title>Parameter Key Words</title>
<para>
The currently recognized parameter key words are:
<variablelist>
<varlistentry id="libpq-connect-host" xreflabel="host">
<term><literal>host</literal></term>
<listitem>
<para>
Name of host to connect to.<indexterm><primary>host
name</primary></indexterm> If a host name looks like an absolute path
name, it specifies Unix-domain communication rather than TCP/IP
communication; the value is the name of the directory in which the
socket file is stored. (On Unix, an absolute path name begins with a
slash. On Windows, paths starting with drive letters are also
recognized.) If the host name starts with <literal>@</literal>, it is
taken as a Unix-domain socket in the abstract namespace (currently
supported on Linux and Windows).
The default behavior when <literal>host</literal> is not
specified, or is empty, is to connect to a Unix-domain
socket<indexterm><primary>Unix domain socket</primary></indexterm> in
<filename>/tmp</filename> (or whatever socket directory was specified
when <productname>PostgreSQL</productname> was built). On Windows,
the default is to connect to <literal>localhost</literal>.
</para>
<para>
A comma-separated list of host names is also accepted, in which case
each host name in the list is tried in order; an empty item in the
list selects the default behavior as explained above. See
<xref linkend="libpq-multiple-hosts"/> for details.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-connect-hostaddr" xreflabel="hostaddr">
<term><literal>hostaddr</literal></term>
<listitem>
<para>
Numeric IP address of host to connect to. This should be in the
standard IPv4 address format, e.g., <literal>172.28.40.9</literal>. If
your machine supports IPv6, you can also use those addresses.
TCP/IP communication is
always used when a nonempty string is specified for this parameter.
If this parameter is not specified, the value of <literal>host</literal>
will be looked up to find the corresponding IP address &mdash; or, if
<literal>host</literal> specifies an IP address, that value will be
used directly.
</para>
<para>
Using <literal>hostaddr</literal> allows the
application to avoid a host name look-up, which might be important
in applications with time constraints. However, a host name is
required for GSSAPI or SSPI authentication
methods, as well as for <literal>verify-full</literal> SSL
certificate verification. The following rules are used:
<itemizedlist>
<listitem>
<para>
If <literal>host</literal> is specified
without <literal>hostaddr</literal>, a host name lookup occurs.
(When using <function>PQconnectPoll</function>, the lookup occurs
when <function>PQconnectPoll</function> first considers this host
name, and it may cause <function>PQconnectPoll</function> to block
for a significant amount of time.)
</para>
</listitem>
<listitem>
<para>
If <literal>hostaddr</literal> is specified without <literal>host</literal>,
the value for <literal>hostaddr</literal> gives the server network address.
The connection attempt will fail if the authentication
method requires a host name.
</para>
</listitem>
<listitem>
<para>
If both <literal>host</literal> and <literal>hostaddr</literal> are specified,
the value for <literal>hostaddr</literal> gives the server network address.
The value for <literal>host</literal> is ignored unless the
authentication method requires it, in which case it will be
used as the host name.
</para>
</listitem>
</itemizedlist>
Note that authentication is likely to fail if <literal>host</literal>
is not the name of the server at network address <literal>hostaddr</literal>.
Also, when both <literal>host</literal> and <literal>hostaddr</literal>
are specified, <literal>host</literal>
is used to identify the connection in a password file (see
<xref linkend="libpq-pgpass"/>).
</para>
<para>
A comma-separated list of <literal>hostaddr</literal> values is also
accepted, in which case each host in the list is tried in order.
An empty item in the list causes the corresponding host name to be
used, or the default host name if that is empty as well. See
<xref linkend="libpq-multiple-hosts"/> for details.
</para>
<para>
Without either a host name or host address,
<application>libpq</application> will connect using a local
Unix-domain socket; or on Windows, it will attempt to connect to
<literal>localhost</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-connect-port" xreflabel="port">
<term><literal>port</literal></term>
<listitem>
<para>
Port number to connect to at the server host, or socket file
name extension for Unix-domain
connections.<indexterm><primary>port</primary></indexterm>
If multiple hosts were given in the <literal>host</literal> or
<literal>hostaddr</literal> parameters, this parameter may specify a
comma-separated list of ports of the same length as the host list, or
it may specify a single port number to be used for all hosts.
An empty string, or an empty item in a comma-separated list,
specifies the default port number established
when <productname>PostgreSQL</productname> was built.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-connect-dbname" xreflabel="dbname">
<term><literal>dbname</literal></term>
<listitem>
<para>
The database name. Defaults to be the same as the user name.
In certain contexts, the value is checked for extended
formats; see <xref linkend="libpq-connstring"/> for more details on
those.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-connect-user" xreflabel="user">
<term><literal>user</literal></term>
<listitem>
<para>
<productname>PostgreSQL</productname> user name to connect as.
Defaults to be the same as the operating system name of the user
running the application.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-connect-password" xreflabel="password">
<term><literal>password</literal></term>
<listitem>
<para>
Password to be used if the server demands password authentication.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-connect-passfile" xreflabel="passfile">
<term><literal>passfile</literal></term>
<listitem>
<para>
Specifies the name of the file used to store passwords
(see <xref linkend="libpq-pgpass"/>).
Defaults to <filename>~/.pgpass</filename>, or
<filename>%APPDATA%\postgresql\pgpass.conf</filename> on Microsoft Windows.
(No error is reported if this file does not exist.)
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-connect-require-auth" xreflabel="require_auth">
<term><literal>require_auth</literal></term>
<listitem>
<para>
Specifies the authentication method that the client requires from the
server. If the server does not use the required method to authenticate
the client, or if the authentication handshake is not fully completed by
the server, the connection will fail. A comma-separated list of methods
may also be provided, of which the server must use exactly one in order
for the connection to succeed. By default, any authentication method is
accepted, and the server is free to skip authentication altogether.
</para>
<para>
Methods may be negated with the addition of a <literal>!</literal>
prefix, in which case the server must <emphasis>not</emphasis> attempt
the listed method; any other method is accepted, and the server is free
not to authenticate the client at all. If a comma-separated list is
provided, the server may not attempt <emphasis>any</emphasis> of the
listed negated methods. Negated and non-negated forms may not be
combined in the same setting.
</para>
<para>
As a final special case, the <literal>none</literal> method requires the
server not to use an authentication challenge. (It may also be negated,
to require some form of authentication.)
</para>
<para>
The following methods may be specified:
<variablelist>
<varlistentry>
<term><literal>password</literal></term>
<listitem>
<para>
The server must request plaintext password authentication.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>md5</literal></term>
<listitem>
<para>
The server must request MD5 hashed password authentication.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>gss</literal></term>
<listitem>
<para>
The server must either request a Kerberos handshake via
<acronym>GSSAPI</acronym> or establish a
<acronym>GSS</acronym>-encrypted channel (see also
<xref linkend="libpq-connect-gssencmode" />).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>sspi</literal></term>
<listitem>
<para>
The server must request Windows <acronym>SSPI</acronym>
authentication.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>scram-sha-256</literal></term>
<listitem>
<para>
The server must successfully complete a SCRAM-SHA-256 authentication
exchange with the client.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>none</literal></term>
<listitem>
<para>
The server must not prompt the client for an authentication
exchange. (This does not prohibit client certificate authentication
via TLS, nor GSS authentication via its encrypted transport.)
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-connect-channel-binding" xreflabel="channel_binding">
<term><literal>channel_binding</literal></term>
<listitem>
<para>
This option controls the client's use of channel binding. A setting
of <literal>require</literal> means that the connection must employ
channel binding, <literal>prefer</literal> means that the client will
choose channel binding if available, and <literal>disable</literal>
prevents the use of channel binding. The default
is <literal>prefer</literal> if
<productname>PostgreSQL</productname> is compiled with SSL support;
otherwise the default is <literal>disable</literal>.
</para>
<para>
Channel binding is a method for the server to authenticate itself to
the client. It is only supported over SSL connections
with <productname>PostgreSQL</productname> 11 or later servers using
the <literal>SCRAM</literal> authentication method.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-connect-connect-timeout" xreflabel="connect_timeout">
<term><literal>connect_timeout</literal></term>
<listitem>
<para>
Maximum time to wait while connecting, in seconds (write as a decimal integer,
e.g., <literal>10</literal>). Zero, negative, or not specified means
wait indefinitely. The minimum allowed timeout is 2 seconds, therefore
a value of <literal>1</literal> is interpreted as <literal>2</literal>.
This timeout applies separately to each host name or IP address.
For example, if you specify two hosts and <literal>connect_timeout</literal>
is 5, each host will time out if no connection is made within 5
seconds, so the total time spent waiting for a connection might be
up to 10 seconds.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-connect-client-encoding" xreflabel="client_encoding">
<term><literal>client_encoding</literal></term>
<listitem>
<para>
This sets the <varname>client_encoding</varname>
configuration parameter for this connection. In addition to
the values accepted by the corresponding server option, you
can use <literal>auto</literal> to determine the right
encoding from the current locale in the client
(<envar>LC_CTYPE</envar> environment variable on Unix
systems).
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-connect-options" xreflabel="options">
<term><literal>options</literal></term>
<listitem>
<para>
Specifies command-line options to send to the server at connection
start. For example, setting this to <literal>-c geqo=off</literal> sets the
session's value of the <varname>geqo</varname> parameter to
<literal>off</literal>. Spaces within this string are considered to
separate command-line arguments, unless escaped with a backslash
(<literal>\</literal>); write <literal>\\</literal> to represent a literal
backslash. For a detailed discussion of the available
options, consult <xref linkend="runtime-config"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-connect-application-name" xreflabel="application_name">
<term><literal>application_name</literal></term>
<listitem>
<para>
Specifies a value for the <xref linkend="guc-application-name"/>
configuration parameter.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-connect-fallback-application-name" xreflabel="fallback_application_name">
<term><literal>fallback_application_name</literal></term>
<listitem>
<para>
Specifies a fallback value for the <xref
linkend="guc-application-name"/> configuration parameter.
This value will be used if no value has been given for
<literal>application_name</literal> via a connection parameter or the
<envar>PGAPPNAME</envar> environment variable. Specifying
a fallback name is useful in generic utility programs that
wish to set a default application name but allow it to be
overridden by the user.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-keepalives" xreflabel="keepalives">
<term><literal>keepalives</literal></term>
<listitem>
<para>
Controls whether client-side TCP keepalives are used. The default
value is 1, meaning on, but you can change this to 0, meaning off,
if keepalives are not wanted. This parameter is ignored for
connections made via a Unix-domain socket.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-keepalives-idle" xreflabel="keepalives_idle">
<term><literal>keepalives_idle</literal></term>
<listitem>
<para>
Controls the number of seconds of inactivity after which TCP should
send a keepalive message to the server. A value of zero uses the
system default. This parameter is ignored for connections made via a
Unix-domain socket, or if keepalives are disabled.
It is only supported on systems where <symbol>TCP_KEEPIDLE</symbol> or
an equivalent socket option is available, and on Windows; on other
systems, it has no effect.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-keepalives-interval" xreflabel="keepalives_interval">
<term><literal>keepalives_interval</literal></term>
<listitem>
<para>
Controls the number of seconds after which a TCP keepalive message
that is not acknowledged by the server should be retransmitted. A
value of zero uses the system default. This parameter is ignored for
connections made via a Unix-domain socket, or if keepalives are disabled.
It is only supported on systems where <symbol>TCP_KEEPINTVL</symbol> or
an equivalent socket option is available, and on Windows; on other
systems, it has no effect.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-keepalives-count" xreflabel="keepalives_count">
<term><literal>keepalives_count</literal></term>
<listitem>
<para>
Controls the number of TCP keepalives that can be lost before the
client's connection to the server is considered dead. A value of
zero uses the system default. This parameter is ignored for
connections made via a Unix-domain socket, or if keepalives are disabled.
It is only supported on systems where <symbol>TCP_KEEPCNT</symbol> or
an equivalent socket option is available; on other systems, it has no
effect.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-tcp-user-timeout" xreflabel="tcp_user_timeout">
<term><literal>tcp_user_timeout</literal></term>
<listitem>
<para>
Controls the number of milliseconds that transmitted data may
remain unacknowledged before a connection is forcibly closed.
A value of zero uses the system default. This parameter is
ignored for connections made via a Unix-domain socket.
It is only supported on systems where <symbol>TCP_USER_TIMEOUT</symbol>
is available; on other systems, it has no effect.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-connect-replication" xreflabel="replication">
<term><literal>replication</literal></term>
<listitem>
<para>
This option determines whether the connection should use the
replication protocol instead of the normal protocol. This is what
PostgreSQL replication connections as well as tools such as
<application>pg_basebackup</application> use internally, but it can
also be used by third-party applications. For a description of the
replication protocol, consult <xref linkend="protocol-replication"/>.
</para>
<para>
The following values, which are case-insensitive, are supported:
<variablelist>
<varlistentry>
<term>
<literal>true</literal>, <literal>on</literal>,
<literal>yes</literal>, <literal>1</literal>
</term>
<listitem>
<para>
The connection goes into physical replication mode.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>database</literal></term>
<listitem>
<para>
The connection goes into logical replication mode, connecting to
the database specified in the <literal>dbname</literal> parameter.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<literal>false</literal>, <literal>off</literal>,
<literal>no</literal>, <literal>0</literal>
</term>
<listitem>
<para>
The connection is a regular one, which is the default behavior.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
<para>
In physical or logical replication mode, only the simple query protocol
can be used.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-connect-gssencmode" xreflabel="gssencmode">
<term><literal>gssencmode</literal></term>
<listitem>
<para>
This option determines whether or with what priority a secure
<acronym>GSS</acronym> TCP/IP connection will be negotiated with the
server. There are three modes:
<variablelist>
<varlistentry>
<term><literal>disable</literal></term>
<listitem>
<para>
only try a non-<acronym>GSSAPI</acronym>-encrypted connection
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>prefer</literal> (default)</term>
<listitem>
<para>
if there are <acronym>GSSAPI</acronym> credentials present (i.e.,
in a credentials cache), first try
a <acronym>GSSAPI</acronym>-encrypted connection; if that fails or
there are no credentials, try a
non-<acronym>GSSAPI</acronym>-encrypted connection. This is the
default when <productname>PostgreSQL</productname> has been
compiled with <acronym>GSSAPI</acronym> support.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>require</literal></term>
<listitem>
<para>
only try a <acronym>GSSAPI</acronym>-encrypted connection
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
<para>
<literal>gssencmode</literal> is ignored for Unix domain socket
communication. If <productname>PostgreSQL</productname> is compiled
without GSSAPI support, using the <literal>require</literal> option
will cause an error, while <literal>prefer</literal> will be accepted
but <application>libpq</application> will not actually attempt
a <acronym>GSSAPI</acronym>-encrypted
connection.<indexterm><primary>GSSAPI</primary><secondary sortas="libpq">with
libpq</secondary></indexterm>
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-connect-sslmode" xreflabel="sslmode">
<term><literal>sslmode</literal></term>
<listitem>
<para>
This option determines whether or with what priority a secure
<acronym>SSL</acronym> TCP/IP connection will be negotiated with the
server. There are six modes:
<variablelist>
<varlistentry>
<term><literal>disable</literal></term>
<listitem>
<para>
only try a non-<acronym>SSL</acronym> connection
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>allow</literal></term>
<listitem>
<para>
first try a non-<acronym>SSL</acronym> connection; if that
fails, try an <acronym>SSL</acronym> connection
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>prefer</literal> (default)</term>
<listitem>
<para>
first try an <acronym>SSL</acronym> connection; if that fails,
try a non-<acronym>SSL</acronym> connection
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>require</literal></term>
<listitem>
<para>
only try an <acronym>SSL</acronym> connection. If a root CA
file is present, verify the certificate in the same way as
if <literal>verify-ca</literal> was specified
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>verify-ca</literal></term>
<listitem>
<para>
only try an <acronym>SSL</acronym> connection, and verify that
the server certificate is issued by a trusted
certificate authority (<acronym>CA</acronym>)
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>verify-full</literal></term>
<listitem>
<para>
only try an <acronym>SSL</acronym> connection, verify that the
server certificate is issued by a
trusted <acronym>CA</acronym> and that the requested server host name
matches that in the certificate
</para>
</listitem>
</varlistentry>
</variablelist>
See <xref linkend="libpq-ssl"/> for a detailed description of how
these options work.
</para>
<para>
<literal>sslmode</literal> is ignored for Unix domain socket
communication.
If <productname>PostgreSQL</productname> is compiled without SSL support,
using options <literal>require</literal>, <literal>verify-ca</literal>, or
<literal>verify-full</literal> will cause an error, while
options <literal>allow</literal> and <literal>prefer</literal> will be
accepted but <application>libpq</application> will not actually attempt
an <acronym>SSL</acronym>
connection.<indexterm><primary>SSL</primary><secondary
sortas="libpq">with libpq</secondary></indexterm>
</para>
<para>
Note that if <acronym>GSSAPI</acronym> encryption is possible,
that will be used in preference to <acronym>SSL</acronym>
encryption, regardless of the value of <literal>sslmode</literal>.
To force use of <acronym>SSL</acronym> encryption in an
environment that has working <acronym>GSSAPI</acronym>
infrastructure (such as a Kerberos server), also set
<literal>gssencmode</literal> to <literal>disable</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-connect-requiressl" xreflabel="requiressl">
<term><literal>requiressl</literal></term>
<listitem>
<para>
This option is deprecated in favor of the <literal>sslmode</literal>
setting.
</para>
<para>
If set to 1, an <acronym>SSL</acronym> connection to the server
is required (this is equivalent to <literal>sslmode</literal>
<literal>require</literal>). <application>libpq</application> will then refuse
to connect if the server does not accept an
<acronym>SSL</acronym> connection. If set to 0 (default),
<application>libpq</application> will negotiate the connection type with
the server (equivalent to <literal>sslmode</literal>
<literal>prefer</literal>). This option is only available if
<productname>PostgreSQL</productname> is compiled with SSL support.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-connect-sslnegotiation" xreflabel="sslnegotiation">
<term><literal>sslnegotiation</literal></term>
<listitem>
<para>
This option controls whether <productname>PostgreSQL</productname>
will perform its protocol negotiation to request encryption from the
server or will just directly make a standard <acronym>SSL</acronym>
connection. Traditional <productname>PostgreSQL</productname>
protocol negotiation is the default and the most flexible with
different server configurations. If the server is known to support
direct <acronym>SSL</acronym> connections then the latter requires one
fewer round trip reducing connection latency and also allows the use
of protocol agnostic SSL network tools.
</para>
<variablelist>
<varlistentry>
<term><literal>postgres</literal></term>
<listitem>
<para>
perform <productname>PostgreSQL</productname> protocol
negotiation. This is the default if the option is not provided.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>direct</literal></term>
<listitem>
<para>
first attempt to establish a standard SSL connection and if that
fails reconnect and perform the negotiation. This fallback
process adds significant latency if the initial SSL connection
fails.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>requiredirect</literal></term>
<listitem>
<para>
attempt to establish a standard SSL connection and if that fails
return a connection failure immediately.
</para>
</listitem>
</varlistentry>
</variablelist>
<para>
Note that if <literal>gssencmode</literal> is set
to <literal>prefer</literal>, a <acronym>GSS</acronym> connection is
attempted first. If the server rejects GSS encryption, SSL is
negotiated over the same TCP connection using the traditional postgres
protocol, regardless of <literal>sslnegotiation</literal>. In other
words, the direct SSL handshake is not used, if a TCP connection has
already been established and can be used for the SSL handshake.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-connect-sslcompression" xreflabel="sslcompression">
<term><literal>sslcompression</literal></term>
<listitem>
<para>
If set to 1, data sent over SSL connections will be compressed. If
set to 0, compression will be disabled. The default is 0. This
parameter is ignored if a connection without SSL is made.
</para>
<para>
SSL compression is nowadays considered insecure and its use is no
longer recommended. <productname>OpenSSL</productname> 1.1.0 disabled
compression by default, and many operating system distributions
disabled it in prior versions as well, so setting this parameter to on
will not have any effect if the server does not accept compression.
<productname>PostgreSQL</productname> 14 disabled compression
completely in the backend.
</para>
<para>
If security is not a primary concern, compression can improve
throughput if the network is the bottleneck. Disabling compression
can improve response time and throughput if CPU performance is the
limiting factor.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-connect-sslcert" xreflabel="sslcert">
<term><literal>sslcert</literal></term>
<listitem>
<para>
This parameter specifies the file name of the client SSL
certificate, replacing the default
<filename>~/.postgresql/postgresql.crt</filename>.
This parameter is ignored if an SSL connection is not made.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-connect-sslkey" xreflabel="sslkey">
<term><literal>sslkey</literal></term>
<listitem>
<para>
This parameter specifies the location for the secret key used for
the client certificate. It can either specify a file name that will
be used instead of the default
<filename>~/.postgresql/postgresql.key</filename>, or it can specify a key
obtained from an external <quote>engine</quote> (engines are
<productname>OpenSSL</productname> loadable modules). An external engine
specification should consist of a colon-separated engine name and
an engine-specific key identifier. This parameter is ignored if an
SSL connection is not made.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-connect-sslpassword" xreflabel="sslpassword">
<term><literal>sslpassword</literal></term>
<listitem>
<para>
This parameter specifies the password for the secret key specified in
<literal>sslkey</literal>, allowing client certificate private keys
to be stored in encrypted form on disk even when interactive passphrase
input is not practical.
</para>
<para>
Specifying this parameter with any non-empty value suppresses the
<literal>Enter PEM pass phrase:</literal>
prompt that <productname>OpenSSL</productname> will emit by default
when an encrypted client certificate key is provided to
<literal>libpq</literal>.
</para>
<para>
If the key is not encrypted this parameter is ignored. The parameter
has no effect on keys specified by <productname>OpenSSL</productname>
engines unless the engine uses the <productname>OpenSSL</productname>
password callback mechanism for prompts.
</para>
<para>
There is no environment variable equivalent to this option, and no
facility for looking it up in <filename>.pgpass</filename>. It can be
used in a service file connection definition. Users with
more sophisticated uses should consider using <productname>OpenSSL</productname> engines and
tools like PKCS#11 or USB crypto offload devices.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-connect-sslcertmode" xreflabel="sslcertmode">
<term><literal>sslcertmode</literal></term>
<listitem>
<para>
This option determines whether a client certificate may be sent to the
server, and whether the server is required to request one. There are
three modes:
<variablelist>
<varlistentry>
<term><literal>disable</literal></term>
<listitem>
<para>
A client certificate is never sent, even if one is available
(default location or provided via
<xref linkend="libpq-connect-sslcert" />).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>allow</literal> (default)</term>
<listitem>
<para>
A certificate may be sent, if the server requests one and the
client has one to send.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>require</literal></term>
<listitem>
<para>
The server <emphasis>must</emphasis> request a certificate. The
connection will fail if the client does not send a certificate and
the server successfully authenticates the client anyway.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
<note>
<para>
<literal>sslcertmode=require</literal> doesn't add any additional
security, since there is no guarantee that the server is validating
the certificate correctly; PostgreSQL servers generally request TLS
certificates from clients whether they validate them or not. The
option may be useful when troubleshooting more complicated TLS
setups.
</para>
</note>
</listitem>
</varlistentry>
<varlistentry id="libpq-connect-sslrootcert" xreflabel="sslrootcert">
<term><literal>sslrootcert</literal></term>
<listitem>
<para>
This parameter specifies the name of a file containing SSL
certificate authority (<acronym>CA</acronym>) certificate(s).
If the file exists, the server's certificate will be verified
to be signed by one of these authorities. The default is
<filename>~/.postgresql/root.crt</filename>.
</para>
<para>
The special value <literal>system</literal> may be specified instead, in
which case the system's trusted CA roots will be loaded. The exact
locations of these root certificates differ by SSL implementation and
platform. For <productname>OpenSSL</productname> in particular, the
locations may be further modified by the <envar>SSL_CERT_DIR</envar>
and <envar>SSL_CERT_FILE</envar> environment variables.
</para>
<note>
<para>
When using <literal>sslrootcert=system</literal>, the default
<literal>sslmode</literal> is changed to <literal>verify-full</literal>,
and any weaker setting will result in an error. In most cases it is
trivial for anyone to obtain a certificate trusted by the system for a
hostname they control, rendering <literal>verify-ca</literal> and all
weaker modes useless.
</para>
<para>
The magic <literal>system</literal> value will take precedence over a
local certificate file with the same name. If for some reason you find
yourself in this situation, use an alternative path like
<literal>sslrootcert=./system</literal> instead.
</para>
</note>
</listitem>
</varlistentry>
<varlistentry id="libpq-connect-sslcrl" xreflabel="sslcrl">
<term><literal>sslcrl</literal></term>
<listitem>
<para>
This parameter specifies the file name of the SSL server certificate
revocation list (CRL). Certificates listed in this file, if it
exists, will be rejected while attempting to authenticate the
server's certificate. If neither
<xref linkend="libpq-connect-sslcrl"/> nor
<xref linkend="libpq-connect-sslcrldir"/> is set, this setting is
taken as
<filename>~/.postgresql/root.crl</filename>.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-connect-sslcrldir" xreflabel="sslcrldir">
<term><literal>sslcrldir</literal></term>
<listitem>
<para>
This parameter specifies the directory name of the SSL server certificate
revocation list (CRL). Certificates listed in the files in this
directory, if it exists, will be rejected while attempting to
authenticate the server's certificate.
</para>
<para>
The directory needs to be prepared with the
<productname>OpenSSL</productname> command
<literal>openssl rehash</literal> or <literal>c_rehash</literal>. See
its documentation for details.
</para>
<para>
Both <literal>sslcrl</literal> and <literal>sslcrldir</literal> can be
specified together.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-connect-sslsni" xreflabel="sslsni">
<term><literal>sslsni</literal><indexterm><primary>Server Name Indication</primary></indexterm></term>
<listitem>
<para>
If set to 1 (default), libpq sets the TLS extension <quote>Server Name
Indication</quote> (<acronym>SNI</acronym>) on SSL-enabled connections.
By setting this parameter to 0, this is turned off.
</para>
<para>
The Server Name Indication can be used by SSL-aware proxies to route
connections without having to decrypt the SSL stream. (Note that
unless the proxy is aware of the PostgreSQL protocol handshake this
would require setting <literal>sslnegotiation</literal>
to <literal>direct</literal> or <literal>requiredirect</literal>.)
However, <acronym>SNI</acronym> makes the destination host name appear
in cleartext in the network traffic, so it might be undesirable in
some cases.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-connect-requirepeer" xreflabel="requirepeer">
<term><literal>requirepeer</literal></term>
<listitem>
<para>
This parameter specifies the operating-system user name of the
server, for example <literal>requirepeer=postgres</literal>.
When making a Unix-domain socket connection, if this
parameter is set, the client checks at the beginning of the
connection that the server process is running under the specified
user name; if it is not, the connection is aborted with an error.
This parameter can be used to provide server authentication similar
to that available with SSL certificates on TCP/IP connections.
(Note that if the Unix-domain socket is in
<filename>/tmp</filename> or another publicly writable location,
any user could start a server listening there. Use this parameter
to ensure that you are connected to a server run by a trusted user.)
This option is only supported on platforms for which the
<literal>peer</literal> authentication method is implemented; see
<xref linkend="auth-peer"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-connect-ssl-min-protocol-version" xreflabel="ssl_min_protocol_version">
<term><literal>ssl_min_protocol_version</literal></term>
<listitem>
<para>
This parameter specifies the minimum SSL/TLS protocol version to allow
for the connection. Valid values are <literal>TLSv1</literal>,
<literal>TLSv1.1</literal>, <literal>TLSv1.2</literal> and
<literal>TLSv1.3</literal>. The supported protocols depend on the
version of <productname>OpenSSL</productname> used, older versions
not supporting the most modern protocol versions. If not specified,
the default is <literal>TLSv1.2</literal>, which satisfies industry
best practices as of this writing.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-connect-ssl-max-protocol-version" xreflabel="ssl_max_protocol_version">
<term><literal>ssl_max_protocol_version</literal></term>
<listitem>
<para>
This parameter specifies the maximum SSL/TLS protocol version to allow
for the connection. Valid values are <literal>TLSv1</literal>,
<literal>TLSv1.1</literal>, <literal>TLSv1.2</literal> and
<literal>TLSv1.3</literal>. The supported protocols depend on the
version of <productname>OpenSSL</productname> used, older versions
not supporting the most modern protocol versions. If not set, this
parameter is ignored and the connection will use the maximum bound
defined by the backend, if set. Setting the maximum protocol version
is mainly useful for testing or if some component has issues working
with a newer protocol.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-connect-krbsrvname" xreflabel="krbsrvname">
<term><literal>krbsrvname</literal></term>
<listitem>
<para>
Kerberos service name to use when authenticating with GSSAPI.
This must match the service name specified in the server
configuration for Kerberos authentication to succeed. (See also
<xref linkend="gssapi-auth"/>.)
The default value is normally <literal>postgres</literal>,
but that can be changed when
building <productname>PostgreSQL</productname> via
the <option>--with-krb-srvnam</option> option
of <application>configure</application>.
In most environments, this parameter never needs to be changed.
Some Kerberos implementations might require a different service name,
such as Microsoft Active Directory which requires the service name
to be in upper case (<literal>POSTGRES</literal>).
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-connect-gsslib" xreflabel="gsslib">
<term><literal>gsslib</literal></term>
<listitem>
<para>
GSS library to use for GSSAPI authentication.
Currently this is disregarded except on Windows builds that include
both GSSAPI and SSPI support. In that case, set
this to <literal>gssapi</literal> to cause libpq to use the GSSAPI
library for authentication instead of the default SSPI.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-connect-gssdelegation" xreflabel="gssdelegation">
<term><literal>gssdelegation</literal></term>
<listitem>
<para>
Forward (delegate) GSS credentials to the server. The default is
<literal>0</literal> which means credentials will not be forwarded
to the server. Set this to <literal>1</literal> to have credentials
forwarded when possible.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-connect-service" xreflabel="service">
<term><literal>service</literal></term>
<listitem>
<para>
Service name to use for additional parameters. It specifies a service
name in <filename>pg_service.conf</filename> that holds additional connection parameters.
This allows applications to specify only a service name so connection parameters
can be centrally maintained. See <xref linkend="libpq-pgservice"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-connect-target-session-attrs" xreflabel="target_session_attrs">
<term><literal>target_session_attrs</literal></term>
<listitem>
<para>
This option determines whether the session must have certain
properties to be acceptable. It's typically used in combination
with multiple host names to select the first acceptable alternative
among several hosts. There are six modes:
<variablelist>
<varlistentry>
<term><literal>any</literal> (default)</term>
<listitem>
<para>
any successful connection is acceptable
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>read-write</literal></term>
<listitem>
<para>
session must accept read-write transactions by default (that
is, the server must not be in hot standby mode and
the <varname>default_transaction_read_only</varname> parameter
must be <literal>off</literal>)
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>read-only</literal></term>
<listitem>
<para>
session must not accept read-write transactions by default (the
converse)
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>primary</literal></term>
<listitem>
<para>
server must not be in hot standby mode
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>standby</literal></term>
<listitem>
<para>
server must be in hot standby mode
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>prefer-standby</literal></term>
<listitem>
<para>
first try to find a standby server, but if none of the listed
hosts is a standby server, try again in <literal>any</literal>
mode
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-connect-load-balance-hosts" xreflabel="load_balance_hosts">
<term><literal>load_balance_hosts</literal></term>
<listitem>
<para>
Controls the order in which the client tries to connect to the available
hosts and addresses. Once a connection attempt is successful no other
hosts and addresses will be tried. This parameter is typically used in
combination with multiple host names or a DNS record that returns
multiple IPs. This parameter can be used in combination with
<xref linkend="libpq-connect-target-session-attrs"/>
to, for example, load balance over standby servers only. Once successfully
connected, subsequent queries on the returned connection will all be
sent to the same server. There are currently two modes:
<variablelist>
<varlistentry>
<term><literal>disable</literal> (default)</term>
<listitem>
<para>
No load balancing across hosts is performed. Hosts are tried in
the order in which they are provided and addresses are tried in
the order they are received from DNS or a hosts file.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>random</literal></term>
<listitem>
<para>
Hosts and addresses are tried in random order. This value is mostly
useful when opening multiple connections at the same time, possibly
from different machines. This way connections can be load balanced
across multiple <productname>PostgreSQL</productname> servers.
</para>
<para>
While random load balancing, due to its random nature, will almost
never result in a completely uniform distribution, it statistically
gets quite close. One important aspect here is that this algorithm
uses two levels of random choices: First the hosts
will be resolved in random order. Then secondly, before resolving
the next host, all resolved addresses for the current host will be
tried in random order. This behaviour can skew the amount of
connections each node gets greatly in certain cases, for instance
when some hosts resolve to more addresses than others. But such a
skew can also be used on purpose, e.g. to increase the number of
connections a larger server gets by providing its hostname multiple
times in the host string.
</para>
<para>
When using this value it's recommended to also configure a reasonable
value for <xref linkend="libpq-connect-connect-timeout"/>. Because then,
if one of the nodes that are used for load balancing is not responding,
a new node will be tried.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</sect2>
</sect1>
<sect1 id="libpq-status">
<title>Connection Status Functions</title>
<para>
These functions can be used to interrogate the status
of an existing database connection object.
</para>
<tip>
<para>
<indexterm><primary>libpq-fe.h</primary></indexterm>
<indexterm><primary>libpq-int.h</primary></indexterm>
<application>libpq</application> application programmers should be careful to
maintain the <structname>PGconn</structname> abstraction. Use the accessor
functions described below to get at the contents of <structname>PGconn</structname>.
Reference to internal <structname>PGconn</structname> fields using
<filename>libpq-int.h</filename> is not recommended because they are subject to change
in the future.
</para>
</tip>
<para>
The following functions return parameter values established at connection.
These values are fixed for the life of the connection. If a multi-host
connection string is used, the values of <xref linkend="libpq-PQhost"/>,
<xref linkend="libpq-PQport"/>, and <xref linkend="libpq-PQpass"/> can change if a new connection
is established using the same <structname>PGconn</structname> object. Other values
are fixed for the lifetime of the <structname>PGconn</structname> object.
<variablelist>
<varlistentry id="libpq-PQdb">
<term><function>PQdb</function><indexterm><primary>PQdb</primary></indexterm></term>
<listitem>
<para>
Returns the database name of the connection.
<synopsis>
char *PQdb(const PGconn *conn);
</synopsis>
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQuser">
<term><function>PQuser</function><indexterm><primary>PQuser</primary></indexterm></term>
<listitem>
<para>
Returns the user name of the connection.
<synopsis>
char *PQuser(const PGconn *conn);
</synopsis>
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQpass">
<term><function>PQpass</function><indexterm><primary>PQpass</primary></indexterm></term>
<listitem>
<para>
Returns the password of the connection.
<synopsis>
char *PQpass(const PGconn *conn);
</synopsis>
</para>
<para>
<xref linkend="libpq-PQpass"/> will return either the password specified
in the connection parameters, or if there was none and the password
was obtained from the <link linkend="libpq-pgpass">password
file</link>, it will return that. In the latter case,
if multiple hosts were specified in the connection parameters, it is
not possible to rely on the result of <xref linkend="libpq-PQpass"/> until
the connection is established. The status of the connection can be
checked using the function <xref linkend="libpq-PQstatus"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQhost">
<term><function>PQhost</function><indexterm><primary>PQhost</primary></indexterm></term>
<listitem>
<para>
Returns the server host name of the active connection.
This can be a host name, an IP address, or a directory path if the
connection is via Unix socket. (The path case can be distinguished
because it will always be an absolute path, beginning
with <literal>/</literal>.)
<synopsis>
char *PQhost(const PGconn *conn);
</synopsis>
</para>
<para>
If the connection parameters specified both <literal>host</literal> and
<literal>hostaddr</literal>, then <xref linkend="libpq-PQhost"/> will
return the <literal>host</literal> information. If only
<literal>hostaddr</literal> was specified, then that is returned.
