libpq - C Librarylibpqlibpq is the C
application programmer's interface to
PostgreSQL. libpq is a set
of library functions that allow client programs to pass queries to the
PostgreSQL backend server and to receive the
results of these queries. libpq is also the
underlying engine for several other PostgreSQL
application interfaces, including libpq++ (C++),
libpgtcl (Tcl), Perl, and
ECPG. So some aspects of libpq>'s behavior will be
important to you if you use one of those packages.
Three short programs are included at the end of this chapter () to show how
to write programs that use libpq. There are also several
complete examples of libpq applications in the
directory src/test/examples in the source code distribution.
Client programs that use libpq must include the
header file libpq-fe.h and must link with the
libpq library.
Database Connection Functions
The following functions deal with making a connection to a
PostgreSQL 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
PGconn> object which is obtained from the function
PQconnectdb> or PQsetdbLogin>. Note that
these functions will always return a non-null object pointer,
unless perhaps there is too little memory even to allocate the
PGconn> object. The PQstatus> function
should be called to check whether a connection was successfully
made before queries are sent via the connection object.
PQconnectdb
Makes a new connection to the database server.
PGconn *PQconnectdb(const char *conninfo);
This function opens a new database connection using the parameters taken
from the string conninfo. Unlike PQsetdbLogin> below,
the parameter set can be extended without changing the function signature,
so use either of this function or the nonblocking analogues PQconnectStart>
and PQconnectPoll is preferred for new application programming.
The passed string
can be empty to use all default parameters, or it can contain one or more
parameter settings separated by whitespace.
Each parameter setting is in the form keyword = value.
(To write an empty value or a value containing
spaces, surround it with single quotes, e.g.,
keyword = 'a value'.
Single quotes and backslashes within the value must be escaped with a
backslash, i.e., \' and \\.)
Spaces around the equal sign are optional.
The currently recognized parameter key words are:
host
Name of host to connect to.
If this begins with a slash, 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.
The default is to connect to a Unix-domain socket in
/tmp.
hostaddr
IP address of host to connect to. This should be in the
standard IPv4 address format, e.g., 172.28.40.9>. If
your machine supports IPv6, you can also use those addresses. If
a nonzero-length string is specified, TCP/IP communication is
used.
Using hostaddr> instead of host> allows the
application to avoid a host name look-up, which may be important in
applications with time constraints. However, Kerberos authentication
requires the host name. The following therefore applies: If
host> is specified without hostaddr>, a host name
lookup is forced. If hostaddr> is specified without
host>, the value for hostaddr> gives the remote
address; if Kerberos is used, this causes a reverse name query. If both
host> and hostaddr> are specified, the value for
hostaddr> gives the remote address; the value for
host> is ignored, unless Kerberos is used, in which case that
value is used for Kerberos authentication. (Note that authentication is
likely to fail if libpq is passed a host name
that is not the name of the machine at hostaddr>.) Also,
host> rather than hostaddr> is used to identify
the connection in $HOME/.pgpass>.
Without either a host name or host address,
libpq will connect using a
local Unix domain socket.
port
Port number to connect to at the server host,
or socket file name extension for Unix-domain connections.
dbname
The database name.
user
User name to connect as.
password
Password to be used if the server demands password authentication.
connect_timeout
Maximum wait for connection, in seconds (write as a decimal integer
string). Zero or not specified means wait indefinitely. It is not
recommended to set the timeout to less than 2 seconds.
options
Command-line options to be sent to the server.
tty
Ignored (formerly, this specified where to send server debug output).
requiressl
If set to 1, an SSL connection to the server is required.
libpq> will then refuse to connect if the server does not
accept an SSL connection.
If set to 0 (default), libpq> will negotiate the connection type with server.
This option is only available if
PostgreSQL> is compiled with SSL support.
service
Service name to use for additional parameters. It specifies a service
name in pg_service.conf that holds additional connection parameters.
This allows applications to specify only a service name so connection parameters
can be centrally maintained. See
PREFIX>/share/pg_service.conf.sample> for
information on how to set up the file.
If any parameter is unspecified, then the corresponding
environment variable (see )
is checked. If the environment variable is not set either,
then hardwired defaults are used.
PQsetdbLogin
Makes a new connection to the database server.
PGconn *PQsetdbLogin(const char *pghost,
const char *pgport,
const char *pgoptions,
const char *pgtty,
const char *dbName,
const char *login,
const char *pwd);
This is the predecessor of PQconnectdb with a fixed
number of parameters. It has the same functionality except that the
missing parameters cannot be specified in the call.
PQsetdb
Makes a new connection to the database server.
PGconn *PQsetdb(char *pghost,
char *pgport,
char *pgoptions,
char *pgtty,
char *dbName);
This is a macro that calls PQsetdbLogin with null pointers
for the login> and pwd> parameters. It is provided
for backward compatibility with very old programs.
PQconnectStartPQconnectPollnonblocking connection
Make a connection to the database server in a nonblocking manner.
PGconn *PQconnectStart(const char *conninfo);
PostgresPollingStatusType PQconnectPoll(PGconn *conn);
These two 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 database connection is made using the parameters taken from the string
conninfo, passed to PQconnectStart. This string is in
the same format as described above for PQconnectdb.
Neither PQconnectStart nor PQconnectPoll will block, as long as a number of
restrictions are met:
The hostaddr> and host> parameters are used appropriately to ensure that
name and reverse name queries are not made. See the documentation of
these parameters under PQconnectdb above for details.
If you call PQtrace, ensure that the stream object into which you trace
will not block.
You ensure for yourself that the socket is in the appropriate state
before calling PQconnectPoll, as described below.
To begin a nonblocking connection request, call conn = PQconnectStart("connection_info_string>").
If conn is null, then libpq> has been unable to allocate a new PGconn>
structure. Otherwise, a valid PGconn> pointer is returned (though not yet
representing a valid connection to the database). On return from
PQconnectStart, call status = PQstatus(conn). If status equals
CONNECTION_BAD, PQconnectStart has failed.
If PQconnectStart> succeeds, the next stage is to poll libpq> so that it may
proceed with the connection sequence. Loop thus: If PQconnectPoll(conn) last returned
PGRES_POLLING_READING, perform a select()> for reading on the socket determined using PQsocket(conn). If
it last returned PGRES_POLLING_WRITING, perform a select()> for writing on
that same socket. If you have yet to call PQconnectPoll, i.e., after the call
to PQconnectStart, behave as if it last returned PGRES_POLLING_WRITING. If
select()> shows that the socket is ready, consider it active. If it has
been decided that this connection is active, call PQconnectPoll(conn)
again. If this call returns PGRES_POLLING_FAILED, the connection procedure
has failed. If this call returns PGRES_POLLING_OK, the connection has been
successfully made.
