libpq
libpq is the C application programmer's interface to
PostgreSQL. libpq is a set
of library routines that allow client programs to pass queries to the
Postgres 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 section to show how
to write programs that use libpq. There are several
complete examples of libpq applications in the
following directories:
../src/test/regress
../src/test/examples
../src/bin/psql
Frontend programs which use libpq must include the
header file libpq-fe.h and must link with the
libpq library.
Database Connection Functions
The following routines deal with making a connection to
a Postgres backend server. The 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 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 routine 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 routine or the non-blocking analogues PQconnectStart
/ PQconnectPoll is prefered for 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 a null value or a value containing
spaces, surround it with single quotes, e.g.,
keyword = 'a value'.
Single quotes within the value must be written as \'.
Spaces around the equal sign are optional.) The currently recognized
parameter keywords are:
host
Name of host to connect to. If a non-zero-length string is specified, TCP/IP
communication is used. Using this parameter causes a hostname look-up.
See hostaddr.
hostaddr
IP address of host to connect to. This should be in standard
numbers-and-dots form, as used by the BSD functions inet_aton et al. If
a non-zero-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 hostname look-up 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 which is not the name of the
machine at hostaddr.
Without both a host name and host address, libpq will connect using a
local Unix domain socket.
port
Port number to connect to at the server host,
or socket filename 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.
options
Trace/debug options to be sent to the server.
tty
A file or tty for optional debug output from the backend.
If any parameter is unspecified, then the corresponding
environment variable (see "Environment Variables" section)
is checked. If the environment variable is not set either,
then hardwired defaults are used.
The return value is a pointer to an abstract struct
representing the connection to the backend.
This function is not thread-safe.
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 but the same functionality.
This function is not thread-safe.
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 primarily
for backward compatibility with old programs.
PQconnectStart
PQconnectPoll
Make a connection to the database server in a non-blocking manner.
PGconn *PQconnectStart(const char *conninfo)
PostgresPollingStatusType *PQconnectPoll(PQconn *conn)
These two routines 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, 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: Consider a connection
'inactive' by default. If PQconnectPoll last returned PGRES_POLLING_ACTIVE,
consider it 'active' instead. If PQconnectPoll(conn) last returned
PGRES_POLLING_READING, perform a select for reading on PQsocket(conn). If
it last returned PGRES_POLLING_WRITING, perform a select for writing on
PQsocket(conn). If you have yet to call PQconnectPoll, i.e. after the call
to PQconnectStart, behave as if it last returned PGRES_POLLING_WRITING. If
the 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 is CONNECTION_BAD, then the
connection procedure has failed; if this is CONNECTION_OK, then the
connection is ready. Either of these states should be equally detectable
from the return value of PQconnectPoll, as above. Other states may be
shown 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 may include:
CONNECTION_STARTED: Waiting for connection to be made.
CONNECTION_MADE: Connection OK; waiting to send.
CONNECTION_AWAITING_RESPONSE: Waiting for a response from the backend.
CONNECTION_AUTH_RESPONSE: Got an authentication response; about to deal
with it.
CONNECTION_ERROR_RESPONSE: Got an error response; about to deal with it.
CONNECTION_AUTH_OK: Received authentication; waiting for ReadyForQuery etc.
CONNECTION_SETENV: Negotiating environment.
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 upon that, 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 USE_SSL
defined. This restriction may be removed in the future.
PQconnectPoll will currently block under Windows, unless libpq is compiled
with WIN32_NON_BLOCKING_CONNECTIONS defined. This code has not yet been
tested under Windows, and so it is currently off by default. This may be
changed in the future.
These functions are not thread-safe.
PQconndefaults Returns the default connection options.
PQconninfoOption *PQconndefaults(void)
struct PQconninfoOption
{
char *keyword; /* The keyword of the option */
char *envvar; /* Fallback environment variable name */
char *compiled; /* Fallback compiled in default value */
char *val; /* Option's value */
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 options - don't
create a field by default */
int dispsize; /* Field size in characters for dialog */
}
Returns the address of the connection options structure. This may
be used to determine all possible PQconnectdb options and their
current default values. The return value points to an array of
PQconninfoOption structs, which ends with an entry having a NULL
keyword pointer. Note that the default values ("val" fields)
will depend on environment variables and other context.