If multiple hosts were specified in the connection parameters,
<xref linkend="libpq-PQhost"/> returns the host actually connected to.
</para>
<para>
<xref linkend="libpq-PQhost"/> returns <symbol>NULL</symbol> if the
<parameter>conn</parameter> argument is <symbol>NULL</symbol>.
Otherwise, if there is an error producing the host information (perhaps
if the connection has not been fully established or there was an
error), it returns an empty string.
</para>
<para>
If multiple hosts were specified in the connection parameters, it is
not possible to rely on the result of <xref linkend="libpq-PQhost"/> until
the connection is established. The status of the connection can be
checked using the function <xref linkend="libpq-PQstatus"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQhostaddr">
<term><function>PQhostaddr</function><indexterm><primary>PQhostaddr</primary></indexterm></term>
<listitem>
<para>
Returns the server IP address of the active connection.
This can be the address that a host name resolved to,
or an IP address provided through the <literal>hostaddr</literal>
parameter.
<synopsis>
char *PQhostaddr(const PGconn *conn);
</synopsis>
</para>
<para>
<xref linkend="libpq-PQhostaddr"/> returns <symbol>NULL</symbol> if the
<parameter>conn</parameter> argument is <symbol>NULL</symbol>.
Otherwise, if there is an error producing the host information
(perhaps if the connection has not been fully established or
there was an error), it returns an empty string.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQport">
<term><function>PQport</function><indexterm><primary>PQport</primary></indexterm></term>
<listitem>
<para>
Returns the port of the active connection.
<synopsis>
char *PQport(const PGconn *conn);
</synopsis>
</para>
<para>
If multiple ports were specified in the connection parameters,
<xref linkend="libpq-PQport"/> returns the port actually connected to.
</para>
<para>
<xref linkend="libpq-PQport"/> returns <symbol>NULL</symbol> if the
<parameter>conn</parameter> argument is <symbol>NULL</symbol>.
Otherwise, if there is an error producing the port information (perhaps
if the connection has not been fully established or there was an
error), it returns an empty string.
</para>
<para>
If multiple ports were specified in the connection parameters, it is
not possible to rely on the result of <xref linkend="libpq-PQport"/> until
the connection is established. The status of the connection can be
checked using the function <xref linkend="libpq-PQstatus"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQtty">
<term><function>PQtty</function><indexterm><primary>PQtty</primary></indexterm></term>
<listitem>
<para>
This function no longer does anything, but it remains for backwards
compatibility. The function always return an empty string, or
<symbol>NULL</symbol> if the <parameter>conn</parameter> argument is
<symbol>NULL</symbol>.
<synopsis>
char *PQtty(const PGconn *conn);
</synopsis>
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQoptions">
<term><function>PQoptions</function><indexterm><primary>PQoptions</primary></indexterm></term>
<listitem>
<para>
Returns the command-line options passed in the connection request.
<synopsis>
char *PQoptions(const PGconn *conn);
</synopsis>
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
<para>
The following functions return status data that can change as operations
are executed on the <structname>PGconn</structname> object.
<variablelist>
<varlistentry id="libpq-PQstatus">
<term><function>PQstatus</function><indexterm><primary>PQstatus</primary></indexterm></term>
<listitem>
<para>
Returns the status of the connection.
<synopsis>
ConnStatusType PQstatus(const PGconn *conn);
</synopsis>
</para>
<para>
The status can be one of a number of values. However, only two of
these are seen outside of an asynchronous connection procedure:
<literal>CONNECTION_OK</literal> and
<literal>CONNECTION_BAD</literal>. A good connection to the database
has the status <literal>CONNECTION_OK</literal>. A failed
connection attempt is signaled by status
<literal>CONNECTION_BAD</literal>. Ordinarily, an OK status will
remain so until <xref linkend="libpq-PQfinish"/>, but a communications
failure might result in the status changing to
<literal>CONNECTION_BAD</literal> prematurely. In that case the
application could try to recover by calling
<xref linkend="libpq-PQreset"/>.
</para>
<para>
See the entry for <xref linkend="libpq-PQconnectStartParams"/>, <function>PQconnectStart</function>
and <function>PQconnectPoll</function> with regards to other status codes that
might be returned.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQtransactionStatus">
<term><function>PQtransactionStatus</function><indexterm><primary>PQtransactionStatus</primary></indexterm></term>
<listitem>
<para>
Returns the current in-transaction status of the server.
<synopsis>
PGTransactionStatusType PQtransactionStatus(const PGconn *conn);
</synopsis>
The status can be <literal>PQTRANS_IDLE</literal> (currently idle),
<literal>PQTRANS_ACTIVE</literal> (a command is in progress),
<literal>PQTRANS_INTRANS</literal> (idle, in a valid transaction block),
or <literal>PQTRANS_INERROR</literal> (idle, in a failed transaction block).
<literal>PQTRANS_UNKNOWN</literal> is reported if the connection is bad.
<literal>PQTRANS_ACTIVE</literal> is reported only when a query
has been sent to the server and not yet completed.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQparameterStatus">
<term><function>PQparameterStatus</function><indexterm><primary>PQparameterStatus</primary></indexterm></term>
<listitem>
<para>
Looks up a current parameter setting of the server.
<synopsis>
const char *PQparameterStatus(const PGconn *conn, const char *paramName);
</synopsis>
Certain parameter values are reported by the server automatically at
connection startup or whenever their values change.
<xref linkend="libpq-PQparameterStatus"/> can be used to interrogate these settings.
It returns the current value of a parameter if known, or <symbol>NULL</symbol>
if the parameter is not known.
</para>
<para>
Parameters reported as of the current release include:
<simplelist type="vert" columns="2">
<member><varname>application_name</varname></member>
<member><varname>client_encoding</varname></member>
<member><varname>DateStyle</varname></member>
<member><varname>default_transaction_read_only</varname></member>
<member><varname>in_hot_standby</varname></member>
<member><varname>integer_datetimes</varname></member>
<member><varname>IntervalStyle</varname></member>
<member><varname>is_superuser</varname></member>
<member><varname>scram_iterations</varname></member>
<member><varname>server_encoding</varname></member>
<member><varname>server_version</varname></member>
<member><varname>session_authorization</varname></member>
<member><varname>standard_conforming_strings</varname></member>
<member><varname>TimeZone</varname></member>
</simplelist>
(<varname>default_transaction_read_only</varname> and
<varname>in_hot_standby</varname> were not reported by releases before
14; <varname>scram_iterations</varname> was not reported by releases
before 16.)
Note that
<varname>server_version</varname>,
<varname>server_encoding</varname> and
<varname>integer_datetimes</varname>
cannot change after startup.
</para>
<para>
If no value for <varname>standard_conforming_strings</varname> is reported,
applications can assume it is <literal>off</literal>, that is, backslashes
are treated as escapes in string literals. Also, the presence of
this parameter can be taken as an indication that the escape string
syntax (<literal>E'...'</literal>) is accepted.
</para>
<para>
Although the returned pointer is declared <literal>const</literal>, it in fact
points to mutable storage associated with the <literal>PGconn</literal> structure.
It is unwise to assume the pointer will remain valid across queries.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQprotocolVersion">
<term><function>PQprotocolVersion</function><indexterm><primary>PQprotocolVersion</primary></indexterm></term>
<listitem>
<para>
Interrogates the frontend/backend protocol being used.
<synopsis>
int PQprotocolVersion(const PGconn *conn);
</synopsis>
Applications might wish to use this function to determine whether certain
features are supported. Currently, the possible values are 3
(3.0 protocol), or zero (connection bad). The protocol version will
not change after connection startup is complete, but it could
theoretically change during a connection reset. The 3.0 protocol is
supported by <productname>PostgreSQL</productname> server versions 7.4
and above.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQserverVersion">
<term><function>PQserverVersion</function><indexterm><primary>PQserverVersion</primary></indexterm></term>
<listitem>
<para>
Returns an integer representing the server version.
<synopsis>
int PQserverVersion(const PGconn *conn);
</synopsis>
</para>
<para>
Applications might use this function to determine the version of the
database server they are connected to. The result is formed by
multiplying the server's major version number by 10000 and adding
the minor version number. For example, version 10.1 will be
returned as 100001, and version 11.0 will be returned as 110000.
Zero is returned if the connection is bad.
</para>
<para>
Prior to major version 10, <productname>PostgreSQL</productname> used
three-part version numbers in which the first two parts together
represented the major version. For those
versions, <xref linkend="libpq-PQserverVersion"/> uses two digits for each
part; for example version 9.1.5 will be returned as 90105, and
version 9.2.0 will be returned as 90200.
</para>
<para>
Therefore, for purposes of determining feature compatibility,
applications should divide the result of <xref linkend="libpq-PQserverVersion"/>
by 100 not 10000 to determine a logical major version number.
In all release series, only the last two digits differ between
minor releases (bug-fix releases).
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQerrorMessage">
<term>
<function>PQerrorMessage</function><indexterm><primary>PQerrorMessage</primary></indexterm>
<indexterm><primary>error message</primary><secondary>in <structname>PGconn</structname></secondary></indexterm>
</term>
<listitem>
<para>
Returns the error message most recently generated by
an operation on the connection.
<synopsis>
char *PQerrorMessage(const PGconn *conn);
</synopsis>
</para>
<para>
Nearly all <application>libpq</application> functions will set a message for
<xref linkend="libpq-PQerrorMessage"/> if they fail. Note that by
<application>libpq</application> convention, a nonempty
<xref linkend="libpq-PQerrorMessage"/> result can consist of multiple lines,
and will include a trailing newline. The caller should not free
the result directly. It will be freed when the associated
<structname>PGconn</structname> handle is passed to
<xref linkend="libpq-PQfinish"/>. The result string should not be
expected to remain the same across operations on the
<literal>PGconn</literal> structure.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQsocket">
<term><function>PQsocket</function><indexterm><primary>PQsocket</primary></indexterm></term>
<listitem>
<para>
Obtains the file descriptor number of the connection socket to
the server. A valid descriptor will be greater than or equal
to 0; a result of -1 indicates that no server connection is
currently open. (This will not change during normal operation,
but could change during connection setup or reset.)
<synopsis>
int PQsocket(const PGconn *conn);
</synopsis>
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQbackendPID">
<term><function>PQbackendPID</function><indexterm><primary>PQbackendPID</primary></indexterm></term>
<listitem>
<para>
Returns the process <acronym>ID</acronym> (PID)<indexterm>
<primary>PID</primary>
<secondary>determining PID of server process</secondary>
<tertiary>in libpq</tertiary>
</indexterm>
of the backend process handling this connection.
<synopsis>
int PQbackendPID(const PGconn *conn);
</synopsis>
</para>
<para>
The backend <acronym>PID</acronym> is useful for debugging
purposes and for comparison to <command>NOTIFY</command>
messages (which include the <acronym>PID</acronym> of the
notifying backend process). Note that the
<acronym>PID</acronym> belongs to a process executing on the
database server host, not the local host!
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQconnectionNeedsPassword">
<term><function>PQconnectionNeedsPassword</function><indexterm><primary>PQconnectionNeedsPassword</primary></indexterm></term>
<listitem>
<para>
Returns true (1) if the connection authentication method
required a password, but none was available.
Returns false (0) if not.
<synopsis>
int PQconnectionNeedsPassword(const PGconn *conn);
</synopsis>
</para>
<para>
This function can be applied after a failed connection attempt
to decide whether to prompt the user for a password.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQconnectionUsedPassword">
<term><function>PQconnectionUsedPassword</function><indexterm><primary>PQconnectionUsedPassword</primary></indexterm></term>
<listitem>
<para>
Returns true (1) if the connection authentication method
used a password. Returns false (0) if not.
<synopsis>
int PQconnectionUsedPassword(const PGconn *conn);
</synopsis>
</para>
<para>
This function can be applied after either a failed or successful
connection attempt to detect whether the server demanded a password.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQconnectionUsedGSSAPI">
<term><function>PQconnectionUsedGSSAPI</function><indexterm><primary>PQconnectionUsedGSSAPI</primary></indexterm></term>
<listitem>
<para>
Returns true (1) if the connection authentication method
used GSSAPI. Returns false (0) if not.
<synopsis>
int PQconnectionUsedGSSAPI(const PGconn *conn);
</synopsis>
</para>
<para>
This function can be applied to detect whether the connection was
authenticated with GSSAPI.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
<para>
The following functions return information related to SSL. This information
usually doesn't change after a connection is established.
<variablelist>
<varlistentry id="libpq-PQsslInUse">
<term><function>PQsslInUse</function><indexterm><primary>PQsslInUse</primary></indexterm></term>
<listitem>
<para>
Returns true (1) if the connection uses SSL, false (0) if not.
<synopsis>
int PQsslInUse(const PGconn *conn);
</synopsis>
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQsslAttribute">
<term><function>PQsslAttribute</function><indexterm><primary>PQsslAttribute</primary></indexterm></term>
<listitem>
<para>
Returns SSL-related information about the connection.
<synopsis>
const char *PQsslAttribute(const PGconn *conn, const char *attribute_name);
</synopsis>
</para>
<para>
The list of available attributes varies depending on the SSL library
being used and the type of connection. Returns NULL if the connection
does not use SSL or the specified attribute name is not defined for the
library in use.
</para>
<para>
The following attributes are commonly available:
<variablelist>
<varlistentry>
<term><literal>library</literal></term>
<listitem>
<para>
Name of the SSL implementation in use. (Currently, only
<literal>"OpenSSL"</literal> is implemented)
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>protocol</literal></term>
<listitem>
<para>
SSL/TLS version in use. Common values
are <literal>"TLSv1"</literal>, <literal>"TLSv1.1"</literal>
and <literal>"TLSv1.2"</literal>, but an implementation may
return other strings if some other protocol is used.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>key_bits</literal></term>
<listitem>
<para>
Number of key bits used by the encryption algorithm.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>cipher</literal></term>
<listitem>
<para>
A short name of the ciphersuite used, e.g.,
<literal>"DHE-RSA-DES-CBC3-SHA"</literal>. The names are specific
to each SSL implementation.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>compression</literal></term>
<listitem>
<para>
Returns "on" if SSL compression is in use, else it returns "off".
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>alpn</literal></term>
<listitem>
<para>
Application protocol selected by the TLS Application-Layer
Protocol Negotiation (ALPN) extension. The only protocol
supported by libpq is <literal>TBD-pgsql</literal>, so this is
mainly useful for checking whether the server supported ALPN or
not. Empty string if ALPN was not used.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
<para>
As a special case, the <literal>library</literal> attribute may be
queried without a connection by passing NULL as
the <literal>conn</literal> argument. The result will be the default
SSL library name, or NULL if <application>libpq</application> was
compiled without any SSL support. (Prior
to <productname>PostgreSQL</productname> version 15, passing NULL as
the <literal>conn</literal> argument always resulted in NULL.
Client programs needing to differentiate between the newer and older
implementations of this case may check the
<literal>LIBPQ_HAS_SSL_LIBRARY_DETECTION</literal> feature macro.)
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQsslAttributeNames">
<term><function>PQsslAttributeNames</function><indexterm><primary>PQsslAttributeNames</primary></indexterm></term>
<listitem>
<para>
Returns an array of SSL attribute names that can be used
in <function>PQsslAttribute()</function>.
The array is terminated by a NULL pointer.
<synopsis>
const char * const * PQsslAttributeNames(const PGconn *conn);
</synopsis>
</para>
<para>
If <literal>conn</literal> is NULL, the attributes available for the
default SSL library are returned, or an empty list
if <application>libpq</application> was compiled without any SSL
support. If <literal>conn</literal> is not NULL, the attributes
available for the SSL library in use for the connection are returned,
or an empty list if the connection is not encrypted.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQsslStruct">
<term><function>PQsslStruct</function><indexterm><primary>PQsslStruct</primary></indexterm></term>
<listitem>
<para>
Returns a pointer to an SSL-implementation-specific object describing
the connection. Returns NULL if the connection is not encrypted
or the requested type of object is not available from the connection's
SSL implementation.
<synopsis>
void *PQsslStruct(const PGconn *conn, const char *struct_name);
</synopsis>
</para>
<para>
The struct(s) available depend on the SSL implementation in use.
For <productname>OpenSSL</productname>, there is one struct,
available under the name <literal>OpenSSL</literal>,
and it returns a pointer to
<productname>OpenSSL</productname>'s <literal>SSL</literal> struct.
To use this function, code along the following lines could be used:
<programlisting><![CDATA[
#include <libpq-fe.h>
#include <openssl/ssl.h>
...
SSL *ssl;
dbconn = PQconnectdb(...);
...
ssl = PQsslStruct(dbconn, "OpenSSL");
if (ssl)
{
/* use OpenSSL functions to access ssl */
}
]]></programlisting>
</para>
<para>
This structure can be used to verify encryption levels, check server
certificates, and more. Refer to the <productname>OpenSSL</productname>
documentation for information about this structure.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQgetssl">
<term><function>PQgetssl</function><indexterm><primary>PQgetssl</primary></indexterm></term>
<listitem>
<para>
<indexterm><primary>SSL</primary><secondary sortas="libpq">in libpq</secondary></indexterm>
Returns the SSL structure used in the connection, or NULL
if SSL is not in use.
<synopsis>
void *PQgetssl(const PGconn *conn);
</synopsis>
</para>
<para>
This function is equivalent to <literal>PQsslStruct(conn, "OpenSSL")</literal>. It should
not be used in new applications, because the returned struct is
specific to <productname>OpenSSL</productname> and will not be
available if another <acronym>SSL</acronym> implementation is used.
To check if a connection uses SSL, call
<xref linkend="libpq-PQsslInUse"/> instead, and for more details about the
connection, use <xref linkend="libpq-PQsslAttribute"/>.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</sect1>
<sect1 id="libpq-exec">
<title>Command Execution Functions</title>
<para>
Once a connection to a database server has been successfully
established, the functions described here are used to perform
SQL queries and commands.
</para>
<sect2 id="libpq-exec-main">
<title>Main Functions</title>
<para>
<variablelist>
<varlistentry id="libpq-PQexec">
<term><function>PQexec</function><indexterm><primary>PQexec</primary></indexterm></term>
<listitem>
<para>
Submits a command to the server and waits for the result.
<synopsis>
PGresult *PQexec(PGconn *conn, const char *command);
</synopsis>
</para>
<para>
Returns a <structname>PGresult</structname> pointer or possibly a null
pointer. A non-null pointer will generally be returned except in
out-of-memory conditions or serious errors such as inability to send
the command to the server. The <xref linkend="libpq-PQresultStatus"/> function
should be called to check the return value for any errors (including
the value of a null pointer, in which case it will return
<symbol>PGRES_FATAL_ERROR</symbol>). Use
<xref linkend="libpq-PQerrorMessage"/> to get more information about such
errors.
</para>
</listitem>
</varlistentry>
</variablelist>
The command string can include multiple SQL commands
(separated by semicolons). Multiple queries sent in a single
<xref linkend="libpq-PQexec"/> call are processed in a single transaction, unless
there are explicit <command>BEGIN</command>/<command>COMMIT</command>
commands included in the query string to divide it into multiple
transactions. (See <xref linkend="protocol-flow-multi-statement"/>
for more details about how the server handles multi-query strings.)
Note however that the returned
<structname>PGresult</structname> structure describes only the result
of the last command executed from the string. Should one of the
commands fail, processing of the string stops with it and the returned
<structname>PGresult</structname> describes the error condition.
</para>
<para>
<variablelist>
<varlistentry id="libpq-PQexecParams">
<term><function>PQexecParams</function><indexterm><primary>PQexecParams</primary></indexterm></term>
<listitem>
<para>
Submits a command to the server and waits for the result,
with the ability to pass parameters separately from the SQL
command text.
<synopsis>
PGresult *PQexecParams(PGconn *conn,
const char *command,
int nParams,
const Oid *paramTypes,
const char * const *paramValues,
const int *paramLengths,
const int *paramFormats,
int resultFormat);
</synopsis>
</para>
<para>
<xref linkend="libpq-PQexecParams"/> is like <xref linkend="libpq-PQexec"/>, but offers additional
functionality: parameter values can be specified separately from the command
string proper, and query results can be requested in either text or binary
format.
</para>
<para>
The function arguments are:
<variablelist>
<varlistentry>
<term><parameter>conn</parameter></term>
<listitem>
<para>
The connection object to send the command through.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><parameter>command</parameter></term>
<listitem>
<para>
The SQL command string to be executed. If parameters are used,
they are referred to in the command string as <literal>$1</literal>,
<literal>$2</literal>, etc.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><parameter>nParams</parameter></term>
<listitem>
<para>
The number of parameters supplied; it is the length of the arrays
<parameter>paramTypes[]</parameter>, <parameter>paramValues[]</parameter>,
<parameter>paramLengths[]</parameter>, and <parameter>paramFormats[]</parameter>. (The
array pointers can be <symbol>NULL</symbol> when <parameter>nParams</parameter>
is zero.)
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><parameter>paramTypes[]</parameter></term>
<listitem>
<para>
Specifies, by OID, the data types to be assigned to the
parameter symbols. If <parameter>paramTypes</parameter> is
<symbol>NULL</symbol>, or any particular element in the array
is zero, the server infers a data type for the parameter symbol
in the same way it would do for an untyped literal string.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><parameter>paramValues[]</parameter></term>
<listitem>
<para>
Specifies the actual values of the parameters. A null pointer
in this array means the corresponding parameter is null;
otherwise the pointer points to a zero-terminated text string
(for text format) or binary data in the format expected by the
server (for binary format).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><parameter>paramLengths[]</parameter></term>
<listitem>
<para>
Specifies the actual data lengths of binary-format parameters.
It is ignored for null parameters and text-format parameters.
The array pointer can be null when there are no binary parameters.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><parameter>paramFormats[]</parameter></term>
<listitem>
<para>
Specifies whether parameters are text (put a zero in the
array entry for the corresponding parameter) or binary (put
a one in the array entry for the corresponding parameter).
If the array pointer is null then all parameters are presumed
to be text strings.
</para>
<para>
Values passed in binary format require knowledge of
the internal representation expected by the backend.
For example, integers must be passed in network byte
order. Passing <type>numeric</type> values requires
knowledge of the server storage format, as implemented
in
<filename>src/backend/utils/adt/numeric.c::numeric_send()</filename> and
<filename>src/backend/utils/adt/numeric.c::numeric_recv()</filename>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><parameter>resultFormat</parameter></term>
<listitem>
<para>
Specify zero to obtain results in text format, or one to obtain
results in binary format. (There is not currently a provision
to obtain different result columns in different formats,
although that is possible in the underlying protocol.)
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
<para>
The primary advantage of <xref linkend="libpq-PQexecParams"/> over
<xref linkend="libpq-PQexec"/> is that parameter values can be separated from the
command string, thus avoiding the need for tedious and error-prone
quoting and escaping.
</para>
<para>
Unlike <xref linkend="libpq-PQexec"/>, <xref linkend="libpq-PQexecParams"/> allows at most
one SQL command in the given string. (There can be semicolons in it,
but not more than one nonempty command.) This is a limitation of the
underlying protocol, but has some usefulness as an extra defense against
SQL-injection attacks.
</para>
<tip>
<para>
Specifying parameter types via OIDs is tedious, particularly if you prefer
not to hard-wire particular OID values into your program. However, you can
avoid doing so even in cases where the server by itself cannot determine the
type of the parameter, or chooses a different type than you want. In the
SQL command text, attach an explicit cast to the parameter symbol to show what
data type you will send. For example:
<programlisting>
SELECT * FROM mytable WHERE x = $1::bigint;
</programlisting>
This forces parameter <literal>$1</literal> to be treated as <type>bigint</type>, whereas
by default it would be assigned the same type as <literal>x</literal>. Forcing the
parameter type decision, either this way or by specifying a numeric type OID,
is strongly recommended when sending parameter values in binary format, because
binary format has less redundancy than text format and so there is less chance
that the server will detect a type mismatch mistake for you.
</para>
</tip>
<para>
<variablelist>
<varlistentry id="libpq-PQprepare">
<term><function>PQprepare</function><indexterm><primary>PQprepare</primary></indexterm></term>
<listitem>
<para>
Submits a request to create a prepared statement with the
given parameters, and waits for completion.
<synopsis>
PGresult *PQprepare(PGconn *conn,
const char *stmtName,
const char *query,
int nParams,
const Oid *paramTypes);
</synopsis>
</para>
<para>
<xref linkend="libpq-PQprepare"/> creates a prepared statement for later
execution with <xref linkend="libpq-PQexecPrepared"/>. This feature allows
commands to be executed repeatedly without being parsed and
planned each time; see <xref linkend="sql-prepare"/> for details.
</para>
<para>
The function creates a prepared statement named
<parameter>stmtName</parameter> from the <parameter>query</parameter> string, which
must contain a single SQL command. <parameter>stmtName</parameter> can be
<literal>""</literal> to create an unnamed statement, in which case any
pre-existing unnamed statement is automatically replaced; otherwise
it is an error if the statement name is already defined in the
current session. If any parameters are used, they are referred
to in the query as <literal>$1</literal>, <literal>$2</literal>, etc.
<parameter>nParams</parameter> is the number of parameters for which types
are pre-specified in the array <parameter>paramTypes[]</parameter>. (The
array pointer can be <symbol>NULL</symbol> when
<parameter>nParams</parameter> is zero.) <parameter>paramTypes[]</parameter>
specifies, by OID, the data types to be assigned to the parameter
symbols. If <parameter>paramTypes</parameter> is <symbol>NULL</symbol>,
or any particular element in the array is zero, the server assigns
a data type to the parameter symbol in the same way it would do
for an untyped literal string. Also, the query can use parameter
symbols with numbers higher than <parameter>nParams</parameter>; data types
will be inferred for these symbols as well. (See
<xref linkend="libpq-PQdescribePrepared"/> for a means to find out
what data types were inferred.)
</para>
<para>
As with <xref linkend="libpq-PQexec"/>, the result is normally a
<structname>PGresult</structname> object whose contents indicate
server-side success or failure. A null result indicates
out-of-memory or inability to send the command at all. Use
<xref linkend="libpq-PQerrorMessage"/> to get more information about
such errors.
</para>
</listitem>
</varlistentry>
</variablelist>
Prepared statements for use with <xref linkend="libpq-PQexecPrepared"/> can also
be created by executing SQL <xref linkend="sql-prepare"/>
statements.
</para>
<para>
<variablelist>
<varlistentry id="libpq-PQexecPrepared">
<term><function>PQexecPrepared</function><indexterm><primary>PQexecPrepared</primary></indexterm></term>
<listitem>
<para>
Sends a request to execute a prepared statement with given
parameters, and waits for the result.
<synopsis>
PGresult *PQexecPrepared(PGconn *conn,
const char *stmtName,
int nParams,
const char * const *paramValues,
const int *paramLengths,
const int *paramFormats,
int resultFormat);
</synopsis>
</para>
<para>
<xref linkend="libpq-PQexecPrepared"/> is like <xref linkend="libpq-PQexecParams"/>,
but the command to be executed is specified by naming a
previously-prepared statement, instead of giving a query string.
This feature allows commands that will be used repeatedly to be
parsed and planned just once, rather than each time they are
executed. The statement must have been prepared previously in
the current session.
</para>
<para>
The parameters are identical to <xref linkend="libpq-PQexecParams"/>, except that the
name of a prepared statement is given instead of a query string, and the
<parameter>paramTypes[]</parameter> parameter is not present (it is not needed since
the prepared statement's parameter types were determined when it was created).
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQdescribePrepared">
<term><function>PQdescribePrepared</function><indexterm><primary>PQdescribePrepared</primary></indexterm></term>
<listitem>
<para>
Submits a request to obtain information about the specified
prepared statement, and waits for completion.
<synopsis>
PGresult *PQdescribePrepared(PGconn *conn, const char *stmtName);
</synopsis>
</para>
<para>
<xref linkend="libpq-PQdescribePrepared"/> allows an application to obtain
information about a previously prepared statement.
</para>
<para>
<parameter>stmtName</parameter> can be <literal>""</literal> or <symbol>NULL</symbol> to reference
the unnamed statement, otherwise it must be the name of an existing
prepared statement. On success, a <structname>PGresult</structname> with
status <literal>PGRES_COMMAND_OK</literal> is returned. The
functions <xref linkend="libpq-PQnparams"/> and
<xref linkend="libpq-PQparamtype"/> can be applied to this
<structname>PGresult</structname> to obtain information about the parameters
of the prepared statement, and the functions
<xref linkend="libpq-PQnfields"/>, <xref linkend="libpq-PQfname"/>,
<xref linkend="libpq-PQftype"/>, etc. provide information about the
result columns (if any) of the statement.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQdescribePortal">
<term><function>PQdescribePortal</function><indexterm><primary>PQdescribePortal</primary></indexterm></term>
<listitem>
<para>
Submits a request to obtain information about the specified
portal, and waits for completion.
<synopsis>
PGresult *PQdescribePortal(PGconn *conn, const char *portalName);
</synopsis>
</para>
<para>
<xref linkend="libpq-PQdescribePortal"/> allows an application to obtain
information about a previously created portal.
(<application>libpq</application> does not provide any direct access to
portals, but you can use this function to inspect the properties
of a cursor created with a <command>DECLARE CURSOR</command> SQL command.)
</para>
<para>
<parameter>portalName</parameter> can be <literal>""</literal> or <symbol>NULL</symbol> to reference
the unnamed portal, otherwise it must be the name of an existing
portal. On success, a <structname>PGresult</structname> with status
<literal>PGRES_COMMAND_OK</literal> is returned. The functions
<xref linkend="libpq-PQnfields"/>, <xref linkend="libpq-PQfname"/>,
<xref linkend="libpq-PQftype"/>, etc. can be applied to the
<structname>PGresult</structname> to obtain information about the result
columns (if any) of the portal.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQclosePrepared">
<term><function>PQclosePrepared</function><indexterm><primary>PQclosePrepared</primary></indexterm></term>
<listitem>
<para>
Submits a request to close the specified prepared statement, and waits
for completion.
<synopsis>
PGresult *PQclosePrepared(PGconn *conn, const char *stmtName);
</synopsis>
</para>
<para>
<xref linkend="libpq-PQclosePrepared"/> allows an application to close
a previously prepared statement. Closing a statement releases all
of its associated resources on the server and allows its name to be
reused.
</para>
<para>
<parameter>stmtName</parameter> can be <literal>""</literal> or
<symbol>NULL</symbol> to reference the unnamed statement. It is fine
if no statement exists with this name, in that case the operation is a
no-op. On success, a <structname>PGresult</structname> with
status <literal>PGRES_COMMAND_OK</literal> is returned.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQclosePortal">
<term><function>PQclosePortal</function><indexterm><primary>PQclosePortal</primary></indexterm></term>
<listitem>
<para>
Submits a request to close the specified portal, and waits for
completion.
<synopsis>
PGresult *PQclosePortal(PGconn *conn, const char *portalName);
</synopsis>
</para>
<para>
<xref linkend="libpq-PQclosePortal"/> allows an application to trigger
a close of a previously created portal. Closing a portal releases all
of its associated resources on the server and allows its name to be
reused. (<application>libpq</application> does not provide any
direct access to portals, but you can use this function to close a
cursor created with a <command>DECLARE CURSOR</command> SQL command.)
</para>
<para>
<parameter>portalName</parameter> can be <literal>""</literal> or
<symbol>NULL</symbol> to reference the unnamed portal. It is fine
if no portal exists with this name, in that case the operation is a
no-op. On success, a <structname>PGresult</structname> with status
<literal>PGRES_COMMAND_OK</literal> is returned.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
<para>
The <structname>PGresult</structname><indexterm><primary>PGresult</primary></indexterm>
structure encapsulates the result returned by the server.
<application>libpq</application> application programmers should be
careful to maintain the <structname>PGresult</structname> abstraction.
Use the accessor functions below to get at the contents of
<structname>PGresult</structname>. Avoid directly referencing the
fields of the <structname>PGresult</structname> structure because they
are subject to change in the future.
<variablelist>
<varlistentry id="libpq-PQresultStatus">
<term><function>PQresultStatus</function><indexterm><primary>PQresultStatus</primary></indexterm></term>
<listitem>
<para>
Returns the result status of the command.
<synopsis>
ExecStatusType PQresultStatus(const PGresult *res);
</synopsis>
</para>
<para>
<xref linkend="libpq-PQresultStatus"/> can return one of the following values:
<variablelist>
<varlistentry id="libpq-pgres-empty-query">
<term><literal>PGRES_EMPTY_QUERY</literal></term>
<listitem>
<para>
The string sent to the server was empty.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-pgres-command-ok">
<term><literal>PGRES_COMMAND_OK</literal></term>
<listitem>
<para>
Successful completion of a command returning no data.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-pgres-tuples-ok">
<term><literal>PGRES_TUPLES_OK</literal></term>
<listitem>
<para>
Successful completion of a command returning data (such as
a <command>SELECT</command> or <command>SHOW</command>).
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-pgres-copy-out">
<term><literal>PGRES_COPY_OUT</literal></term>
<listitem>
<para>
Copy Out (from server) data transfer started.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-pgres-copy-in">
<term><literal>PGRES_COPY_IN</literal></term>
<listitem>
<para>
Copy In (to server) data transfer started.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-pgres-bad-response">
<term><literal>PGRES_BAD_RESPONSE</literal></term>
<listitem>
<para>
The server's response was not understood.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-pgres-nonfatal-error">
<term><literal>PGRES_NONFATAL_ERROR</literal></term>
<listitem>
<para>
A nonfatal error (a notice or warning) occurred.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-pgres-fatal-error">
<term><literal>PGRES_FATAL_ERROR</literal></term>
<listitem>
<para>
A fatal error occurred.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-pgres-copy-both">
<term><literal>PGRES_COPY_BOTH</literal></term>
<listitem>
<para>
Copy In/Out (to and from server) data transfer started. This
feature is currently used only for streaming replication,
so this status should not occur in ordinary applications.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-pgres-single-tuple">
<term><literal>PGRES_SINGLE_TUPLE</literal></term>
<listitem>
<para>
The <structname>PGresult</structname> contains a single result tuple
from the current command. This status occurs only when
single-row mode has been selected for the query
(see <xref linkend="libpq-single-row-mode"/>).
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-pgres-tuples-chunk">
<term><literal>PGRES_TUPLES_CHUNK</literal></term>
<listitem>
<para>
The <structname>PGresult</structname> contains several result tuples
from the current command. This status occurs only when
chunked mode has been selected for the query
(see <xref linkend="libpq-single-row-mode"/>).
The number of tuples will not exceed the limit passed to
<xref linkend="libpq-PQsetChunkedRowsMode"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-pgres-pipeline-sync">
<term><literal>PGRES_PIPELINE_SYNC</literal></term>
<listitem>
<para>
The <structname>PGresult</structname> represents a
synchronization point in pipeline mode, requested by either
<xref linkend="libpq-PQpipelineSync"/> or
<xref linkend="libpq-PQsendPipelineSync"/>.
This status occurs only when pipeline mode has been selected.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-pgres-pipeline-aborted">
<term><literal>PGRES_PIPELINE_ABORTED</literal></term>
<listitem>
<para>
The <structname>PGresult</structname> represents a pipeline that has
received an error from the server. <function>PQgetResult</function>
must be called repeatedly, and each time it will return this status code
until the end of the current pipeline, at which point it will return
<literal>PGRES_PIPELINE_SYNC</literal> and normal processing can
resume.
</para>
</listitem>
</varlistentry>
</variablelist>
If the result status is <literal>PGRES_TUPLES_OK</literal>,
<literal>PGRES_SINGLE_TUPLE</literal>, or
<literal>PGRES_TUPLES_CHUNK</literal>, then
the functions described below can be used to retrieve the rows
returned by the query. Note that a <command>SELECT</command>
command that happens to retrieve zero rows still shows
<literal>PGRES_TUPLES_OK</literal>.