Note that the use of select() to ensure that the socket is ready is merely
a (likely) example; those with other facilities available, such as a
poll() call, may of course use that instead.
At any time during connection, the status of the connection may be
checked, by calling PQstatus>. If this gives CONNECTION_BAD>, then the
connection procedure has failed; if it gives CONNECTION_OK>, then the
connection is ready. Both of these states are equally detectable
from the return value of PQconnectPoll>, described above. Other states may also occur
during (and only during) an asynchronous connection procedure. These
indicate the current stage of the connection procedure and may be useful
to provide feedback to the user for example. These statuses are:
CONNECTION_STARTED
Waiting for connection to be made.
CONNECTION_MADE
Connection OK; waiting to send.
CONNECTION_AWAITING_RESPONSE
Waiting for a response from the server.
CONNECTION_AUTH_OK
Received authentication; waiting for connection start-up to continue.
CONNECTION_SETENV
Negotiating environment (part of the connection start-up).
Note that, although these constants will remain (in order to maintain
compatibility), an application should never rely upon these appearing in a
particular order, or at all, or on the status always being one of these
documented values. An application may do something like this:
switch(PQstatus(conn))
{
case CONNECTION_STARTED:
feedback = "Connecting...";
break;
case CONNECTION_MADE:
feedback = "Connected to server...";
break;
.
.
.
default:
feedback = "Connecting...";
}
Note that if PQconnectStart returns a non-null pointer, you must call
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 a
call to PQconnectStart or PQconnectPoll failed.
PQconnectPoll will currently block if
libpq> is compiled with SSL support. This restriction may be removed in the future.
Finally, these functions leave the connection in a nonblocking state as if
PQsetnonblocking had been called.
PQconndefaults
Returns the default connection options.
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; /* Character to display for 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;
Returns a connection options array. This may
be used to determine all possible PQconnectdb options and their
current default values. The return value points to an array of
PQconninfoOption structures, which ends with an entry having a null
key-word pointer. Note that the current default values (val fields)
will depend on environment variables and other context.
Callers must treat the connection options data as read-only.
After processing the options array, free it by passing it to
PQconninfoFree. If this is not done, a small amount of memory
is leaked for each call to PQconndefaults.
In PostgreSQL versions before 7.0, PQconndefaults returned a pointer
to a static array, rather than a dynamically allocated array. That
was not thread-safe, so the behavior has been changed.
PQfinish
Closes the connection to the server. Also frees
memory used by the PGconn object.
void PQfinish(PGconn *conn);
Note that even if the server connection attempt fails (as
indicated by PQstatus), the application should call PQfinish
to free the memory used by the PGconn object.
The PGconn> pointer should not be used after PQfinish has been called.
PQreset
Resets the communication channel to the server.
void PQreset(PGconn *conn);
This function will close the connection
to the server and attempt to reestablish a new
connection to the same server, using all the same
parameters previously used. This may be useful for
error recovery if a working connection is lost.
PQresetStartPQresetPoll
Reset the communication channel to the server, in a nonblocking manner.
int PQresetStart(PGconn *conn);
PostgresPollingStatusType PQresetPoll(PGconn *conn);
These functions will close the connection to the server and attempt to
reestablish a new connection to the same server, using all the same
parameters previously used. This may be useful for error recovery if a
working connection is lost. They differ from PQreset (above) in that they
act in a nonblocking manner. These functions suffer from the same
restrictions as PQconnectStart> and PQconnectPoll>.
To initiate a connection reset, call PQresetStart. If it returns 0, the reset has failed. If it returns 1,
poll the reset using PQresetPoll in exactly the same way as you would
create the connection using PQconnectPoll.
libpq-fe.h>>
libpq-int.h>>
libpq application programmers should be careful to
maintain the PGconn abstraction. Use the accessor functions below to get
at the contents of PGconn. Avoid directly referencing the fields of the
PGconn> structure because they are subject to change in the future.
(Beginning in PostgreSQL release 6.4, the
definition of the struct behind PGconn> is not even provided in libpq-fe.h.
If you have old code that accesses PGconn fields directly, you can keep using it
by including libpq-int.h too, but you are encouraged to fix the code
soon.)
PQdb
Returns the database name of the connection.
char *PQdb(const PGconn *conn);
PQdb> and the next several functions return the values established
at connection. These values are fixed for the life of the PGconn>
object.
PQuser
Returns the user name of the connection.
char *PQuser(const PGconn *conn);
PQpass
Returns the password of the connection.
char *PQpass(const PGconn *conn);
PQhost
Returns the server host name of the connection.
char *PQhost(const PGconn *conn);
PQport
Returns the port of the connection.
char *PQport(const PGconn *conn);
PQtty
Returns the debug TTY of the connection.
(This is obsolete, since the server no longer pays attention
to the TTY setting, but the function remains
for backwards compatibility.)
char *PQtty(const PGconn *conn);
PQoptions
Returns the command-line options passed in the connection request.
char *PQoptions(const PGconn *conn);
PQstatus
Returns the status of the connection.
ConnStatusType PQstatus(const PGconn *conn);
The status can be one of a number of values.
However, only two of these are
seen outside of an asynchronous connection procedure:
CONNECTION_OK and
CONNECTION_BAD. A good
connection to the database has the status CONNECTION_OK.
A failed connection
attempt is signaled by status
CONNECTION_BAD.
Ordinarily, an OK status will remain so until
PQfinish, but a
communications failure might result in the status changing to
CONNECTION_BAD prematurely.
In that case the application
could try to recover by calling PQreset.
See the entry for PQconnectStart> and PQconnectPoll> with regards
to other status codes
that might be seen.
PQerrorMessageerror message>>
Returns the error message most recently generated by
an operation on the connection.
char *PQerrorMessage(const PGconn* conn);
Nearly all libpq> functions will set a message for
PQerrorMessage if they fail.
Note that by libpq convention, a nonempty
PQerrorMessage result will
include a trailing newline.
PQsocket
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.
int PQsocket(const PGconn *conn);
PQbackendPID
Returns the process ID of the backend server process
handling this connection.
int PQbackendPID(const PGconn *conn);
The backend PID is useful for debugging
purposes and for comparison to NOTIFY
messages (which include the PID of the
notifying backend process). Note that the
PID belongs to a process executing on the
database server host, not the local host!
PQgetsslSSL>>
Returns the SSL structure used in the connection, or null
if SSL is not in use.
SSL *PQgetssl(const PGconn *conn);
This structure can be used to verify encryption levels, check
server certificates, and more. Refer to the OpenSSL> documentation
for information about this structure.
You must define USE_SSL in order to get the
prototype for this function. Doing this will also
automatically include ssl.h from OpenSSL.
Command Execution Functions
Once a connection to a database server has been successfully
established, the functions described here are used to perform
SQL queries and commands.