Callers must treat the connection options data as read-only.
This function is not thread-safe.
PQfinish
Close the connection to the backend. Also frees
memory used by the PGconn object.
void PQfinish(PGconn *conn)
Note that even if the backend 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
Reset the communication port with the backend.
void PQreset(PGconn *conn)
This function will close the connection
to the backend and attempt to reestablish a new
connection to the same postmaster, using all the same
parameters previously used. This may be useful for
error recovery if a working connection is lost.
PQresetStart
PQresetPoll
Reset the communication port with the backend, in a non-blocking manner.
int PQresetStart(PGconn *conn);
PostgresPollingStatusType PQresetPoll(PGconn *conn);
These functions will close the connection to the backend and attempt to
reestablish a new connection to the same postmaster, 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 non-blocking manner. These functions suffer from the same
restrictions as PQconnectStart and PQconnectPoll.
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 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 struct 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.
const 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.
const char *PQuser(const PGconn *conn)
PQpass
Returns the password of the connection.
const char *PQpass(const PGconn *conn)
PQhost
Returns the server host name of the connection.
const char *PQhost(const PGconn *conn)
PQport
Returns the port of the connection.
const char *PQport(const PGconn *conn)
PQtty
Returns the debug tty of the connection.
const char *PQtty(const PGconn *conn)
PQoptions
Returns the backend options used in the connection.
const 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 or 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.
PQerrorMessage
Returns the error message most recently generated by
an operation on the connection.
const char *PQerrorMessage(const PGconn* conn);
Nearly all libpq functions will set PQerrorMessage if they fail.
Note that by libpq convention, a non-empty PQerrorMessage will
include a trailing newline.
PQbackendPID
Returns the process ID of the backend server 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).
Note that the PID belongs to a process executing on the database
server host, not the local host!
PQsetenvStart
PQsetenvPoll
PQsetenvAbort
Perform an environment negotiation.
PGsetenvHandle *PQsetenvStart(PGconn *conn)
PostgresPollingStatusType *PQsetenvPoll(PGsetenvHandle handle)
void PQsetenvAbort(PGsetenvHandle handle)
These two routines can be used to re-perform the environment negotiation
that occurs during the opening of a connection to a database server. I have
no idea why this might be useful (XXX anyone?) but it might prove useful
for users to be able to reconfigure their character encodings on-the-fly,
for example.
These functions will not block, subject to the restrictions applied to
PQconnectStart and PQconnectPoll.
To begin, call handle=PQsetenvStart(conn), where conn is an open connection
to the database server. If handle is NULL, then libpq has been unable to
allocate a new PGsetenvHandle structure. Otherwise, a valid handle is
returned. This handle is intended to be opaque - you may only use it to
call other functions in libpq (PQsetenvPoll, for example).
Poll the procedure using PQsetenvPoll, in exactly the same way as you would
create a connection using PQconnectPoll.
The procedure may be aborted at any time by calling PQsetenvAbort(handle).
These functions are not thread-safe.
PQsetenv
Perform an environment negotiation.
int PQsetenv(PGconn *conn)
This function performs the same duties as PQsetenvStart and PQsetenvPoll, but
blocks to do so. It returns 1 on success and 0 on failure.
Query 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.
PQexec
Submit a query to Postgres
and wait for the result.
PGresult *PQexec(PGconn *conn,
const char *query);
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 query to the backend.
If a NULL 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 query result
returned by the backend.
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.
(Beginning in Postgres release 6.4, the
definition of struct PGresult is not even provided in libpq-fe.h. If you
have old code that accesses PGresult fields directly, you can keep using it
by including libpq-int.h too, but you are encouraged to fix the code
soon.)
PQresultStatus
Returns the result status of the query.
ExecStatusType PQresultStatus(const PGresult *res)
PQresultStatus can return one of the following values:
PGRES_EMPTY_QUERY -- The string sent to the backend was empty.
PGRES_COMMAND_OK -- Successful completion of a command returning no data
PGRES_TUPLES_OK -- The query successfully executed
PGRES_COPY_OUT -- Copy Out (from server) data transfer started
PGRES_COPY_IN -- Copy In (to server) data transfer started
PGRES_BAD_RESPONSE -- The server's response was not understood
PGRES_NONFATAL_ERROR
PGRES_FATAL_ERROR
If the result status is PGRES_TUPLES_OK, then the
routines described below can be used to retrieve the
tuples returned by the query. Note that a SELECT that
happens to retrieve zero tuples still shows PGRES_TUPLES_OK.