<literal>PGRES_COMMAND_OK</literal> is for commands that can never
return rows (<command>INSERT</command> or <command>UPDATE</command>
without a <literal>RETURNING</literal> clause,
etc.). A response of <literal>PGRES_EMPTY_QUERY</literal> might
indicate a bug in the client software.
</para>
<para>
A result of status <symbol>PGRES_NONFATAL_ERROR</symbol> will
never be returned directly by <xref linkend="libpq-PQexec"/> or other
query execution functions; results of this kind are instead passed
to the notice processor (see <xref
linkend="libpq-notice-processing"/>).
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQresStatus">
<term><function>PQresStatus</function><indexterm><primary>PQresStatus</primary></indexterm></term>
<listitem>
<para>
Converts the enumerated type returned by
<xref linkend="libpq-PQresultStatus"/> into a string constant describing the
status code. The caller should not free the result.
<synopsis>
char *PQresStatus(ExecStatusType status);
</synopsis>
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQresultErrorMessage">
<term><function>PQresultErrorMessage</function><indexterm><primary>PQresultErrorMessage</primary></indexterm></term>
<listitem>
<para>
Returns the error message associated with the command, or an empty string
if there was no error.
<synopsis>
char *PQresultErrorMessage(const PGresult *res);
</synopsis>
If there was an error, the returned string will include a trailing
newline. The caller should not free the result directly. It will
be freed when the associated <structname>PGresult</structname> handle is
passed to <xref linkend="libpq-PQclear"/>.
</para>
<para>
Immediately following a <xref linkend="libpq-PQexec"/> or
<xref linkend="libpq-PQgetResult"/> call,
<xref linkend="libpq-PQerrorMessage"/> (on the connection) will return
the same string as <xref linkend="libpq-PQresultErrorMessage"/> (on
the result). However, a <structname>PGresult</structname> will
retain its error message until destroyed, whereas the connection's
error message will change when subsequent operations are done.
Use <xref linkend="libpq-PQresultErrorMessage"/> when you want to
know the status associated with a particular
<structname>PGresult</structname>; use
<xref linkend="libpq-PQerrorMessage"/> when you want to know the
status from the latest operation on the connection.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQresultVerboseErrorMessage">
<term><function>PQresultVerboseErrorMessage</function><indexterm><primary>PQresultVerboseErrorMessage</primary></indexterm></term>
<listitem>
<para>
Returns a reformatted version of the error message associated with
a <structname>PGresult</structname> object.
<synopsis>
char *PQresultVerboseErrorMessage(const PGresult *res,
PGVerbosity verbosity,
PGContextVisibility show_context);
</synopsis>
In some situations a client might wish to obtain a more detailed
version of a previously-reported error.
<xref linkend="libpq-PQresultVerboseErrorMessage"/> addresses this need
by computing the message that would have been produced
by <xref linkend="libpq-PQresultErrorMessage"/> if the specified
verbosity settings had been in effect for the connection when the
given <structname>PGresult</structname> was generated. If
the <structname>PGresult</structname> is not an error result,
<quote>PGresult is not an error result</quote> is reported instead.
The returned string includes a trailing newline.
</para>
<para>
Unlike most other functions for extracting data from
a <structname>PGresult</structname>, the result of this function is a freshly
allocated string. The caller must free it
using <function>PQfreemem()</function> when the string is no longer needed.
</para>
<para>
A NULL return is possible if there is insufficient memory.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQresultErrorField">
<term><function>PQresultErrorField</function><indexterm><primary>PQresultErrorField</primary></indexterm></term>
<listitem>
<para>
Returns an individual field of an error report.
<synopsis>
char *PQresultErrorField(const PGresult *res, int fieldcode);
</synopsis>
<parameter>fieldcode</parameter> is an error field identifier; see the symbols
listed below. <symbol>NULL</symbol> is returned if the
<structname>PGresult</structname> is not an error or warning result,
or does not include the specified field. Field values will normally
not include a trailing newline. The caller should not free the
result directly. It will be freed when the
associated <structname>PGresult</structname> handle is passed to
<xref linkend="libpq-PQclear"/>.
</para>
<para>
The following field codes are available:
<variablelist>
<varlistentry id="libpq-pg-diag-severity">
<term><symbol>PG_DIAG_SEVERITY</symbol></term>
<listitem>
<para>
The severity; the field contents are <literal>ERROR</literal>,
<literal>FATAL</literal>, or <literal>PANIC</literal> (in an error message),
or <literal>WARNING</literal>, <literal>NOTICE</literal>, <literal>DEBUG</literal>,
<literal>INFO</literal>, or <literal>LOG</literal> (in a notice message), or
a localized translation of one of these. Always present.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PG-diag-severity-nonlocalized">
<term><symbol>PG_DIAG_SEVERITY_NONLOCALIZED</symbol></term>
<listitem>
<para>
The severity; the field contents are <literal>ERROR</literal>,
<literal>FATAL</literal>, or <literal>PANIC</literal> (in an error message),
or <literal>WARNING</literal>, <literal>NOTICE</literal>, <literal>DEBUG</literal>,
<literal>INFO</literal>, or <literal>LOG</literal> (in a notice message).
This is identical to the <symbol>PG_DIAG_SEVERITY</symbol> field except
that the contents are never localized. This is present only in
reports generated by <productname>PostgreSQL</productname> versions 9.6
and later.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-pg-diag-sqlstate">
<term><symbol>PG_DIAG_SQLSTATE</symbol><indexterm
><primary>error codes</primary><secondary>libpq</secondary></indexterm></term>
<listitem>
<para>
The SQLSTATE code for the error. The SQLSTATE code identifies
the type of error that has occurred; it can be used by
front-end applications to perform specific operations (such
as error handling) in response to a particular database error.
For a list of the possible SQLSTATE codes, see <xref
linkend="errcodes-appendix"/>. This field is not localizable,
and is always present.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-pg-diag-message-primary">
<term><symbol>PG_DIAG_MESSAGE_PRIMARY</symbol></term>
<listitem>
<para>
The primary human-readable error message (typically one line).
Always present.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-pg-diag-message-detail">
<term><symbol>PG_DIAG_MESSAGE_DETAIL</symbol></term>
<listitem>
<para>
Detail: an optional secondary error message carrying more
detail about the problem. Might run to multiple lines.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-pg-diag-message-hint">
<term><symbol>PG_DIAG_MESSAGE_HINT</symbol></term>
<listitem>
<para>
Hint: an optional suggestion what to do about the problem.
This is intended to differ from detail in that it offers advice
(potentially inappropriate) rather than hard facts. Might
run to multiple lines.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-pg-diag-statement-position">
<term><symbol>PG_DIAG_STATEMENT_POSITION</symbol></term>
<listitem>
<para>
A string containing a decimal integer indicating an error cursor
position as an index into the original statement string. The
first character has index 1, and positions are measured in
characters not bytes.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-pg-diag-internal-position">
<term><symbol>PG_DIAG_INTERNAL_POSITION</symbol></term>
<listitem>
<para>
This is defined the same as the
<symbol>PG_DIAG_STATEMENT_POSITION</symbol> field, but it is used
when the cursor position refers to an internally generated
command rather than the one submitted by the client. The
<symbol>PG_DIAG_INTERNAL_QUERY</symbol> field will always appear when
this field appears.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-pg-diag-internal-query">
<term><symbol>PG_DIAG_INTERNAL_QUERY</symbol></term>
<listitem>
<para>
The text of a failed internally-generated command. This could
be, for example, an SQL query issued by a PL/pgSQL function.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-pg-diag-context">
<term><symbol>PG_DIAG_CONTEXT</symbol></term>
<listitem>
<para>
An indication of the context in which the error occurred.
Presently this includes a call stack traceback of active
procedural language functions and internally-generated queries.
The trace is one entry per line, most recent first.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-pg-diag-schema-name">
<term><symbol>PG_DIAG_SCHEMA_NAME</symbol></term>
<listitem>
<para>
If the error was associated with a specific database object,
the name of the schema containing that object, if any.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-pg-diag-table-name">
<term><symbol>PG_DIAG_TABLE_NAME</symbol></term>
<listitem>
<para>
If the error was associated with a specific table, the name of the
table. (Refer to the schema name field for the name of the
table's schema.)
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-pg-diag-column-name">
<term><symbol>PG_DIAG_COLUMN_NAME</symbol></term>
<listitem>
<para>
If the error was associated with a specific table column, the name
of the column. (Refer to the schema and table name fields to
identify the table.)
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-pg-diag-datatype-name">
<term><symbol>PG_DIAG_DATATYPE_NAME</symbol></term>
<listitem>
<para>
If the error was associated with a specific data type, the name of
the data type. (Refer to the schema name field for the name of
the data type's schema.)
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-pg-diag-constraint-name">
<term><symbol>PG_DIAG_CONSTRAINT_NAME</symbol></term>
<listitem>
<para>
If the error was associated with a specific constraint, the name
of the constraint. Refer to fields listed above for the
associated table or domain. (For this purpose, indexes are
treated as constraints, even if they weren't created with
constraint syntax.)
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-pg-diag-source-file">
<term><symbol>PG_DIAG_SOURCE_FILE</symbol></term>
<listitem>
<para>
The file name of the source-code location where the error was
reported.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-pg-diag-source-line">
<term><symbol>PG_DIAG_SOURCE_LINE</symbol></term>
<listitem>
<para>
The line number of the source-code location where the error
was reported.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-pg-diag-source-function">
<term><symbol>PG_DIAG_SOURCE_FUNCTION</symbol></term>
<listitem>
<para>
The name of the source-code function reporting the error.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
<note>
<para>
The fields for schema name, table name, column name, data type name,
and constraint name are supplied only for a limited number of error
types; see <xref linkend="errcodes-appendix"/>. Do not assume that
the presence of any of these fields guarantees the presence of
another field. Core error sources observe the interrelationships
noted above, but user-defined functions may use these fields in other
ways. In the same vein, do not assume that these fields denote
contemporary objects in the current database.
</para>
</note>
<para>
The client is responsible for formatting displayed information to meet
its needs; in particular it should break long lines as needed.
Newline characters appearing in the error message fields should be
treated as paragraph breaks, not line breaks.
</para>
<para>
Errors generated internally by <application>libpq</application> will
have severity and primary message, but typically no other fields.
</para>
<para>
Note that error fields are only available from
<structname>PGresult</structname> objects, not
<structname>PGconn</structname> objects; there is no
<function>PQerrorField</function> function.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQclear">
<term><function>PQclear</function><indexterm><primary>PQclear</primary></indexterm></term>
<listitem>
<para>
Frees the storage associated with a
<structname>PGresult</structname>. Every command result should be
freed via <xref linkend="libpq-PQclear"/> when it is no longer
needed.
<synopsis>
void PQclear(PGresult *res);
</synopsis>
If the argument is a <symbol>NULL</symbol> pointer, no operation is
performed.
</para>
<para>
You can keep a <structname>PGresult</structname> object around for
as long as you need it; it does not go away when you issue a new
command, nor even if you close the connection. To get rid of it,
you must call <xref linkend="libpq-PQclear"/>. Failure to do this
will result in memory leaks in your application.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</sect2>
<sect2 id="libpq-exec-select-info">
<title>Retrieving Query Result Information</title>
<para>
These functions are used to extract information from a
<structname>PGresult</structname> object that represents a successful
query result (that is, one that has status
<literal>PGRES_TUPLES_OK</literal>,
<literal>PGRES_SINGLE_TUPLE</literal>, or
<literal>PGRES_TUPLES_CHUNK</literal>).
They can also be used to extract
information from a successful Describe operation: a Describe's result
has all the same column information that actual execution of the query
would provide, but it has zero rows. For objects with other status values,
these functions will act as though the result has zero rows and zero columns.
</para>
<variablelist>
<varlistentry id="libpq-PQntuples">
<term><function>PQntuples</function><indexterm><primary>PQntuples</primary></indexterm></term>
<listitem>
<para>
Returns the number of rows (tuples) in the query result.
(Note that <structname>PGresult</structname> objects are limited to no more
than <literal>INT_MAX</literal> rows, so an <type>int</type> result is
sufficient.)
<synopsis>
int PQntuples(const PGresult *res);
</synopsis>
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQnfields">
<term><function>PQnfields</function><indexterm><primary>PQnfields</primary></indexterm></term>
<listitem>
<para>
Returns the number of columns (fields) in each row of the query
result.
<synopsis>
int PQnfields(const PGresult *res);
</synopsis>
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQfname">
<term><function>PQfname</function><indexterm><primary>PQfname</primary></indexterm></term>
<listitem>
<para>
Returns the column name associated with the given column number.
Column numbers start at 0. The caller should not free the result
directly. It will be freed when the associated
<structname>PGresult</structname> handle is passed to
<xref linkend="libpq-PQclear"/>.
<synopsis>
char *PQfname(const PGresult *res,
int column_number);
</synopsis>
</para>
<para>
<symbol>NULL</symbol> is returned if the column number is out of range.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQfnumber">
<term><function>PQfnumber</function><indexterm><primary>PQfnumber</primary></indexterm></term>
<listitem>
<para>
Returns the column number associated with the given column name.
<synopsis>
int PQfnumber(const PGresult *res,
const char *column_name);
</synopsis>
</para>
<para>
-1 is returned if the given name does not match any column.
</para>
<para>
The given name is treated like an identifier in an SQL command,
that is, it is downcased unless double-quoted. For example, given
a query result generated from the SQL command:
<programlisting>
SELECT 1 AS FOO, 2 AS "BAR";
</programlisting>
we would have the results:
<programlisting>
PQfname(res, 0) <lineannotation>foo</lineannotation>
PQfname(res, 1) <lineannotation>BAR</lineannotation>
PQfnumber(res, "FOO") <lineannotation>0</lineannotation>
PQfnumber(res, "foo") <lineannotation>0</lineannotation>
PQfnumber(res, "BAR") <lineannotation>-1</lineannotation>
PQfnumber(res, "\"BAR\"") <lineannotation>1</lineannotation>
</programlisting>
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQftable">
<term><function>PQftable</function><indexterm><primary>PQftable</primary></indexterm></term>
<listitem>
<para>
Returns the OID of the table from which the given column was
fetched. Column numbers start at 0.
<synopsis>
Oid PQftable(const PGresult *res,
int column_number);
</synopsis>
</para>
<para>
<literal>InvalidOid</literal> is returned if the column number is out of range,
or if the specified column is not a simple reference to a table column.
You can query the system table <literal>pg_class</literal> to determine
exactly which table is referenced.
</para>
<para>
The type <type>Oid</type> and the constant
<literal>InvalidOid</literal> will be defined when you include
the <application>libpq</application> header file. They will both
be some integer type.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQftablecol">
<term><function>PQftablecol</function><indexterm><primary>PQftablecol</primary></indexterm></term>
<listitem>
<para>
Returns the column number (within its table) of the column making
up the specified query result column. Query-result column numbers
start at 0, but table columns have nonzero numbers.
<synopsis>
int PQftablecol(const PGresult *res,
int column_number);
</synopsis>
</para>
<para>
Zero is returned if the column number is out of range, or if the
specified column is not a simple reference to a table column.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQfformat">
<term><function>PQfformat</function><indexterm><primary>PQfformat</primary></indexterm></term>
<listitem>
<para>
Returns the format code indicating the format of the given
column. Column numbers start at 0.
<synopsis>
int PQfformat(const PGresult *res,
int column_number);
</synopsis>
</para>
<para>
Format code zero indicates textual data representation, while format
code one indicates binary representation. (Other codes are reserved
for future definition.)
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQftype">
<term><function>PQftype</function><indexterm><primary>PQftype</primary></indexterm></term>
<listitem>
<para>
Returns the data type associated with the given column number.
The integer returned is the internal OID number of the type.
Column numbers start at 0.
<synopsis>
Oid PQftype(const PGresult *res,
int column_number);
</synopsis>
</para>
<para>
You can query the system table <literal>pg_type</literal> to
obtain the names and properties of the various data types. The
<acronym>OID</acronym>s of the built-in data types are defined
in the file <filename>catalog/pg_type_d.h</filename>
in the <productname>PostgreSQL</productname>
installation's <filename>include</filename> directory.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQfmod">
<term><function>PQfmod</function><indexterm><primary>PQfmod</primary></indexterm></term>
<listitem>
<para>
Returns the type modifier of the column associated with the
given column number. Column numbers start at 0.
<synopsis>
int PQfmod(const PGresult *res,
int column_number);
</synopsis>
</para>
<para>
The interpretation of modifier values is type-specific; they
typically indicate precision or size limits. The value -1 is
used to indicate <quote>no information available</quote>. Most data
types do not use modifiers, in which case the value is always
-1.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQfsize">
<term><function>PQfsize</function><indexterm><primary>PQfsize</primary></indexterm></term>
<listitem>
<para>
Returns the size in bytes of the column associated with the
given column number. Column numbers start at 0.
<synopsis>
int PQfsize(const PGresult *res,
int column_number);
</synopsis>
</para>
<para>
<xref linkend="libpq-PQfsize"/> returns the space allocated for this column
in a database row, in other words the size of the server's
internal representation of the data type. (Accordingly, it is
not really very useful to clients.) A negative value indicates
the data type is variable-length.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQbinaryTuples">
<term><function>PQbinaryTuples</function><indexterm><primary>PQbinaryTuples</primary></indexterm></term>
<listitem>
<para>
Returns 1 if the <structname>PGresult</structname> contains binary data
and 0 if it contains text data.
<synopsis>
int PQbinaryTuples(const PGresult *res);
</synopsis>
</para>
<para>
This function is deprecated (except for its use in connection with
<command>COPY</command>), because it is possible for a single
<structname>PGresult</structname> to contain text data in some columns and
binary data in others. <xref linkend="libpq-PQfformat"/> is preferred.
<xref linkend="libpq-PQbinaryTuples"/> returns 1 only if all columns of the
result are binary (format 1).
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQgetvalue">
<term><function>PQgetvalue</function><indexterm><primary>PQgetvalue</primary></indexterm></term>
<listitem>
<para>
Returns a single field value of one row of a
<structname>PGresult</structname>. Row and column numbers start
at 0. The caller should not free the result directly. It will
be freed when the associated <structname>PGresult</structname> handle is
passed to <xref linkend="libpq-PQclear"/>.
<synopsis>
char *PQgetvalue(const PGresult *res,
int row_number,
int column_number);
</synopsis>
</para>
<para>
For data in text format, the value returned by
<xref linkend="libpq-PQgetvalue"/> is a null-terminated character
string representation of the field value. For data in binary
format, the value is in the binary representation determined by
the data type's <function>typsend</function> and <function>typreceive</function>
functions. (The value is actually followed by a zero byte in
this case too, but that is not ordinarily useful, since the
value is likely to contain embedded nulls.)
</para>
<para>
An empty string is returned if the field value is null. See
<xref linkend="libpq-PQgetisnull"/> to distinguish null values from
empty-string values.
</para>
<para>
The pointer returned by <xref linkend="libpq-PQgetvalue"/> points
to storage that is part of the <structname>PGresult</structname>
structure. One should not modify the data it points to, and one
must explicitly copy the data into other storage if it is to be
used past the lifetime of the <structname>PGresult</structname>
structure itself.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQgetisnull">
<term><function>PQgetisnull</function><indexterm
><primary>PQgetisnull</primary></indexterm><indexterm
><primary>null value</primary><secondary sortas="libpq">in libpq</secondary></indexterm></term>
<listitem>
<para>
Tests a field for a null value. Row and column numbers start
at 0.
<synopsis>
int PQgetisnull(const PGresult *res,
int row_number,
int column_number);
</synopsis>
</para>
<para>
This function returns 1 if the field is null and 0 if it
contains a non-null value. (Note that
<xref linkend="libpq-PQgetvalue"/> will return an empty string,
not a null pointer, for a null field.)
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQgetlength">
<term><function>PQgetlength</function><indexterm><primary>PQgetlength</primary></indexterm></term>
<listitem>
<para>
Returns the actual length of a field value in bytes. Row and
column numbers start at 0.
<synopsis>
int PQgetlength(const PGresult *res,
int row_number,
int column_number);
</synopsis>
</para>
<para>
This is the actual data length for the particular data value,
that is, the size of the object pointed to by
<xref linkend="libpq-PQgetvalue"/>. For text data format this is
the same as <function>strlen()</function>. For binary format this is
essential information. Note that one should <emphasis>not</emphasis>
rely on <xref linkend="libpq-PQfsize"/> to obtain the actual data
length.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQnparams">
<term><function>PQnparams</function><indexterm><primary>PQnparams</primary></indexterm></term>
<listitem>
<para>
Returns the number of parameters of a prepared statement.
<synopsis>
int PQnparams(const PGresult *res);
</synopsis>
</para>
<para>
This function is only useful when inspecting the result of
<xref linkend="libpq-PQdescribePrepared"/>. For other types of results it
will return zero.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQparamtype">
<term><function>PQparamtype</function><indexterm><primary>PQparamtype</primary></indexterm></term>
<listitem>
<para>
Returns the data type of the indicated statement parameter.
Parameter numbers start at 0.
<synopsis>
Oid PQparamtype(const PGresult *res, int param_number);
</synopsis>
</para>
<para>
This function is only useful when inspecting the result of
<xref linkend="libpq-PQdescribePrepared"/>. For other types of results it
will return zero.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQprint">
<term><function>PQprint</function><indexterm><primary>PQprint</primary></indexterm></term>
<listitem>
<para>
Prints out all the rows and, optionally, the column names to
the specified output stream.
<synopsis>
void PQprint(FILE *fout, /* output stream */
const PGresult *res,
const PQprintOpt *po);
typedef struct
{
pqbool header; /* print output field headings and row count */
pqbool align; /* fill align the fields */
pqbool standard; /* old brain dead format */
pqbool html3; /* output HTML tables */
pqbool expanded; /* expand tables */
pqbool pager; /* use pager for output if needed */
char *fieldSep; /* field separator */
char *tableOpt; /* attributes for HTML table element */
char *caption; /* HTML table caption */
char **fieldName; /* null-terminated array of replacement field names */
} PQprintOpt;
</synopsis>
</para>
<para>
This function was formerly used by <application>psql</application>
to print query results, but this is no longer the case. Note
that it assumes all the data is in text format.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="libpq-exec-nonselect">
<title>Retrieving Other Result Information</title>
<para>
These functions are used to extract other information from
<structname>PGresult</structname> objects.
</para>
<variablelist>
<varlistentry id="libpq-PQcmdStatus">
<term><function>PQcmdStatus</function><indexterm><primary>PQcmdStatus</primary></indexterm></term>
<listitem>
<para>
Returns the command status tag from the SQL command that generated
the <structname>PGresult</structname>.
<synopsis>
char *PQcmdStatus(PGresult *res);
</synopsis>
</para>
<para>
Commonly this is just the name of the command, but it might include
additional data such as the number of rows processed. The caller
should not free the result directly. It will be freed when the
associated <structname>PGresult</structname> handle is passed to
<xref linkend="libpq-PQclear"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQcmdTuples">
<term><function>PQcmdTuples</function><indexterm><primary>PQcmdTuples</primary></indexterm></term>
<listitem>
<para>
Returns the number of rows affected by the SQL command.
<synopsis>
char *PQcmdTuples(PGresult *res);
</synopsis>
</para>
<para>
This function returns a string containing the number of rows
affected by the <acronym>SQL</acronym> statement that generated the
<structname>PGresult</structname>. This function can only be used following
the execution of a <command>SELECT</command>, <command>CREATE TABLE AS</command>,
<command>INSERT</command>, <command>UPDATE</command>, <command>DELETE</command>,
<command>MERGE</command>, <command>MOVE</command>, <command>FETCH</command>,
or <command>COPY</command> statement, or an <command>EXECUTE</command> of a
prepared query that contains an <command>INSERT</command>,
<command>UPDATE</command>, <command>DELETE</command>,
or <command>MERGE</command> statement.
If the command that generated the <structname>PGresult</structname> was anything
else, <xref linkend="libpq-PQcmdTuples"/> returns an empty string. The caller
should not free the return value directly. It will be freed when
the associated <structname>PGresult</structname> handle is passed to
<xref linkend="libpq-PQclear"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQoidValue">
<term><function>PQoidValue</function><indexterm><primary>PQoidValue</primary></indexterm></term>
<listitem>
<para>
Returns the OID<indexterm><primary>OID</primary><secondary>in libpq</secondary></indexterm>
of the inserted row, if the <acronym>SQL</acronym> command was an
<command>INSERT</command> that inserted exactly one row into a table that
has OIDs, or a <command>EXECUTE</command> of a prepared query containing
a suitable <command>INSERT</command> statement. Otherwise, this function
returns <literal>InvalidOid</literal>. This function will also
return <literal>InvalidOid</literal> if the table affected by the
<command>INSERT</command> statement does not contain OIDs.
<synopsis>
Oid PQoidValue(const PGresult *res);
</synopsis>
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQoidStatus">
<term><function>PQoidStatus</function><indexterm><primary>PQoidStatus</primary></indexterm></term>
<listitem>
<para>
This function is deprecated in favor of
<xref linkend="libpq-PQoidValue"/> and is not thread-safe.
It returns a string with the OID of the inserted row, while
<xref linkend="libpq-PQoidValue"/> returns the OID value.
<synopsis>
char *PQoidStatus(const PGresult *res);
</synopsis>
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="libpq-exec-escape-string">
<title>Escaping Strings for Inclusion in SQL Commands</title>
<indexterm zone="libpq-exec-escape-string">
<primary>escaping strings</primary>
<secondary>in libpq</secondary>
</indexterm>
<variablelist>
<varlistentry id="libpq-PQescapeLiteral">
<term><function>PQescapeLiteral</function><indexterm><primary>PQescapeLiteral</primary></indexterm></term>
<listitem>
<para>
<synopsis>
char *PQescapeLiteral(PGconn *conn, const char *str, size_t length);
</synopsis>
</para>
<para>
<xref linkend="libpq-PQescapeLiteral"/> escapes a string for
use within an SQL command. This is useful when inserting data
values as literal constants in SQL commands. Certain characters
(such as quotes and backslashes) must be escaped to prevent them
from being interpreted specially by the SQL parser.
<xref linkend="libpq-PQescapeLiteral"/> performs this operation.
</para>
<para>
<xref linkend="libpq-PQescapeLiteral"/> returns an escaped version of the
<parameter>str</parameter> parameter in memory allocated with
<function>malloc()</function>. This memory should be freed using
<function>PQfreemem()</function> when the result is no longer needed.
A terminating zero byte is not required, and should not be
counted in <parameter>length</parameter>. (If a terminating zero byte is found
before <parameter>length</parameter> bytes are processed,
<xref linkend="libpq-PQescapeLiteral"/> stops at the zero; the behavior is
thus rather like <function>strncpy</function>.) The
return string has all special characters replaced so that they can
be properly processed by the <productname>PostgreSQL</productname>
string literal parser. A terminating zero byte is also added. The
single quotes that must surround <productname>PostgreSQL</productname>
string literals are included in the result string.
</para>
<para>
On error, <xref linkend="libpq-PQescapeLiteral"/> returns <symbol>NULL</symbol> and a suitable
message is stored in the <parameter>conn</parameter> object.
</para>
<tip>
<para>
It is especially important to do proper escaping when handling
strings that were received from an untrustworthy source.
Otherwise there is a security risk: you are vulnerable to
<quote>SQL injection</quote> attacks wherein unwanted SQL commands are
fed to your database.
</para>
</tip>
<para>
Note that it is neither necessary nor correct to do escaping when a data
value is passed as a separate parameter in <xref linkend="libpq-PQexecParams"/> or
its sibling routines.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQescapeIdentifier">
<term><function>PQescapeIdentifier</function><indexterm><primary>PQescapeIdentifier</primary></indexterm></term>
<listitem>
<para>
<synopsis>
char *PQescapeIdentifier(PGconn *conn, const char *str, size_t length);
</synopsis>
</para>
<para>
<xref linkend="libpq-PQescapeIdentifier"/> escapes a string for
use as an SQL identifier, such as a table, column, or function name.
This is useful when a user-supplied identifier might contain
special characters that would otherwise not be interpreted as part
of the identifier by the SQL parser, or when the identifier might
contain upper case characters whose case should be preserved.
</para>
<para>
<xref linkend="libpq-PQescapeIdentifier"/> returns a version of the
<parameter>str</parameter> parameter escaped as an SQL identifier
in memory allocated with <function>malloc()</function>. This memory must be
freed using <function>PQfreemem()</function> when the result is no longer
needed. A terminating zero byte is not required, and should not be
counted in <parameter>length</parameter>. (If a terminating zero byte is found
before <parameter>length</parameter> bytes are processed,
<xref linkend="libpq-PQescapeIdentifier"/> stops at the zero; the behavior is
thus rather like <function>strncpy</function>.) The
return string has all special characters replaced so that it
will be properly processed as an SQL identifier. A terminating zero byte
is also added. The return string will also be surrounded by double
quotes.
</para>
<para>
On error, <xref linkend="libpq-PQescapeIdentifier"/> returns <symbol>NULL</symbol> and a suitable
message is stored in the <parameter>conn</parameter> object.
</para>
<tip>
<para>
As with string literals, to prevent SQL injection attacks,
SQL identifiers must be escaped when they are received from an
untrustworthy source.
</para>
</tip>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQescapeStringConn">
<term><function>PQescapeStringConn</function><indexterm><primary>PQescapeStringConn</primary></indexterm></term>
<listitem>
<para>
<synopsis>
size_t PQescapeStringConn(PGconn *conn,
char *to, const char *from, size_t length,
int *error);
</synopsis>
</para>
<para>
<xref linkend="libpq-PQescapeStringConn"/> escapes string literals, much like
<xref linkend="libpq-PQescapeLiteral"/>. Unlike <xref linkend="libpq-PQescapeLiteral"/>,
the caller is responsible for providing an appropriately sized buffer.
Furthermore, <xref linkend="libpq-PQescapeStringConn"/> does not generate the
single quotes that must surround <productname>PostgreSQL</productname> string
literals; they should be provided in the SQL command that the
result is inserted into. The parameter <parameter>from</parameter> points to
the first character of the string that is to be escaped, and the
<parameter>length</parameter> parameter gives the number of bytes in this
string. A terminating zero byte is not required, and should not be
counted in <parameter>length</parameter>. (If a terminating zero byte is found
before <parameter>length</parameter> bytes are processed,
<xref linkend="libpq-PQescapeStringConn"/> stops at the zero; the behavior is
thus rather like <function>strncpy</function>.) <parameter>to</parameter> shall point
to a buffer that is able to hold at least one more byte than twice
the value of <parameter>length</parameter>, otherwise the behavior is undefined.
Behavior is likewise undefined if the <parameter>to</parameter> and
<parameter>from</parameter> strings overlap.
</para>
<para>
If the <parameter>error</parameter> parameter is not <symbol>NULL</symbol>, then
<literal>*error</literal> is set to zero on success, nonzero on error.
Presently the only possible error conditions involve invalid multibyte
encoding in the source string. The output string is still generated
on error, but it can be expected that the server will reject it as
malformed. On error, a suitable message is stored in the
<parameter>conn</parameter> object, whether or not <parameter>error</parameter> is <symbol>NULL</symbol>.
</para>
<para>
<xref linkend="libpq-PQescapeStringConn"/> returns the number of bytes written
to <parameter>to</parameter>, not including the terminating zero byte.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQescapeString">
<term><function>PQescapeString</function><indexterm><primary>PQescapeString</primary></indexterm></term>
<listitem>
<para>
<xref linkend="libpq-PQescapeString"/> is an older, deprecated version of
<xref linkend="libpq-PQescapeStringConn"/>.
<synopsis>
size_t PQescapeString (char *to, const char *from, size_t length);
</synopsis>
</para>
<para>
The only difference from <xref linkend="libpq-PQescapeStringConn"/> is that
<xref linkend="libpq-PQescapeString"/> does not take <structname>PGconn</structname>
or <parameter>error</parameter> parameters.
Because of this, it cannot adjust its behavior depending on the
connection properties (such as character encoding) and therefore
<emphasis>it might give the wrong results</emphasis>. Also, it has no way
to report error conditions.
</para>
<para>
<xref linkend="libpq-PQescapeString"/> can be used safely in
client programs that work with only one <productname>PostgreSQL</productname>
connection at a time (in this case it can find out what it needs to
know <quote>behind the scenes</quote>). In other contexts it is a security
hazard and should be avoided in favor of
<xref linkend="libpq-PQescapeStringConn"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQescapeByteaConn">
<term><function>PQescapeByteaConn</function><indexterm><primary>PQescapeByteaConn</primary></indexterm></term>
<listitem>
<para>
Escapes binary data for use within an SQL command with the type
<type>bytea</type>. As with <xref linkend="libpq-PQescapeStringConn"/>,
this is only used when inserting data directly into an SQL command string.
<synopsis>
unsigned char *PQescapeByteaConn(PGconn *conn,
const unsigned char *from,
size_t from_length,
size_t *to_length);
</synopsis>
</para>
<para>
Certain byte values must be escaped when used as part of a
<type>bytea</type> literal in an <acronym>SQL</acronym> statement.
<xref linkend="libpq-PQescapeByteaConn"/> escapes bytes using
either hex encoding or backslash escaping. See <xref
linkend="datatype-binary"/> for more information.
</para>
<para>
The <parameter>from</parameter> parameter points to the first
byte of the string that is to be escaped, and the
<parameter>from_length</parameter> parameter gives the number of
bytes in this binary string. (A terminating zero byte is
neither necessary nor counted.) The <parameter>to_length</parameter>
parameter points to a variable that will hold the resultant
escaped string length. This result string length includes the terminating
zero byte of the result.
</para>
<para>
<xref linkend="libpq-PQescapeByteaConn"/> returns an escaped version of the
<parameter>from</parameter> parameter binary string in memory
allocated with <function>malloc()</function>. This memory should be freed using
<function>PQfreemem()</function> when the result is no longer needed. The
return string has all special characters replaced so that they can
be properly processed by the <productname>PostgreSQL</productname>
string literal parser, and the <type>bytea</type> input function. A
terminating zero byte is also added. The single quotes that must
surround <productname>PostgreSQL</productname> string literals are
not part of the result string.
</para>
<para>
On error, a null pointer is returned, and a suitable error message
is stored in the <parameter>conn</parameter> object. Currently, the only
possible error is insufficient memory for the result string.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQescapeBytea">
<term><function>PQescapeBytea</function><indexterm><primary>PQescapeBytea</primary></indexterm></term>
<listitem>
<para>
<xref linkend="libpq-PQescapeBytea"/> is an older, deprecated version of
<xref linkend="libpq-PQescapeByteaConn"/>.
<synopsis>
unsigned char *PQescapeBytea(const unsigned char *from,
size_t from_length,
size_t *to_length);
</synopsis>
</para>
<para>
The only difference from <xref linkend="libpq-PQescapeByteaConn"/> is that
<xref linkend="libpq-PQescapeBytea"/> does not take a <structname>PGconn</structname>
parameter. Because of this, <xref linkend="libpq-PQescapeBytea"/> can
only be used safely in client programs that use a single
<productname>PostgreSQL</productname> connection at a time (in this case
it can find out what it needs to know <quote>behind the
scenes</quote>). It <emphasis>might give the wrong results</emphasis> if
used in programs that use multiple database connections (use
<xref linkend="libpq-PQescapeByteaConn"/> in such cases).
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQunescapeBytea">
<term><function>PQunescapeBytea</function><indexterm><primary>PQunescapeBytea</primary></indexterm></term>
<listitem>
<para>
Converts a string representation of binary data into binary data
&mdash; the reverse of <xref linkend="libpq-PQescapeBytea"/>. This
is needed when retrieving <type>bytea</type> data in text format,
but not when retrieving it in binary format.