Main FunctionsPQexec
Submits a command to the server
and waits for the result.
PGresult *PQexec(PGconn *conn,
const char *command);
Returns a PGresult 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.
If a null pointer is returned, it
should be treated like a PGRES_FATAL_ERROR result. Use
PQerrorMessage to get more information about the error.
The PGresult structure encapsulates the result
returned by the server.
libpq application programmers should be careful to
maintain the PGresult abstraction. Use the accessor functions below to get
at the contents of PGresult. Avoid directly referencing the fields of the
PGresult structure because they are subject to change in the future.
Multiple queries sent in a single
function call are processed in a single transaction, unless there are explicit
BEGIN/COMMIT commands included in the query string to divide it into multiple
transactions.
PQresultStatus
Returns the result status of the command.
ExecStatusType PQresultStatus(const PGresult *res);
PQresultStatus can return one of the following values:
PGRES_EMPTY_QUERYThe string sent to the server was empty.PGRES_COMMAND_OKSuccessful completion of a command returning no data.PGRES_TUPLES_OKThe query successfully executed.PGRES_COPY_OUTCopy Out (from server) data transfer started.PGRES_COPY_INCopy In (to server) data transfer started.PGRES_BAD_RESPONSEThe server's response was not understood.PGRES_NONFATAL_ERRORA nonfatal error occurred.PGRES_FATAL_ERRORA fatal error occurred.
If the result status is PGRES_TUPLES_OK, then the
functions described below can be used to retrieve the rows returned by
the query. Note that a SELECT command that happens
to retrieve zero rows still shows PGRES_TUPLES_OK.
PGRES_COMMAND_OK is for commands that can never
return rows (INSERT, UPDATE,
etc.). A response of PGRES_EMPTY_QUERY often
exposes a bug in the client software.
PQresStatus
Converts the enumerated type returned by PQresultStatus> into
a string constant describing the status code.
char *PQresStatus(ExecStatusType status);
PQresultErrorMessage
Returns the error message associated with the command, or an empty string
if there was no error.
char *PQresultErrorMessage(const PGresult *res);
Immediately following a PQexec or PQgetResult
call, PQerrorMessage (on the connection) will return the same
string as PQresultErrorMessage (on the result). However, a
PGresult will retain its error message
until destroyed, whereas the connection's error message will change when
subsequent operations are done. Use PQresultErrorMessage when you want to
know the status associated with a particular PGresult; use PQerrorMessage
when you want to know the status from the latest operation on the connection.
PQclear
Frees the storage associated with a PGresult.
Every command result should be freed via PQclear when
it is no longer needed.
void PQclear(PQresult *res);
You can keep a PGresult 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 PQclear. Failure to do this will
result in memory leaks in your client application.
PQmakeEmptyPGresult
Constructs an empty PGresult object with the given status.
PGresult* PQmakeEmptyPGresult(PGconn *conn, ExecStatusType status);
This is libpq>'s internal function to allocate and initialize an empty
PGresult object. It is exported because some applications find it
useful to generate result objects (particularly objects with error
status) themselves. If conn is not null and status> indicates an error,
the current error message of the specified connection is copied into the PGresult.
Note that PQclear should eventually be called on the object, just
as with a PGresult returned by libpq itself.
Escaping Strings for Inclusion in SQL Commandsescaping strings>>
PQescapeString escapes a string for use within an SQL commmand.
size_t PQescapeString (char *to, const char *from, size_t length);
If you want to use strings that have been received
from a source that is not trustworthy (for example, because a random user
entered them), you should not directly include them in SQL
commands for security reasons. Instead, you have to escape certain
characters that are otherwise interpreted specially by the SQL parser.
PQescapeString> performs this operation.
The parameter from> points to the first character of the string that
is to be escaped, and the length> parameter counts the
number of characters in this string. (A terminating zero byte is
neither necessary nor counted.) to> shall point to a
buffer that is able to hold at least one more character than twice
the value of length>, otherwise the behavior is
undefined. A call to PQescapeString> writes an escaped
version of the from> string to the to>
buffer, replacing special characters so that they cannot cause any
harm, and adding a terminating zero byte. The single quotes that
must surround PostgreSQL> string literals are not part of the result
string.
PQescapeString> returns the number of characters written
to to>, not including the terminating zero byte.
Behavior is undefined when the to> and from>
strings overlap.
Escaping Binary Strings for Inclusion in SQL Commandsescaping binary stringsPQescapeBytea
Escapes binary data for use within an SQL command with the type bytea.
unsigned char *PQescapeBytea(const unsigned char *from,
size_t from_length,
size_t *to_length);
Certain byte values must be escaped (but all
byte values may be escaped) when used as part
of a bytea literal in an SQL
statement. In general, to escape a byte, it is converted into the
three digit octal number equal to the octet value, and preceded by
two backslashes. The single quote ('>) and backslash
(\>) characters have special alternative escape
sequences. See for more
information. PQescapeBytea performs this
operation, escaping only the minimally required bytes.
The from parameter points to the first
byte of the string that is to be escaped, and the
from_length parameter reflects the number of
bytes in this binary string. (A terminating zero byte is
neither necessary nor counted.) The to_length
parameter points to a variable that will hold the resultant
escaped string length. The result string length includes the terminating
zero byte of the result.
PQescapeBytea> returns an escaped version of the
from parameter binary string in memory
allocated with malloc()>, and must be freed using
PQfreemem()>.
The return string has all special characters replaced
so that they can be properly processed by the PostgreSQL string literal
parser, and the bytea input function. A terminating zero
byte is also added. The single quotes that must surround
PostgreSQL string literals are not part of the result string.
PQunescapeBytea
Converts an escaped string representation of binary data into binary
data --- the reverse of PQescapeBytea.
unsigned char *PQunescapeBytea(const unsigned char *from, size_t *to_length);
The from parameter points to an escaped string
such as might be returned by PQgetvalue when applied to a
bytea column. PQunescapeBytea converts
this string representation into its binary representation.
It returns a pointer to a buffer allocated with
malloc(), or null on error, and puts the size of
the buffer in to_length. The memory must be
freed using PQfreemem()>.
PQfreemem
Frees memory allocated by libpq>
void PQfreemem(void *ptr);
Frees memory allocated by libpq>, particularly
PQescapeBytea,
PQunescapeBytea,
and PQnotifies.
It is needed by Win32, which can not free memory across
DLL's, unless multithreaded DLL's (/MD in VC6) are used.
Retrieving Query Result InformationPQntuples
Returns the number of rows (tuples)
in the query result.
int PQntuples(const PGresult *res);
PQnfields
Returns the number of columns (fields)
in each row of the query result.
int PQnfields(const PGresult *res);
PQfname
Returns the column name associated with the given column number.