PGRES_COMMAND_OK is for commands that can never return tuples
(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.
const char *PQresStatus(ExecStatusType status);
PQresultErrorMessage
returns the error message associated with the query, or an empty string
if there was no error.
const 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.
PQntuples
Returns the number of tuples (instances)
in the query result.
int PQntuples(const PGresult *res);
PQnfields
Returns the number of fields
(attributes) in each tuple of the query result.
int PQnfields(const PGresult *res);
PQbinaryTuples
Returns 1 if the PGresult contains binary tuple data,
0 if it contains ASCII data.
int PQbinaryTuples(const PGresult *res);
Currently, binary tuple data can only be returned by a query that
extracts data from a BINARY cursor.
PQfname
Returns the field (attribute) name associated with the given field index.
Field indices start at 0.
const char *PQfname(const PGresult *res,
int field_index);
PQfnumber
Returns the field (attribute) index
associated with the given field name.
int PQfnumber(const PGresult *res,
const char *field_name);
-1 is returned if the given name does not match any field.
PQftype
Returns the field type associated with the
given field index. The integer returned is an
internal coding of the type. Field indices start
at 0.
Oid PQftype(const PGresult *res,
int field_num);
You can query the system table pg_type to obtain
the name and properties of the various datatypes. The OIDs
of the built-in datatypes are defined in src/include/catalog/pg_type.h
in the source tree.
PQfsize
Returns the size in bytes of the field
associated with the given field index.
Field indices start at 0.
int PQfsize(const PGresult *res,
int field_index);
PQfsize returns the space allocated for this field in a database
tuple, in other words the size of the server's binary representation
of the data type. -1 is returned if the field is variable size.
PQfmod
Returns the type-specific modification data of the field
associated with the given field index.
Field indices start at 0.
int PQfmod(const PGresult *res,
int field_index);
PQgetvalue
Returns a single field (attribute) value of one tuple
of a PGresult.
Tuple and field indices start at 0.
const char* PQgetvalue(const PGresult *res,
int tup_num,
int field_num);
For most queries, the value returned by PQgetvalue
is a null-terminated ASCII string representation
of the attribute value. But if PQbinaryTuples() is 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 field 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 it,
and one must explicitly
copy the value into other storage if it is to
be used past the lifetime of the PGresult structure itself.
PQgetlength
Returns the length of a field (attribute) in bytes.
Tuple and field indices start at 0.
int PQgetlength(const PGresult *res,
int tup_num,
int field_num);
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 ASCII-represented
values, this size has little to do with the binary size reported by PQfsize.
PQgetisnull
Tests a field for a NULL entry.
Tuple and field indices start at 0.
int PQgetisnull(const PGresult *res,
int tup_num,
int field_num);
This function returns 1 if the field contains a NULL, 0 if
it contains a non-null value. (Note that PQgetvalue
will return an empty string, not a null pointer, for a NULL
field.)
PQcmdStatus
Returns the command status string from the SQL command that
generated the PGresult.
const char * PQcmdStatus(const PGresult *res);
PQcmdTuples
Returns the number of rows affected by the SQL command.
const char * PQcmdTuples(const 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 object id of the tuple
inserted, if the SQL command was an INSERT.
Otherwise, returns InvalidOid.
Oid PQoidValue(const PGresult *res);
PQoidStatus
Returns a string with the object id of the tuple
inserted, if the SQL command was an INSERT.
Otherwise, returns an empty string.
const char * PQoidStatus(const PGresult *res);
The function is deprecated in favor of PQoidValue.
PQprint
Prints out all the tuples and, optionally, the
attribute names to the specified output stream.
void PQprint(FILE* fout, /* output stream */
const PGresult *res,
const PQprintOpt *po);
struct _PQprintOpt {
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; /* insert to HTML <table ...> */
char *caption; /* HTML <caption> */
char **fieldName; /* null terminated array of replacement field names */
}
This function is intended to replace PQprintTuples(), which is
now obsolete. The psql program uses
PQprint() to display query results.