<synopsis>
unsigned char *PQunescapeBytea(const unsigned char *from, size_t *to_length);
</synopsis>
</para>
<para>
The <parameter>from</parameter> parameter points to a string
such as might be returned by <xref linkend="libpq-PQgetvalue"/> when applied
to a <type>bytea</type> column. <xref linkend="libpq-PQunescapeBytea"/>
converts this string representation into its binary representation.
It returns a pointer to a buffer allocated with
<function>malloc()</function>, or <symbol>NULL</symbol> on error, and puts the size of
the buffer in <parameter>to_length</parameter>. The result must be
freed using <xref linkend="libpq-PQfreemem"/> when it is no longer needed.
</para>
<para>
This conversion is not exactly the inverse of
<xref linkend="libpq-PQescapeBytea"/>, because the string is not expected
to be <quote>escaped</quote> when received from <xref linkend="libpq-PQgetvalue"/>.
In particular this means there is no need for string quoting considerations,
and so no need for a <structname>PGconn</structname> parameter.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
</sect1>
<sect1 id="libpq-async">
<title>Asynchronous Command Processing</title>
<indexterm zone="libpq-async">
<primary>nonblocking connection</primary>
</indexterm>
<para>
The <xref linkend="libpq-PQexec"/> function is adequate for submitting
commands in normal, synchronous applications. It has a few
deficiencies, however, that can be of importance to some users:
<itemizedlist>
<listitem>
<para>
<xref linkend="libpq-PQexec"/> waits for the command to be completed.
The application might have other work to do (such as maintaining a
user interface), in which case it won't want to block waiting for
the response.
</para>
</listitem>
<listitem>
<para>
Since the execution of the client application is suspended while it
waits for the result, it is hard for the application to decide that
it would like to try to cancel the ongoing command. (It can be done
from a signal handler, but not otherwise.)
</para>
</listitem>
<listitem>
<para>
<xref linkend="libpq-PQexec"/> can return only one
<structname>PGresult</structname> structure. If the submitted command
string contains multiple <acronym>SQL</acronym> commands, all but
the last <structname>PGresult</structname> are discarded by
<xref linkend="libpq-PQexec"/>.
</para>
</listitem>
<listitem>
<para>
<xref linkend="libpq-PQexec"/> always collects the command's entire result,
buffering it in a single <structname>PGresult</structname>. While
this simplifies error-handling logic for the application, it can be
impractical for results containing many rows.
</para>
</listitem>
</itemizedlist>
</para>
<para>
Applications that do not like these limitations can instead use the
underlying functions that <xref linkend="libpq-PQexec"/> is built from:
<xref linkend="libpq-PQsendQuery"/> and <xref linkend="libpq-PQgetResult"/>.
There are also
<xref linkend="libpq-PQsendQueryParams"/>,
<xref linkend="libpq-PQsendPrepare"/>,
<xref linkend="libpq-PQsendQueryPrepared"/>,
<xref linkend="libpq-PQsendDescribePrepared"/>,
<xref linkend="libpq-PQsendDescribePortal"/>,
<xref linkend="libpq-PQsendClosePrepared"/>, and
<xref linkend="libpq-PQsendClosePortal"/>,
which can be used with <xref linkend="libpq-PQgetResult"/> to duplicate
the functionality of
<xref linkend="libpq-PQexecParams"/>,
<xref linkend="libpq-PQprepare"/>,
<xref linkend="libpq-PQexecPrepared"/>,
<xref linkend="libpq-PQdescribePrepared"/>,
<xref linkend="libpq-PQdescribePortal"/>
<xref linkend="libpq-PQclosePrepared"/>, and
<xref linkend="libpq-PQclosePortal"/>
respectively.
<variablelist>
<varlistentry id="libpq-PQsendQuery">
<term><function>PQsendQuery</function><indexterm><primary>PQsendQuery</primary></indexterm></term>
<listitem>
<para>
Submits a command to the server without waiting for the result(s).
1 is returned if the command was successfully dispatched and 0 if
not (in which case, use <xref linkend="libpq-PQerrorMessage"/> to get more
information about the failure).
<synopsis>
int PQsendQuery(PGconn *conn, const char *command);
</synopsis>
After successfully calling <xref linkend="libpq-PQsendQuery"/>, call
<xref linkend="libpq-PQgetResult"/> one or more times to obtain the
results. <xref linkend="libpq-PQsendQuery"/> cannot be called again
(on the same connection) until <xref linkend="libpq-PQgetResult"/>
has returned a null pointer, indicating that the command is done.
</para>
<para>
In pipeline mode, this function is disallowed.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQsendQueryParams">
<term><function>PQsendQueryParams</function><indexterm><primary>PQsendQueryParams</primary></indexterm></term>
<listitem>
<para>
Submits a command and separate parameters to the server without
waiting for the result(s).
<synopsis>
int PQsendQueryParams(PGconn *conn,
const char *command,
int nParams,
const Oid *paramTypes,
const char * const *paramValues,
const int *paramLengths,
const int *paramFormats,
int resultFormat);
</synopsis>
This is equivalent to <xref linkend="libpq-PQsendQuery"/> except that
query parameters can be specified separately from the query string.
The function's parameters are handled identically to
<xref linkend="libpq-PQexecParams"/>. Like
<xref linkend="libpq-PQexecParams"/>, it allows only one command in the
query string.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQsendPrepare">
<term><function>PQsendPrepare</function><indexterm><primary>PQsendPrepare</primary></indexterm></term>
<listitem>
<para>
Sends a request to create a prepared statement with the given
parameters, without waiting for completion.
<synopsis>
int PQsendPrepare(PGconn *conn,
const char *stmtName,
const char *query,
int nParams,
const Oid *paramTypes);
</synopsis>
This is an asynchronous version of <xref linkend="libpq-PQprepare"/>: it
returns 1 if it was able to dispatch the request, and 0 if not.
After a successful call, call <xref linkend="libpq-PQgetResult"/> to
determine whether the server successfully created the prepared
statement. The function's parameters are handled identically to
<xref linkend="libpq-PQprepare"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQsendQueryPrepared">
<term><function>PQsendQueryPrepared</function><indexterm><primary>PQsendQueryPrepared</primary></indexterm></term>
<listitem>
<para>
Sends a request to execute a prepared statement with given
parameters, without waiting for the result(s).
<synopsis>
int PQsendQueryPrepared(PGconn *conn,
const char *stmtName,
int nParams,
const char * const *paramValues,
const int *paramLengths,
const int *paramFormats,
int resultFormat);
</synopsis>
This is similar to <xref linkend="libpq-PQsendQueryParams"/>, but
the command to be executed is specified by naming a
previously-prepared statement, instead of giving a query string.
The function's parameters are handled identically to
<xref linkend="libpq-PQexecPrepared"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQsendDescribePrepared">
<term><function>PQsendDescribePrepared</function><indexterm><primary>PQsendDescribePrepared</primary></indexterm></term>
<listitem>
<para>
Submits a request to obtain information about the specified
prepared statement, without waiting for completion.
<synopsis>
int PQsendDescribePrepared(PGconn *conn, const char *stmtName);
</synopsis>
This is an asynchronous version of <xref linkend="libpq-PQdescribePrepared"/>:
it returns 1 if it was able to dispatch the request, and 0 if not.
After a successful call, call <xref linkend="libpq-PQgetResult"/> to
obtain the results. The function's parameters are handled
identically to <xref linkend="libpq-PQdescribePrepared"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQsendDescribePortal">
<term><function>PQsendDescribePortal</function><indexterm><primary>PQsendDescribePortal</primary></indexterm></term>
<listitem>
<para>
Submits a request to obtain information about the specified
portal, without waiting for completion.
<synopsis>
int PQsendDescribePortal(PGconn *conn, const char *portalName);
</synopsis>
This is an asynchronous version of <xref linkend="libpq-PQdescribePortal"/>:
it returns 1 if it was able to dispatch the request, and 0 if not.
After a successful call, call <xref linkend="libpq-PQgetResult"/> to
obtain the results. The function's parameters are handled
identically to <xref linkend="libpq-PQdescribePortal"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQsendClosePrepared">
<term><function>PQsendClosePrepared</function><indexterm><primary>PQsendClosePrepared</primary></indexterm></term>
<listitem>
<para>
Submits a request to close the specified prepared statement, without
waiting for completion.
<synopsis>
int PQsendClosePrepared(PGconn *conn, const char *stmtName);
</synopsis>
This is an asynchronous version of <xref linkend="libpq-PQclosePrepared"/>:
it returns 1 if it was able to dispatch the request, and 0 if not.
After a successful call, call <xref linkend="libpq-PQgetResult"/> to
obtain the results. The function's parameters are handled
identically to <xref linkend="libpq-PQclosePrepared"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQsendClosePortal">
<term><function>PQsendClosePortal</function><indexterm><primary>PQsendClosePortal</primary></indexterm></term>
<listitem>
<para>
Submits a request to close specified portal, without waiting for
completion.
<synopsis>
int PQsendClosePortal(PGconn *conn, const char *portalName);
</synopsis>
This is an asynchronous version of <xref linkend="libpq-PQclosePortal"/>:
it returns 1 if it was able to dispatch the request, and 0 if not.
After a successful call, call <xref linkend="libpq-PQgetResult"/> to
obtain the results. The function's parameters are handled
identically to <xref linkend="libpq-PQclosePortal"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQgetResult">
<term><function>PQgetResult</function><indexterm><primary>PQgetResult</primary></indexterm></term>
<listitem>
<para>
Waits for the next result from a prior
<xref linkend="libpq-PQsendQuery"/>,
<xref linkend="libpq-PQsendQueryParams"/>,
<xref linkend="libpq-PQsendPrepare"/>,
<xref linkend="libpq-PQsendQueryPrepared"/>,
<xref linkend="libpq-PQsendDescribePrepared"/>,
<xref linkend="libpq-PQsendDescribePortal"/>,
<xref linkend="libpq-PQsendClosePrepared"/>,
<xref linkend="libpq-PQsendClosePortal"/>,
<xref linkend="libpq-PQsendPipelineSync"/>, or
<xref linkend="libpq-PQpipelineSync"/>
call, and returns it.
A null pointer is returned when the command is complete and there
will be no more results.
<synopsis>
PGresult *PQgetResult(PGconn *conn);
</synopsis>
</para>
<para>
<xref linkend="libpq-PQgetResult"/> must be called repeatedly until
it returns a null pointer, indicating that the command is done.
(If called when no command is active,
<xref linkend="libpq-PQgetResult"/> will just return a null pointer
at once.) Each non-null result from
<xref linkend="libpq-PQgetResult"/> should be processed using the
same <structname>PGresult</structname> accessor functions previously
described. Don't forget to free each result object with
<xref linkend="libpq-PQclear"/> when done with it. Note that
<xref linkend="libpq-PQgetResult"/> will block only if a command is
active and the necessary response data has not yet been read by
<xref linkend="libpq-PQconsumeInput"/>.
</para>
<para>
In pipeline mode, <function>PQgetResult</function> will return normally
unless an error occurs; for any subsequent query sent after the one
that caused the error until (and excluding) the next synchronization point,
a special result of type <literal>PGRES_PIPELINE_ABORTED</literal> will
be returned, and a null pointer will be returned after it.
When the pipeline synchronization point is reached, a result of type
<literal>PGRES_PIPELINE_SYNC</literal> will be returned.
The result of the next query after the synchronization point follows
immediately (that is, no null pointer is returned after
the synchronization point.)
</para>
<note>
<para>
Even when <xref linkend="libpq-PQresultStatus"/> indicates a fatal
error, <xref linkend="libpq-PQgetResult"/> should be called until it
returns a null pointer, to allow <application>libpq</application> to
process the error information completely.
</para>
</note>
</listitem>
</varlistentry>
</variablelist>
</para>
<para>
Using <xref linkend="libpq-PQsendQuery"/> and
<xref linkend="libpq-PQgetResult"/> solves one of
<xref linkend="libpq-PQexec"/>'s problems: If a command string contains
multiple <acronym>SQL</acronym> commands, the results of those commands
can be obtained individually. (This allows a simple form of overlapped
processing, by the way: the client can be handling the results of one
command while the server is still working on later queries in the same
command string.)
</para>
<para>
Another frequently-desired feature that can be obtained with
<xref linkend="libpq-PQsendQuery"/> and <xref linkend="libpq-PQgetResult"/>
is retrieving large query results a limited number of rows at a time.
This is discussed
in <xref linkend="libpq-single-row-mode"/>.
</para>
<para>
By itself, calling <xref linkend="libpq-PQgetResult"/>
will still cause the client to block until the server completes the
next <acronym>SQL</acronym> command. This can be avoided by proper
use of two more functions:
<variablelist>
<varlistentry id="libpq-PQconsumeInput">
<term><function>PQconsumeInput</function><indexterm><primary>PQconsumeInput</primary></indexterm>
</term>
<listitem>
<para>
If input is available from the server, consume it.
<synopsis>
int PQconsumeInput(PGconn *conn);
</synopsis>
</para>
<para>
<xref linkend="libpq-PQconsumeInput"/> normally returns 1 indicating
<quote>no error</quote>, but returns 0 if there was some kind of
trouble (in which case <xref linkend="libpq-PQerrorMessage"/> can be
consulted). Note that the result does not say whether any input
data was actually collected. After calling
<xref linkend="libpq-PQconsumeInput"/>, the application can check
<xref linkend="libpq-PQisBusy"/> and/or
<function>PQnotifies</function> to see if their state has changed.
</para>
<para>
<xref linkend="libpq-PQconsumeInput"/> can be called even if the
application is not prepared to deal with a result or notification
just yet. The function will read available data and save it in
a buffer, thereby causing a <function>select()</function>
read-ready indication to go away. The application can thus use
<xref linkend="libpq-PQconsumeInput"/> to clear the
<function>select()</function> condition immediately, and then
examine the results at leisure.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQisBusy">
<term><function>PQisBusy</function><indexterm><primary>PQisBusy</primary></indexterm></term>
<listitem>
<para>
Returns 1 if a command is busy, that is,
<xref linkend="libpq-PQgetResult"/> would block waiting for input.
A 0 return indicates that <xref linkend="libpq-PQgetResult"/> can be
called with assurance of not blocking.
<synopsis>
int PQisBusy(PGconn *conn);
</synopsis>
</para>
<para>
<xref linkend="libpq-PQisBusy"/> will not itself attempt to read data
from the server; therefore <xref linkend="libpq-PQconsumeInput"/>
must be invoked first, or the busy state will never end.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
<para>
A typical application using these functions will have a main loop that
uses <function>select()</function> or <function>poll()</function> to wait for
all the conditions that it must respond to. One of the conditions
will be input available from the server, which in terms of
<function>select()</function> means readable data on the file
descriptor identified by <xref linkend="libpq-PQsocket"/>. When the main
loop detects input ready, it should call
<xref linkend="libpq-PQconsumeInput"/> to read the input. It can then
call <xref linkend="libpq-PQisBusy"/>, followed by
<xref linkend="libpq-PQgetResult"/> if <xref linkend="libpq-PQisBusy"/>
returns false (0). It can also call <function>PQnotifies</function>
to detect <command>NOTIFY</command> messages (see <xref
linkend="libpq-notify"/>).
</para>
<para>
A client that uses
<xref linkend="libpq-PQsendQuery"/>/<xref linkend="libpq-PQgetResult"/>
can also attempt to cancel a command that is still being processed
by the server; see <xref linkend="libpq-cancel"/>. But regardless of
the return value of <xref linkend="libpq-PQcancelBlocking"/>, the application
must continue with the normal result-reading sequence using
<xref linkend="libpq-PQgetResult"/>. A successful cancellation will
simply cause the command to terminate sooner than it would have
otherwise.
</para>
<para>
By using the functions described above, it is possible to avoid
blocking while waiting for input from the database server. However,
it is still possible that the application will block waiting to send
output to the server. This is relatively uncommon but can happen if
very long SQL commands or data values are sent. (It is much more
probable if the application sends data via <command>COPY IN</command>,
however.) To prevent this possibility and achieve completely
nonblocking database operation, the following additional functions
can be used.
<variablelist>
<varlistentry id="libpq-PQsetnonblocking">
<term><function>PQsetnonblocking</function><indexterm><primary>PQsetnonblocking</primary></indexterm></term>
<listitem>
<para>
Sets the nonblocking status of the connection.
<synopsis>
int PQsetnonblocking(PGconn *conn, int arg);
</synopsis>
</para>
<para>
Sets the state of the connection to nonblocking if
<parameter>arg</parameter> is 1, or blocking if
<parameter>arg</parameter> is 0. Returns 0 if OK, -1 if error.
</para>
<para>
In the nonblocking state, successful calls to
<xref linkend="libpq-PQsendQuery"/>, <xref linkend="libpq-PQputline"/>,
<xref linkend="libpq-PQputnbytes"/>, <xref linkend="libpq-PQputCopyData"/>,
and <xref linkend="libpq-PQendcopy"/> will not block; their changes
are stored in the local output buffer until they are flushed.
Unsuccessful calls will return an error and must be retried.
</para>
<para>
Note that <xref linkend="libpq-PQexec"/> does not honor nonblocking
mode; if it is called, it will act in blocking fashion anyway.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQisnonblocking">
<term><function>PQisnonblocking</function><indexterm><primary>PQisnonblocking</primary></indexterm></term>
<listitem>
<para>
Returns the blocking status of the database connection.
<synopsis>
int PQisnonblocking(const PGconn *conn);
</synopsis>
</para>
<para>
Returns 1 if the connection is set to nonblocking mode and 0 if
blocking.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQflush">
<term><function>PQflush</function><indexterm><primary>PQflush</primary></indexterm></term>
<listitem>
<para>
Attempts to flush any queued output data to the server. Returns
0 if successful (or if the send queue is empty), -1 if it failed
for some reason, or 1 if it was unable to send all the data in
the send queue yet (this case can only occur if the connection
is nonblocking).
<synopsis>
int PQflush(PGconn *conn);
</synopsis>
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
<para>
After sending any command or data on a nonblocking connection, call
<xref linkend="libpq-PQflush"/>. If it returns 1, wait for the socket
to become read- or write-ready. If it becomes write-ready, call
<xref linkend="libpq-PQflush"/> again. If it becomes read-ready, call
<xref linkend="libpq-PQconsumeInput"/>, then call
<xref linkend="libpq-PQflush"/> again. Repeat until
<xref linkend="libpq-PQflush"/> returns 0. (It is necessary to check for
read-ready and drain the input with <xref linkend="libpq-PQconsumeInput"/>,
because the server can block trying to send us data, e.g., NOTICE
messages, and won't read our data until we read its.) Once
<xref linkend="libpq-PQflush"/> returns 0, wait for the socket to be
read-ready and then read the response as described above.
</para>
</sect1>
<sect1 id="libpq-pipeline-mode">
<title>Pipeline Mode</title>
<indexterm zone="libpq-pipeline-mode">
<primary>libpq</primary>
<secondary>pipeline mode</secondary>
</indexterm>
<indexterm zone="libpq-pipeline-mode">
<primary>pipelining</primary>
<secondary>in libpq</secondary>
</indexterm>
<indexterm zone="libpq-pipeline-mode">
<primary>batch mode</primary>
<secondary>in libpq</secondary>
</indexterm>
<para>
<application>libpq</application> pipeline mode allows applications to
send a query without having to read the result of the previously
sent query. Taking advantage of the pipeline mode, a client will wait
less for the server, since multiple queries/results can be
sent/received in a single network transaction.
</para>
<para>
While pipeline mode provides a significant performance boost, writing
clients using the pipeline mode is more complex because it involves
managing a queue of pending queries and finding which result
corresponds to which query in the queue.
</para>
<para>
Pipeline mode also generally consumes more memory on both the client and server,
though careful and aggressive management of the send/receive queue can mitigate
this. This applies whether or not the connection is in blocking or non-blocking
mode.
</para>
<para>
While <application>libpq</application>'s pipeline API was introduced in
<productname>PostgreSQL</productname> 14, it is a client-side feature
which doesn't require special server support and works on any server
that supports the v3 extended query protocol. For more information see
<xref linkend="protocol-flow-pipelining"/>.
</para>
<sect2 id="libpq-pipeline-using">
<title>Using Pipeline Mode</title>
<para>
To issue pipelines, the application must switch the connection
into pipeline mode,
which is done with <xref linkend="libpq-PQenterPipelineMode"/>.
<xref linkend="libpq-PQpipelineStatus"/> can be used
to test whether pipeline mode is active.
In pipeline mode, only <link linkend="libpq-async">asynchronous operations</link>
that utilize the extended query protocol
are permitted, command strings containing multiple SQL commands are
disallowed, and so is <literal>COPY</literal>.
Using synchronous command execution functions
such as <function>PQfn</function>,
<function>PQexec</function>,
<function>PQexecParams</function>,
<function>PQprepare</function>,
<function>PQexecPrepared</function>,
<function>PQdescribePrepared</function>,
<function>PQdescribePortal</function>,
<function>PQclosePrepared</function>,
<function>PQclosePortal</function>,
is an error condition.
<function>PQsendQuery</function> is
also disallowed, because it uses the simple query protocol.
Once all dispatched commands have had their results processed, and
the end pipeline result has been consumed, the application may return
to non-pipelined mode with <xref linkend="libpq-PQexitPipelineMode"/>.
</para>
<note>
<para>
It is best to use pipeline mode with <application>libpq</application> in
<link linkend="libpq-PQsetnonblocking">non-blocking mode</link>. If used
in blocking mode it is possible for a client/server deadlock to occur.
<footnote>
<para>
The client will block trying to send queries to the server, but the
server will block trying to send results to the client from queries
it has already processed. This only occurs when the client sends
enough queries to fill both its output buffer and the server's receive
buffer before it switches to processing input from the server,
but it's hard to predict exactly when that will happen.
</para>
</footnote>
</para>
</note>
<sect3 id="libpq-pipeline-sending">
<title>Issuing Queries</title>
<para>
After entering pipeline mode, the application dispatches requests using
<xref linkend="libpq-PQsendQueryParams"/>
or its prepared-query sibling
<xref linkend="libpq-PQsendQueryPrepared"/>.
These requests are queued on the client-side until flushed to the server;
this occurs when <xref linkend="libpq-PQpipelineSync"/> is used to
establish a synchronization point in the pipeline,
or when <xref linkend="libpq-PQflush"/> is called.
The functions <xref linkend="libpq-PQsendPrepare"/>,
<xref linkend="libpq-PQsendDescribePrepared"/>,
<xref linkend="libpq-PQsendDescribePortal"/>,
<xref linkend="libpq-PQsendClosePrepared"/>, and
<xref linkend="libpq-PQsendClosePortal"/> also work in pipeline mode.
Result processing is described below.
</para>
<para>
The server executes statements, and returns results, in the order the
client sends them. The server will begin executing the commands in the
pipeline immediately, not waiting for the end of the pipeline.
Note that results are buffered on the server side; the server flushes
that buffer when a synchronization point is established with either
<function>PQpipelineSync</function> or
<function>PQsendPipelineSync</function>, or when
<function>PQsendFlushRequest</function> is called.
If any statement encounters an error, the server aborts the current
transaction and does not execute any subsequent command in the queue
until the next synchronization point;
a <literal>PGRES_PIPELINE_ABORTED</literal> result is produced for
each such command.
(This remains true even if the commands in the pipeline would rollback
the transaction.)
Query processing resumes after the synchronization point.
</para>
<para>
It's fine for one operation to depend on the results of a
prior one; for example, one query may define a table that the next
query in the same pipeline uses. Similarly, an application may
create a named prepared statement and execute it with later
statements in the same pipeline.
</para>
</sect3>
<sect3 id="libpq-pipeline-results">
<title>Processing Results</title>
<para>
To process the result of one query in a pipeline, the application calls
<function>PQgetResult</function> repeatedly and handles each result
until <function>PQgetResult</function> returns null.
The result from the next query in the pipeline may then be retrieved using
<function>PQgetResult</function> again and the cycle repeated.
The application handles individual statement results as normal.
When the results of all the queries in the pipeline have been
returned, <function>PQgetResult</function> returns a result
containing the status value <literal>PGRES_PIPELINE_SYNC</literal>
</para>
<para>
The client may choose to defer result processing until the complete
pipeline has been sent, or interleave that with sending further
queries in the pipeline; see <xref linkend="libpq-pipeline-interleave"/>.
</para>
<para>
<function>PQgetResult</function> behaves the same as for normal
asynchronous processing except that it may contain the new
<type>PGresult</type> types <literal>PGRES_PIPELINE_SYNC</literal>
and <literal>PGRES_PIPELINE_ABORTED</literal>.
<literal>PGRES_PIPELINE_SYNC</literal> is reported exactly once for each
<function>PQpipelineSync</function> or
<function>PQsendPipelineSync</function> at the corresponding point
in the pipeline.
<literal>PGRES_PIPELINE_ABORTED</literal> is emitted in place of a normal
query result for the first error and all subsequent results
until the next <literal>PGRES_PIPELINE_SYNC</literal>;
see <xref linkend="libpq-pipeline-errors"/>.
</para>
<para>
<function>PQisBusy</function>, <function>PQconsumeInput</function>, etc
operate as normal when processing pipeline results. In particular,
a call to <function>PQisBusy</function> in the middle of a pipeline
returns 0 if the results for all the queries issued so far have been
consumed.
</para>
<para>
<application>libpq</application> does not provide any information to the
application about the query currently being processed (except that
<function>PQgetResult</function> returns null to indicate that we start
returning the results of next query). The application must keep track
of the order in which it sent queries, to associate them with their
corresponding results.
Applications will typically use a state machine or a FIFO queue for this.
</para>
</sect3>
<sect3 id="libpq-pipeline-errors">
<title>Error Handling</title>
<para>
From the client's perspective, after <function>PQresultStatus</function>
returns <literal>PGRES_FATAL_ERROR</literal>,
the pipeline is flagged as aborted.
<function>PQresultStatus</function> will report a
<literal>PGRES_PIPELINE_ABORTED</literal> result for each remaining queued
operation in an aborted pipeline. The result for
<function>PQpipelineSync</function> or
<function>PQsendPipelineSync</function> is reported as
<literal>PGRES_PIPELINE_SYNC</literal> to signal the end of the aborted pipeline
and resumption of normal result processing.
</para>
<para>
The client <emphasis>must</emphasis> process results with
<function>PQgetResult</function> during error recovery.
</para>
<para>
If the pipeline used an implicit transaction, then operations that have
already executed are rolled back and operations that were queued to follow
the failed operation are skipped entirely. The same behavior holds if the
pipeline starts and commits a single explicit transaction (i.e. the first
statement is <literal>BEGIN</literal> and the last is
<literal>COMMIT</literal>) except that the session remains in an aborted
transaction state at the end of the pipeline. If a pipeline contains
<emphasis>multiple explicit transactions</emphasis>, all transactions that
committed prior to the error remain committed, the currently in-progress
transaction is aborted, and all subsequent operations are skipped completely,
including subsequent transactions. If a pipeline synchronization point
occurs with an explicit transaction block in aborted state, the next pipeline
will become aborted immediately unless the next command puts the transaction
in normal mode with <command>ROLLBACK</command>.
</para>
<note>
<para>
The client must not assume that work is committed when it
<emphasis>sends</emphasis> a <literal>COMMIT</literal> &mdash; only when the
corresponding result is received to confirm the commit is complete.
Because errors arrive asynchronously, the application needs to be able to
restart from the last <emphasis>received</emphasis> committed change and
resend work done after that point if something goes wrong.
</para>
</note>
</sect3>
<sect3 id="libpq-pipeline-interleave">
<title>Interleaving Result Processing and Query Dispatch</title>
<para>
To avoid deadlocks on large pipelines the client should be structured
around a non-blocking event loop using operating system facilities
such as <function>select</function>, <function>poll</function>,
<function>WaitForMultipleObjectEx</function>, etc.
</para>
<para>
The client application should generally maintain a queue of work
remaining to be dispatched and a queue of work that has been dispatched
but not yet had its results processed. When the socket is writable
it should dispatch more work. When the socket is readable it should
read results and process them, matching them up to the next entry in
its corresponding results queue. Based on available memory, results from the
socket should be read frequently: there's no need to wait until the
pipeline end to read the results. Pipelines should be scoped to logical
units of work, usually (but not necessarily) one transaction per pipeline.
There's no need to exit pipeline mode and re-enter it between pipelines,
or to wait for one pipeline to finish before sending the next.
</para>
<para>
An example using <function>select()</function> and a simple state
machine to track sent and received work is in
<filename>src/test/modules/libpq_pipeline/libpq_pipeline.c</filename>
in the PostgreSQL source distribution.
</para>
</sect3>
</sect2>
<sect2 id="libpq-pipeline-functions">
<title>Functions Associated with Pipeline Mode</title>
<variablelist>
<varlistentry id="libpq-PQpipelineStatus">
<term><function>PQpipelineStatus</function><indexterm><primary>PQpipelineStatus</primary></indexterm></term>
<listitem>
<para>
Returns the current pipeline mode status of the
<application>libpq</application> connection.
<synopsis>
PGpipelineStatus PQpipelineStatus(const PGconn *conn);
</synopsis>
</para>
<para>
<function>PQpipelineStatus</function> can return one of the following values:
<variablelist>
<varlistentry>
<term>
<literal>PQ_PIPELINE_ON</literal>
</term>
<listitem>
<para>
The <application>libpq</application> connection is in
pipeline mode.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<literal>PQ_PIPELINE_OFF</literal>
</term>
<listitem>
<para>
The <application>libpq</application> connection is
<emphasis>not</emphasis> in pipeline mode.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<literal>PQ_PIPELINE_ABORTED</literal>
</term>
<listitem>
<para>
The <application>libpq</application> connection is in pipeline
mode and an error occurred while processing the current pipeline.
The aborted flag is cleared when <function>PQgetResult</function>
returns a result of type <literal>PGRES_PIPELINE_SYNC</literal>.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQenterPipelineMode">
<term><function>PQenterPipelineMode</function><indexterm><primary>PQenterPipelineMode</primary></indexterm></term>
<listitem>
<para>
Causes a connection to enter pipeline mode if it is currently idle or
already in pipeline mode.
<synopsis>
int PQenterPipelineMode(PGconn *conn);
</synopsis>
</para>
<para>
Returns 1 for success.
Returns 0 and has no effect if the connection is not currently
idle, i.e., it has a result ready, or it is waiting for more
input from the server, etc.
This function does not actually send anything to the server,
it just changes the <application>libpq</application> connection
state.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQexitPipelineMode">
<term><function>PQexitPipelineMode</function><indexterm><primary>PQexitPipelineMode</primary></indexterm></term>
<listitem>
<para>
Causes a connection to exit pipeline mode if it is currently in pipeline mode
with an empty queue and no pending results.
<synopsis>
int PQexitPipelineMode(PGconn *conn);
</synopsis>
</para>
<para>
Returns 1 for success. Returns 1 and takes no action if not in
pipeline mode. If the current statement isn't finished processing,
or <function>PQgetResult</function> has not been called to collect
results from all previously sent query, returns 0 (in which case,
use <xref linkend="libpq-PQerrorMessage"/> to get more information
about the failure).
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQpipelineSync">
<term><function>PQpipelineSync</function><indexterm><primary>PQpipelineSync</primary></indexterm></term>
<listitem>
<para>
Marks a synchronization point in a pipeline by sending a
<link linkend="protocol-flow-ext-query">sync message</link>
and flushing the send buffer. This serves as
the delimiter of an implicit transaction and an error recovery
point; see <xref linkend="libpq-pipeline-errors"/>.
<synopsis>
int PQpipelineSync(PGconn *conn);
</synopsis>
</para>
<para>
Returns 1 for success. Returns 0 if the connection is not in
pipeline mode or sending a
<link linkend="protocol-flow-ext-query">sync message</link>
failed.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQsendPipelineSync">
<term><function>PQsendPipelineSync</function><indexterm><primary>PQsendPipelineSync</primary></indexterm></term>
<listitem>
<para>
Marks a synchronization point in a pipeline by sending a
<link linkend="protocol-flow-ext-query">sync message</link>
without flushing the send buffer. This serves as
the delimiter of an implicit transaction and an error recovery
point; see <xref linkend="libpq-pipeline-errors"/>.
<synopsis>
int PQsendPipelineSync(PGconn *conn);
</synopsis>
</para>
<para>
Returns 1 for success. Returns 0 if the connection is not in
pipeline mode or sending a
<link linkend="protocol-flow-ext-query">sync message</link>
failed.
Note that the message is not itself flushed to the server automatically;
use <function>PQflush</function> if necessary.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQsendFlushRequest">
<term><function>PQsendFlushRequest</function><indexterm><primary>PQsendFlushRequest</primary></indexterm></term>
<listitem>
<para>
Sends a request for the server to flush its output buffer.
<synopsis>
int PQsendFlushRequest(PGconn *conn);
</synopsis>
</para>
<para>
Returns 1 for success. Returns 0 on any failure.
</para>
<para>
The server flushes its output buffer automatically as a result of
<function>PQpipelineSync</function> being called, or
on any request when not in pipeline mode; this function is useful
to cause the server to flush its output buffer in pipeline mode
without establishing a synchronization point.
Note that the request is not itself flushed to the server automatically;
use <function>PQflush</function> if necessary.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="libpq-pipeline-tips">
<title>When to Use Pipeline Mode</title>
<para>
Much like asynchronous query mode, there is no meaningful performance
overhead when using pipeline mode. It increases client application complexity,
and extra caution is required to prevent client/server deadlocks, but
pipeline mode can offer considerable performance improvements, in exchange for
increased memory usage from leaving state around longer.
</para>
<para>
Pipeline mode is most useful when the server is distant, i.e., network latency
(<quote>ping time</quote>) is high, and also when many small operations
are being performed in rapid succession. There is usually less benefit
in using pipelined commands when each query takes many multiples of the client/server
round-trip time to execute. A 100-statement operation run on a server
300 ms round-trip-time away would take 30 seconds in network latency alone
without pipelining; with pipelining it may spend as little as 0.3 s waiting for
results from the server.
</para>
<para>
Use pipelined commands when your application does lots of small
<literal>INSERT</literal>, <literal>UPDATE</literal> and
<literal>DELETE</literal> operations that can't easily be transformed
into operations on sets, or into a <literal>COPY</literal> operation.
</para>
<para>
Pipeline mode is not useful when information from one operation is required by
the client to produce the next operation. In such cases, the client
would have to introduce a synchronization point and wait for a full client/server
round-trip to get the results it needs. However, it's often possible to
adjust the client design to exchange the required information server-side.
Read-modify-write cycles are especially good candidates; for example:
<programlisting>
BEGIN;
SELECT x FROM mytable WHERE id = 42 FOR UPDATE;
-- result: x=2
-- client adds 1 to x:
UPDATE mytable SET x = 3 WHERE id = 42;
COMMIT;
</programlisting>
could be much more efficiently done with:
<programlisting>
UPDATE mytable SET x = x + 1 WHERE id = 42;
</programlisting>
</para>
<para>
Pipelining is less useful, and more complex, when a single pipeline contains
multiple transactions (see <xref linkend="libpq-pipeline-errors"/>).
</para>
</sect2>
</sect1>
<!-- keep this not-too-apropos sect1 ID for stability of doc URLs -->
<sect1 id="libpq-single-row-mode">
<title>Retrieving Query Results in Chunks</title>
<indexterm zone="libpq-single-row-mode">
<primary>libpq</primary>
<secondary>single-row mode</secondary>
</indexterm>
<indexterm zone="libpq-single-row-mode">
<primary>libpq</primary>
<secondary>chunked mode</secondary>
</indexterm>
<para>
Ordinarily, <application>libpq</application> collects an SQL command's
entire result and returns it to the application as a single
<structname>PGresult</structname>. This can be unworkable for commands
that return a large number of rows. For such cases, applications can use
<xref linkend="libpq-PQsendQuery"/> and <xref linkend="libpq-PQgetResult"/> in
<firstterm>single-row mode</firstterm> or <firstterm>chunked
mode</firstterm>. In these modes, result row(s) are returned to the
application as they are received from the server, one at a time for
single-row mode or in groups for chunked mode.