Column numbers start at 0.
char *PQfname(const PGresult *res,
int column_number);
PQfnumber
Returns the column number
associated with the given column name.
int PQfnumber(const PGresult *res,
const char *column_name);
-1 is returned if the given name does not match any column.
PQftype
Returns the column data type associated with the
given column number. The integer returned is the
internal OID number of the type. Column numbers start
at 0.
Oid PQftype(const PGresult *res,
int column_number);
You can query the system table pg_type to obtain
the name and properties of the various data types. The OIDs
of the built-in data types are defined in the file src/include/catalog/pg_type.h
in the source tree.
PQfmod
Returns the type-specific modification data of the column
associated with the given column number.
Column numbers start at 0.
int PQfmod(const PGresult *res,
int column_number);
PQfsize
Returns the size in bytes of the column
associated with the given column number.
Column numbers start at 0.
int PQfsize(const PGresult *res,
int column_number);
PQfsize> returns the space allocated for this column in a database
row, in other words the size of the server's binary representation
of the data type. -1 is returned if the column has a variable size.
PQbinaryTuples
Returns 1 if the PGresult> contains binary row data
and 0 if it contains text data.
int PQbinaryTuples(const PGresult *res);
Currently, binary row data can only be returned by a query that
extracts data from a binary cursor.
Retrieving Query Result ValuesPQgetvalue
Returns a single column value of one row
of a PGresult.
Row and colums indices start at 0.
char* PQgetvalue(const PGresult *res,
int row_number,
int column_number);
For most queries, the value returned by PQgetvalue
is a null-terminated character string representation
of the column value. But if PQbinaryTuples returns 1,
the value returned by PQgetvalue is the binary
representation of the
type in the internal format of the backend server
(but not including the size word, if the column is variable-length).
It is then the programmer's responsibility to cast and
convert the data to the correct C type.
The pointer
returned by PQgetvalue points to storage that is
part of the PGresult 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 PGresult structure itself.
PQgetisnull
Tests a column for a null value.
Row and column numbers start at 0.
int PQgetisnull(const PGresult *res,
int row_number,
int column_number);
This function returns 1 if the column is null and 0 if
it contains a non-null value. (Note that PQgetvalue
will return an empty string, not a null pointer, for a null
column.)
PQgetlength
Returns the length of a column value in bytes.
Row and column numbers start at 0.
int PQgetlength(const PGresult *res,
int row_number,
int column_number);
This is the actual data length for the particular data value, that is, the
size of the object pointed to by PQgetvalue. Note that for character-represented
values, this size has little to do with the binary size reported by PQfsize.
PQprint
Prints out all the rows and, optionally, the
column names to the specified output stream.
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;
This function was formerly used by psql
to print query results, but this is no longer the case and this
function is no longer actively supported.
Retrieving Result Information for Other CommandsPQcmdStatus
Returns the command status string from the SQL command that
generated the PGresult.
char * PQcmdStatus(PGresult *res);
PQcmdTuples
Returns the number of rows affected by the SQL command.
char * PQcmdTuples(PGresult *res);
If the SQL command that generated the
PGresult was INSERT>, UPDATE>, or DELETE, this returns a
string containing the number of rows affected. If the
command was anything else, it returns the empty string.
PQoidValue
Returns the OID of the inserted row, if the
SQL command was an INSERT
that inserted exactly one row into a table that has OIDs.
Otherwise, returns InvalidOid.
Oid PQoidValue(const PGresult *res);
The type Oid and the constant
InvalidOid will be defined if you include
the libpq header file. They will
both be some integer type.
PQoidStatus
Returns a string with the OID of the inserted row, if the
SQL command was an
INSERT. (The string will be
0> if the INSERT did not
insert exactly one row, or if the target table does not have
OIDs.) If the command was not an INSERT,
returns an empty string.
char * PQoidStatus(const PGresult *res);
This function is deprecated in favor of PQoidValue
and is not thread-safe.
Asynchronous Command Processingnonblocking connection>>
The PQexec function is adequate for submitting commands in
normal, synchronous
applications. It has a couple of deficiencies, however, that can be of importance to some users:
PQexec waits for the command to be completed. The application may have other
work to do (such as maintaining a user interface), in which case it won't
want to block waiting for the response.
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.)
PQexec can return only one PGresult structure. If the submitted command
string contains multiple SQL commands, all but the last PGresult are
discarded by PQexec.
Applications that do not like these limitations can instead use the
underlying functions that PQexec is built from:
PQsendQuery and PQgetResult.
Older programs that used this functionality as well as
PQputline and PQputnbytes
could block waiting to send data to the server. To
address that issue, the function PQsetnonblocking
was added.
Old applications can neglect to use PQsetnonblocking
and get the old potentially blocking behavior. Newer programs can use
PQsetnonblocking to achieve a completely nonblocking
connection to the server.
PQsetnonblocking
Sets the nonblocking status of the connection.
int PQsetnonblocking(PGconn *conn, int arg);
Sets the state of the connection to nonblocking if arg is 1 and
blocking if arg is 0. Returns 0 if OK, -1 if error.
In the nonblocking state, calls to
PQputline, PQputnbytes,
PQsendQuery, and PQendcopy
will not block but instead return an error if they need to be called
again.
When a database connection has been set to nonblocking mode and
PQexec is called, it will temporarily set the state
of the connection to blocking until the PQexec call
completes.
More of libpq is expected to be made safe for
the nonblocking mode in the future.
PQisnonblocking
Returns the blocking status of the database connection.
int PQisnonblocking(const PGconn *conn);
Returns 1 if the connection is set to nonblocking mode and
0 if blocking.
PQsendQuery
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
PQerrorMessage> to get more information about the failure).
int PQsendQuery(PGconn *conn,
const char *command);
After successfully calling PQsendQuery, call
PQgetResult one or more
times to obtain the results. PQsendQuery may not be called
again (on the same connection) until PQgetResult has returned a null pointer,
indicating that the command is done.
PQgetResult
Waits for the next result from a prior PQsendQuery,
and return it. A null pointer is returned when the command is complete
and there will be no more results.
PGresult *PQgetResult(PGconn *conn);
PQgetResult must be called repeatedly until it returns a null pointer,
indicating that the command is done. (If called when no command is
active, PQgetResult will just return a null pointer at once.)
Each non-null result from PQgetResult should be processed using
the same PGresult> accessor functions previously described.
Don't forget to free each result object with PQclear when done with it.
Note that PQgetResult will block only if a command is active and the
necessary response data has not yet been read by PQconsumeInput.