PQprintTuples
Prints out all the tuples and, optionally, the
attribute names to the specified output stream.
void PQprintTuples(const PGresult *res,
FILE *fout, /* output stream */
int printAttName,/* print attribute names or not*/
int terseOutput, /* delimiter bars or not?*/
int width); /* width of column, variable width if 0*/
PQdisplayTuples
Prints out all the tuples and, optionally, the
attribute names to the specified output stream.
void PQdisplayTuples(const PGresult* res,
FILE *fout, /* output stream */
int fillAlign, /* space fill to align columns */
const char *fieldSep, /* field separator */
int printHeader, /* display headers? */
int quiet); /* suppress print of row count at end */
PQdisplayTuples() was intended to supersede
PQprintTuples(), and
is in turn superseded by PQprint().
PQclear
Frees the storage associated with the PGresult.
Every query 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 query,
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 the frontend application.
PQmakeEmptyPGresult
Constructs an empty PGresult object with the given status.
PGresult* PQmakeEmptyPGresult(PGconn *conn, ExecStatusType status);
This is libpq's internal routine 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 connection's current errorMessage is copied into the PGresult.
Note that PQclear should eventually be called on the object, just
as with a PGresult returned by libpq itself.
Asynchronous Query Processing
The PQexec function is adequate for submitting queries in
simple synchronous
applications. It has a couple of major deficiencies however:
PQexec waits for the query 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 control is buried inside PQexec, it is hard for the frontend
to decide it would like to try to cancel the ongoing query. (It can be
done from a signal handler, but not otherwise.)
PQexec can return only one PGresult structure. If the submitted query
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.
PQsendQuery
Submit a query to Postgres without
waiting for the result(s). TRUE is returned if the query was
successfully dispatched, FALSE if not (in which case, use
PQerrorMessage to get more information about the failure).
int PQsendQuery(PGconn *conn,
const char *query);
After successfully calling PQsendQuery, call
PQgetResult one or more
times to obtain the query results. PQsendQuery may not be called
again (on the same connection) until PQgetResult has returned NULL,
indicating that the query is done.
PQgetResult
Wait for the next result from a prior PQsendQuery,
and return it. NULL is returned when the query is complete
and there will be no more results.
PGresult *PQgetResult(PGconn *conn);
PQgetResult must be called repeatedly until it returns NULL,
indicating that the query is done. (If called when no query is
active, PQgetResult will just return NULL 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 query 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 query 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 frontend can be handling the
results of one query while the backend is still working on later
queries in the same query string.) However, calling PQgetResult will
still cause the frontend to block until the backend completes the
next SQL command. This can be avoided by proper use of three more
functions:
PQconsumeInput
If input is available from the backend, 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 is set). 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
routine will read available data and save it in a buffer, thereby
causing a select(2) 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 query 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 backend;
therefore PQconsumeInput must be invoked first, or the busy
state will never end.
PQsocket
Obtain the file descriptor number for the backend connection socket.
A valid descriptor will be >= 0; a result of -1 indicates that
no backend connection is currently open.
int PQsocket(const PGconn *conn);
PQsocket should be used to obtain the backend socket descriptor
in preparation for executing select(2). This allows an
application to wait for either backend responses or other conditions.
If the result of select(2) indicates that data can be read from
the backend socket, then PQconsumeInput should be called to read the
data; after which, PQisBusy, PQgetResult,
and/or PQnotifies can be used to process the response.
A typical frontend using these functions will have a main loop that uses
select(2) to wait for all the conditions that it must
respond to. One of the conditions will be input available from the backend,
which in select's terms is 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 "Asynchronous
Notification", below).
A frontend that uses PQsendQuery/PQgetResult
can also attempt to cancel a query that is still being processed by the backend.
PQrequestCancel
Request that Postgres abandon
processing of the current query.
int PQrequestCancel(PGconn *conn);
The return value is 1 if the cancel request was successfully
dispatched, 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 query will terminate early and return
an error result. If the cancellation fails (say, because the
backend was already done processing the query), then there will
be no visible result at all.
Note that if the current query is part of a transaction, 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 queries that it issues through PQexec.
Note that PQrequestCancel will have no effect if the connection
is not currently open or the backend is not currently processing a query.