</para>
<para>
To enter one of these modes, call <xref linkend="libpq-PQsetSingleRowMode"/>
or <xref linkend="libpq-PQsetChunkedRowsMode"/>
immediately after a successful call of <xref linkend="libpq-PQsendQuery"/>
(or a sibling function). This mode selection is effective only for the
currently executing query. Then call <xref linkend="libpq-PQgetResult"/>
repeatedly, until it returns null, as documented in <xref
linkend="libpq-async"/>. If the query returns any rows, they are returned
as one or more <structname>PGresult</structname> objects, which look like
normal query results except for having status code
<literal>PGRES_SINGLE_TUPLE</literal> for single-row mode or
<literal>PGRES_TUPLES_CHUNK</literal> for chunked mode, instead of
<literal>PGRES_TUPLES_OK</literal>. There is exactly one result row in
each <literal>PGRES_SINGLE_TUPLE</literal> object, while
a <literal>PGRES_TUPLES_CHUNK</literal> object contains at least one
row but not more than the specified number of rows per chunk.
After the last row, or immediately if
the query returns zero rows, a zero-row object with status
<literal>PGRES_TUPLES_OK</literal> is returned; this is the signal that no
more rows will arrive. (But note that it is still necessary to continue
calling <xref linkend="libpq-PQgetResult"/> until it returns null.) All of
these <structname>PGresult</structname> objects will contain the same row
description data (column names, types, etc.) that an ordinary
<structname>PGresult</structname> object for the query would have.
Each object should be freed with <xref linkend="libpq-PQclear"/> as usual.
</para>
<para>
When using pipeline mode, single-row or chunked mode needs to be
activated for each query in the pipeline before retrieving results for
that query with <function>PQgetResult</function>.
See <xref linkend="libpq-pipeline-mode"/> for more information.
</para>
<para>
<variablelist>
<varlistentry id="libpq-PQsetSingleRowMode">
<term><function>PQsetSingleRowMode</function><indexterm><primary>PQsetSingleRowMode</primary></indexterm></term>
<listitem>
<para>
Select single-row mode for the currently-executing query.
<synopsis>
int PQsetSingleRowMode(PGconn *conn);
</synopsis>
</para>
<para>
This function can only be called immediately after
<xref linkend="libpq-PQsendQuery"/> or one of its sibling functions,
before any other operation on the connection such as
<xref linkend="libpq-PQconsumeInput"/> or
<xref linkend="libpq-PQgetResult"/>. If called at the correct time,
the function activates single-row mode for the current query and
returns 1. Otherwise the mode stays unchanged and the function
returns 0. In any case, the mode reverts to normal after
completion of the current query.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQsetChunkedRowsMode">
<term><function>PQsetChunkedRowsMode</function><indexterm><primary>PQsetChunkedRowsMode</primary></indexterm></term>
<listitem>
<para>
Select chunked mode for the currently-executing query.
<synopsis>
int PQsetChunkedRowsMode(PGconn *conn, int chunkSize);
</synopsis>
</para>
<para>
This function is similar to
<xref linkend="libpq-PQsetSingleRowMode"/>, except that it
specifies retrieval of up to <replaceable>chunkSize</replaceable> rows
per <structname>PGresult</structname>, not necessarily just one row.
This function can only be called immediately after
<xref linkend="libpq-PQsendQuery"/> or one of its sibling functions,
before any other operation on the connection such as
<xref linkend="libpq-PQconsumeInput"/> or
<xref linkend="libpq-PQgetResult"/>. If called at the correct time,
the function activates chunked mode for the current query and
returns 1. Otherwise the mode stays unchanged and the function
returns 0. In any case, the mode reverts to normal after
completion of the current query.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
<caution>
<para>
While processing a query, the server may return some rows and then
encounter an error, causing the query to be aborted. Ordinarily,
<application>libpq</application> discards any such rows and reports only the
error. But in single-row or chunked mode, some rows may have already
been returned to the application. Hence, the application will see some
<literal>PGRES_SINGLE_TUPLE</literal> or <literal>PGRES_TUPLES_CHUNK</literal>
<structname>PGresult</structname>
objects followed by a <literal>PGRES_FATAL_ERROR</literal> object. For
proper transactional behavior, the application must be designed to
discard or undo whatever has been done with the previously-processed
rows, if the query ultimately fails.
</para>
</caution>
</sect1>
<sect1 id="libpq-cancel">
<title>Canceling Queries in Progress</title>
<indexterm zone="libpq-cancel">
<primary>canceling SQL queries</primary>
</indexterm>
<indexterm zone="libpq-cancel">
<primary>query cancellation</primary>
</indexterm>
<sect2 id="libpq-cancel-functions">
<title>Functions for Sending Cancel Requests</title>
<variablelist>
<varlistentry id="libpq-PQcancelCreate">
<term><function>PQcancelCreate</function><indexterm><primary>PQcancelCreate</primary></indexterm></term>
<listitem>
<para>
Prepares a connection over which a cancel request can be sent.
<synopsis>
PGcancelConn *PQcancelCreate(PGconn *conn);
</synopsis>
</para>
<para>
<xref linkend="libpq-PQcancelCreate"/> creates a
<structname>PGcancelConn</structname><indexterm><primary>PGcancelConn</primary></indexterm>
object, but it won't instantly start sending a cancel request over this
connection. A cancel request can be sent over this connection in a
blocking manner using <xref linkend="libpq-PQcancelBlocking"/> and in a
non-blocking manner using <xref linkend="libpq-PQcancelStart"/>.
The return value can be passed to <xref linkend="libpq-PQcancelStatus"/>
to check if the <structname>PGcancelConn</structname> object was
created successfully. The <structname>PGcancelConn</structname> object
is an opaque structure that is not meant to be accessed directly by the
application. This <structname>PGcancelConn</structname> object can be
used to cancel the query that's running on the original connection in a
thread-safe way.
</para>
<para>
Many connection parameters of the original client will be reused when
setting up the connection for the cancel request. Importantly, if the
original connection requires encryption of the connection and/or
verification of the target host (using <literal>sslmode</literal> or
<literal>gssencmode</literal>), then the connection for the cancel
request is made with these same requirements. Any connection options
that are only used during authentication or after authentication of the
client are ignored though, because cancellation requests do not require
authentication and the connection is closed right after the cancellation
request is submitted.
</para>
<para>
Note that when <function>PQcancelCreate</function> returns a non-null
pointer, you must call <xref linkend="libpq-PQcancelFinish"/> when you
are finished with it, in order to dispose of the structure and any
associated memory blocks. This must be done even if the cancel request
failed or was abandoned.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQcancelBlocking">
<term><function>PQcancelBlocking</function><indexterm><primary>PQcancelBlocking</primary></indexterm></term>
<listitem>
<para>
Requests that the server abandons processing of the current command
in a blocking manner.
<synopsis>
int PQcancelBlocking(PGcancelConn *cancelConn);
</synopsis>
</para>
<para>
The request is made over the given <structname>PGcancelConn</structname>,
which needs to be created with <xref linkend="libpq-PQcancelCreate"/>.
The return value of <xref linkend="libpq-PQcancelBlocking"/>
is 1 if the cancel request was successfully
dispatched and 0 if not. If it was unsuccessful, the error message can be
retrieved using <xref linkend="libpq-PQcancelErrorMessage"/>.
</para>
<para>
Successful dispatch of the cancellation is no guarantee that the request
will have any effect, however. If the cancellation is effective, the
command being canceled will terminate early and return an error result.
If the cancellation fails (say, because the server was already done
processing the command), then there will be no visible result at all.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQcancelStart">
<term><function>PQcancelStart</function><indexterm><primary>PQcancelStart</primary></indexterm></term>
<term><function>PQcancelPoll</function><indexterm><primary>PQcancelPoll</primary></indexterm></term>
<listitem>
<para>
Requests that the server abandons processing of the current command
in a non-blocking manner.
<synopsis>
int PQcancelStart(PGcancelConn *cancelConn);
PostgresPollingStatusType PQcancelPoll(PGcancelConn *cancelConn);
</synopsis>
</para>
<para>
The request is made over the given <structname>PGcancelConn</structname>,
which needs to be created with <xref linkend="libpq-PQcancelCreate"/>.
The return value of <xref linkend="libpq-PQcancelStart"/>
is 1 if the cancellation request could be started and 0 if not.
If it was unsuccessful, the error message can be
retrieved using <xref linkend="libpq-PQcancelErrorMessage"/>.
</para>
<para>
If <function>PQcancelStart</function> succeeds, the next stage
is to poll <application>libpq</application> so that it can proceed with
the cancel connection sequence.
Use <xref linkend="libpq-PQcancelSocket"/> to obtain the descriptor of the
socket underlying the database connection.
(Caution: do not assume that the socket remains the same
across <function>PQcancelPoll</function> calls.)
Loop thus: If <function>PQcancelPoll(cancelConn)</function> last returned
<symbol>PGRES_POLLING_READING</symbol>, wait until the socket is ready to
read (as indicated by <function>select()</function>,
<function>poll()</function>, or similar system function).
Then call <function>PQcancelPoll(cancelConn)</function> again.
Conversely, if <function>PQcancelPoll(cancelConn)</function> last returned
<symbol>PGRES_POLLING_WRITING</symbol>, wait until the socket is ready
to write, then call <function>PQcancelPoll(cancelConn)</function> again.
On the first iteration, i.e., if you have yet to call
<function>PQcancelPoll(cancelConn)</function>, behave as if it last returned
<symbol>PGRES_POLLING_WRITING</symbol>. Continue this loop until
<function>PQcancelPoll(cancelConn)</function> returns
<symbol>PGRES_POLLING_FAILED</symbol>, indicating the connection procedure
has failed, or <symbol>PGRES_POLLING_OK</symbol>, indicating cancel
request was successfully dispatched.
</para>
<para>
Successful dispatch of the cancellation is no guarantee that the request
will have any effect, however. If the cancellation is effective, the
command being canceled will terminate early and return an error result.
If the cancellation fails (say, because the server was already done
processing the command), then there will be no visible result at all.
</para>
<para>
At any time during connection, the status of the connection can be
checked by calling <xref linkend="libpq-PQcancelStatus"/>.
If this call returns <symbol>CONNECTION_BAD</symbol>, then
the cancel procedure has failed; if the call returns
<function>CONNECTION_OK</function>, then cancel request was
successfully dispatched.
Both of these states are equally detectable from the return value of
<function>PQcancelPoll</function>, described above.
Other states might also occur during (and only during) an asynchronous
connection procedure.
These indicate the current stage of the connection procedure and might
be useful to provide feedback to the user for example.
These statuses are:
<variablelist>
<varlistentry id="libpq-cancel-connection-allocated">
<term><symbol>CONNECTION_ALLOCATED</symbol></term>
<listitem>
<para>
Waiting for a call to <xref linkend="libpq-PQcancelStart"/> or
<xref linkend="libpq-PQcancelBlocking"/>, to actually open the
socket. This is the connection state right after
calling <xref linkend="libpq-PQcancelCreate"/>
or <xref linkend="libpq-PQcancelReset"/>. No connection to the
server has been initiated yet at this point. To actually start
sending the cancel request use <xref linkend="libpq-PQcancelStart"/> or
<xref linkend="libpq-PQcancelBlocking"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-cancel-connection-started">
<term><symbol>CONNECTION_STARTED</symbol></term>
<listitem>
<para>
Waiting for connection to be made.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-cancel-connection-made">
<term><symbol>CONNECTION_MADE</symbol></term>
<listitem>
<para>
Connection OK; waiting to send.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-cancel-connection-awaiting-response">
<term><symbol>CONNECTION_AWAITING_RESPONSE</symbol></term>
<listitem>
<para>
Waiting for a response from the server.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-cancel-connection-ssl-startup">
<term><symbol>CONNECTION_SSL_STARTUP</symbol></term>
<listitem>
<para>
Negotiating SSL encryption.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-cancel-connection-gss-startup">
<term><symbol>CONNECTION_GSS_STARTUP</symbol></term>
<listitem>
<para>
Negotiating GSS encryption.
</para>
</listitem>
</varlistentry>
</variablelist>
Note that, although these constants will remain (in order to maintain
compatibility), an application should never rely upon these occurring in a
particular order, or at all, or on the status always being one of these
documented values. An application might do something like this:
<programlisting>
switch(PQcancelStatus(conn))
{
case CONNECTION_STARTED:
feedback = "Connecting...";
break;
case CONNECTION_MADE:
feedback = "Connected to server...";
break;
.
.
.
default:
feedback = "Connecting...";
}
</programlisting>
</para>
<para>
The <literal>connect_timeout</literal> connection parameter is ignored
when using <function>PQcancelPoll</function>; it is the application's
responsibility to decide whether an excessive amount of time has elapsed.
Otherwise, <function>PQcancelStart</function> followed by a
<function>PQcancelPoll</function> loop is equivalent to
<xref linkend="libpq-PQcancelBlocking"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQcancelStatus">
<term><function>PQcancelStatus</function><indexterm><primary>PQcancelStatus</primary></indexterm></term>
<listitem>
<para>
Returns the status of the cancel connection.
<synopsis>
ConnStatusType PQcancelStatus(const PGcancelConn *cancelConn);
</synopsis>
</para>
<para>
The status can be one of a number of values. However, only three of
these are seen outside of an asynchronous cancel procedure:
<literal>CONNECTION_ALLOCATED</literal>,
<literal>CONNECTION_OK</literal> and
<literal>CONNECTION_BAD</literal>. The initial state of a
<function>PGcancelConn</function> that's successfully created using
<xref linkend="libpq-PQcancelCreate"/> is <literal>CONNECTION_ALLOCATED</literal>.
A cancel request that was successfully dispatched
has the status <literal>CONNECTION_OK</literal>. A failed
cancel attempt is signaled by status
<literal>CONNECTION_BAD</literal>. An OK status will
remain so until <xref linkend="libpq-PQcancelFinish"/> or
<xref linkend="libpq-PQcancelReset"/> is called.
</para>
<para>
See the entry for <xref linkend="libpq-PQcancelStart"/> with regards
to other status codes that might be returned.
</para>
<para>
Successful dispatch of the cancellation is no guarantee that the request
will have any effect, however. If the cancellation is effective, the
command being canceled will terminate early and return an error result.
If the cancellation fails (say, because the server was already done
processing the command), then there will be no visible result at all.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQcancelSocket">
<term><function>PQcancelSocket</function><indexterm><primary>PQcancelSocket</primary></indexterm></term>
<listitem>
<para>
Obtains the file descriptor number of the cancel connection socket to
the server.
<synopsis>
int PQcancelSocket(const PGcancelConn *cancelConn);
</synopsis>
</para>
<para>
A valid descriptor will be greater than or equal to 0;
a result of -1 indicates that no server connection is currently open.
This might change as a result of calling any of the functions
in this section on the <structname>PQcancelConn</structname>
(except for <xref linkend="libpq-PQcancelErrorMessage"/> and
<function>PQcancelSocket</function> itself).
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQcancelErrorMessage">
<term>
<function>PQcancelErrorMessage</function><indexterm><primary>PQcancelErrorMessage</primary></indexterm>
<indexterm><primary>error message</primary><secondary>in <structname>PGcancelConn</structname></secondary></indexterm>
</term>
<listitem>
<para>
Returns the error message most recently generated by an
operation on the cancel connection.
<synopsis>
char *PQcancelErrorMessage(const PGcancelConn *cancelconn);
</synopsis>
</para>
<para>
Nearly all <application>libpq</application> functions that take a
<structname>PGcancelConn</structname> will set a message for
<xref linkend="libpq-PQcancelErrorMessage"/> if they fail.
Note that by <application>libpq</application> convention,
a nonempty <xref linkend="libpq-PQcancelErrorMessage"/> result
can consist of multiple lines, and will include a trailing newline.
The caller should not free the result directly.
It will be freed when the associated
<structname>PGcancelConn</structname> handle is passed to
<xref linkend="libpq-PQcancelFinish"/>. The result string should not be
expected to remain the same across operations on the
<literal>PGcancelConn</literal> structure.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQcancelFinish">
<term><function>PQcancelFinish</function><indexterm><primary>PQcancelFinish</primary></indexterm></term>
<listitem>
<para>
Closes the cancel connection (if it did not finish sending the
cancel request yet). Also frees memory used by the
<structname>PGcancelConn</structname> object.
<synopsis>
void PQcancelFinish(PGcancelConn *cancelConn);
</synopsis>
</para>
<para>
Note that even if the cancel attempt fails (as
indicated by <xref linkend="libpq-PQcancelStatus"/>), the
application should call <xref linkend="libpq-PQcancelFinish"/>
to free the memory used by the <structname>PGcancelConn</structname>
object.
The <structname>PGcancelConn</structname> pointer must not be used
again after <xref linkend="libpq-PQcancelFinish"/> has been called.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQcancelReset">
<term><function>PQcancelReset</function><indexterm><primary>PQcancelReset</primary></indexterm></term>
<listitem>
<para>
Resets the <symbol>PGcancelConn</symbol> so it can be reused for a new
cancel connection.
<synopsis>
void PQcancelReset(PGcancelConn *cancelConn);
</synopsis>
</para>
<para>
If the <symbol>PGcancelConn</symbol> is currently used to send a cancel
request, then this connection is closed. It will then prepare the
<symbol>PGcancelConn</symbol> object such that it can be used to send a
new cancel request.
</para>
<para>
This can be used to create one <structname>PGcancelConn</structname>
for a <structname>PGconn</structname> and reuse it multiple times
throughout the lifetime of the original <structname>PGconn</structname>.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="libpq-cancel-deprecated">
<title>Obsolete Functions for Sending Cancel Requests</title>
<para>
These functions represent older methods of sending cancel requests.
Although they still work, they are deprecated due to not sending the cancel
requests in an encrypted manner, even when the original connection
specified <literal>sslmode</literal> or <literal>gssencmode</literal> to
require encryption. Thus these older methods are heavily discouraged from
being used in new code, and it is recommended to change existing code to
use the new functions instead.
</para>
<variablelist>
<varlistentry id="libpq-PQgetCancel">
<term><function>PQgetCancel</function><indexterm><primary>PQgetCancel</primary></indexterm></term>
<listitem>
<para>
Creates a data structure containing the information needed to cancel
a command using <xref linkend="libpq-PQcancel"/>.
<synopsis>
PGcancel *PQgetCancel(PGconn *conn);
</synopsis>
</para>
<para>
<xref linkend="libpq-PQgetCancel"/> creates a
<structname>PGcancel</structname><indexterm><primary>PGcancel</primary></indexterm>
object given a <structname>PGconn</structname> connection object.
It will return <symbol>NULL</symbol> if the given <parameter>conn</parameter>
is <symbol>NULL</symbol> or an invalid connection.
The <structname>PGcancel</structname> object is an opaque
structure that is not meant to be accessed directly by the
application; it can only be passed to <xref linkend="libpq-PQcancel"/>
or <xref linkend="libpq-PQfreeCancel"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQfreeCancel">
<term><function>PQfreeCancel</function><indexterm><primary>PQfreeCancel</primary></indexterm></term>
<listitem>
<para>
Frees a data structure created by <xref linkend="libpq-PQgetCancel"/>.
<synopsis>
void PQfreeCancel(PGcancel *cancel);
</synopsis>
</para>
<para>
<xref linkend="libpq-PQfreeCancel"/> frees a data object previously created
by <xref linkend="libpq-PQgetCancel"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQcancel">
<term><function>PQcancel</function><indexterm><primary>PQcancel</primary></indexterm></term>
<listitem>
<para>
<xref linkend="libpq-PQcancel"/> is a deprecated and insecure
variant of <xref linkend="libpq-PQcancelBlocking"/>, but one that can be
used safely from within a signal handler.
<synopsis>
int PQcancel(PGcancel *cancel, char *errbuf, int errbufsize);
</synopsis>
</para>
<para>
<xref linkend="libpq-PQcancel"/> only exists because of backwards
compatibility reasons. <xref linkend="libpq-PQcancelBlocking"/> should be
used instead. The only benefit that <xref linkend="libpq-PQcancel"/> has
is that it can be safely invoked from a signal handler, if the
<parameter>errbuf</parameter> is a local variable in the signal handler.
However, this is generally not considered a big enough benefit to be
worth the security issues that this function has.
</para>
<para>
The <structname>PGcancel</structname> object is read-only as far as
<xref linkend="libpq-PQcancel"/> is concerned, so it can also be invoked
from a thread that is separate from the one manipulating the
<structname>PGconn</structname> object.
</para>
<para>
The return value of <xref linkend="libpq-PQcancel"/> is 1 if the
cancel request was successfully dispatched and 0 if not.
If not, <parameter>errbuf</parameter> is filled with an explanatory
error message.
<parameter>errbuf</parameter> must be a char array of size
<parameter>errbufsize</parameter> (the recommended size is 256 bytes).
</para>
</listitem>
</varlistentry>
</variablelist>
<variablelist>
<varlistentry id="libpq-PQrequestCancel">
<term><function>PQrequestCancel</function><indexterm><primary>PQrequestCancel</primary></indexterm></term>
<listitem>
<para>
<xref linkend="libpq-PQrequestCancel"/> is a deprecated and insecure
variant of <xref linkend="libpq-PQcancelBlocking"/>.
<synopsis>
int PQrequestCancel(PGconn *conn);
</synopsis>
</para>
<para>
<xref linkend="libpq-PQrequestCancel"/> only exists because of backwards
compatibility reasons. <xref linkend="libpq-PQcancelBlocking"/> should be
used instead. There is no benefit to using
<xref linkend="libpq-PQrequestCancel"/> over
<xref linkend="libpq-PQcancelBlocking"/>.
</para>
<para>
Requests that the server abandon processing of the current
command. It operates directly on the
<structname>PGconn</structname> object, and in case of failure stores the
error message in the <structname>PGconn</structname> object (whence it can
be retrieved by <xref linkend="libpq-PQerrorMessage"/>). Although
the functionality is the same, this approach is not safe within
multiple-thread programs or signal handlers, since it is possible
that overwriting the <structname>PGconn</structname>'s error message will
mess up the operation currently in progress on the connection.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
</sect1>
<sect1 id="libpq-fastpath">
<title>The Fast-Path Interface</title>
<indexterm zone="libpq-fastpath">
<primary>fast path</primary>
</indexterm>
<para>
<productname>PostgreSQL</productname> provides a fast-path interface
to send simple function calls to the server.
</para>
<tip>
<para>
This interface is somewhat obsolete, as one can achieve similar
performance and greater functionality by setting up a prepared
statement to define the function call. Then, executing the statement
with binary transmission of parameters and results substitutes for a
fast-path function call.
</para>
</tip>
<para>
The function <function id="libpq-PQfn">PQfn</function><indexterm><primary>PQfn</primary></indexterm>
requests execution of a server function via the fast-path interface:
<synopsis>
PGresult *PQfn(PGconn *conn,
int fnid,
int *result_buf,
int *result_len,
int result_is_int,
const PQArgBlock *args,
int nargs);
typedef struct
{
int len;
int isint;
union
{
int *ptr;
int integer;
} u;
} PQArgBlock;
</synopsis>
</para>
<para>
The <parameter>fnid</parameter> argument is the OID of the function to be
executed. <parameter>args</parameter> and <parameter>nargs</parameter> define the
parameters to be passed to the function; they must match the declared
function argument list. When the <parameter>isint</parameter> field of a
parameter structure is true, the <parameter>u.integer</parameter> value is sent
to the server as an integer of the indicated length (this must be
2 or 4 bytes); proper byte-swapping occurs. When <parameter>isint</parameter>
is false, the indicated number of bytes at <parameter>*u.ptr</parameter> are
sent with no processing; the data must be in the format expected by
the server for binary transmission of the function's argument data
type. (The declaration of <parameter>u.ptr</parameter> as being of
type <type>int *</type> is historical; it would be better to consider
it <type>void *</type>.)
<parameter>result_buf</parameter> points to the buffer in which to place
the function's return value. The caller must have allocated sufficient
space to store the return value. (There is no check!) The actual result
length in bytes will be returned in the integer pointed to by
<parameter>result_len</parameter>. If a 2- or 4-byte integer result
is expected, set <parameter>result_is_int</parameter> to 1, otherwise
set it to 0. Setting <parameter>result_is_int</parameter> to 1 causes
<application>libpq</application> to byte-swap the value if necessary, so that it
is delivered as a proper <type>int</type> value for the client machine;
note that a 4-byte integer is delivered into <parameter>*result_buf</parameter>
for either allowed result size.
When <parameter>result_is_int</parameter> is 0, the binary-format byte string
sent by the server is returned unmodified. (In this case it's better
to consider <parameter>result_buf</parameter> as being of
type <type>void *</type>.)
</para>
<para>
<function>PQfn</function> always returns a valid
<structname>PGresult</structname> pointer, with
status <literal>PGRES_COMMAND_OK</literal> for success
or <literal>PGRES_FATAL_ERROR</literal> if some problem was encountered.
The result status should be
checked before the result is used. The caller is responsible for
freeing the <structname>PGresult</structname> with
<xref linkend="libpq-PQclear"/> when it is no longer needed.
</para>
<para>
To pass a NULL argument to the function, set
the <parameter>len</parameter> field of that parameter structure
to <literal>-1</literal>; the <parameter>isint</parameter>
and <parameter>u</parameter> fields are then irrelevant.
</para>
<para>
If the function returns NULL, <parameter>*result_len</parameter> is set
to <literal>-1</literal>, and <parameter>*result_buf</parameter> is not
modified.
</para>
<para>
Note that it is not possible to handle set-valued results when using
this interface. Also, the function must be a plain function, not an
aggregate, window function, or procedure.
</para>
</sect1>
<sect1 id="libpq-notify">
<title>Asynchronous Notification</title>
<indexterm zone="libpq-notify">
<primary>NOTIFY</primary>
<secondary>in libpq</secondary>
</indexterm>
<para>
<productname>PostgreSQL</productname> offers asynchronous notification
via the <command>LISTEN</command> and <command>NOTIFY</command>
commands. A client session registers its interest in a particular
notification channel with the <command>LISTEN</command> command (and
can stop listening with the <command>UNLISTEN</command> command). All
sessions listening on a particular channel will be notified
asynchronously when a <command>NOTIFY</command> command with that
channel name is executed by any session. A <quote>payload</quote> string can
be passed to communicate additional data to the listeners.
</para>
<para>
<application>libpq</application> applications submit
<command>LISTEN</command>, <command>UNLISTEN</command>,
and <command>NOTIFY</command> commands as
ordinary SQL commands. The arrival of <command>NOTIFY</command>
messages can subsequently be detected by calling
<function id="libpq-PQnotifies">PQnotifies</function>.<indexterm><primary>PQnotifies</primary></indexterm>
</para>
<para>
The function <function>PQnotifies</function> returns the next notification
from a list of unhandled notification messages received from the server.
It returns a null pointer if there are no pending notifications. Once a
notification is returned from <function>PQnotifies</function>, it is considered
handled and will be removed from the list of notifications.
<synopsis>
PGnotify *PQnotifies(PGconn *conn);
typedef struct pgNotify
{
char *relname; /* notification channel name */
int be_pid; /* process ID of notifying server process */
char *extra; /* notification payload string */
} PGnotify;
</synopsis>
After processing a <structname>PGnotify</structname> object returned
by <function>PQnotifies</function>, be sure to free it with
<xref linkend="libpq-PQfreemem"/>. It is sufficient to free the
<structname>PGnotify</structname> pointer; the
<structfield>relname</structfield> and <structfield>extra</structfield>
fields do not represent separate allocations. (The names of these fields
are historical; in particular, channel names need not have anything to
do with relation names.)
</para>
<para>
<xref linkend="libpq-example-2"/> gives a sample program that illustrates
the use of asynchronous notification.
</para>
<para>
<function>PQnotifies</function> does not actually read data from the
server; it just returns messages previously absorbed by another
<application>libpq</application> function. In ancient releases of
<application>libpq</application>, the only way to ensure timely receipt
of <command>NOTIFY</command> messages was to constantly submit commands, even
empty ones, and then check <function>PQnotifies</function> after each
<xref linkend="libpq-PQexec"/>. While this still works, it is deprecated
as a waste of processing power.
</para>
<para>
A better way to check for <command>NOTIFY</command> messages when you have no
useful commands to execute is to call
<xref linkend="libpq-PQconsumeInput"/>, then check
<function>PQnotifies</function>. You can use
<function>select()</function> to wait for data to arrive from the
server, thereby using no <acronym>CPU</acronym> power unless there is
something to do. (See <xref linkend="libpq-PQsocket"/> to obtain the file
descriptor number to use with <function>select()</function>.) Note that
this will work OK whether you submit commands with
<xref linkend="libpq-PQsendQuery"/>/<xref linkend="libpq-PQgetResult"/> or
simply use <xref linkend="libpq-PQexec"/>. You should, however, remember
to check <function>PQnotifies</function> after each
<xref linkend="libpq-PQgetResult"/> or <xref linkend="libpq-PQexec"/>, to
see if any notifications came in during the processing of the command.
</para>
</sect1>
<sect1 id="libpq-copy">
<title>Functions Associated with the <command>COPY</command> Command</title>
<indexterm zone="libpq-copy">
<primary>COPY</primary>
<secondary>with libpq</secondary>
</indexterm>
<para>
The <command>COPY</command> command in
<productname>PostgreSQL</productname> has options to read from or write
to the network connection used by <application>libpq</application>.
The functions described in this section allow applications to take
advantage of this capability by supplying or consuming copied data.
</para>
<para>
The overall process is that the application first issues the SQL
<command>COPY</command> command via <xref linkend="libpq-PQexec"/> or one
of the equivalent functions. The response to this (if there is no
error in the command) will be a <structname>PGresult</structname> object bearing
a status code of <literal>PGRES_COPY_OUT</literal> or
<literal>PGRES_COPY_IN</literal> (depending on the specified copy
direction). The application should then use the functions of this
section to receive or transmit data rows. When the data transfer is
complete, another <structname>PGresult</structname> object is returned to indicate
success or failure of the transfer. Its status will be
<literal>PGRES_COMMAND_OK</literal> for success or
<literal>PGRES_FATAL_ERROR</literal> if some problem was encountered.
At this point further SQL commands can be issued via
<xref linkend="libpq-PQexec"/>. (It is not possible to execute other SQL
commands using the same connection while the <command>COPY</command>
operation is in progress.)
</para>
<para>
If a <command>COPY</command> command is issued via
<xref linkend="libpq-PQexec"/> in a string that could contain additional
commands, the application must continue fetching results via
<xref linkend="libpq-PQgetResult"/> after completing the <command>COPY</command>
sequence. Only when <xref linkend="libpq-PQgetResult"/> returns
<symbol>NULL</symbol> is it certain that the <xref linkend="libpq-PQexec"/>
command string is done and it is safe to issue more commands.
</para>
<para>
The functions of this section should be executed only after obtaining
a result status of <literal>PGRES_COPY_OUT</literal> or
<literal>PGRES_COPY_IN</literal> from <xref linkend="libpq-PQexec"/> or
<xref linkend="libpq-PQgetResult"/>.
</para>
<para>
A <structname>PGresult</structname> object bearing one of these status values
carries some additional data about the <command>COPY</command> operation
that is starting. This additional data is available using functions
that are also used in connection with query results:
<variablelist>
<varlistentry id="libpq-PQnfields-1">
<term><function>PQnfields</function><indexterm
><primary>PQnfields</primary><secondary>with COPY</secondary></indexterm></term>
<listitem>
<para>
Returns the number of columns (fields) to be copied.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQbinaryTuples-1">
<term><function>PQbinaryTuples</function><indexterm
><primary>PQbinaryTuples</primary><secondary>with COPY</secondary></indexterm></term>
<listitem>
<para>
0 indicates the overall copy format is textual (rows separated by
newlines, columns separated by separator characters, etc.). 1
indicates the overall copy format is binary. See <xref
linkend="sql-copy"/> for more information.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQfformat-1">
<term><function>PQfformat</function><indexterm
><primary>PQfformat</primary><secondary>with COPY</secondary></indexterm></term>
<listitem>
<para>
Returns the format code (0 for text, 1 for binary) associated with
each column of the copy operation. The per-column format codes
will always be zero when the overall copy format is textual, but
the binary format can support both text and binary columns.
(However, as of the current implementation of <command>COPY</command>,
only binary columns appear in a binary copy; so the per-column
formats always match the overall format at present.)
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
<sect2 id="libpq-copy-send">
<title>Functions for Sending <command>COPY</command> Data</title>
<para>
These functions are used to send data during <literal>COPY FROM
STDIN</literal>. They will fail if called when the connection is not in
<literal>COPY_IN</literal> state.
</para>
<variablelist>
<varlistentry id="libpq-PQputCopyData">
<term><function>PQputCopyData</function><indexterm><primary>PQputCopyData</primary></indexterm></term>
<listitem>
<para>
Sends data to the server during <literal>COPY_IN</literal> state.
<synopsis>
int PQputCopyData(PGconn *conn,
const char *buffer,
int nbytes);
</synopsis>
</para>
<para>
Transmits the <command>COPY</command> data in the specified
<parameter>buffer</parameter>, of length <parameter>nbytes</parameter>, to the server.
The result is 1 if the data was queued, zero if it was not queued
because of full buffers (this will only happen in nonblocking mode),
or -1 if an error occurred.
(Use <xref linkend="libpq-PQerrorMessage"/> to retrieve details if
the return value is -1. If the value is zero, wait for write-ready
and try again.)
</para>
<para>
The application can divide the <command>COPY</command> data stream
into buffer loads of any convenient size. Buffer-load boundaries
have no semantic significance when sending. The contents of the
data stream must match the data format expected by the
<command>COPY</command> command; see <xref linkend="sql-copy"/> for details.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQputCopyEnd">
<term><function>PQputCopyEnd</function><indexterm><primary>PQputCopyEnd</primary></indexterm></term>
<listitem>
<para>
Sends end-of-data indication to the server during <literal>COPY_IN</literal> state.
<synopsis>
int PQputCopyEnd(PGconn *conn,
const char *errormsg);
</synopsis>
</para>
<para>
Ends the <literal>COPY_IN</literal> operation successfully if
<parameter>errormsg</parameter> is <symbol>NULL</symbol>. If
<parameter>errormsg</parameter> is not <symbol>NULL</symbol> then the
<command>COPY</command> is forced to fail, with the string pointed to by
<parameter>errormsg</parameter> used as the error message. (One should not
assume that this exact error message will come back from the server,
however, as the server might have already failed the
<command>COPY</command> for its own reasons.)
</para>
<para>
The result is 1 if the termination message was sent; or in
nonblocking mode, this may only indicate that the termination
message was successfully queued. (In nonblocking mode, to be
certain that the data has been sent, you should next wait for
write-ready and call <xref linkend="libpq-PQflush"/>, repeating until it
returns zero.) Zero indicates that the function could not queue
the termination message because of full buffers; this will only
happen in nonblocking mode. (In this case, wait for
write-ready and try the <xref linkend="libpq-PQputCopyEnd"/> call
again.) If a hard error occurs, -1 is returned; you can use
<xref linkend="libpq-PQerrorMessage"/> to retrieve details.
</para>
<para>
After successfully calling <xref linkend="libpq-PQputCopyEnd"/>, call
<xref linkend="libpq-PQgetResult"/> to obtain the final result status of the
<command>COPY</command> command. One can wait for this result to be
available in the usual way. Then return to normal operation.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="libpq-copy-receive">
<title>Functions for Receiving <command>COPY</command> Data</title>
<para>
These functions are used to receive data during <literal>COPY TO
STDOUT</literal>. They will fail if called when the connection is not in
<literal>COPY_OUT</literal> state.
</para>
<variablelist>
<varlistentry id="libpq-PQgetCopyData">
<term><function>PQgetCopyData</function><indexterm><primary>PQgetCopyData</primary></indexterm></term>
<listitem>
<para>
Receives data from the server during <literal>COPY_OUT</literal> state.