Using PQsendQuery and PQgetResult
solves one of PQexec's problems:
If a command string contains multiple SQL 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.) However, calling PQgetResult will
still cause the client to block until the server completes the
next SQL command. This can be avoided by proper use of three more
functions:
PQconsumeInput
If input is available from the server, consume it.
int PQconsumeInput(PGconn *conn);
PQconsumeInput normally returns 1 indicating no error,
but returns 0 if there was some kind of trouble (in which case
PQerrorMessage can be used). Note that the result does not say
whether any input data was actually collected. After calling
PQconsumeInput, the application may check
PQisBusy and/or PQnotifies to see if
their state has changed.
PQconsumeInput may 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 select() read-ready indication to go away. The
application can thus use PQconsumeInput to clear the
select() condition immediately, and then examine the results at leisure.
PQisBusy
Returns 1 if a command is busy, that is, PQgetResult would block
waiting for input. A 0 return indicates that PQgetResult can
be called with assurance of not blocking.
int PQisBusy(PGconn *conn);
PQisBusy will not itself attempt to read data from the server;
therefore PQconsumeInput must be invoked first, or the busy
state will never end.
PQflush
Attempts to flush any data queued 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).
int PQflush(PGconn *conn);
PQflush needs to be called on a nonblocking connection
before calling select() to determine if a response has
arrived. If 0 is returned it ensures that there is no data queued to the
server that has not actually been sent. Only applications that have used
PQsetnonblocking have a need for this.
A typical application using these functions will have a main loop that uses
select() 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 select() means readable data on the file
descriptor identified by PQsocket.
When the main loop detects input ready, it should call
PQconsumeInput to read the input. It can then call
PQisBusy, followed by PQgetResult
if PQisBusy returns false (0). It can also call
PQnotifies to detect NOTIFY> messages (see ).
Nonblocking connections (that have used PQsetnonblocking)
should not use select() until PQflush
has returned 0 indicating that there is no buffered data waiting to be sent
to the server.
A client that uses PQsendQuery/PQgetResult
can also attempt to cancel a command that is still being processed by the server.
PQrequestCancel
Requests that the server abandon
processing of the current command.
int PQrequestCancel(PGconn *conn);
The return value is 1 if the cancel request was successfully
dispatched and 0 if not. (If not, PQerrorMessage tells why not.)
Successful dispatch is no guarantee that the request will have any
effect, however. Regardless of the return value of PQrequestCancel,
the application must continue with the normal result-reading
sequence using PQgetResult. If the cancellation
is effective, the current command 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.
Note that if the current command is part of a transaction block, cancellation
will abort the whole transaction.
PQrequestCancel can safely be invoked from a signal handler.
So, it is also possible to use it in conjunction with plain
PQexec, if the decision to cancel can be made in a signal
handler. For example, psql invokes
PQrequestCancel from a SIGINT> signal handler, thus allowing
interactive cancellation of commands that it issues through PQexec.
The Fast-Path InterfacePostgreSQL provides a fast-path interface to send
function calls to the server. This is a trapdoor into system internals and
can be a potential security hole. Most users will not need this feature.
The function PQfn requests execution of a server
function via the fast-path interface:
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;
The fnid> argument is the OID of the function to be
executed.
result_buf is the buffer in which
to place the return value. The caller must have allocated
sufficient space to store the return value. (There is no check!)
The actual result length will be returned in the integer pointed
to by result_len. If a 4-byte integer result is expected, set
result_is_int to 1, otherwise set it to 0. (Setting result_is_int to 1
tells libpq> to byte-swap the value if necessary, so that it is
delivered as a proper int value for the client machine. When
result_is_int> is 0, the byte string sent by the server is returned
unmodified.)
args> and nargs> specify the arguments to be passed to the function.
PQfn always returns a valid PGresult pointer. The result status
should be checked before the result is used. The
caller is responsible for freeing the PGresult with
PQclear when it is no longer needed.
Asynchronous NotificationNOTIFYPostgreSQL offers asynchronous notification via the
LISTEN and NOTIFY commands. A server-side session registers its interest in a particular
notification condition with the LISTEN command (and can stop listening
with the UNLISTEN command). All sessions listening on a
particular condition will be notified asynchronously when a NOTIFY command with that
condition name is executed by any session. No additional information is
passed from the notifier to the listener. Thus, typically, any actual data
that needs to be communicated is transferred through a database table.
Commonly, the condition name is the same as the associated table, but it is
not necessary for there to be any associated table.
libpq applications submit LISTEN and UNLISTEN
commands as ordinary SQL command. The arrival of NOTIFY
messages can subsequently be detected by calling PQnotifies.
The function PQnotifies
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 PQnotifies>, it is considered handled and will be
removed from the list of notifications.
PGnotify* PQnotifies(PGconn *conn);
typedef struct pgNotify {
char *relname; /* notification name */
int be_pid; /* process ID of server process */
} PGnotify;
After processing a PGnotify object returned by PQnotifies,
be sure to free it with PQfreemem().
In PostgreSQL 6.4 and later,
the be_pid is that of the notifying backend process,
whereas in earlier versions it was always the PID of your own backend process.
gives a sample program that illustrates the use
of asynchronous notification.
PQnotifies() does not actually read data from the server; it just
returns messages previously absorbed by another libpq
function. In prior releases of libpq, the only way
to ensure timely receipt of NOTIFY> messages was to constantly submit commands,
even empty ones, and then check PQnotifies() after each
PQexec(). While this still works, it is
deprecated as a waste of processing power.
A better way to check for NOTIFY>
messages when you have no useful commands to execute is to call
PQconsumeInput(), then check
PQnotifies().
You can use select() to wait for data to
arrive from the server, thereby using no CPU power unless there is something
to do. (See PQsocket() to obtain the file descriptor
number to use with select().)
Note that this will work OK whether you submit commands with
PQsendQuery/PQgetResult or simply
use PQexec. You should, however, remember to
check PQnotifies() after each
PQgetResult or PQexec, to see
if any notifications came in during the processing of the command.
Functions Associated with the COPY CommandCOPYwith libpq
The COPY command in PostgreSQL has options to read from
or write to the network connection used by libpq.
Therefore, functions are necessary to access this network
connection directly so applications may take advantage of this capability.
These functions should be executed only after obtaining a result
status of PGRES_COPY_OUT or
PGRES_COPY_IN from PQexec or
PQgetResult.
PQgetline
Reads a newline-terminated line of characters
(transmitted by the server) into a buffer
string of size length>.
int PQgetline(PGconn *conn,
char *buffer,
int length);
This function copies up to length>-1 characters
into the buffer and converts
the terminating newline into a zero byte.
PQgetline returns EOF 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.
Note that the application must check to see if a
new line consists of the two characters \.,
which indicates that the server has finished sending
the results of the COPY command.
If the application might
receive lines that are more than length>-1 characters long,
care is needed to be sure it recognizes the \. line correctly
(and does not, for example, mistake the end of a long data line
for a terminator line).