Fast Path
PostgreSQL provides a fast path interface to send
function calls to the backend. This is a trapdoor into system internals and
can be a potential security hole. Most users will not need this feature.
PQfn
Request execution of a backend 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);
The fnid argument is the object identifier 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 backend is returned
unmodified.)
args and nargs specify the arguments to be passed to the function.
typedef struct {
int len;
int isint;
union {
int *ptr;
int integer;
} u;
} PQArgBlock;
PQfn always returns a valid PGresult*. The resultStatus
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 Notification
PostgreSQL supports asynchronous notification via the
LISTEN and NOTIFY commands. A backend registers its interest in a particular
notification condition with the LISTEN command (and can stop listening
with the UNLISTEN command). All backends listening on a
particular condition will be notified asynchronously when a NOTIFY of that
condition name is executed by any backend. 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 relation.
Commonly the condition name is the same as the associated relation, but it is
not necessary for there to be any associated relation.
libpq applications submit LISTEN and UNLISTEN
commands as ordinary SQL queries. Subsequently, arrival of NOTIFY
messages can be detected by calling PQnotifies().
PQnotifies
Returns the next notification from a list of unhandled
notification messages received from the backend. Returns NULL 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[NAMEDATALEN]; /* name of relation
* containing data */
int be_pid; /* process id of backend */
} PGnotify;
After processing a PGnotify object returned by PQnotifies,
be sure to free it with free() to avoid a memory leak.
In PostgreSQL 6.4 and later,
the be_pid is the notifying backend's,
whereas in earlier versions it was always your own backend's PID.
The second sample program gives an example of the use
of asynchronous notification.
PQnotifies() does not actually read backend data; 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 queries,
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 queries to make is to call
PQconsumeInput(), then check PQnotifies().
You can use select(2) to wait for backend data to
arrive, thereby using no CPU power unless there is something
to do. Note that this will work OK whether you use PQsendQuery/
PQgetResult or simply PQexec for
queries. You should, however, remember to check PQnotifies()
after each PQgetResult or PQexec to see
if any notifications came in during the processing of the query.
Functions Associated with the COPY Command
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 PGRES_COPY_OUT
or PGRES_COPY_IN result object from PQexec
or PQgetResult.
PQgetline
Reads a newline-terminated line of characters
(transmitted by the backend server) into a buffer
string of size length.
int PQgetline(PGconn *conn,
char *string,
int length)
Like fgets(3), this routine copies up to length-1 characters
into string. It is like gets(3), however, in that it converts
the terminating newline into a null character.
PQgetline returns EOF at EOF, 0 if the
entire line has been read, and 1 if the buffer is full but the
terminating newline has not yet been read.
Notice that the application must check to see if a
new line consists of the two characters "\.",
which indicates that the backend 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 one recognizes the "\." line correctly
(and does not, for example, mistake the end of a long data line
for a terminator line).
The code in
src/bin/psql/copy.c
contains example routines that correctly handle the copy protocol.
PQgetlineAsync
Reads a newline-terminated line of characters
(transmitted by the backend server) into a buffer
without blocking.
int PQgetlineAsync(PGconn *conn,
char *buffer,
int bufsize)
This routine 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 routine takes
responsibility for detecting end-of-data.
On each call, PQgetlineAsync will return data if a complete newline-
terminated data line is available in libpq's input buffer, or if the
incoming data line is too long to fit in the buffer offered by the caller.
Otherwise, no data is returned until the rest of the line arrives.
The routine 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 newline character. If possible
a whole line will be returned at one time. But if the buffer offered by
the caller is too small to hold a line sent by the backend, then a partial
data line will be returned. This can be detected by testing whether the
last returned byte is \n
or not.
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 backend server.
Returns 0 if OK, EOF if unable to send the string.
int PQputline(PGconn *conn,
const char *string);
Note the application must explicitly send the two
characters \.
on a final line to indicate to
the backend that it has finished sending its data.
PQputnbytes
Sends a non-null-terminated string to the backend server.
Returns 0 if OK, 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.
PQendcopy
Syncs with the backend. This function waits until
the backend has finished the copy. It should
either be issued when the last string has been
sent to the backend using PQputline or when the
last string has been received from the backend
using PGgetline. It must be issued or the backend
may get out of sync
with the frontend. Upon
return from this function, the backend is ready to
receive the next query.