<synopsis>
int PQgetCopyData(PGconn *conn,
char **buffer,
int async);
</synopsis>
</para>
<para>
Attempts to obtain another row of data from the server during a
<command>COPY</command>. Data is always returned one data row at
a time; if only a partial row is available, it is not returned.
Successful return of a data row involves allocating a chunk of
memory to hold the data. The <parameter>buffer</parameter> parameter must
be non-<symbol>NULL</symbol>. <parameter>*buffer</parameter> is set to
point to the allocated memory, or to <symbol>NULL</symbol> in cases
where no buffer is returned. A non-<symbol>NULL</symbol> result
buffer should be freed using <xref linkend="libpq-PQfreemem"/> when no longer
needed.
</para>
<para>
When a row is successfully returned, the return value is the number
of data bytes in the row (this will always be greater than zero).
The returned string is always null-terminated, though this is
probably only useful for textual <command>COPY</command>. A result
of zero indicates that the <command>COPY</command> is still in
progress, but no row is yet available (this is only possible when
<parameter>async</parameter> is true). A result of -1 indicates that the
<command>COPY</command> is done. A result of -2 indicates that an
error occurred (consult <xref linkend="libpq-PQerrorMessage"/> for the reason).
</para>
<para>
When <parameter>async</parameter> is true (not zero),
<xref linkend="libpq-PQgetCopyData"/> will not block waiting for input; it
will return zero if the <command>COPY</command> is still in progress
but no complete row is available. (In this case wait for read-ready
and then call <xref linkend="libpq-PQconsumeInput"/> before calling
<xref linkend="libpq-PQgetCopyData"/> again.) When <parameter>async</parameter> is
false (zero), <xref linkend="libpq-PQgetCopyData"/> will block until data is
available or the operation completes.
</para>
<para>
After <xref linkend="libpq-PQgetCopyData"/> returns -1, call
<xref linkend="libpq-PQgetResult"/> to obtain the final result status of the
<command>COPY</command> command. One can wait for this result to be
available in the usual way. Then return to normal operation.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="libpq-copy-deprecated">
<title>Obsolete Functions for <command>COPY</command></title>
<para>
These functions represent older methods of handling <command>COPY</command>.
Although they still work, they are deprecated due to poor error handling,
inconvenient methods of detecting end-of-data, and lack of support for binary
or nonblocking transfers.
</para>
<variablelist>
<varlistentry id="libpq-PQgetline">
<term><function>PQgetline</function><indexterm><primary>PQgetline</primary></indexterm></term>
<listitem>
<para>
Reads a newline-terminated line of characters (transmitted
by the server) into a buffer string of size <parameter>length</parameter>.
<synopsis>
int PQgetline(PGconn *conn,
char *buffer,
int length);
</synopsis>
</para>
<para>
This function copies up to <parameter>length</parameter>-1 characters into
the buffer and converts the terminating newline into a zero byte.
<xref linkend="libpq-PQgetline"/> returns <symbol>EOF</symbol> at the
end of input, 0 if the entire line has been read, and 1 if the
buffer is full but the terminating newline has not yet been read.
</para>
<para>
Note that the application must check to see if a new line consists
of the two characters <literal>\.</literal>, which indicates
that the server has finished sending the results of the
<command>COPY</command> command. If the application might receive
lines that are more than <parameter>length</parameter>-1 characters long,
care is needed to be sure it recognizes the <literal>\.</literal>
line correctly (and does not, for example, mistake the end of a
long data line for a terminator line).
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQgetlineAsync">
<term><function>PQgetlineAsync</function><indexterm><primary>PQgetlineAsync</primary></indexterm></term>
<listitem>
<para>
Reads a row of <command>COPY</command> data (transmitted by the
server) into a buffer without blocking.
<synopsis>
int PQgetlineAsync(PGconn *conn,
char *buffer,
int bufsize);
</synopsis>
</para>
<para>
This function is similar to <xref linkend="libpq-PQgetline"/>, but it can be used
by applications
that must read <command>COPY</command> data asynchronously, that is, without blocking.
Having issued the <command>COPY</command> command and gotten a <literal>PGRES_COPY_OUT</literal>
response, the
application should call <xref linkend="libpq-PQconsumeInput"/> and
<xref linkend="libpq-PQgetlineAsync"/> until the
end-of-data signal is detected.
</para>
<para>
Unlike <xref linkend="libpq-PQgetline"/>, this function takes
responsibility for detecting end-of-data.
</para>
<para>
On each call, <xref linkend="libpq-PQgetlineAsync"/> will return data if a
complete data row is available in <application>libpq</application>'s input buffer.
Otherwise, no data is returned until the rest of the row arrives.
The function returns -1 if the end-of-copy-data marker has been recognized,
or 0 if no data is available, or a positive number giving the number of
bytes of data returned. If -1 is returned, the caller must next call
<xref linkend="libpq-PQendcopy"/>, and then return to normal processing.
</para>
<para>
The data returned will not extend beyond a data-row boundary. If possible
a whole row will be returned at one time. But if the buffer offered by
the caller is too small to hold a row sent by the server, then a partial
data row will be returned. With textual data this can be detected by testing
whether the last returned byte is <literal>\n</literal> or not. (In a binary
<command>COPY</command>, actual parsing of the <command>COPY</command> data format will be needed to make the
equivalent determination.)
The returned string is not null-terminated. (If you want to add a
terminating null, be sure to pass a <parameter>bufsize</parameter> one smaller
than the room actually available.)
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQputline">
<term><function>PQputline</function><indexterm><primary>PQputline</primary></indexterm></term>
<listitem>
<para>
Sends a null-terminated string to the server. Returns 0 if
OK and <symbol>EOF</symbol> if unable to send the string.
<synopsis>
int PQputline(PGconn *conn,
const char *string);
</synopsis>
</para>
<para>
The <command>COPY</command> data stream sent by a series of calls
to <xref linkend="libpq-PQputline"/> has the same format as that
returned by <xref linkend="libpq-PQgetlineAsync"/>, except that
applications are not obliged to send exactly one data row per
<xref linkend="libpq-PQputline"/> call; it is okay to send a partial
line or multiple lines per call.
</para>
<note>
<para>
Before <productname>PostgreSQL</productname> protocol 3.0, it was necessary
for the application to explicitly send the two characters
<literal>\.</literal> as a final line to indicate to the server that it had
finished sending <command>COPY</command> data. While this still works, it is deprecated and the
special meaning of <literal>\.</literal> can be expected to be removed in a
future release. It is sufficient to call <xref linkend="libpq-PQendcopy"/> after
having sent the actual data.
</para>
</note>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQputnbytes">
<term><function>PQputnbytes</function><indexterm><primary>PQputnbytes</primary></indexterm></term>
<listitem>
<para>
Sends a non-null-terminated string to the server. Returns
0 if OK and <symbol>EOF</symbol> if unable to send the string.
<synopsis>
int PQputnbytes(PGconn *conn,
const char *buffer,
int nbytes);
</synopsis>
</para>
<para>
This is exactly like <xref linkend="libpq-PQputline"/>, except that the data
buffer need not be null-terminated since the number of bytes to send is
specified directly. Use this procedure when sending binary data.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQendcopy">
<term><function>PQendcopy</function><indexterm><primary>PQendcopy</primary></indexterm></term>
<listitem>
<para>
Synchronizes with the server.
<synopsis>
int PQendcopy(PGconn *conn);
</synopsis>
This function waits until the server has finished the copying.
It should either be issued when the last string has been sent
to the server using <xref linkend="libpq-PQputline"/> or when the
last string has been received from the server using
<function>PQgetline</function>. It must be issued or the server
will get <quote>out of sync</quote> with the client. Upon return
from this function, the server is ready to receive the next SQL
command. The return value is 0 on successful completion,
nonzero otherwise. (Use <xref linkend="libpq-PQerrorMessage"/> to
retrieve details if the return value is nonzero.)
</para>
<para>
When using <xref linkend="libpq-PQgetResult"/>, the application should
respond to a <literal>PGRES_COPY_OUT</literal> result by executing
<xref linkend="libpq-PQgetline"/> repeatedly, followed by
<xref linkend="libpq-PQendcopy"/> after the terminator line is seen.
It should then return to the <xref linkend="libpq-PQgetResult"/> loop
until <xref linkend="libpq-PQgetResult"/> returns a null pointer.
Similarly a <literal>PGRES_COPY_IN</literal> result is processed
by a series of <xref linkend="libpq-PQputline"/> calls followed by
<xref linkend="libpq-PQendcopy"/>, then return to the
<xref linkend="libpq-PQgetResult"/> loop. This arrangement will
ensure that a <command>COPY</command> command embedded in a series
of <acronym>SQL</acronym> commands will be executed correctly.
</para>
<para>
Older applications are likely to submit a <command>COPY</command>
via <xref linkend="libpq-PQexec"/> and assume that the transaction
is done after <xref linkend="libpq-PQendcopy"/>. This will work
correctly only if the <command>COPY</command> is the only
<acronym>SQL</acronym> command in the command string.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
</sect1>
<sect1 id="libpq-control">
<title>Control Functions</title>
<para>
These functions control miscellaneous details of <application>libpq</application>'s
behavior.
</para>
<variablelist>
<varlistentry id="libpq-PQclientEncoding">
<term><function>PQclientEncoding</function><indexterm><primary>PQclientEncoding</primary></indexterm></term>
<listitem>
<para>
Returns the client encoding.
<synopsis>
int PQclientEncoding(const PGconn *<replaceable>conn</replaceable>);
</synopsis>
Note that it returns the encoding ID, not a symbolic string
such as <literal>EUC_JP</literal>. If unsuccessful, it returns -1.
To convert an encoding ID to an encoding name, you
can use:
<synopsis>
char *pg_encoding_to_char(int <replaceable>encoding_id</replaceable>);
</synopsis>
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQsetClientEncoding">
<term><function>PQsetClientEncoding</function><indexterm><primary>PQsetClientEncoding</primary></indexterm></term>
<listitem>
<para>
Sets the client encoding.
<synopsis>
int PQsetClientEncoding(PGconn *<replaceable>conn</replaceable>, const char *<replaceable>encoding</replaceable>);
</synopsis>
<replaceable>conn</replaceable> is a connection to the server,
and <replaceable>encoding</replaceable> is the encoding you want to
use. If the function successfully sets the encoding, it returns 0,
otherwise -1. The current encoding for this connection can be
determined by using <xref linkend="libpq-PQclientEncoding"/>.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQsetErrorVerbosity">
<term><function>PQsetErrorVerbosity</function><indexterm><primary>PQsetErrorVerbosity</primary></indexterm></term>
<listitem>
<para>
Determines the verbosity of messages returned by
<xref linkend="libpq-PQerrorMessage"/> and <xref linkend="libpq-PQresultErrorMessage"/>.
<synopsis>
typedef enum
{
PQERRORS_TERSE,
PQERRORS_DEFAULT,
PQERRORS_VERBOSE,
PQERRORS_SQLSTATE
} PGVerbosity;
PGVerbosity PQsetErrorVerbosity(PGconn *conn, PGVerbosity verbosity);
</synopsis>
<xref linkend="libpq-PQsetErrorVerbosity"/> sets the verbosity mode,
returning the connection's previous setting.
In <firstterm>TERSE</firstterm> mode, returned messages include
severity, primary text, and position only; this will normally fit on a
single line. The <firstterm>DEFAULT</firstterm> mode produces messages
that include the above plus any detail, hint, or context fields (these
might span multiple lines). The <firstterm>VERBOSE</firstterm> mode
includes all available fields. The <firstterm>SQLSTATE</firstterm>
mode includes only the error severity and the <symbol>SQLSTATE</symbol>
error code, if one is available (if not, the output is like
<firstterm>TERSE</firstterm> mode).
</para>
<para>
Changing the verbosity setting does not affect the messages available
from already-existing <structname>PGresult</structname> objects, only
subsequently-created ones.
(But see <xref linkend="libpq-PQresultVerboseErrorMessage"/> if you
want to print a previous error with a different verbosity.)
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQsetErrorContextVisibility">
<term><function>PQsetErrorContextVisibility</function><indexterm><primary>PQsetErrorContextVisibility</primary></indexterm></term>
<listitem>
<para>
Determines the handling of <literal>CONTEXT</literal> fields in messages
returned by <xref linkend="libpq-PQerrorMessage"/>
and <xref linkend="libpq-PQresultErrorMessage"/>.
<synopsis>
typedef enum
{
PQSHOW_CONTEXT_NEVER,
PQSHOW_CONTEXT_ERRORS,
PQSHOW_CONTEXT_ALWAYS
} PGContextVisibility;
PGContextVisibility PQsetErrorContextVisibility(PGconn *conn, PGContextVisibility show_context);
</synopsis>
<xref linkend="libpq-PQsetErrorContextVisibility"/> sets the context display mode,
returning the connection's previous setting. This mode controls
whether the <literal>CONTEXT</literal> field is included in messages.
The <firstterm>NEVER</firstterm> mode
never includes <literal>CONTEXT</literal>, while <firstterm>ALWAYS</firstterm> always
includes it if available. In <firstterm>ERRORS</firstterm> mode (the
default), <literal>CONTEXT</literal> fields are included only in error
messages, not in notices and warnings.
(However, if the verbosity setting is <firstterm>TERSE</firstterm>
or <firstterm>SQLSTATE</firstterm>, <literal>CONTEXT</literal> fields
are omitted regardless of the context display mode.)
</para>
<para>
Changing this mode does not
affect the messages available from
already-existing <structname>PGresult</structname> objects, only
subsequently-created ones.
(But see <xref linkend="libpq-PQresultVerboseErrorMessage"/> if you
want to print a previous error with a different display mode.)
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQtrace">
<term><function>PQtrace</function><indexterm><primary>PQtrace</primary></indexterm></term>
<listitem>
<para>
Enables tracing of the client/server communication to a debugging file
stream.
<synopsis>
void PQtrace(PGconn *conn, FILE *stream);
</synopsis>
</para>
<para>
Each line consists of: an optional timestamp, a direction indicator
(<literal>F</literal> for messages from client to server
or <literal>B</literal> for messages from server to client),
message length, message type, and message contents.
Non-message contents fields (timestamp, direction, length and message type)
are separated by a tab. Message contents are separated by a space.
Protocol strings are enclosed in double quotes, while strings used as data
values are enclosed in single quotes. Non-printable chars are printed as
hexadecimal escapes.
Further message-type-specific detail can be found in
<xref linkend="protocol-message-formats"/>.
</para>
<note>
<para>
On Windows, if the <application>libpq</application> library and an application are
compiled with different flags, this function call will crash the
application because the internal representation of the <literal>FILE</literal>
pointers differ. Specifically, multithreaded/single-threaded,
release/debug, and static/dynamic flags should be the same for the
library and all applications using that library.
</para>
</note>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQsetTraceFlags">
<term><function>PQsetTraceFlags</function><indexterm><primary>PQsetTraceFlags</primary></indexterm></term>
<listitem>
<para>
Controls the tracing behavior of client/server communication.
<synopsis>
void PQsetTraceFlags(PGconn *conn, int flags);
</synopsis>
</para>
<para>
<literal>flags</literal> contains flag bits describing the operating mode
of tracing.
If <literal>flags</literal> contains <literal>PQTRACE_SUPPRESS_TIMESTAMPS</literal>,
then the timestamp is not included when printing each message.
If <literal>flags</literal> contains <literal>PQTRACE_REGRESS_MODE</literal>,
then some fields are redacted when printing each message, such as object
OIDs, to make the output more convenient to use in testing frameworks.
This function must be called after calling <function>PQtrace</function>.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQuntrace">
<term><function>PQuntrace</function><indexterm><primary>PQuntrace</primary></indexterm></term>
<listitem>
<para>
Disables tracing started by <xref linkend="libpq-PQtrace"/>.
<synopsis>
void PQuntrace(PGconn *conn);
</synopsis>
</para>
</listitem>
</varlistentry>
</variablelist>
</sect1>
<sect1 id="libpq-misc">
<title>Miscellaneous Functions</title>
<para>
As always, there are some functions that just don't fit anywhere.
</para>
<variablelist>
<varlistentry id="libpq-PQfreemem">
<term><function>PQfreemem</function><indexterm><primary>PQfreemem</primary></indexterm></term>
<listitem>
<para>
Frees memory allocated by <application>libpq</application>.
<synopsis>
void PQfreemem(void *ptr);
</synopsis>
</para>
<para>
Frees memory allocated by <application>libpq</application>, particularly
<xref linkend="libpq-PQescapeByteaConn"/>,
<xref linkend="libpq-PQescapeBytea"/>,
<xref linkend="libpq-PQunescapeBytea"/>,
and <function>PQnotifies</function>.
It is particularly important that this function, rather than
<function>free()</function>, be used on Microsoft Windows. This is because
allocating memory in a DLL and releasing it in the application works
only if multithreaded/single-threaded, release/debug, and static/dynamic
flags are the same for the DLL and the application. On non-Microsoft
Windows platforms, this function is the same as the standard library
function <function>free()</function>.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQconninfoFree">
<term><function>PQconninfoFree</function><indexterm><primary>PQconninfoFree</primary></indexterm></term>
<listitem>
<para>
Frees the data structures allocated by
<xref linkend="libpq-PQconndefaults"/> or <xref linkend="libpq-PQconninfoParse"/>.
<synopsis>
void PQconninfoFree(PQconninfoOption *connOptions);
</synopsis>
If the argument is a <symbol>NULL</symbol> pointer, no operation is
performed.
</para>
<para>
A simple <xref linkend="libpq-PQfreemem"/> will not do for this, since
the array contains references to subsidiary strings.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQencryptPasswordConn">
<term><function>PQencryptPasswordConn</function><indexterm><primary>PQencryptPasswordConn</primary></indexterm></term>
<listitem>
<para>
Prepares the encrypted form of a <productname>PostgreSQL</productname> password.
<synopsis>
char *PQencryptPasswordConn(PGconn *conn, const char *passwd, const char *user, const char *algorithm);
</synopsis>
This function is intended to be used by client applications that
wish to send commands like <literal>ALTER USER joe PASSWORD
'pwd'</literal>. It is good practice not to send the original cleartext
password in such a command, because it might be exposed in command
logs, activity displays, and so on. Instead, use this function to
convert the password to encrypted form before it is sent.
</para>
<para>
The <parameter>passwd</parameter> and <parameter>user</parameter> arguments
are the cleartext password, and the SQL name of the user it is for.
<parameter>algorithm</parameter> specifies the encryption algorithm
to use to encrypt the password. Currently supported algorithms are
<literal>md5</literal> and <literal>scram-sha-256</literal> (<literal>on</literal> and
<literal>off</literal> are also accepted as aliases for <literal>md5</literal>, for
compatibility with older server versions). Note that support for
<literal>scram-sha-256</literal> was introduced in <productname>PostgreSQL</productname>
version 10, and will not work correctly with older server versions. If
<parameter>algorithm</parameter> is <symbol>NULL</symbol>, this function will query
the server for the current value of the
<xref linkend="guc-password-encryption"/> setting. That can block, and
will fail if the current transaction is aborted, or if the connection
is busy executing another query. If you wish to use the default
algorithm for the server but want to avoid blocking, query
<varname>password_encryption</varname> yourself before calling
<xref linkend="libpq-PQencryptPasswordConn"/>, and pass that value as the
<parameter>algorithm</parameter>.
</para>
<para>
The return value is a string allocated by <function>malloc</function>.
The caller can assume the string doesn't contain any special characters
that would require escaping. Use <xref linkend="libpq-PQfreemem"/> to free the
result when done with it. On error, returns <symbol>NULL</symbol>, and
a suitable message is stored in the connection object.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQchangePassword">
<term><function>PQchangePassword</function><indexterm><primary>PQchangePassword</primary></indexterm></term>
<listitem>
<para>
Changes a <productname>PostgreSQL</productname> password.
<synopsis>
PGresult *PQchangePassword(PGconn *conn, const char *user, const char *passwd);
</synopsis>
This function uses <function>PQencryptPasswordConn</function>
to build and execute the command <literal>ALTER USER ... PASSWORD
'...'</literal>, thereby changing the user's password. It exists for
the same reason as <function>PQencryptPasswordConn</function>, but
is more convenient as it both builds and runs the command for you.
<xref linkend="libpq-PQencryptPasswordConn"/> is passed a
<symbol>NULL</symbol> for the algorithm argument, hence encryption is
done according to the server's <xref linkend="guc-password-encryption"/>
setting.
</para>
<para>
The <parameter>user</parameter> and <parameter>passwd</parameter> arguments
are the SQL name of the target user, and the new cleartext password.
</para>
<para>
Returns a <structname>PGresult</structname> pointer representing
the result of the <literal>ALTER USER</literal> command, or
a null pointer if the routine failed before issuing any command.
The <xref linkend="libpq-PQresultStatus"/> function should be called
to check the return value for any errors (including the value of a null
pointer, in which case it will return
<symbol>PGRES_FATAL_ERROR</symbol>). Use
<xref linkend="libpq-PQerrorMessage"/> to get more information about
such errors.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQencryptPassword">
<term><function>PQencryptPassword</function><indexterm><primary>PQencryptPassword</primary></indexterm></term>
<listitem>
<para>
Prepares the md5-encrypted form of a <productname>PostgreSQL</productname> password.
<synopsis>
char *PQencryptPassword(const char *passwd, const char *user);
</synopsis>
<xref linkend="libpq-PQencryptPassword"/> is an older, deprecated version of
<xref linkend="libpq-PQencryptPasswordConn"/>. The difference is that
<xref linkend="libpq-PQencryptPassword"/> does not
require a connection object, and <literal>md5</literal> is always used as the
encryption algorithm.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQmakeEmptyPGresult">
<term><function>PQmakeEmptyPGresult</function><indexterm><primary>PQmakeEmptyPGresult</primary></indexterm></term>
<listitem>
<para>
Constructs an empty <structname>PGresult</structname> object with the given status.
<synopsis>
PGresult *PQmakeEmptyPGresult(PGconn *conn, ExecStatusType status);
</synopsis>
</para>
<para>
This is <application>libpq</application>'s internal function to allocate and
initialize an empty <structname>PGresult</structname> object. This
function returns <symbol>NULL</symbol> if memory could not be allocated. It is
exported because some applications find it useful to generate result
objects (particularly objects with error status) themselves. If
<parameter>conn</parameter> is not null and <parameter>status</parameter>
indicates an error, the current error message of the specified
connection is copied into the <structname>PGresult</structname>.
Also, if <parameter>conn</parameter> is not null, any event procedures
registered in the connection are copied into the
<structname>PGresult</structname>. (They do not get
<literal>PGEVT_RESULTCREATE</literal> calls, but see
<xref linkend="libpq-PQfireResultCreateEvents"/>.)
Note that <xref linkend="libpq-PQclear"/> should eventually be called
on the object, just as with a <structname>PGresult</structname>
returned by <application>libpq</application> itself.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQfireResultCreateEvents">
<term><function>PQfireResultCreateEvents</function><indexterm><primary>PQfireResultCreateEvents</primary></indexterm></term>
<listitem>
<para>
Fires a <literal>PGEVT_RESULTCREATE</literal> event (see <xref
linkend="libpq-events"/>) for each event procedure registered in the
<structname>PGresult</structname> object. Returns non-zero for success,
zero if any event procedure fails.
<synopsis>
int PQfireResultCreateEvents(PGconn *conn, PGresult *res);
</synopsis>
</para>
<para>
The <literal>conn</literal> argument is passed through to event procedures
but not used directly. It can be <symbol>NULL</symbol> if the event
procedures won't use it.
</para>
<para>
Event procedures that have already received a
<literal>PGEVT_RESULTCREATE</literal> or <literal>PGEVT_RESULTCOPY</literal> event
for this object are not fired again.
</para>
<para>
The main reason that this function is separate from
<xref linkend="libpq-PQmakeEmptyPGresult"/> is that it is often appropriate
to create a <structname>PGresult</structname> and fill it with data
before invoking the event procedures.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQcopyResult">
<term><function>PQcopyResult</function><indexterm><primary>PQcopyResult</primary></indexterm></term>
<listitem>
<para>
Makes a copy of a <structname>PGresult</structname> object. The copy is
not linked to the source result in any way and
<xref linkend="libpq-PQclear"/> must be called when the copy is no longer
needed. If the function fails, <symbol>NULL</symbol> is returned.
<synopsis>
PGresult *PQcopyResult(const PGresult *src, int flags);
</synopsis>
</para>
<para>
This is not intended to make an exact copy. The returned result is
always put into <literal>PGRES_TUPLES_OK</literal> status, and does not
copy any error message in the source. (It does copy the command status
string, however.) The <parameter>flags</parameter> argument determines
what else is copied. It is a bitwise OR of several flags.
<literal>PG_COPYRES_ATTRS</literal> specifies copying the source
result's attributes (column definitions).
<literal>PG_COPYRES_TUPLES</literal> specifies copying the source
result's tuples. (This implies copying the attributes, too.)
<literal>PG_COPYRES_NOTICEHOOKS</literal> specifies
copying the source result's notify hooks.
<literal>PG_COPYRES_EVENTS</literal> specifies copying the source
result's events. (But any instance data associated with the source
is not copied.)
The event procedures receive <literal>PGEVT_RESULTCOPY</literal> events.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQsetResultAttrs">
<term><function>PQsetResultAttrs</function><indexterm><primary>PQsetResultAttrs</primary></indexterm></term>
<listitem>
<para>
Sets the attributes of a <structname>PGresult</structname> object.
<synopsis>
int PQsetResultAttrs(PGresult *res, int numAttributes, PGresAttDesc *attDescs);
</synopsis>
</para>
<para>
The provided <parameter>attDescs</parameter> are copied into the result.
If the <parameter>attDescs</parameter> pointer is <symbol>NULL</symbol> or
<parameter>numAttributes</parameter> is less than one, the request is
ignored and the function succeeds. If <parameter>res</parameter>
already contains attributes, the function will fail. If the function
fails, the return value is zero. If the function succeeds, the return
value is non-zero.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQsetvalue">
<term><function>PQsetvalue</function><indexterm><primary>PQsetvalue</primary></indexterm></term>
<listitem>
<para>
Sets a tuple field value of a <structname>PGresult</structname> object.
<synopsis>
int PQsetvalue(PGresult *res, int tup_num, int field_num, char *value, int len);
</synopsis>
</para>
<para>
The function will automatically grow the result's internal tuples array
as needed. However, the <parameter>tup_num</parameter> argument must be
less than or equal to <xref linkend="libpq-PQntuples"/>, meaning this
function can only grow the tuples array one tuple at a time. But any
field of any existing tuple can be modified in any order. If a value at
<parameter>field_num</parameter> already exists, it will be overwritten.
If <parameter>len</parameter> is -1 or
<parameter>value</parameter> is <symbol>NULL</symbol>, the field value
will be set to an SQL null value. The
<parameter>value</parameter> is copied into the result's private storage,
thus is no longer needed after the function
returns. If the function fails, the return value is zero. If the
function succeeds, the return value is non-zero.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQresultAlloc">
<term><function>PQresultAlloc</function><indexterm><primary>PQresultAlloc</primary></indexterm></term>
<listitem>
<para>
Allocate subsidiary storage for a <structname>PGresult</structname> object.
<synopsis>
void *PQresultAlloc(PGresult *res, size_t nBytes);
</synopsis>
</para>
<para>
Any memory allocated with this function will be freed when
<parameter>res</parameter> is cleared. If the function fails,
the return value is <symbol>NULL</symbol>. The result is
guaranteed to be adequately aligned for any type of data,
just as for <function>malloc</function>.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQresultMemorySize">
<term><function>PQresultMemorySize</function><indexterm><primary>PQresultMemorySize</primary></indexterm></term>
<listitem>
<para>
Retrieves the number of bytes allocated for
a <structname>PGresult</structname> object.
<synopsis>
size_t PQresultMemorySize(const PGresult *res);
</synopsis>
</para>
<para>
This value is the sum of all <function>malloc</function> requests
associated with the <structname>PGresult</structname> object, that is,
all the memory that will be freed by <xref linkend="libpq-PQclear"/>.
This information can be useful for managing memory consumption.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQlibVersion">
<term><function>PQlibVersion</function><indexterm
><primary>PQlibVersion</primary><seealso>PQserverVersion</seealso></indexterm></term>
<listitem>
<para>
Return the version of <productname>libpq</productname> that is being used.
<synopsis>
int PQlibVersion(void);
</synopsis>
</para>
<para>
The result of this function can be used to determine, at
run time, whether specific functionality is available in the currently
loaded version of libpq. The function can be used, for example,
to determine which connection options are available in
<xref linkend="libpq-PQconnectdb"/>.
</para>
<para>
The result is formed by multiplying the library's major version
number by 10000 and adding the minor version number. For example,
version 10.1 will be returned as 100001, and version 11.0 will be
returned as 110000.
</para>
<para>
Prior to major version 10, <productname>PostgreSQL</productname> used
three-part version numbers in which the first two parts together
represented the major version. For those
versions, <xref linkend="libpq-PQlibVersion"/> uses two digits for each
part; for example version 9.1.5 will be returned as 90105, and
version 9.2.0 will be returned as 90200.
</para>
<para>
Therefore, for purposes of determining feature compatibility,
applications should divide the result of <xref linkend="libpq-PQlibVersion"/>
by 100 not 10000 to determine a logical major version number.
In all release series, only the last two digits differ between
minor releases (bug-fix releases).
</para>
<note>
<para>
This function appeared in <productname>PostgreSQL</productname> version 9.1, so
it cannot be used to detect required functionality in earlier
versions, since calling it will create a link dependency
on version 9.1 or later.
</para>
</note>
</listitem>
</varlistentry>
</variablelist>
</sect1>
<sect1 id="libpq-notice-processing">
<title>Notice Processing</title>
<indexterm zone="libpq-notice-processing">
<primary>notice processing</primary>
<secondary>in libpq</secondary>
</indexterm>
<para>
Notice and warning messages generated by the server are not returned
by the query execution functions, since they do not imply failure of
the query. Instead they are passed to a notice handling function, and
execution continues normally after the handler returns. The default
notice handling function prints the message on
<filename>stderr</filename>, but the application can override this
behavior by supplying its own handling function.
</para>
<para>
For historical reasons, there are two levels of notice handling, called
the notice receiver and notice processor. The default behavior is for
the notice receiver to format the notice and pass a string to the notice
processor for printing. However, an application that chooses to provide
its own notice receiver will typically ignore the notice processor
layer and just do all the work in the notice receiver.
</para>
<para>
The function <function id="libpq-PQsetNoticeReceiver">PQsetNoticeReceiver</function>
<indexterm><primary>notice receiver</primary></indexterm>
<indexterm><primary>PQsetNoticeReceiver</primary></indexterm> sets or
examines the current notice receiver for a connection object.
Similarly, <function id="libpq-PQsetNoticeProcessor">PQsetNoticeProcessor</function>
<indexterm><primary>notice processor</primary></indexterm>
<indexterm><primary>PQsetNoticeProcessor</primary></indexterm> sets or
examines the current notice processor.
<synopsis>
typedef void (*PQnoticeReceiver) (void *arg, const PGresult *res);
PQnoticeReceiver
PQsetNoticeReceiver(PGconn *conn,
PQnoticeReceiver proc,
void *arg);
typedef void (*PQnoticeProcessor) (void *arg, const char *message);
PQnoticeProcessor
PQsetNoticeProcessor(PGconn *conn,
PQnoticeProcessor proc,
void *arg);
</synopsis>
Each of these functions returns the previous notice receiver or
processor function pointer, and sets the new value. If you supply a
null function pointer, no action is taken, but the current pointer is
returned.
</para>
<para>
When a notice or warning message is received from the server, or
generated internally by <application>libpq</application>, the notice
receiver function is called. It is passed the message in the form of
a <symbol>PGRES_NONFATAL_ERROR</symbol>
<structname>PGresult</structname>. (This allows the receiver to extract
individual fields using <xref linkend="libpq-PQresultErrorField"/>, or obtain a
complete preformatted message using <xref linkend="libpq-PQresultErrorMessage"/>
or <xref linkend="libpq-PQresultVerboseErrorMessage"/>.) The same
void pointer passed to <function>PQsetNoticeReceiver</function> is also
passed. (This pointer can be used to access application-specific state
if needed.)
</para>
<para>
The default notice receiver simply extracts the message (using
<xref linkend="libpq-PQresultErrorMessage"/>) and passes it to the notice
processor.
</para>
<para>
The notice processor is responsible for handling a notice or warning
message given in text form. It is passed the string text of the message
(including a trailing newline), plus a void pointer that is the same
one passed to <function>PQsetNoticeProcessor</function>. (This pointer
can be used to access application-specific state if needed.)
</para>
<para>
The default notice processor is simply:
<programlisting>
static void
defaultNoticeProcessor(void *arg, const char *message)
{
fprintf(stderr, "%s", message);
}
</programlisting>
</para>
<para>
Once you have set a notice receiver or processor, you should expect
that that function could be called as long as either the
<structname>PGconn</structname> object or <structname>PGresult</structname> objects made
from it exist. At creation of a <structname>PGresult</structname>, the
<structname>PGconn</structname>'s current notice handling pointers are copied
into the <structname>PGresult</structname> for possible use by functions like
<xref linkend="libpq-PQgetvalue"/>.
</para>
</sect1>
<sect1 id="libpq-events">
<title>Event System</title>
<para>
<application>libpq</application>'s event system is designed to notify
registered event handlers about interesting
<application>libpq</application> events, such as the creation or
destruction of <structname>PGconn</structname> and
<structname>PGresult</structname> objects. A principal use case is that
this allows applications to associate their own data with a
<structname>PGconn</structname> or <structname>PGresult</structname>
and ensure that that data is freed at an appropriate time.
</para>
<para>
Each registered event handler is associated with two pieces of data,
known to <application>libpq</application> only as opaque <literal>void *</literal>
pointers. There is a <firstterm>pass-through</firstterm> pointer that is provided
by the application when the event handler is registered with a
<structname>PGconn</structname>. The pass-through pointer never changes for the
life of the <structname>PGconn</structname> and all <structname>PGresult</structname>s
generated from it; so if used, it must point to long-lived data.
In addition there is an <firstterm>instance data</firstterm> pointer, which starts
out <symbol>NULL</symbol> in every <structname>PGconn</structname> and <structname>PGresult</structname>.
This pointer can be manipulated using the
<xref linkend="libpq-PQinstanceData"/>,
<xref linkend="libpq-PQsetInstanceData"/>,
<xref linkend="libpq-PQresultInstanceData"/> and
<xref linkend="libpq-PQresultSetInstanceData"/> functions. Note that
unlike the pass-through pointer, instance data of a <structname>PGconn</structname>
is not automatically inherited by <structname>PGresult</structname>s created from
it. <application>libpq</application> does not know what pass-through
and instance data pointers point to (if anything) and will never attempt
to free them &mdash; that is the responsibility of the event handler.
</para>
<sect2 id="libpq-events-types">
<title>Event Types</title>
<para>
The enum <literal>PGEventId</literal> names the types of events handled by
the event system. All its values have names beginning with
<literal>PGEVT</literal>. For each event type, there is a corresponding
event info structure that carries the parameters passed to the event
handlers. The event types are:
</para>
<variablelist>
<varlistentry id="libpq-pgevt-register">
<term><literal>PGEVT_REGISTER</literal></term>
<listitem>
<para>
The register event occurs when <xref linkend="libpq-PQregisterEventProc"/>
is called. It is the ideal time to initialize any
<literal>instanceData</literal> an event procedure may need. Only one
register event will be fired per event handler per connection. If the
event procedure fails (returns zero), the registration is canceled.