The code in the file
src/bin/psql/copy.c
contains example functions that correctly handle the COPY protocol.
PQgetlineAsync
Reads a row of COPY data
(transmitted by the server) into a buffer
without blocking.
int PQgetlineAsync(PGconn *conn,
char *buffer,
int bufsize);
This function is similar to PQgetline, but it can be used
by applications
that must read COPY data asynchronously, that is, without blocking.
Having issued the COPY command and gotten a PGRES_COPY_OUT
response, the
application should call PQconsumeInput and
PQgetlineAsync until the
end-of-data signal is detected.
Unlike PQgetline, this function takes
responsibility for detecting end-of-data.
On each call, PQgetlineAsync will return data if a
complete data row is available in libpq>'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
PQendcopy, and then return to normal processing.
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 \n or not. (In a binary
COPY, actual parsing of the COPY 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 bufsize one smaller
than the room actually available.)
PQputline
Sends a null-terminated string to the server.
Returns 0 if OK and EOF if unable to send the string.
int PQputline(PGconn *conn,
const char *string);
The COPY datastream sent by a series of calls to
PQputline has the same format as that returned by
PQgetlineAsync, except that applications are not
obliged to send exactly one data row per PQputline
call; it is okay to send a partial line or multiple lines per call.
Before PostgreSQL 7.4, it was necessary for the
application to explicitly send the two characters \. as a
final line to indicate to the server that it had finished sending COPY data.
While this still works, it is deprecated and the special meaning of
\. can be expected to be removed in a future release.
It is sufficient to call PQendcopy after having sent the
actual data.
PQputnbytes
Sends a non-null-terminated string to the server.
Returns 0 if OK and EOF if unable to send the string.
int PQputnbytes(PGconn *conn,
const char *buffer,
int nbytes);
This is exactly like 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.
PQendcopy
Synchronizes with the server.
int PQendcopy(PGconn *conn);
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 PQputline or when the
last string has been received from the server
using PGgetline. It must be issued or the server
will get out of sync 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 PQerrorMessage to retrieve
details if the return value is nonzero.)
When using PQgetResult, the application should respond to
a PGRES_COPY_OUT result by executing PQgetline
repeatedly, followed by PQendcopy after the terminator line is seen.
It should then return to the PQgetResult loop until
PQgetResult returns a null pointer. Similarly a PGRES_COPY_IN
result is processed by a series of PQputline calls followed by
PQendcopy, then return to the PQgetResult loop.
This arrangement will ensure that
a COPY command embedded in a series of SQL commands
will be executed correctly.
Older applications are likely to submit a COPY
via PQexec and assume that the transaction is done after
PQendcopy.
This will work correctly only if the COPY is the only
SQL command in the command string.
An example:
PQexec(conn, "CREATE TABLE foo (a integer, b varchar(16), d double precision);");
PQexec(conn, "COPY foo FROM STDIN;");
PQputline(conn, "3\thello world\t4.5\n");
PQputline(conn, "4\tgoodbye world\t7.11\n");
...
PQendcopy(conn);
Tracing FunctionsPQtrace
Enables tracing of the client/server communication to a debugging file stream.
void PQtrace(PGconn *conn
FILE *stream);
PQuntrace
Disables tracing started by PQtrace.
void PQuntrace(PGconn *conn);
Notice Processing
The function PQsetNoticeProcessornotice processor>>
controls the reporting of notice and warning messages generated by the server.
typedef void (*PQnoticeProcessor) (void *arg, const char *message);
PQnoticeProcessor
PQsetNoticeProcessor(PGconn *conn,
PQnoticeProcessor proc,
void *arg);
By default, libpq prints notice messages
from the server, as well as a few error messages that it generates by
itself, on stderr.
This behavior can be overridden by supplying a callback function that
does something else with the messages, a so-called notice processor.
The callback function is passed
the text of the message (which includes a trailing newline), plus
a void pointer that is the same one passed to
PQsetNoticeProcessor.
(This pointer can be used to access application-specific state if needed.)
The default notice processor is simply
static void
defaultNoticeProcessor(void * arg, const char * message)
{
fprintf(stderr, "%s", message);
}
To use a special notice processor, call
PQsetNoticeProcessor just after
creation of a new PGconn> object.
The return value is the pointer to the previous notice processor.
If you supply a null callback function pointer, no action is taken,
but the current pointer is returned.
Once you have set a notice processor, you should expect that that function
could be called as long as either the PGconn> object or PGresult> objects
made from it exist. At creation of a PGresult>, the PGconn>'s current
notice processor pointer is copied into the PGresult> for possible use by
functions like PQgetvalue.
Environment Variablesenvironment variables
The following environment variables can be used to select default
connection parameter values, which will be used by
PQconnectdb>, PQsetdbLogin> and
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.
PGHOSTPGHOST sets the database server name.
If this begins with a slash, 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 (default /tmp).
PGHOSTADDRPGHOSTADDR specifies the numeric IP address of the database
server. This can be set instead of PGHOST to avoid DNS
lookup overhead. See the documentation of
these parameters, under PQconnectdb above, for details
on their interaction.
PGPORTPGPORT sets the TCP port number or Unix-domain
socket file extension for communicating with the
PostgreSQL server.
PGDATABASEPGDATABASE sets the
PostgreSQL database name.
PGUSERPGUSER
sets the user name used to connect to the database.
PGPASSWORDPGPASSWORD
sets the password used if the server demands password
authentication. This environment variable is deprecated for security
reasons; consider migrating to use the $HOME/.pgpass>
file (see ).
PGSERVICEPGSERVICE
sets the service name to be looked up in pg_service.conf.
This offers a shorthand way of setting all the parameters.
PGREALMPGREALM sets the Kerberos realm to use with
PostgreSQL, if it is different from the local realm.
If PGREALM is set, libpq
applications will attempt authentication with servers for this realm and use
separate ticket files to avoid conflicts with local
ticket files. This environment variable is only
used if Kerberos authentication is selected by the server.
PGOPTIONSPGOPTIONS sets additional run-time options for
the PostgreSQL server.
PGREQUIRESSLPGREQUIRESSL sets whether or not the connection must be
made over SSL. If set to
1, libpq>
will refuse to connect if the server does not accept
an SSL connection.
This option is only available if
PostgreSQL> is compiled with SSL support.
PGCONNECT_TIMEOUTPGCONNECT_TIMEOUT sets the maximum number of seconds
that libpq will wait when attempting to
connect to the PostgreSQL server. If unset
or set to zero, libpq will wait indefinitely.
It is not recommended to set the timeout to less than 2 seconds.
The following environment variables can be used to specify default
behavior for every PostgreSQL session.
PGDATESTYLEPGDATESTYLE
sets the default style of date/time representation.