The return value is 0 on successful completion,
nonzero otherwise.
int PQendcopy(PGconn *conn);
As an example:
PQexec(conn, "create table foo (a int4, b char(16), d float8)");
PQexec(conn, "copy foo from stdin");
PQputline(conn, "3\thello world\t4.5\n");
PQputline(conn,"4\tgoodbye world\t7.11\n");
...
PQputline(conn,"\\.\n");
PQendcopy(conn);
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 NULL. 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 in or copy out command embedded in a series of SQL commands
will be executed correctly.
Older applications are likely to submit a copy in or copy out
via PQexec and assume that the transaction is done after
PQendcopy.
This will work correctly only if the copy in/out is the only
SQL command in the query string.
<Application>libpq</Application> Tracing Functions
PQtrace
Enable tracing of the frontend/backend communication to a debugging file stream.
void PQtrace(PGconn *conn
FILE *debug_port)
PQuntrace
Disable tracing started by PQtrace
void PQuntrace(PGconn *conn)
<Application>libpq</Application> Control Functions
PQsetNoticeProcessor
Control reporting of notice and warning messages generated by libpq.
typedef void (*PQnoticeProcessor) (void *arg, const char *message);
PQnoticeProcessor
PQsetNoticeProcessor(PGconn *conn,
PQnoticeProcessor proc,
void *arg);
By default, libpq prints notice
messages
from the backend 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. The callback function is passed
the text of the error 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 callback
function pointer of NULL, no action is taken, but the current pointer is returned.
Environment Variables
The following environment variables can be used to select default
connection parameter values, which will be used by PQconnectdb or
PQsetdbLogin if no value is directly specified by the calling code.
These are useful to avoid hard-coding database names into simple
application programs.
PGHOST sets the default server name.
If a non-zero-length string is specified, TCP/IP communication is used.
Without a host name, libpq will connect using a local Unix domain socket.
PGPORT sets the default port or local Unix domain socket
file extension for communicating with the PostgreSQL
backend.
PGDATABASE sets the default
PostgreSQL database name.
PGUSER
sets the username used to connect to the database and for authentication.
PGPASSWORD
sets the password used if the backend demands password authentication.
PGREALM sets the Kerberos realm to use with
PostgreSQL, if it is different from the local realm.
If PGREALM is set, PostgreSQL
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 backend.
PGOPTIONS sets additional runtime options for
the PostgreSQL backend.
PGTTY sets the file or tty on which debugging
messages from the backend server are displayed.
The following environment variables can be used to specify user-level default
behavior for every Postgres session:
PGDATESTYLE
sets the default style of date/time representation.
PGTZ
sets the default time zone.
The following environment variables can be used to specify default internal
behavior for every Postgres session:
PGGEQO
sets the default mode for the genetic optimizer.
PGRPLANS
sets the default mode to allow or disable right-sided plans in the optimizer.
PGCOSTHEAP
sets the default cost for heap searches for the optimizer.
PGCOSTINDEX
sets the default cost for indexed searches for the optimizer.
Refer to the SET SQL command
for information on correct values for these environment variables.
Caveats
The query buffer is 8192 bytes long, and queries over
that length will be rejected.
Sample Programs
Sample Program 1
/*
* testlibpq.c Test the C version of Libpq, the Postgres 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; /* debugging tty for the backend server */
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 instances 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 instances */
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); */
}
Sample 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; /* debugging tty for the backend server */
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);
free(notify);
}
}
/* close the connection to the database and cleanup */
PQfinish(conn);
}
Sample Program 3
/*
* testlibpq3.c Test the C version of Libpq, the Postgres frontend
* library. tests the binary cursor interface
*
*
*
* populate a database by doing the following:
*
* CREATE TABLE test1 (i int4, d float4, p polygon);
*
* INSERT INTO test1 values (1, 3.567, '(3.0, 4.0, 1.0,
* 2.0)'::polygon);
*
* INSERT INTO test1 values (2, 89.05, '(4.0, 3.0, 2.0,
* 1.0)'::polygon);
*
* 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; /* debugging tty for the backend server */
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 instances 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);
}