<synopsis>
typedef struct
{
PGconn *conn;
} PGEventRegister;
</synopsis>
When a <literal>PGEVT_REGISTER</literal> event is received, the
<parameter>evtInfo</parameter> pointer should be cast to a
<structname>PGEventRegister *</structname>. This structure contains a
<structname>PGconn</structname> that should be in the
<literal>CONNECTION_OK</literal> status; guaranteed if one calls
<xref linkend="libpq-PQregisterEventProc"/> right after obtaining a good
<structname>PGconn</structname>. When returning a failure code, all
cleanup must be performed as no <literal>PGEVT_CONNDESTROY</literal>
event will be sent.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-pgevt-connreset">
<term><literal>PGEVT_CONNRESET</literal></term>
<listitem>
<para>
The connection reset event is fired on completion of
<xref linkend="libpq-PQreset"/> or <function>PQresetPoll</function>. In
both cases, the event is only fired if the reset was successful.
The return value of the event procedure is ignored
in <productname>PostgreSQL</productname> v15 and later.
With earlier versions, however, it's important to return success
(nonzero) or the connection will be aborted.
<synopsis>
typedef struct
{
PGconn *conn;
} PGEventConnReset;
</synopsis>
When a <literal>PGEVT_CONNRESET</literal> event is received, the
<parameter>evtInfo</parameter> pointer should be cast to a
<structname>PGEventConnReset *</structname>. Although the contained
<structname>PGconn</structname> was just reset, all event data remains
unchanged. This event should be used to reset/reload/requery any
associated <literal>instanceData</literal>. Note that even if the
event procedure fails to process <literal>PGEVT_CONNRESET</literal>, it will
still receive a <literal>PGEVT_CONNDESTROY</literal> event when the connection
is closed.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-pgevt-conndestroy">
<term><literal>PGEVT_CONNDESTROY</literal></term>
<listitem>
<para>
The connection destroy event is fired in response to
<xref linkend="libpq-PQfinish"/>. It is the event procedure's
responsibility to properly clean up its event data as libpq has no
ability to manage this memory. Failure to clean up will lead
to memory leaks.
<synopsis>
typedef struct
{
PGconn *conn;
} PGEventConnDestroy;
</synopsis>
When a <literal>PGEVT_CONNDESTROY</literal> event is received, the
<parameter>evtInfo</parameter> pointer should be cast to a
<structname>PGEventConnDestroy *</structname>. This event is fired
prior to <xref linkend="libpq-PQfinish"/> performing any other cleanup.
The return value of the event procedure is ignored since there is no
way of indicating a failure from <xref linkend="libpq-PQfinish"/>. Also,
an event procedure failure should not abort the process of cleaning up
unwanted memory.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-pgevt-resultcreate">
<term><literal>PGEVT_RESULTCREATE</literal></term>
<listitem>
<para>
The result creation event is fired in response to any query execution
function that generates a result, including
<xref linkend="libpq-PQgetResult"/>. This event will only be fired after
the result has been created successfully.
<synopsis>
typedef struct
{
PGconn *conn;
PGresult *result;
} PGEventResultCreate;
</synopsis>
When a <literal>PGEVT_RESULTCREATE</literal> event is received, the
<parameter>evtInfo</parameter> pointer should be cast to a
<structname>PGEventResultCreate *</structname>. The
<parameter>conn</parameter> is the connection used to generate the
result. This is the ideal place to initialize any
<literal>instanceData</literal> that needs to be associated with the
result. If an event procedure fails (returns zero), that event
procedure will be ignored for the remaining lifetime of the result;
that is, it will not receive <literal>PGEVT_RESULTCOPY</literal>
or <literal>PGEVT_RESULTDESTROY</literal> events for this result or
results copied from it.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-pgevt-resultcopy">
<term><literal>PGEVT_RESULTCOPY</literal></term>
<listitem>
<para>
The result copy event is fired in response to
<xref linkend="libpq-PQcopyResult"/>. This event will only be fired after
the copy is complete. Only event procedures that have
successfully handled the <literal>PGEVT_RESULTCREATE</literal>
or <literal>PGEVT_RESULTCOPY</literal> event for the source result
will receive <literal>PGEVT_RESULTCOPY</literal> events.
<synopsis>
typedef struct
{
const PGresult *src;
PGresult *dest;
} PGEventResultCopy;
</synopsis>
When a <literal>PGEVT_RESULTCOPY</literal> event is received, the
<parameter>evtInfo</parameter> pointer should be cast to a
<structname>PGEventResultCopy *</structname>. The
<parameter>src</parameter> result is what was copied while the
<parameter>dest</parameter> result is the copy destination. This event
can be used to provide a deep copy of <literal>instanceData</literal>,
since <literal>PQcopyResult</literal> cannot do that. If an event
procedure fails (returns zero), that event procedure will be
ignored for the remaining lifetime of the new result; that is, it
will not receive <literal>PGEVT_RESULTCOPY</literal>
or <literal>PGEVT_RESULTDESTROY</literal> events for that result or
results copied from it.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-pgevt-resultdestroy">
<term><literal>PGEVT_RESULTDESTROY</literal></term>
<listitem>
<para>
The result destroy event is fired in response to a
<xref linkend="libpq-PQclear"/>. It is the event procedure's
responsibility to properly clean up its event data as libpq has no
ability to manage this memory. Failure to clean up will lead
to memory leaks.
<synopsis>
typedef struct
{
PGresult *result;
} PGEventResultDestroy;
</synopsis>
When a <literal>PGEVT_RESULTDESTROY</literal> event is received, the
<parameter>evtInfo</parameter> pointer should be cast to a
<structname>PGEventResultDestroy *</structname>. This event is fired
prior to <xref linkend="libpq-PQclear"/> performing any other cleanup.
The return value of the event procedure is ignored since there is no
way of indicating a failure from <xref linkend="libpq-PQclear"/>. Also,
an event procedure failure should not abort the process of cleaning up
unwanted memory.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="libpq-events-proc">
<title>Event Callback Procedure</title>
<variablelist>
<varlistentry id="libpq-PGEventProc">
<term><literal>PGEventProc</literal><indexterm><primary>PGEventProc</primary></indexterm></term>
<listitem>
<para>
<literal>PGEventProc</literal> is a typedef for a pointer to an
event procedure, that is, the user callback function that receives
events from libpq. The signature of an event procedure must be
<synopsis>
int eventproc(PGEventId evtId, void *evtInfo, void *passThrough)
</synopsis>
The <parameter>evtId</parameter> parameter indicates which
<literal>PGEVT</literal> event occurred. The
<parameter>evtInfo</parameter> pointer must be cast to the appropriate
structure type to obtain further information about the event.
The <parameter>passThrough</parameter> parameter is the pointer
provided to <xref linkend="libpq-PQregisterEventProc"/> when the event
procedure was registered. The function should return a non-zero value
if it succeeds and zero if it fails.
</para>
<para>
A particular event procedure can be registered only once in any
<structname>PGconn</structname>. This is because the address of the procedure
is used as a lookup key to identify the associated instance data.
</para>
<caution>
<para>
On Windows, functions can have two different addresses: one visible
from outside a DLL and another visible from inside the DLL. One
should be careful that only one of these addresses is used with
<application>libpq</application>'s event-procedure functions, else confusion will
result. The simplest rule for writing code that will work is to
ensure that event procedures are declared <literal>static</literal>. If the
procedure's address must be available outside its own source file,
expose a separate function to return the address.
</para>
</caution>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="libpq-events-funcs">
<title>Event Support Functions</title>
<variablelist>
<varlistentry id="libpq-PQregisterEventProc">
<term><function>PQregisterEventProc</function><indexterm><primary>PQregisterEventProc</primary></indexterm></term>
<listitem>
<para>
Registers an event callback procedure with libpq.
<synopsis>
int PQregisterEventProc(PGconn *conn, PGEventProc proc,
const char *name, void *passThrough);
</synopsis>
</para>
<para>
An event procedure must be registered once on each
<structname>PGconn</structname> you want to receive events about. There is no
limit, other than memory, on the number of event procedures that
can be registered with a connection. The function returns a non-zero
value if it succeeds and zero if it fails.
</para>
<para>
The <parameter>proc</parameter> argument will be called when a libpq
event is fired. Its memory address is also used to lookup
<literal>instanceData</literal>. The <parameter>name</parameter>
argument is used to refer to the event procedure in error messages.
This value cannot be <symbol>NULL</symbol> or a zero-length string. The name string is
copied into the <structname>PGconn</structname>, so what is passed need not be
long-lived. The <parameter>passThrough</parameter> pointer is passed
to the <parameter>proc</parameter> whenever an event occurs. This
argument can be <symbol>NULL</symbol>.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQsetInstanceData">
<term><function>PQsetInstanceData</function><indexterm><primary>PQsetInstanceData</primary></indexterm></term>
<listitem>
<para>
Sets the connection <parameter>conn</parameter>'s <literal>instanceData</literal>
for procedure <parameter>proc</parameter> to <parameter>data</parameter>. This
returns non-zero for success and zero for failure. (Failure is
only possible if <parameter>proc</parameter> has not been properly
registered in <parameter>conn</parameter>.)
<synopsis>
int PQsetInstanceData(PGconn *conn, PGEventProc proc, void *data);
</synopsis>
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQinstanceData">
<term><function>PQinstanceData</function><indexterm><primary>PQinstanceData</primary></indexterm></term>
<listitem>
<para>
Returns the
connection <parameter>conn</parameter>'s <literal>instanceData</literal>
associated with procedure <parameter>proc</parameter>,
or <symbol>NULL</symbol> if there is none.
<synopsis>
void *PQinstanceData(const PGconn *conn, PGEventProc proc);
</synopsis>
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQresultSetInstanceData">
<term><function>PQresultSetInstanceData</function><indexterm><primary>PQresultSetInstanceData</primary></indexterm></term>
<listitem>
<para>
Sets the result's <literal>instanceData</literal>
for <parameter>proc</parameter> to <parameter>data</parameter>. This returns
non-zero for success and zero for failure. (Failure is only
possible if <parameter>proc</parameter> has not been properly registered
in the result.)
<synopsis>
int PQresultSetInstanceData(PGresult *res, PGEventProc proc, void *data);
</synopsis>
</para>
<para>
Beware that any storage represented by <parameter>data</parameter>
will not be accounted for by <xref linkend="libpq-PQresultMemorySize"/>,
unless it is allocated using <xref linkend="libpq-PQresultAlloc"/>.
(Doing so is recommendable because it eliminates the need to free
such storage explicitly when the result is destroyed.)
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQresultInstanceData">
<term><function>PQresultInstanceData</function><indexterm><primary>PQresultInstanceData</primary></indexterm></term>
<listitem>
<para>
Returns the result's <literal>instanceData</literal> associated with <parameter>proc</parameter>, or <symbol>NULL</symbol>
if there is none.
<synopsis>
void *PQresultInstanceData(const PGresult *res, PGEventProc proc);
</synopsis>
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="libpq-events-example">
<title>Event Example</title>
<para>
Here is a skeleton example of managing private data associated with
libpq connections and results.
</para>
<programlisting>
<![CDATA[
/* required header for libpq events (note: includes libpq-fe.h) */
#include <libpq-events.h>
/* The instanceData */
typedef struct
{
int n;
char *str;
} mydata;
/* PGEventProc */
static int myEventProc(PGEventId evtId, void *evtInfo, void *passThrough);
int
main(void)
{
mydata *data;
PGresult *res;
PGconn *conn =
PQconnectdb("dbname=postgres options=-csearch_path=");
if (PQstatus(conn) != CONNECTION_OK)
{
/* PQerrorMessage's result includes a trailing newline */
fprintf(stderr, "%s", PQerrorMessage(conn));
PQfinish(conn);
return 1;
}
/* called once on any connection that should receive events.
* Sends a PGEVT_REGISTER to myEventProc.
*/
if (!PQregisterEventProc(conn, myEventProc, "mydata_proc", NULL))
{
fprintf(stderr, "Cannot register PGEventProc\n");
PQfinish(conn);
return 1;
}
/* conn instanceData is available */
data = PQinstanceData(conn, myEventProc);
/* Sends a PGEVT_RESULTCREATE to myEventProc */
res = PQexec(conn, "SELECT 1 + 1");
/* result instanceData is available */
data = PQresultInstanceData(res, myEventProc);
/* If PG_COPYRES_EVENTS is used, sends a PGEVT_RESULTCOPY to myEventProc */
res_copy = PQcopyResult(res, PG_COPYRES_TUPLES | PG_COPYRES_EVENTS);
/* result instanceData is available if PG_COPYRES_EVENTS was
* used during the PQcopyResult call.
*/
data = PQresultInstanceData(res_copy, myEventProc);
/* Both clears send a PGEVT_RESULTDESTROY to myEventProc */
PQclear(res);
PQclear(res_copy);
/* Sends a PGEVT_CONNDESTROY to myEventProc */
PQfinish(conn);
return 0;
}
static int
myEventProc(PGEventId evtId, void *evtInfo, void *passThrough)
{
switch (evtId)
{
case PGEVT_REGISTER:
{
PGEventRegister *e = (PGEventRegister *)evtInfo;
mydata *data = get_mydata(e->conn);
/* associate app specific data with connection */
PQsetInstanceData(e->conn, myEventProc, data);
break;
}
case PGEVT_CONNRESET:
{
PGEventConnReset *e = (PGEventConnReset *)evtInfo;
mydata *data = PQinstanceData(e->conn, myEventProc);
if (data)
memset(data, 0, sizeof(mydata));
break;
}
case PGEVT_CONNDESTROY:
{
PGEventConnDestroy *e = (PGEventConnDestroy *)evtInfo;
mydata *data = PQinstanceData(e->conn, myEventProc);
/* free instance data because the conn is being destroyed */
if (data)
free_mydata(data);
break;
}
case PGEVT_RESULTCREATE:
{
PGEventResultCreate *e = (PGEventResultCreate *)evtInfo;
mydata *conn_data = PQinstanceData(e->conn, myEventProc);
mydata *res_data = dup_mydata(conn_data);
/* associate app specific data with result (copy it from conn) */
PQresultSetInstanceData(e->result, myEventProc, res_data);
break;
}
case PGEVT_RESULTCOPY:
{
PGEventResultCopy *e = (PGEventResultCopy *)evtInfo;
mydata *src_data = PQresultInstanceData(e->src, myEventProc);
mydata *dest_data = dup_mydata(src_data);
/* associate app specific data with result (copy it from a result) */
PQresultSetInstanceData(e->dest, myEventProc, dest_data);
break;
}
case PGEVT_RESULTDESTROY:
{
PGEventResultDestroy *e = (PGEventResultDestroy *)evtInfo;
mydata *data = PQresultInstanceData(e->result, myEventProc);
/* free instance data because the result is being destroyed */
if (data)
free_mydata(data);
break;
}
/* unknown event ID, just return true. */
default:
break;
}
return true; /* event processing succeeded */
}
]]>
</programlisting>
</sect2>
</sect1>
<sect1 id="libpq-envars">
<title>Environment Variables</title>
<indexterm zone="libpq-envars">
<primary>environment variable</primary>
</indexterm>
<para>
The following environment variables can be used to select default
connection parameter values, which will be used by
<xref linkend="libpq-PQconnectdb"/>, <xref linkend="libpq-PQsetdbLogin"/> and
<xref linkend="libpq-PQsetdb"/> if no value is directly specified by the calling
code. These are useful to avoid hard-coding database connection
information into simple client applications, for example.
<itemizedlist>
<listitem>
<para>
<indexterm>
<primary><envar>PGHOST</envar></primary>
</indexterm>
<envar>PGHOST</envar> behaves the same as the <xref
linkend="libpq-connect-host"/> connection parameter.
</para>
</listitem>
<listitem>
<para>
<indexterm>
<primary><envar>PGSSLNEGOTIATION</envar></primary>
</indexterm>
<envar>PGSSLNEGOTIATION</envar> behaves the same as the <xref
linkend="libpq-connect-sslnegotiation"/> connection parameter.
</para>
</listitem>
<listitem>
<para>
<indexterm>
<primary><envar>PGHOSTADDR</envar></primary>
</indexterm>
<envar>PGHOSTADDR</envar> behaves the same as the <xref
linkend="libpq-connect-hostaddr"/> connection parameter.
This can be set instead of or in addition to <envar>PGHOST</envar>
to avoid DNS lookup overhead.
</para>
</listitem>
<listitem>
<para>
<indexterm>
<primary><envar>PGPORT</envar></primary>
</indexterm>
<envar>PGPORT</envar> behaves the same as the <xref
linkend="libpq-connect-port"/> connection parameter.
</para>
</listitem>
<listitem>
<para>
<indexterm>
<primary><envar>PGDATABASE</envar></primary>
</indexterm>
<envar>PGDATABASE</envar> behaves the same as the <xref
linkend="libpq-connect-dbname"/> connection parameter.
</para>
</listitem>
<listitem>
<para>
<indexterm>
<primary><envar>PGUSER</envar></primary>
</indexterm>
<envar>PGUSER</envar> behaves the same as the <xref
linkend="libpq-connect-user"/> connection parameter.
</para>
</listitem>
<listitem>
<para>
<indexterm>
<primary><envar>PGPASSWORD</envar></primary>
</indexterm>
<envar>PGPASSWORD</envar> behaves the same as the <xref
linkend="libpq-connect-password"/> connection parameter.
Use of this environment variable
is not recommended for security reasons, as some operating systems
allow non-root users to see process environment variables via
<application>ps</application>; instead consider using a password file
(see <xref linkend="libpq-pgpass"/>).
</para>
</listitem>
<listitem>
<para>
<indexterm>
<primary><envar>PGPASSFILE</envar></primary>
</indexterm>
<envar>PGPASSFILE</envar> behaves the same as the <xref
linkend="libpq-connect-passfile"/> connection parameter.
</para>
</listitem>
<listitem>
<para>
<indexterm>
<primary><envar>PGREQUIREAUTH</envar></primary>
</indexterm>
<envar>PGREQUIREAUTH</envar> behaves the same as the <xref
linkend="libpq-connect-require-auth"/> connection parameter.
</para>
</listitem>
<listitem>
<para>
<indexterm>
<primary><envar>PGCHANNELBINDING</envar></primary>
</indexterm>
<envar>PGCHANNELBINDING</envar> behaves the same as the <xref
linkend="libpq-connect-channel-binding"/> connection parameter.
</para>
</listitem>
<listitem>
<para>
<indexterm>
<primary><envar>PGSERVICE</envar></primary>
</indexterm>
<envar>PGSERVICE</envar> behaves the same as the <xref
linkend="libpq-connect-service"/> connection parameter.
</para>
</listitem>
<listitem>
<para>
<indexterm>
<primary><envar>PGSERVICEFILE</envar></primary>
</indexterm>
<envar>PGSERVICEFILE</envar> specifies the name of the per-user
connection service file
(see <xref linkend="libpq-pgservice"/>).
Defaults to <filename>~/.pg_service.conf</filename>, or
<filename>%APPDATA%\postgresql\.pg_service.conf</filename> on
Microsoft Windows.
</para>
</listitem>
<listitem>
<para>
<indexterm>
<primary><envar>PGOPTIONS</envar></primary>
</indexterm>
<envar>PGOPTIONS</envar> behaves the same as the <xref
linkend="libpq-connect-options"/> connection parameter.
</para>
</listitem>
<listitem>
<para>
<indexterm>
<primary><envar>PGAPPNAME</envar></primary>
</indexterm>
<envar>PGAPPNAME</envar> behaves the same as the <xref
linkend="libpq-connect-application-name"/> connection parameter.
</para>
</listitem>
<listitem>
<para>
<indexterm>
<primary><envar>PGSSLMODE</envar></primary>
</indexterm>
<envar>PGSSLMODE</envar> behaves the same as the <xref
linkend="libpq-connect-sslmode"/> connection parameter.
</para>
</listitem>
<listitem>
<para>
<indexterm>
<primary><envar>PGREQUIRESSL</envar></primary>
</indexterm>
<envar>PGREQUIRESSL</envar> behaves the same as the <xref
linkend="libpq-connect-requiressl"/> connection parameter.
This environment variable is deprecated in favor of the
<envar>PGSSLMODE</envar> variable; setting both variables suppresses the
effect of this one.
</para>
</listitem>
<listitem>
<para>
<indexterm>
<primary><envar>PGSSLCOMPRESSION</envar></primary>
</indexterm>
<envar>PGSSLCOMPRESSION</envar> behaves the same as the <xref
linkend="libpq-connect-sslcompression"/> connection parameter.
</para>
</listitem>
<listitem>
<para>
<indexterm>
<primary><envar>PGSSLCERT</envar></primary>
</indexterm>
<envar>PGSSLCERT</envar> behaves the same as the <xref
linkend="libpq-connect-sslcert"/> connection parameter.
</para>
</listitem>
<listitem>
<para>
<indexterm>
<primary><envar>PGSSLKEY</envar></primary>
</indexterm>
<envar>PGSSLKEY</envar> behaves the same as the <xref
linkend="libpq-connect-sslkey"/> connection parameter.
</para>
</listitem>
<listitem>
<para>
<indexterm>
<primary><envar>PGSSLCERTMODE</envar></primary>
</indexterm>
<envar>PGSSLCERTMODE</envar> behaves the same as the <xref
linkend="libpq-connect-sslcertmode"/> connection parameter.
</para>
</listitem>
<listitem>
<para>
<indexterm>
<primary><envar>PGSSLROOTCERT</envar></primary>
</indexterm>
<envar>PGSSLROOTCERT</envar> behaves the same as the <xref
linkend="libpq-connect-sslrootcert"/> connection parameter.
</para>
</listitem>
<listitem>
<para>
<indexterm>
<primary><envar>PGSSLCRL</envar></primary>
</indexterm>
<envar>PGSSLCRL</envar> behaves the same as the <xref
linkend="libpq-connect-sslcrl"/> connection parameter.
</para>
</listitem>
<listitem>
<para>
<indexterm>
<primary><envar>PGSSLCRLDIR</envar></primary>
</indexterm>
<envar>PGSSLCRLDIR</envar> behaves the same as the <xref
linkend="libpq-connect-sslcrldir"/> connection parameter.
</para>
</listitem>
<listitem>
<para>
<indexterm>
<primary><envar>PGSSLSNI</envar></primary>
</indexterm>
<envar>PGSSLSNI</envar> behaves the same as the <xref
linkend="libpq-connect-sslsni"/> connection parameter.
</para>
</listitem>
<listitem>
<para>
<indexterm>
<primary><envar>PGREQUIREPEER</envar></primary>
</indexterm>
<envar>PGREQUIREPEER</envar> behaves the same as the <xref
linkend="libpq-connect-requirepeer"/> connection parameter.
</para>
</listitem>
<listitem>
<para>
<indexterm>
<primary><envar>PGSSLMINPROTOCOLVERSION</envar></primary>
</indexterm>
<envar>PGSSLMINPROTOCOLVERSION</envar> behaves the same as the <xref
linkend="libpq-connect-ssl-min-protocol-version"/> connection parameter.
</para>
</listitem>
<listitem>
<para>
<indexterm>
<primary><envar>PGSSLMAXPROTOCOLVERSION</envar></primary>
</indexterm>
<envar>PGSSLMAXPROTOCOLVERSION</envar> behaves the same as the <xref
linkend="libpq-connect-ssl-max-protocol-version"/> connection parameter.
</para>
</listitem>
<listitem>
<para>
<indexterm>
<primary><envar>PGGSSENCMODE</envar></primary>
</indexterm>
<envar>PGGSSENCMODE</envar> behaves the same as the <xref
linkend="libpq-connect-gssencmode"/> connection parameter.
</para>
</listitem>
<listitem>
<para>
<indexterm>
<primary><envar>PGKRBSRVNAME</envar></primary>
</indexterm>
<envar>PGKRBSRVNAME</envar> behaves the same as the <xref
linkend="libpq-connect-krbsrvname"/> connection parameter.
</para>
</listitem>
<listitem>
<para>
<indexterm>
<primary><envar>PGGSSLIB</envar></primary>
</indexterm>
<envar>PGGSSLIB</envar> behaves the same as the <xref
linkend="libpq-connect-gsslib"/> connection parameter.
</para>
</listitem>
<listitem>
<para>
<indexterm>
<primary><envar>PGGSSDELEGATION</envar></primary>
</indexterm>
<envar>PGGSSDELEGATION</envar> behaves the same as the <xref
linkend="libpq-connect-gssdelegation"/> connection parameter.
</para>
</listitem>
<listitem>
<para>
<indexterm>
<primary><envar>PGCONNECT_TIMEOUT</envar></primary>
</indexterm>
<envar>PGCONNECT_TIMEOUT</envar> behaves the same as the <xref
linkend="libpq-connect-connect-timeout"/> connection parameter.
</para>
</listitem>
<listitem>
<para>
<indexterm>
<primary><envar>PGCLIENTENCODING</envar></primary>
</indexterm>
<envar>PGCLIENTENCODING</envar> behaves the same as the <xref
linkend="libpq-connect-client-encoding"/> connection parameter.
</para>
</listitem>
<listitem>
<para>
<indexterm>
<primary><envar>PGTARGETSESSIONATTRS</envar></primary>
</indexterm>
<envar>PGTARGETSESSIONATTRS</envar> behaves the same as the <xref
linkend="libpq-connect-target-session-attrs"/> connection parameter.
</para>
</listitem>
<listitem>
<para>
<indexterm>
<primary><envar>PGLOADBALANCEHOSTS</envar></primary>
</indexterm>
<envar>PGLOADBALANCEHOSTS</envar> behaves the same as the <xref
linkend="libpq-connect-load-balance-hosts"/> connection parameter.
</para>
</listitem>
</itemizedlist>
</para>
<para>
The following environment variables can be used to specify default
behavior for each <productname>PostgreSQL</productname> session. (See
also the <xref linkend="sql-alterrole"/>
and <xref linkend="sql-alterdatabase"/>
commands for ways to set default behavior on a per-user or per-database
basis.)
<itemizedlist>
<listitem>
<para>
<indexterm>
<primary><envar>PGDATESTYLE</envar></primary>
</indexterm>
<envar>PGDATESTYLE</envar> sets the default style of date/time
representation. (Equivalent to <literal>SET datestyle TO
...</literal>.)
</para>
</listitem>
<listitem>
<para>
<indexterm>
<primary><envar>PGTZ</envar></primary>
</indexterm>
<envar>PGTZ</envar> sets the default time zone. (Equivalent to
<literal>SET timezone TO ...</literal>.)
</para>
</listitem>
<listitem>
<para>
<indexterm>
<primary><envar>PGGEQO</envar></primary>
</indexterm>
<envar>PGGEQO</envar> sets the default mode for the genetic query
optimizer. (Equivalent to <literal>SET geqo TO ...</literal>.)
</para>
</listitem>
</itemizedlist>
Refer to the <acronym>SQL</acronym> command <xref linkend="sql-set"/>
for information on correct values for these
environment variables.
</para>
<para>
The following environment variables determine internal behavior of
<application>libpq</application>; they override compiled-in defaults.
<itemizedlist>
<listitem>
<para>
<indexterm>
<primary><envar>PGSYSCONFDIR</envar></primary>
</indexterm>
<envar>PGSYSCONFDIR</envar> sets the directory containing the
<filename>pg_service.conf</filename> file and in a future version
possibly other system-wide configuration files.
</para>
</listitem>
<listitem>
<para>
<indexterm>
<primary><envar>PGLOCALEDIR</envar></primary>
</indexterm>
<envar>PGLOCALEDIR</envar> sets the directory containing the
<literal>locale</literal> files for message localization.
</para>
</listitem>
</itemizedlist>
</para>
</sect1>
<sect1 id="libpq-pgpass">
<title>The Password File</title>
<indexterm zone="libpq-pgpass">
<primary>password file</primary>
</indexterm>
<indexterm zone="libpq-pgpass">
<primary>.pgpass</primary>
</indexterm>
<para>
The file <filename>.pgpass</filename> in a user's home directory can
contain passwords to
be used if the connection requires a password (and no password has been
specified otherwise). On Microsoft Windows the file is named
<filename>%APPDATA%\postgresql\pgpass.conf</filename> (where
<filename>%APPDATA%</filename> refers to the Application Data subdirectory in
the user's profile).
Alternatively, the password file to use can be specified
using the connection parameter <xref linkend="libpq-connect-passfile"/>
or the environment variable <envar>PGPASSFILE</envar>.
</para>
<para>
This file should contain lines of the following format:
<synopsis>
<replaceable>hostname</replaceable>:<replaceable>port</replaceable>:<replaceable>database</replaceable>:<replaceable>username</replaceable>:<replaceable>password</replaceable>
</synopsis>
(You can add a reminder comment to the file by copying the line above and
preceding it with <literal>#</literal>.)
Each of the first four fields can be a literal value, or
<literal>*</literal>, which matches anything. The password field from
the first line that matches the current connection parameters will be
used. (Therefore, put more-specific entries first when you are using
wildcards.) If an entry needs to contain <literal>:</literal> or
<literal>\</literal>, escape this character with <literal>\</literal>.
The host name field is matched to the <literal>host</literal> connection
parameter if that is specified, otherwise to
the <literal>hostaddr</literal> parameter if that is specified; if neither
are given then the host name <literal>localhost</literal> is searched for.
The host name <literal>localhost</literal> is also searched for when
the connection is a Unix-domain socket connection and
the <literal>host</literal> parameter
matches <application>libpq</application>'s default socket directory path.
In a standby server, a database field of <literal>replication</literal>
matches streaming replication connections made to the primary server.
The database field is of limited usefulness otherwise, because users have
the same password for all databases in the same cluster.
</para>
<para>
On Unix systems, the permissions on a password file must
disallow any access to world or group; achieve this by a command such as
<command>chmod 0600 ~/.pgpass</command>. If the permissions are less
strict than this, the file will be ignored. On Microsoft Windows, it
is assumed that the file is stored in a directory that is secure, so
no special permissions check is made.
</para>
</sect1>
<sect1 id="libpq-pgservice">
<title>The Connection Service File</title>
<indexterm zone="libpq-pgservice">
<primary>connection service file</primary>
</indexterm>
<indexterm zone="libpq-pgservice">
<primary>pg_service.conf</primary>
</indexterm>
<indexterm zone="libpq-pgservice">
<primary>.pg_service.conf</primary>
</indexterm>
<para>
The connection service file allows libpq connection parameters to be
associated with a single service name. That service name can then be
specified in a libpq connection string, and the associated settings will be
used. This allows connection parameters to be modified without requiring
a recompile of the libpq-using application. The service name can also be
specified using the <envar>PGSERVICE</envar> environment variable.
</para>
<para>
Service names can be defined in either a per-user service file or a
system-wide file. If the same service name exists in both the user
and the system file, the user file takes precedence.
By default, the per-user service file is named
<filename>~/.pg_service.conf</filename>.
On Microsoft Windows, it is named
<filename>%APPDATA%\postgresql\.pg_service.conf</filename> (where
<filename>%APPDATA%</filename> refers to the Application Data subdirectory
in the user's profile). A different file name can be specified by
setting the environment variable <envar>PGSERVICEFILE</envar>.
The system-wide file is named <filename>pg_service.conf</filename>.
By default it is sought in the <filename>etc</filename> directory
of the <productname>PostgreSQL</productname> installation
(use <literal>pg_config --sysconfdir</literal> to identify this
directory precisely). Another directory, but not a different file
name, can be specified by setting the environment variable
<envar>PGSYSCONFDIR</envar>.
</para>
<para>
Either service file uses an <quote>INI file</quote> format where the section
name is the service name and the parameters are connection
parameters; see <xref linkend="libpq-paramkeywords"/> for a list. For
example:
<programlisting>
# comment
[mydb]
host=somehost
port=5433
user=admin
</programlisting>
An example file is provided in
the <productname>PostgreSQL</productname> installation at
<filename>share/pg_service.conf.sample</filename>.
</para>
<para>
Connection parameters obtained from a service file are combined with
parameters obtained from other sources. A service file setting
overrides the corresponding environment variable, and in turn can be
overridden by a value given directly in the connection string.
For example, using the above service file, a connection string
<literal>service=mydb port=5434</literal> will use
host <literal>somehost</literal>, port <literal>5434</literal>,
user <literal>admin</literal>, and other parameters as set by
environment variables or built-in defaults.
</para>
</sect1>
<sect1 id="libpq-ldap">
<title>LDAP Lookup of Connection Parameters</title>
<indexterm zone="libpq-ldap">
<primary>LDAP connection parameter lookup</primary>
</indexterm>
<para>
If <application>libpq</application> has been compiled with LDAP support (option
<literal><option>--with-ldap</option></literal> for <command>configure</command>)
it is possible to retrieve connection options like <literal>host</literal>
or <literal>dbname</literal> via LDAP from a central server.
The advantage is that if the connection parameters for a database change,
the connection information doesn't have to be updated on all client machines.
</para>
<para>
LDAP connection parameter lookup uses the connection service file
<filename>pg_service.conf</filename> (see <xref
linkend="libpq-pgservice"/>). A line in a
<filename>pg_service.conf</filename> stanza that starts with
<literal>ldap://</literal> will be recognized as an LDAP URL and an
LDAP query will be performed. The result must be a list of
<literal>keyword = value</literal> pairs which will be used to set
connection options. The URL must conform to
<ulink url="https://datatracker.ietf.org/doc/html/rfc1959">RFC 1959</ulink>
and be of the form
<synopsis>
ldap://[<replaceable>hostname</replaceable>[:<replaceable>port</replaceable>]]/<replaceable>search_base</replaceable>?<replaceable>attribute</replaceable>?<replaceable>search_scope</replaceable>?<replaceable>filter</replaceable>
</synopsis>
where <replaceable>hostname</replaceable> defaults to
<literal>localhost</literal> and <replaceable>port</replaceable>
defaults to 389.
</para>
<para>
Processing of <filename>pg_service.conf</filename> is terminated after
a successful LDAP lookup, but is continued if the LDAP server cannot
be contacted. This is to provide a fallback with further LDAP URL
lines that point to different LDAP servers, classical <literal>keyword
= value</literal> pairs, or default connection options. If you would
rather get an error message in this case, add a syntactically incorrect
line after the LDAP URL.
</para>
<para>
A sample LDAP entry that has been created with the LDIF file
<programlisting>
version:1
dn:cn=mydatabase,dc=mycompany,dc=com
changetype:add
objectclass:top
objectclass:device
cn:mydatabase
description:host=dbserver.mycompany.com
description:port=5439
description:dbname=mydb
description:user=mydb_user
description:sslmode=require
</programlisting>
might be queried with the following LDAP URL:
<programlisting>
ldap://ldap.mycompany.com/dc=mycompany,dc=com?description?one?(cn=mydatabase)
</programlisting>
</para>
<para>
You can also mix regular service file entries with LDAP lookups.
A complete example for a stanza in <filename>pg_service.conf</filename>
would be:
<programlisting>
# only host and port are stored in LDAP, specify dbname and user explicitly
[customerdb]
dbname=customer
user=appuser
ldap://ldap.acme.com/cn=dbserver,cn=hosts?pgconnectinfo?base?(objectclass=*)
</programlisting>
</para>
</sect1>
<sect1 id="libpq-ssl">
<title>SSL Support</title>
<indexterm zone="libpq-ssl">
<primary>SSL</primary>
<secondary>TLS</secondary>
</indexterm>
<para>
<productname>PostgreSQL</productname> has native support for using <acronym>SSL</acronym>
connections to encrypt client/server communications using
<acronym>TLS</acronym> protocols for increased security.
See <xref linkend="ssl-tcp"/> for details about the server-side
<acronym>SSL</acronym> functionality.
</para>
<para>
<application>libpq</application> reads the system-wide
<productname>OpenSSL</productname> configuration file. By default, this
file is named <filename>openssl.cnf</filename> and is located in the
directory reported by <literal>openssl version -d</literal>. This default
can be overridden by setting environment variable
<envar>OPENSSL_CONF</envar> to the name of the desired configuration
file.