(Equivalent to SET datestyle TO ....)
PGTZPGTZ
sets the default time zone.
(Equivalent to SET timezone TO ....)
PGCLIENTENCODINGPGCLIENTENCODING
sets the default client character set encoding.
(Equivalent to SET client_encoding TO ....)
PGGEQOPGGEQO
sets the default mode for the genetic query optimizer.
(Equivalent to SET geqo TO ....)
Refer to the SQL command SET
for information on correct values for these environment variables.
The Password Filepassword file.pgpass
The file .pgpass in a user's home directory is a file
that can contain passwords to be used if the connection requires a
password (and no password has been specified otherwise).
This file should have lines of the following format:
hostname:port:database:username:password
Each of the first four fields may be a literal value, or *,
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 : or
\, escape this character with \.
The permissions on .pgpass must disallow any
access to world or group; achieve this by the command
chmod 0600 ~/.pgpass.
If the permissions are less strict than this, the file will be ignored.
Threading Behaviorthreadswith libpqlibpq is thread-safe as of
PostgreSQL 7.0, so long as no two threads
attempt to manipulate the same PGconn> 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, start up multiple connections.)
PGresult> objects are read-only after creation, and so can be passed around
freely between threads.
The deprecated functions PQoidStatus and
fe_setauthsvc are not thread-safe and should not be
used in multithread programs. PQoidStatus can be
replaced by PQoidValue. There is no good reason to
call fe_setauthsvc at all.
libpq applications that use the crypt
authentication method rely on the crypt() operating
system function, which is often not thread-safe. It is better to use the
md5 method, which is thread-safe on all
platforms.
Building libpq Programs
To build (i.e., compile and link) your libpq programs you need to
do all of the following things:
Include the libpq-fe.h header file:
#include <libpq-fe.h>
If you failed to do that then you will normally get error
messages from your compiler similar to
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)
Point your compiler to the directory where the PostgreSQL> header
files were installed, by supplying the
-Idirectory 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:
cc -c -I/usr/local/pgsql/include testprog.c
If you are using makefiles then add the option to the
CPPFLAGS variable:
CPPFLAGS += -I/usr/local/pgsql/include
pg_config>>
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
pg_config to find out where the header files
are on the local system:
$ pg_config --includedir
/usr/local/include
Failure to specify the correct option to the compiler will
result in an error message such as
testlibpq.c:8:22: libpq-fe.h: No such file or directory
When linking the final program, specify the option
-lpq so that the libpq library gets pulled
in, as well as the option
-Ldirectory to
point the compiler to the directory where the libpq library resides. (Again, the
compiler will search some directories by default.) For maximum
portability, put the option before the
option. For example:
cc -o testprog testprog1.o testprog2.o -L/usr/local/pgsql/lib -lpq
You can find out the library directory using
pg_config as well:
$ pg_config --libdir
/usr/local/pgsql/lib
Error messages that point to problems in this area could look
like the following.
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'
This means you forgot .
/usr/bin/ld: cannot find -lpq
This means you forgot the option or did not specify
the right directory.
libpq-int.h>>
If your codes references the header file
libpq-int.h and you refuse to fix your code to
not use it, starting in PostgreSQL> 7.2, this file will be found in
includedir/postgresql/internal/libpq-int.h,
so you need to add the appropriate option to
your compiler command line.
Example Programslibpq Example Program 1
/*
* testlibpq.c
*
* Test the C version of libpq, the PostgreSQL> frontend
* library.
*/
#include <stdio.h>
#include <libpq-fe.h>
void
exit_nicely(PGconn *conn)
{
PQfinish(conn);
exit(1);
}
main()
{
char *pghost,
*pgport,
*pgoptions,
*pgtty;
char *dbName;
int nFields;
int i,
j;
/* FILE *debug; */
PGconn *conn;
PGresult *res;
/*
* begin, by setting the parameters for a backend connection if the
* parameters are null, then the system will try to use reasonable
* defaults by looking up environment variables or, failing that,
* using hardwired constants
*/
pghost = NULL; /* host name of the backend server */
pgport = NULL; /* port of the backend server */
pgoptions = NULL; /* special options to start up the backend
* server */
pgtty = NULL; /* unused */
dbName = "template1";
/* make a connection to the database */
conn = PQsetdb(pghost, pgport, pgoptions, pgtty, dbName);
/*
* check to see that the backend connection was successfully made
*/
if (PQstatus(conn) == CONNECTION_BAD)
{
fprintf(stderr, "Connection to database '%s' failed.\n", dbName);
fprintf(stderr, "%s", PQerrorMessage(conn));
exit_nicely(conn);
}
/* debug = fopen("/tmp/trace.out","w"); */
/* PQtrace(conn, debug); */
/* start a transaction block */
res = PQexec(conn, "BEGIN");
if (!res || PQresultStatus(res) != PGRES_COMMAND_OK)
{
fprintf(stderr, "BEGIN command failed\n");
PQclear(res);
exit_nicely(conn);
}
/*
* should PQclear PGresult whenever it is no longer needed to avoid
* memory leaks
*/
PQclear(res);
/*
* fetch rows from the pg_database, the system catalog of
* databases
*/
res = PQexec(conn, "DECLARE mycursor CURSOR FOR SELECT * FROM pg_database");
if (!res || PQresultStatus(res) != PGRES_COMMAND_OK)
{
fprintf(stderr, "DECLARE CURSOR command failed\n");
PQclear(res);
exit_nicely(conn);
}
PQclear(res);
res = PQexec(conn, "FETCH ALL in mycursor");
if (!res || PQresultStatus(res) != PGRES_TUPLES_OK)
{
fprintf(stderr, "FETCH ALL command didn't return tuples properly\n");
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 cursor */
res = PQexec(conn, "CLOSE mycursor");
PQclear(res);
/* commit the transaction */
res = PQexec(conn, "COMMIT");
PQclear(res);
/* close the connection to the database and cleanup */
PQfinish(conn);
/* fclose(debug); */
return 0;
}
libpq Example Program 2
/*
* testlibpq2.c
* Test of the asynchronous notification interface
*
* Start this program, then from psql in another window do
* NOTIFY TBL2;
*
* Or, if you want to get fancy, try this:
* Populate a database with the following:
*
* 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);
*
* and do
*
* INSERT INTO TBL1 values (10);
*
*/
#include <stdio.h>
#include "libpq-fe.h"
void
exit_nicely(PGconn *conn)
{
PQfinish(conn);
exit(1);
}
main()
{
char *pghost,
*pgport,
*pgoptions,
*pgtty;
char *dbName;
int nFields;
int i,
j;
PGconn *conn;
PGresult *res;
PGnotify *notify;
/*
* begin, by setting the parameters for a backend connection if the
* parameters are null, then the system will try to use reasonable
* defaults by looking up environment variables or, failing that,
* using hardwired constants
*/
pghost = NULL; /* host name of the backend server */
pgport = NULL; /* port of the backend server */
pgoptions = NULL; /* special options to start up the backend
* server */
pgtty = NULL; /* unused */
dbName = getenv("USER"); /* change this to the name of your test
* database */
/* make a connection to the database */
conn = PQsetdb(pghost, pgport, pgoptions, pgtty, dbName);
/*
* check to see that the backend connection was successfully made
*/
if (PQstatus(conn) == CONNECTION_BAD)
{
fprintf(stderr, "Connection to database '%s' failed.\n", dbName);
fprintf(stderr, "%s", PQerrorMessage(conn));
exit_nicely(conn);
}
res = PQexec(conn, "LISTEN TBL2");
if (!res || PQresultStatus(res) != PGRES_COMMAND_OK)
{
fprintf(stderr, "LISTEN command failed\n");
PQclear(res);
exit_nicely(conn);
}
/*
* should PQclear PGresult whenever it is no longer needed to avoid
* memory leaks
*/
PQclear(res);
while (1)
{
/*
* wait a little bit between checks; waiting with select()
* would be more efficient.
*/
sleep(1);
/* collect any asynchronous backend messages */
PQconsumeInput(conn);
/* check for asynchronous notify messages */
while ((notify = PQnotifies(conn)) != NULL)
{
fprintf(stderr,
"ASYNC NOTIFY of '%s' from backend pid '%d' received\n",
notify->relname, notify->be_pid);
PQfreemem(notify);
}
}
/* close the connection to the database and cleanup */
PQfinish(conn);
return 0;
}
libpq Example Program 3>
/*
* testlibpq3.c Test the C version of Libpq, the PostgreSQL> frontend
* library. tests the binary cursor interface
*
*
*
* populate a database by doing the following:
*
* CREATE TABLE test1 (i int4, d real, p polygon);
*
* INSERT INTO test1 values (1, 3.567, polygon '(3.0, 4.0, 1.0, 2.0)');
*
* INSERT INTO test1 values (2, 89.05, polygon '(4.0, 3.0, 2.0, 1.0)');
*
* the expected output is:
*
* tuple 0: got i = (4 bytes) 1, d = (4 bytes) 3.567000, p = (4
* bytes) 2 points boundbox = (hi=3.000000/4.000000, lo =
* 1.000000,2.000000) tuple 1: got i = (4 bytes) 2, d = (4 bytes)
* 89.050003, p = (4 bytes) 2 points boundbox =
* (hi=4.000000/3.000000, lo = 2.000000,1.000000)
*
*
*/
#include <stdio.h>
#include "libpq-fe.h"
#include "utils/geo_decls.h" /* for the POLYGON type */
void
exit_nicely(PGconn *conn)
{
PQfinish(conn);
exit(1);
}
main()
{
char *pghost,
*pgport,
*pgoptions,
*pgtty;
char *dbName;
int nFields;
int i,
j;
int i_fnum,
d_fnum,
p_fnum;
PGconn *conn;
PGresult *res;
/*
* begin, by setting the parameters for a backend connection if the
* parameters are null, then the system will try to use reasonable
* defaults by looking up environment variables or, failing that,
* using hardwired constants
*/
pghost = NULL; /* host name of the backend server */
pgport = NULL; /* port of the backend server */
pgoptions = NULL; /* special options to start up the backend
* server */
pgtty = NULL; /* unused */
dbName = getenv("USER"); /* change this to the name of your test
* database */
/* make a connection to the database */
conn = PQsetdb(pghost, pgport, pgoptions, pgtty, dbName);
/*
* check to see that the backend connection was successfully made
*/
if (PQstatus(conn) == CONNECTION_BAD)
{
fprintf(stderr, "Connection to database '%s' failed.\n", dbName);
fprintf(stderr, "%s", PQerrorMessage(conn));
exit_nicely(conn);
}
/* start a transaction block */
res = PQexec(conn, "BEGIN");
if (!res || PQresultStatus(res) != PGRES_COMMAND_OK)
{
fprintf(stderr, "BEGIN command failed\n");
PQclear(res);
exit_nicely(conn);
}
/*
* should PQclear PGresult whenever it is no longer needed to avoid
* memory leaks
*/
PQclear(res);
/*
* fetch rows from the pg_database, the system catalog of
* databases
*/
res = PQexec(conn, "DECLARE mycursor BINARY CURSOR FOR SELECT * FROM test1");
if (!res || PQresultStatus(res) != PGRES_COMMAND_OK)
{
fprintf(stderr, "DECLARE CURSOR command failed\n");
PQclear(res);
exit_nicely(conn);
}
PQclear(res);
res = PQexec(conn, "FETCH ALL in mycursor");
if (!res || PQresultStatus(res) != PGRES_TUPLES_OK)
{
fprintf(stderr, "FETCH ALL command didn't return tuples properly\n");
PQclear(res);
exit_nicely(conn);
}
i_fnum = PQfnumber(res, "i");
d_fnum = PQfnumber(res, "d");
p_fnum = PQfnumber(res, "p");
for (i = 0; i < 3; i++)
{
printf("type[%d] = %d, size[%d] = %d\n",
i, PQftype(res, i),
i, PQfsize(res, i));
}
for (i = 0; i < PQntuples(res); i++)
{
int *ival;
float *dval;
int plen;
POLYGON *pval;
/* we hard-wire this to the 3 fields we know about */
ival = (int *) PQgetvalue(res, i, i_fnum);
dval = (float *) PQgetvalue(res, i, d_fnum);
plen = PQgetlength(res, i, p_fnum);
/*
* plen doesn't include the length field so need to
* increment by VARHDSZ
*/
pval = (POLYGON *) malloc(plen + VARHDRSZ);
pval->size = plen;
memmove((char *) &pval->npts, PQgetvalue(res, i, p_fnum), plen);
printf("tuple %d: got\n", i);
printf(" i = (%d bytes) %d,\n",
PQgetlength(res, i, i_fnum), *ival);
printf(" d = (%d bytes) %f,\n",
PQgetlength(res, i, d_fnum), *dval);
printf(" p = (%d bytes) %d points \tboundbox = (hi=%f/%f, lo = %f,%f)\n",
PQgetlength(res, i, d_fnum),
pval->npts,
pval->boundbox.xh,
pval->boundbox.yh,
pval->boundbox.xl,
pval->boundbox.yl);
}
PQclear(res);
/* close the cursor */
res = PQexec(conn, "CLOSE mycursor");
PQclear(res);
/* commit the transaction */
res = PQexec(conn, "COMMIT");
PQclear(res);
/* close the connection to the database and cleanup */
PQfinish(conn);
return 0;
}