</para>
<sect2 id="libq-ssl-certificates">
<title>Client Verification of Server Certificates</title>
<para>
By default, <productname>PostgreSQL</productname> will not perform any verification of
the server certificate. This means that it is possible to spoof the server
identity (for example by modifying a DNS record or by taking over the server
IP address) without the client knowing. In order to prevent spoofing,
the client must be able to verify the server's identity via a chain of
trust. A chain of trust is established by placing a root (self-signed)
certificate authority (<acronym>CA</acronym>) certificate on one
computer and a leaf certificate <emphasis>signed</emphasis> by the
root certificate on another computer. It is also possible to use an
<quote>intermediate</quote> certificate which is signed by the root
certificate and signs leaf certificates.
</para>
<para>
To allow the client to verify the identity of the server, place a root
certificate on the client and a leaf certificate signed by the root
certificate on the server. To allow the server to verify the identity
of the client, place a root certificate on the server and a leaf
certificate signed by the root certificate on the client. One or more
intermediate certificates (usually stored with the leaf certificate)
can also be used to link the leaf certificate to the root certificate.
</para>
<para>
Once a chain of trust has been established, there are two ways for
the client to validate the leaf certificate sent by the server.
If the parameter <literal>sslmode</literal> is set to <literal>verify-ca</literal>,
libpq will verify that the server is trustworthy by checking the
certificate chain up to the root certificate stored on the client.
If <literal>sslmode</literal> is set to <literal>verify-full</literal>,
libpq will <emphasis>also</emphasis> verify that the server host
name matches the name stored in the server certificate. The
SSL connection will fail if the server certificate cannot be
verified. <literal>verify-full</literal> is recommended in most
security-sensitive environments.
</para>
<para>
In <literal>verify-full</literal> mode, the host name is matched against the
certificate's Subject Alternative Name attribute(s) (SAN), or against the
Common Name attribute if no SAN of type <literal>dNSName</literal> is
present. If the certificate's name attribute starts with an asterisk
(<literal>*</literal>), the asterisk will be treated as
a wildcard, which will match all characters <emphasis>except</emphasis> a dot
(<literal>.</literal>). This means the certificate will not match subdomains.
If the connection is made using an IP address instead of a host name, the
IP address will be matched (without doing any DNS lookups) against SANs of
type <literal>iPAddress</literal> or <literal>dNSName</literal>. If no
<literal>iPAddress</literal> SAN is present and no
matching <literal>dNSName</literal> SAN is present, the host IP address is
matched against the Common Name attribute.
</para>
<note>
<para>
For backward compatibility with earlier versions of PostgreSQL, the host
IP address is verified in a manner different
from <ulink url="https://datatracker.ietf.org/doc/html/rfc6125">RFC 6125</ulink>.
The host IP address is always matched against <literal>dNSName</literal>
SANs as well as <literal>iPAddress</literal> SANs, and can be matched
against the Common Name attribute if no relevant SANs exist.
</para>
</note>
<para>
To allow server certificate verification, one or more root certificates
must be placed in the file <filename>~/.postgresql/root.crt</filename>
in the user's home directory. (On Microsoft Windows the file is named
<filename>%APPDATA%\postgresql\root.crt</filename>.) Intermediate
certificates should also be added to the file if they are needed to link
the certificate chain sent by the server to the root certificates
stored on the client.
</para>
<para>
Certificate Revocation List (CRL) entries are also checked
if the file <filename>~/.postgresql/root.crl</filename> exists
(<filename>%APPDATA%\postgresql\root.crl</filename> on Microsoft
Windows).
</para>
<para>
The location of the root certificate file and the CRL can be changed by
setting
the connection parameters <literal>sslrootcert</literal> and <literal>sslcrl</literal>
or the environment variables <envar>PGSSLROOTCERT</envar> and <envar>PGSSLCRL</envar>.
<literal>sslcrldir</literal> or the environment variable <envar>PGSSLCRLDIR</envar>
can also be used to specify a directory containing CRL files.
</para>
<note>
<para>
For backwards compatibility with earlier versions of PostgreSQL, if a
root CA file exists, the behavior of
<literal>sslmode</literal>=<literal>require</literal> will be the same
as that of <literal>verify-ca</literal>, meaning the server certificate
is validated against the CA. Relying on this behavior is discouraged,
and applications that need certificate validation should always use
<literal>verify-ca</literal> or <literal>verify-full</literal>.
</para>
</note>
</sect2>
<sect2 id="libpq-ssl-clientcert">
<title>Client Certificates</title>
<para>
If the server attempts to verify the identity of the
client by requesting the client's leaf certificate,
<application>libpq</application> will send the certificate(s) stored in
file <filename>~/.postgresql/postgresql.crt</filename> in the user's home
directory. The certificates must chain to the root certificate trusted
by the server. A matching
private key file <filename>~/.postgresql/postgresql.key</filename> must also
be present.
On Microsoft Windows these files are named
<filename>%APPDATA%\postgresql\postgresql.crt</filename> and
<filename>%APPDATA%\postgresql\postgresql.key</filename>.
The location of the certificate and key files can be overridden by the
connection parameters <literal>sslcert</literal>
and <literal>sslkey</literal>, or by the
environment variables <envar>PGSSLCERT</envar> and <envar>PGSSLKEY</envar>.
</para>
<para>
On Unix systems, the permissions on the private key file must disallow
any access to world or group; achieve this by a command such as
<command>chmod 0600 ~/.postgresql/postgresql.key</command>.
Alternatively, the file can be owned by root and have group read access
(that is, <literal>0640</literal> permissions). That setup is intended
for installations where certificate and key files are managed by the
operating system. The user of <application>libpq</application> should
then be made a member of the group that has access to those certificate
and key files. (On Microsoft Windows, there is no file permissions
check, since the <filename>%APPDATA%\postgresql</filename> directory is
presumed secure.)
</para>
<para>
The first certificate in <filename>postgresql.crt</filename> must be the
client's certificate because it must match the client's private key.
<quote>Intermediate</quote> certificates can be optionally appended
to the file &mdash; doing so avoids requiring storage of intermediate
certificates on the server (<xref linkend="guc-ssl-ca-file"/>).
</para>
<para>
The certificate and key may be in PEM or ASN.1 DER format.
</para>
<para>
The key may be
stored in cleartext or encrypted with a passphrase using any algorithm
supported by <productname>OpenSSL</productname>, like AES-128. If the key
is stored encrypted, then the passphrase may be provided in the
<xref linkend="libpq-connect-sslpassword"/> connection option. If an
encrypted key is supplied and the <literal>sslpassword</literal> option
is absent or blank, a password will be prompted for interactively by
<productname>OpenSSL</productname> with a
<literal>Enter PEM pass phrase:</literal> prompt if a TTY is available.
Applications can override the client certificate prompt and the handling
of the <literal>sslpassword</literal> parameter by supplying their own
key password callback; see
<xref linkend="libpq-pqsetsslkeypasshook-openssl"/>.
</para>
<para>
For instructions on creating certificates, see <xref
linkend="ssl-certificate-creation"/>.
</para>
</sect2>
<sect2 id="libpq-ssl-protection">
<title>Protection Provided in Different Modes</title>
<para>
The different values for the <literal>sslmode</literal> parameter provide different
levels of protection. SSL can provide
protection against three types of attacks:
<variablelist>
<varlistentry>
<term>Eavesdropping</term>
<listitem>
<para>If a third party can examine the network traffic between the
client and the server, it can read both connection information (including
the user name and password) and the data that is passed. <acronym>SSL</acronym>
uses encryption to prevent this.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Man-in-the-middle (<acronym>MITM</acronym>)</term>
<listitem>
<para>If a third party can modify the data while passing between the
client and server, it can pretend to be the server and therefore see and
modify data <emphasis>even if it is encrypted</emphasis>. The third party can then
forward the connection information and data to the original server,
making it impossible to detect this attack. Common vectors to do this
include DNS poisoning and address hijacking, whereby the client is directed
to a different server than intended. There are also several other
attack methods that can accomplish this. <acronym>SSL</acronym> uses certificate
verification to prevent this, by authenticating the server to the client.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Impersonation</term>
<listitem>
<para>If a third party can pretend to be an authorized client, it can
simply access data it should not have access to. Typically this can
happen through insecure password management. <acronym>SSL</acronym> uses
client certificates to prevent this, by making sure that only holders
of valid certificates can access the server.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
<para>
For a connection to be known SSL-secured, SSL usage must be configured
on <emphasis>both the client and the server</emphasis> before the connection
is made. If it is only configured on the server, the client may end up
sending sensitive information (e.g., passwords) before
it knows that the server requires high security. In libpq, secure
connections can be ensured
by setting the <literal>sslmode</literal> parameter to <literal>verify-full</literal> or
<literal>verify-ca</literal>, and providing the system with a root certificate to
verify against. This is analogous to using an <literal>https</literal>
<acronym>URL</acronym> for encrypted web browsing.
</para>
<para>
Once the server has been authenticated, the client can pass sensitive data.
This means that up until this point, the client does not need to know if
certificates will be used for authentication, making it safe to specify that
only in the server configuration.
</para>
<para>
All <acronym>SSL</acronym> options carry overhead in the form of encryption and
key-exchange, so there is a trade-off that has to be made between performance
and security. <xref linkend="libpq-ssl-sslmode-statements"/>
illustrates the risks the different <literal>sslmode</literal> values
protect against, and what statement they make about security and overhead.
</para>
<table id="libpq-ssl-sslmode-statements">
<title>SSL Mode Descriptions</title>
<tgroup cols="4">
<colspec colname="col1" colwidth="1*"/>
<colspec colname="col2" colwidth="1*"/>
<colspec colname="col3" colwidth="1*"/>
<colspec colname="col4" colwidth="2*"/>
<thead>
<row>
<entry><literal>sslmode</literal></entry>
<entry>Eavesdropping protection</entry>
<entry><acronym>MITM</acronym> protection</entry>
<entry>Statement</entry>
</row>
</thead>
<tbody>
<row>
<entry><literal>disable</literal></entry>
<entry>No</entry>
<entry>No</entry>
<entry>I don't care about security, and I don't want to pay the overhead
of encryption.
</entry>
</row>
<row>
<entry><literal>allow</literal></entry>
<entry>Maybe</entry>
<entry>No</entry>
<entry>I don't care about security, but I will pay the overhead of
encryption if the server insists on it.
</entry>
</row>
<row>
<entry><literal>prefer</literal></entry>
<entry>Maybe</entry>
<entry>No</entry>
<entry>I don't care about encryption, but I wish to pay the overhead of
encryption if the server supports it.
</entry>
</row>
<row>
<entry><literal>require</literal></entry>
<entry>Yes</entry>
<entry>No</entry>
<entry>I want my data to be encrypted, and I accept the overhead. I trust
that the network will make sure I always connect to the server I want.
</entry>
</row>
<row>
<entry><literal>verify-ca</literal></entry>
<entry>Yes</entry>
<entry>Depends on CA policy</entry>
<entry>I want my data encrypted, and I accept the overhead. I want to be
sure that I connect to a server that I trust.
</entry>
</row>
<row>
<entry><literal>verify-full</literal></entry>
<entry>Yes</entry>
<entry>Yes</entry>
<entry>I want my data encrypted, and I accept the overhead. I want to be
sure that I connect to a server I trust, and that it's the one I
specify.
</entry>
</row>
</tbody>
</tgroup>
</table>
<para>
The difference between <literal>verify-ca</literal> and <literal>verify-full</literal>
depends on the policy of the root <acronym>CA</acronym>. If a public
<acronym>CA</acronym> is used, <literal>verify-ca</literal> allows connections to a server
that <emphasis>somebody else</emphasis> may have registered with the <acronym>CA</acronym>.
In this case, <literal>verify-full</literal> should always be used. If
a local <acronym>CA</acronym> is used, or even a self-signed certificate, using
<literal>verify-ca</literal> often provides enough protection.
</para>
<para>
The default value for <literal>sslmode</literal> is <literal>prefer</literal>. As is shown
in the table, this makes no sense from a security point of view, and it only
promises performance overhead if possible. It is only provided as the default
for backward compatibility, and is not recommended in secure deployments.
</para>
</sect2>
<sect2 id="libpq-ssl-fileusage">
<title>SSL Client File Usage</title>
<para>
<xref linkend="libpq-ssl-file-usage"/> summarizes the files that are
relevant to the SSL setup on the client.
</para>
<table id="libpq-ssl-file-usage">
<title>Libpq/Client SSL File Usage</title>
<tgroup cols="3">
<thead>
<row>
<entry>File</entry>
<entry>Contents</entry>
<entry>Effect</entry>
</row>
</thead>
<tbody>
<row>
<entry><filename>~/.postgresql/postgresql.crt</filename></entry>
<entry>client certificate</entry>
<entry>sent to server</entry>
</row>
<row>
<entry><filename>~/.postgresql/postgresql.key</filename></entry>
<entry>client private key</entry>
<entry>proves client certificate sent by owner; does not indicate
certificate owner is trustworthy</entry>
</row>
<row>
<entry><filename>~/.postgresql/root.crt</filename></entry>
<entry>trusted certificate authorities</entry>
<entry>checks that server certificate is signed by a trusted certificate
authority</entry>
</row>
<row>
<entry><filename>~/.postgresql/root.crl</filename></entry>
<entry>certificates revoked by certificate authorities</entry>
<entry>server certificate must not be on this list</entry>
</row>
</tbody>
</tgroup>
</table>
</sect2>
<sect2 id="libpq-ssl-initialize">
<title>SSL Library Initialization</title>
<para>
If your application initializes <literal>libssl</literal> and/or
<literal>libcrypto</literal> libraries and <application>libpq</application>
is built with <acronym>SSL</acronym> support, you should call
<xref linkend="libpq-PQinitOpenSSL"/> to tell <application>libpq</application>
that the <literal>libssl</literal> and/or <literal>libcrypto</literal> libraries
have been initialized by your application, so that
<application>libpq</application> will not also initialize those libraries.
However, this is unnecessary when using <productname>OpenSSL</productname>
version 1.1.0 or later, as duplicate initializations are no longer problematic.
</para>
<para>
<variablelist>
<varlistentry id="libpq-PQinitOpenSSL">
<term><function>PQinitOpenSSL</function><indexterm><primary>PQinitOpenSSL</primary></indexterm></term>
<listitem>
<para>
Allows applications to select which security libraries to initialize.
<synopsis>
void PQinitOpenSSL(int do_ssl, int do_crypto);
</synopsis>
</para>
<para>
When <parameter>do_ssl</parameter> is non-zero, <application>libpq</application>
will initialize the <productname>OpenSSL</productname> library before first
opening a database connection. When <parameter>do_crypto</parameter> is
non-zero, the <literal>libcrypto</literal> library will be initialized. By
default (if <xref linkend="libpq-PQinitOpenSSL"/> is not called), both libraries
are initialized. When SSL support is not compiled in, this function is
present but does nothing.
</para>
<para>
If your application uses and initializes either <productname>OpenSSL</productname>
or its underlying <literal>libcrypto</literal> library, you <emphasis>must</emphasis>
call this function with zeroes for the appropriate parameter(s)
before first opening a database connection. Also be sure that you
have done that initialization before opening a database connection.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQinitSSL">
<term><function>PQinitSSL</function><indexterm><primary>PQinitSSL</primary></indexterm></term><listitem>
<para>
Allows applications to select which security libraries to initialize.
<synopsis>
void PQinitSSL(int do_ssl);
</synopsis>
</para>
<para>
This function is equivalent to
<literal>PQinitOpenSSL(do_ssl, do_ssl)</literal>.
It is sufficient for applications that initialize both or neither
of <productname>OpenSSL</productname> and <literal>libcrypto</literal>.
</para>
<para>
<xref linkend="libpq-PQinitSSL"/> has been present since
<productname>PostgreSQL</productname> 8.0, while <xref linkend="libpq-PQinitOpenSSL"/>
was added in <productname>PostgreSQL</productname> 8.4, so <xref linkend="libpq-PQinitSSL"/>
might be preferable for applications that need to work with older
versions of <application>libpq</application>.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</sect2>
</sect1>
<sect1 id="libpq-threading">
<title>Behavior in Threaded Programs</title>
<indexterm zone="libpq-threading">
<primary>threads</primary>
<secondary>with libpq</secondary>
</indexterm>
<para>
As of version 17, <application>libpq</application> is always reentrant and thread-safe.
However, one restriction is that no two threads attempt to manipulate
the same <structname>PGconn</structname> object at the same time. In particular,
you cannot issue concurrent commands from different threads through
the same connection object. (If you need to run concurrent commands,
use multiple connections.)
</para>
<para>
<structname>PGresult</structname> objects are normally read-only after creation,
and so can be passed around freely between threads. However, if you use
any of the <structname>PGresult</structname>-modifying functions described in
<xref linkend="libpq-misc"/> or <xref linkend="libpq-events"/>, it's up
to you to avoid concurrent operations on the same <structname>PGresult</structname>,
too.
</para>
<para>
In earlier versions, <application>libpq</application> could be compiled
with or without thread support, depending on compiler options. This
function allows the querying of <application>libpq</application>'s
thread-safe status:
</para>
<variablelist>
<varlistentry id="libpq-PQisthreadsafe">
<term><function>PQisthreadsafe</function><indexterm><primary>PQisthreadsafe</primary></indexterm></term>
<listitem>
<para>
Returns the thread safety status of the
<application>libpq</application> library.
<synopsis>
int PQisthreadsafe();
</synopsis>
</para>
<para>
Returns 1 if the <application>libpq</application> is thread-safe
and 0 if it is not. Always returns 1 on version 17 and above.
</para>
</listitem>
</varlistentry>
</variablelist>
<para>
The deprecated functions <xref linkend="libpq-PQrequestCancel"/> and
<xref linkend="libpq-PQoidStatus"/> are not thread-safe and should not be
used in multithread programs. <xref linkend="libpq-PQrequestCancel"/>
can be replaced by <xref linkend="libpq-PQcancelBlocking"/>.
<xref linkend="libpq-PQoidStatus"/> can be replaced by
<xref linkend="libpq-PQoidValue"/>.
</para>
<para>
If you are using Kerberos inside your application (in addition to inside
<application>libpq</application>), you will need to do locking around
Kerberos calls because Kerberos functions are not thread-safe. See
function <function>PQregisterThreadLock</function> in the
<application>libpq</application> source code for a way to do cooperative
locking between <application>libpq</application> and your application.
</para>
</sect1>
<sect1 id="libpq-build">
<title>Building <application>libpq</application> Programs</title>
<indexterm zone="libpq-build">
<primary>compiling</primary>
<secondary>libpq applications</secondary>
</indexterm>
<para>
To build (i.e., compile and link) a program using
<application>libpq</application> you need to do all of the following
things:
<itemizedlist>
<listitem>
<para>
Include the <filename>libpq-fe.h</filename> header file:
<programlisting>
#include &lt;libpq-fe.h&gt;
</programlisting>
If you failed to do that then you will normally get error messages
from your compiler similar to:
<screen>
foo.c: In function `main':
foo.c:34: `PGconn' undeclared (first use in this function)
foo.c:35: `PGresult' undeclared (first use in this function)
foo.c:54: `CONNECTION_BAD' undeclared (first use in this function)
foo.c:68: `PGRES_COMMAND_OK' undeclared (first use in this function)
foo.c:95: `PGRES_TUPLES_OK' undeclared (first use in this function)
</screen>
</para>
</listitem>
<listitem>
<para>
Point your compiler to the directory where the <productname>PostgreSQL</productname> header
files were installed, by supplying the
<literal>-I<replaceable>directory</replaceable></literal> option
to your compiler. (In some cases the compiler will look into
the directory in question by default, so you can omit this
option.) For instance, your compile command line could look
like:
<programlisting>
cc -c -I/usr/local/pgsql/include testprog.c
</programlisting>
If you are using makefiles then add the option to the
<varname>CPPFLAGS</varname> variable:
<programlisting>
CPPFLAGS += -I/usr/local/pgsql/include
</programlisting>
</para>
<para>
If there is any chance that your program might be compiled by
other users then you should not hardcode the directory location
like that. Instead, you can run the utility
<command>pg_config</command><indexterm><primary>pg_config</primary><secondary
sortas="libpq">with libpq</secondary></indexterm> to find out where the header
files are on the local system:
<screen>
<prompt>$</prompt> pg_config --includedir
<computeroutput>/usr/local/include</computeroutput>
</screen>
</para>
<para>
If you
have <command>pkg-config</command><indexterm><primary>pkg-config</primary><secondary sortas="libpq">with
libpq</secondary></indexterm> installed, you can run instead:
<screen>
<prompt>$</prompt> pkg-config --cflags libpq
<computeroutput>-I/usr/local/include</computeroutput>
</screen>
Note that this will already include the <option>-I</option> in front of
the path.
</para>
<para>
Failure to specify the correct option to the compiler will
result in an error message such as:
<screen>
testlibpq.c:8:22: libpq-fe.h: No such file or directory
</screen>
</para>
</listitem>
<listitem>
<para>
When linking the final program, specify the option
<literal>-lpq</literal> so that the <application>libpq</application>
library gets pulled in, as well as the option
<literal>-L<replaceable>directory</replaceable></literal> to point
the compiler to the directory where the
<application>libpq</application> library resides. (Again, the
compiler will search some directories by default.) For maximum
portability, put the <option>-L</option> option before the
<option>-lpq</option> option. For example:
<programlisting>
cc -o testprog testprog1.o testprog2.o -L/usr/local/pgsql/lib -lpq
</programlisting>
</para>
<para>
You can find out the library directory using
<command>pg_config</command> as well:
<screen>
<prompt>$</prompt> pg_config --libdir
<computeroutput>/usr/local/pgsql/lib</computeroutput>
</screen>
</para>
<para>
Or again use <command>pkg-config</command>:
<screen>
<prompt>$</prompt> pkg-config --libs libpq
<computeroutput>-L/usr/local/pgsql/lib -lpq</computeroutput>
</screen>
Note again that this prints the full options, not only the path.
</para>
<para>
Error messages that point to problems in this area could look like
the following:
<screen>
testlibpq.o: In function `main':
testlibpq.o(.text+0x60): undefined reference to `PQsetdbLogin'
testlibpq.o(.text+0x71): undefined reference to `PQstatus'
testlibpq.o(.text+0xa4): undefined reference to `PQerrorMessage'
</screen>
This means you forgot <option>-lpq</option>.
<screen>
/usr/bin/ld: cannot find -lpq
</screen>
This means you forgot the <option>-L</option> option or did not
specify the right directory.
</para>
</listitem>
</itemizedlist>
</para>
</sect1>
<sect1 id="libpq-example">
<title>Example Programs</title>
<para>
These examples and others can be found in the
directory <filename>src/test/examples</filename> in the source code
distribution.
</para>
<example id="libpq-example-1">
<title><application>libpq</application> Example Program 1</title>
<programlisting>
<![CDATA[
/*
* src/test/examples/testlibpq.c
*
*
* testlibpq.c
*
* Test the C version of libpq, the PostgreSQL frontend library.
*/
#include <stdio.h>
#include <stdlib.h>
#include "libpq-fe.h"
static void
exit_nicely(PGconn *conn)
{
PQfinish(conn);
exit(1);
}
int
main(int argc, char **argv)
{
const char *conninfo;
PGconn *conn;
PGresult *res;
int nFields;
int i,
j;
/*
* If the user supplies a parameter on the command line, use it as the
* conninfo string; otherwise default to setting dbname=postgres and using
* environment variables or defaults for all other connection parameters.
*/
if (argc > 1)
conninfo = argv[1];
else
conninfo = "dbname = postgres";
/* Make a connection to the database */
conn = PQconnectdb(conninfo);
/* Check to see that the backend connection was successfully made */
if (PQstatus(conn) != CONNECTION_OK)
{
fprintf(stderr, "%s", PQerrorMessage(conn));
exit_nicely(conn);
}
/* Set always-secure search path, so malicious users can't take control. */
res = PQexec(conn,
"SELECT pg_catalog.set_config('search_path', '', false)");
if (PQresultStatus(res) != PGRES_TUPLES_OK)
{
fprintf(stderr, "SET failed: %s", PQerrorMessage(conn));
PQclear(res);
exit_nicely(conn);
}
/*
* Should PQclear PGresult whenever it is no longer needed to avoid memory
* leaks
*/
PQclear(res);
/*
* Our test case here involves using a cursor, for which we must be inside
* a transaction block. We could do the whole thing with a single
* PQexec() of "select * from pg_database", but that's too trivial to make
* a good example.
*/
/* Start a transaction block */
res = PQexec(conn, "BEGIN");
if (PQresultStatus(res) != PGRES_COMMAND_OK)
{
fprintf(stderr, "BEGIN command failed: %s", PQerrorMessage(conn));
PQclear(res);
exit_nicely(conn);
}
PQclear(res);
/*
* Fetch rows from pg_database, the system catalog of databases
*/
res = PQexec(conn, "DECLARE myportal CURSOR FOR select * from pg_database");
if (PQresultStatus(res) != PGRES_COMMAND_OK)
{
fprintf(stderr, "DECLARE CURSOR failed: %s", PQerrorMessage(conn));
PQclear(res);
exit_nicely(conn);
}
PQclear(res);
res = PQexec(conn, "FETCH ALL in myportal");
if (PQresultStatus(res) != PGRES_TUPLES_OK)
{
fprintf(stderr, "FETCH ALL failed: %s", PQerrorMessage(conn));
PQclear(res);
exit_nicely(conn);
}
/* first, print out the attribute names */
nFields = PQnfields(res);
for (i = 0; i < nFields; i++)
printf("%-15s", PQfname(res, i));
printf("\n\n");
/* next, print out the rows */
for (i = 0; i < PQntuples(res); i++)
{
for (j = 0; j < nFields; j++)
printf("%-15s", PQgetvalue(res, i, j));
printf("\n");
}
PQclear(res);
/* close the portal ... we don't bother to check for errors ... */
res = PQexec(conn, "CLOSE myportal");
PQclear(res);
/* end the transaction */
res = PQexec(conn, "END");
PQclear(res);
/* close the connection to the database and cleanup */
PQfinish(conn);
return 0;
}
]]>
</programlisting>
</example>
<example id="libpq-example-2">
<title><application>libpq</application> Example Program 2</title>
<programlisting>
<![CDATA[
/*
* src/test/examples/testlibpq2.c
*
*
* testlibpq2.c
* Test of the asynchronous notification interface
*
* Start this program, then from psql in another window do
* NOTIFY TBL2;
* Repeat four times to get this program to exit.
*
* Or, if you want to get fancy, try this:
* populate a database with the following commands
* (provided in src/test/examples/testlibpq2.sql):
*
* CREATE SCHEMA TESTLIBPQ2;
* SET search_path = TESTLIBPQ2;
* CREATE TABLE TBL1 (i int4);
* CREATE TABLE TBL2 (i int4);
* CREATE RULE r1 AS ON INSERT TO TBL1 DO
* (INSERT INTO TBL2 VALUES (new.i); NOTIFY TBL2);
*
* Start this program, then from psql do this four times:
*
* INSERT INTO TESTLIBPQ2.TBL1 VALUES (10);
*/
#ifdef WIN32
#include <windows.h>
#endif
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <sys/select.h>
#include <sys/time.h>
#include <sys/types.h>
#include "libpq-fe.h"
static void
exit_nicely(PGconn *conn)
{
PQfinish(conn);
exit(1);
}
int
main(int argc, char **argv)
{
const char *conninfo;
PGconn *conn;
PGresult *res;
PGnotify *notify;
int nnotifies;
/*
* If the user supplies a parameter on the command line, use it as the
* conninfo string; otherwise default to setting dbname=postgres and using
* environment variables or defaults for all other connection parameters.
*/
if (argc > 1)
conninfo = argv[1];
else
conninfo = "dbname = postgres";
/* Make a connection to the database */
conn = PQconnectdb(conninfo);
/* Check to see that the backend connection was successfully made */
if (PQstatus(conn) != CONNECTION_OK)
{
fprintf(stderr, "%s", PQerrorMessage(conn));
exit_nicely(conn);
}
/* Set always-secure search path, so malicious users can't take control. */
res = PQexec(conn,
"SELECT pg_catalog.set_config('search_path', '', false)");
if (PQresultStatus(res) != PGRES_TUPLES_OK)
{
fprintf(stderr, "SET failed: %s", PQerrorMessage(conn));
PQclear(res);
exit_nicely(conn);
}
/*
* Should PQclear PGresult whenever it is no longer needed to avoid memory
* leaks
*/
PQclear(res);
/*
* Issue LISTEN command to enable notifications from the rule's NOTIFY.
*/
res = PQexec(conn, "LISTEN TBL2");
if (PQresultStatus(res) != PGRES_COMMAND_OK)
{
fprintf(stderr, "LISTEN command failed: %s", PQerrorMessage(conn));
PQclear(res);
exit_nicely(conn);
}
PQclear(res);
/* Quit after four notifies are received. */
nnotifies = 0;
while (nnotifies < 4)
{
/*
* Sleep until something happens on the connection. We use select(2)
* to wait for input, but you could also use poll() or similar
* facilities.
*/
int sock;
fd_set input_mask;
sock = PQsocket(conn);
if (sock < 0)
break; /* shouldn't happen */
FD_ZERO(&input_mask);
FD_SET(sock, &input_mask);
if (select(sock + 1, &input_mask, NULL, NULL, NULL) < 0)
{
fprintf(stderr, "select() failed: %s\n", strerror(errno));
exit_nicely(conn);
}
/* Now check for input */
PQconsumeInput(conn);
while ((notify = PQnotifies(conn)) != NULL)
{
fprintf(stderr,
"ASYNC NOTIFY of '%s' received from backend PID %d\n",
notify->relname, notify->be_pid);
PQfreemem(notify);
nnotifies++;
PQconsumeInput(conn);
}
}
fprintf(stderr, "Done.\n");
/* close the connection to the database and cleanup */
PQfinish(conn);
return 0;
}
]]>
</programlisting>
</example>
<example id="libpq-example-3">
<title><application>libpq</application> Example Program 3</title>
<programlisting>
<![CDATA[
/*
* src/test/examples/testlibpq3.c
*
*
* testlibpq3.c
* Test out-of-line parameters and binary I/O.
*
* Before running this, populate a database with the following commands
* (provided in src/test/examples/testlibpq3.sql):
*
* CREATE SCHEMA testlibpq3;
* SET search_path = testlibpq3;
* SET standard_conforming_strings = ON;
* CREATE TABLE test1 (i int4, t text, b bytea);
* INSERT INTO test1 values (1, 'joe''s place', '\000\001\002\003\004');
* INSERT INTO test1 values (2, 'ho there', '\004\003\002\001\000');
*
* The expected output is:
*
* tuple 0: got
* i = (4 bytes) 1
* t = (11 bytes) 'joe's place'
* b = (5 bytes) \000\001\002\003\004
*
* tuple 0: got
* i = (4 bytes) 2
* t = (8 bytes) 'ho there'
* b = (5 bytes) \004\003\002\001\000
*/
#ifdef WIN32
#include <windows.h>
#endif
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <string.h>
#include <sys/types.h>
#include "libpq-fe.h"
/* for ntohl/htonl */
#include <netinet/in.h>
#include <arpa/inet.h>
static void
exit_nicely(PGconn *conn)
{
PQfinish(conn);
exit(1);
}
/*
* This function prints a query result that is a binary-format fetch from
* a table defined as in the comment above. We split it out because the
* main() function uses it twice.
*/
static void
show_binary_results(PGresult *res)
{
int i,
j;
int i_fnum,
t_fnum,
b_fnum;
/* Use PQfnumber to avoid assumptions about field order in result */
i_fnum = PQfnumber(res, "i");
t_fnum = PQfnumber(res, "t");
b_fnum = PQfnumber(res, "b");
for (i = 0; i < PQntuples(res); i++)
{
char *iptr;
char *tptr;
char *bptr;
int blen;
int ival;
/* Get the field values (we ignore possibility they are null!) */
iptr = PQgetvalue(res, i, i_fnum);
tptr = PQgetvalue(res, i, t_fnum);
bptr = PQgetvalue(res, i, b_fnum);
/*
* The binary representation of INT4 is in network byte order, which
* we'd better coerce to the local byte order.
*/
ival = ntohl(*((uint32_t *) iptr));
/*
* The binary representation of TEXT is, well, text, and since libpq
* was nice enough to append a zero byte to it, it'll work just fine
* as a C string.
*
* The binary representation of BYTEA is a bunch of bytes, which could
* include embedded nulls so we have to pay attention to field length.
*/
blen = PQgetlength(res, i, b_fnum);
printf("tuple %d: got\n", i);
printf(" i = (%d bytes) %d\n",
PQgetlength(res, i, i_fnum), ival);
printf(" t = (%d bytes) '%s'\n",
PQgetlength(res, i, t_fnum), tptr);
printf(" b = (%d bytes) ", blen);
for (j = 0; j < blen; j++)
printf("\\%03o", bptr[j]);
printf("\n\n");
}
}
int
main(int argc, char **argv)
{
const char *conninfo;
PGconn *conn;
PGresult *res;
const char *paramValues[1];
int paramLengths[1];
int paramFormats[1];
uint32_t binaryIntVal;
/*
* If the user supplies a parameter on the command line, use it as the
* conninfo string; otherwise default to setting dbname=postgres and using
* environment variables or defaults for all other connection parameters.
*/
if (argc > 1)
conninfo = argv[1];
else
conninfo = "dbname = postgres";
/* Make a connection to the database */
conn = PQconnectdb(conninfo);
/* Check to see that the backend connection was successfully made */
if (PQstatus(conn) != CONNECTION_OK)
{
fprintf(stderr, "%s", PQerrorMessage(conn));
exit_nicely(conn);
}
/* Set always-secure search path, so malicious users can't take control. */
res = PQexec(conn, "SET search_path = testlibpq3");
if (PQresultStatus(res) != PGRES_COMMAND_OK)
{
fprintf(stderr, "SET failed: %s", PQerrorMessage(conn));
PQclear(res);
exit_nicely(conn);
}
PQclear(res);
/*
* The point of this program is to illustrate use of PQexecParams() with
* out-of-line parameters, as well as binary transmission of data.
*
* This first example transmits the parameters as text, but receives the
* results in binary format. By using out-of-line parameters we can avoid
* a lot of tedious mucking about with quoting and escaping, even though
* the data is text. Notice how we don't have to do anything special with
* the quote mark in the parameter value.
*/
/* Here is our out-of-line parameter value */
paramValues[0] = "joe's place";
res = PQexecParams(conn,
"SELECT * FROM test1 WHERE t = $1",
1, /* one param */
NULL, /* let the backend deduce param type */
paramValues,
NULL, /* don't need param lengths since text */
NULL, /* default to all text params */
1); /* ask for binary results */
if (PQresultStatus(res) != PGRES_TUPLES_OK)
{
fprintf(stderr, "SELECT failed: %s", PQerrorMessage(conn));
PQclear(res);
exit_nicely(conn);
}
show_binary_results(res);
PQclear(res);
/*
* In this second example we transmit an integer parameter in binary form,
* and again retrieve the results in binary form.
*
* Although we tell PQexecParams we are letting the backend deduce
* parameter type, we really force the decision by casting the parameter
* symbol in the query text. This is a good safety measure when sending
* binary parameters.
*/
/* Convert integer value "2" to network byte order */
binaryIntVal = htonl((uint32_t) 2);
/* Set up parameter arrays for PQexecParams */
paramValues[0] = (char *) &binaryIntVal;
paramLengths[0] = sizeof(binaryIntVal);
paramFormats[0] = 1; /* binary */
res = PQexecParams(conn,
"SELECT * FROM test1 WHERE i = $1::int4",
1, /* one param */
NULL, /* let the backend deduce param type */
paramValues,
paramLengths,
paramFormats,
1); /* ask for binary results */
if (PQresultStatus(res) != PGRES_TUPLES_OK)
{
fprintf(stderr, "SELECT failed: %s", PQerrorMessage(conn));
PQclear(res);
exit_nicely(conn);
}
show_binary_results(res);
PQclear(res);
/* close the connection to the database and cleanup */
PQfinish(conn);
return 0;
}
]]>
</programlisting>
</example>
</sect1>
</chapter>
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