LIBPQ(INTRO)
DESCRIPTION
Libpq is the programmer's interface to Postgres. Libpq is
a set of library routines that allows queries to pass to
the Postgres backend and instances to return through an
IPC channel.
This version of the documentation describes the C inter-
face library. Three short programs are included at the
end of this section to show how to write programs that use
Libpq.
There are several 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.
Control and Initialization
The following environment variables can be used to set up
default environment values to avoid hard-coding database
names into an application program:
+o PGHOST sets the default server name. If it is set to a
non-zero-length string, it causes TCP/IP communication to
be used, rather than the default local Unix domain sock-
ets.
+o PGOPTIONS sets additional runtime options for the Post-
gres backend.
+o PGPORT sets the default port or local Unix domain socket
file extension for communicating with the Postgres back-
end.
+o PGTTY sets the file or tty on which debugging messages
from the backend server are displayed.
+o PGDATABASE sets the default Postgres database name.
+o PGREALM sets the Kerberos realm to use with Postgres, if
it is different from the local realm. If PGREALM is set,
Postgres 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
enabled.
The following environment variables can be used to specify
user-level default behavior for every Postgres session:
+o PGDATESTYLE sets the default style of date/time repre-
sentation.
+o PGTZ sets the default time zone.
The following environment variables can be used to specify
default internal behavior for every Postgres session:
+o PGGEQO sets the default mode for the genetic optimizer.
+o PGRPLANS sets the default mode to allow or disable
right-sided plans in the optimizer.
+o PGCOSTHEAP sets the default cost for heap searches for
the optimizer.
+o PGCOSTINDEX sets the default cost for indexed searches
for the optimizer.
See the set(l) man page for information on the arguments
for these environment variables.
Database Connection Functions
The following routines deal with making a connection to a
backend from a C program.
PQsetdb
Makes a new connection to a backend.
PGconn *PQsetdb(char *pghost,
char *pgport,
char *pgoptions,
char *pgtty,
char *dbName);
If any argument is NULL, then the corresponding
environment variable is checked. If the environ-
ment variable is also not set, then hardwired
defaults are used.
PQsetdb always returns a valid PGconn pointer. The
PQstatus (see below) command should be called to
ensure that a connection was properly made before
queries are sent via the connection. Libpq pro-
grammers should be careful to maintain the PGconn
abstraction. Use the accessor functions below to
get at the contents of PGconn. Avoid directly ref-
erencing the fields of the PGconn structure as they
are subject to change in the future.
PQdb returns the database name of the connection.
char *PQdb(PGconn *conn)
PQhost returns the host name of the connection.
char *PQhost(PGconn *conn)
PQoptions returns the pgoptions used in the connec-
tion.
char *PQoptions(PGconn *conn)
PQport returns the pgport of the connection.
char *PQport(PGconn *conn)
PQtty returns the pgtty of the connection.
char *PQtty(PGconn *conn)
PQstatus Returns the status of the connection. The
status can be CONNECTION_OK or CONNECTION_BAD.
ConnStatusType *PQstatus(PGconn *conn)
PQerrorMessage returns the error message associated
with the connection
char *PQerrorMessage(PGconn* conn);
PQfinish
Close the connection to the backend. Also frees
memory used by the PGconn structure. The PGconn
pointer should not be used after PQfinish has been
called.
void PQfinish(PGconn *conn)
PQreset
Reset the communication port with the backend.
This function will close the IPC socket connection
to the backend and attempt to reestablish a new
connection to the same backend.
void PQreset(PGconn *conn)
Query Execution Functions
PQexec
Submit a query to Postgres. Returns a PGresult
pointer if the query was successful or a NULL oth-
erwise. If a NULL is returned, PQerrorMessage can
be used to get more information about the error.
PGresult *PQexec(PGconn *conn,
char *query);
The PGresult structure encapsulates the query
result returned by the backend. Libpq programmers
should be careful to maintain the PGresult abstrac-
tion. Use the accessor functions described below to
retrieve the results of the query. Avoid directly
referencing the fields of the PGresult structure as
they are subject to change in the future.
PQresultStatus
Returns the result status of the query. PQresult-
Status can return one of the following values:
PGRES_EMPTY_QUERY,
PGRES_COMMAND_OK, /* the query was a command */
PGRES_TUPLES_OK, /* the query successfully returned tuples */
PGRES_COPY_OUT,
PGRES_COPY_IN,
PGRES_BAD_RESPONSE, /* an unexpected response was received */
PGRES_NONFATAL_ERROR,
PGRES_FATAL_ERROR
If the result status is PGRES_TUPLES_OK, then the
following routines can be used to retrieve the
tuples returned by the query.
PQntuples returns the number of tuples (instances)
in the query result.
int PQntuples(PGresult *res);
PQcmdTuples returns the number of tuples
(instances) affected by INSERT, UPDATE, and DELETE
queries.
char *PQcmdTuples(PGresult *res);
PQnfields returns the number of fields (attributes)
in the query result.
int PQnfields(PGresult *res);
PQfname returns the field (attribute) name associ-
ated with the given field index. Field indices
start at 0.
char *PQfname(PGresult *res,
int field_index);
PQfnumber returns the field (attribute) index asso-
ciated with the given field name.
int PQfnumber(PGresult *res,
char* field_name);
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(PGresult *res,
int field_num);
PQfsize returns the size in bytes of the field
associated with the given field index. If the size
returned is -1, the field is a variable length
field. Field indices start at 0.
int2 PQfsize(PGresult *res,
int field_index);
PQgetvalue returns the field (attribute) value.
For most queries, the value returned by PQgetvalue
is a null-terminated ASCII string representation of
the attribute value. If the query was a result of
a BINARY cursor, then the value returned by PQget-
value is the binary representation of the type in
the internal format of the backend server. It is
the programmer's responsibility to cast and convert
the data to the correct C type. The value returned
by PQgetvalue points to storage that is part of the
PGresult structure. One must explicitly copy the
value into other storage if it is to be used past
the lifetime of the PGresult structure itself.
char* PQgetvalue(PGresult *res,
int tup_num,
int field_num);
PQgetlength returns the length of a field
(attribute) in bytes. If the field is a struct
varlena , the length returned here does not include
the size field of the varlena, i.e., it is 4 bytes
less.
int PQgetlength(PGresult *res,
int tup_num,
int field_num);
PQgetisnull returns the NULL status of a field.
int PQgetisnull(PGresult *res,
int tup_num,
int field_num);
PQcmdStatus
Returns the command status associated with the last
query command.
char *PQcmdStatus(PGresult *res);
PQoidStatus
Returns a string with the object id of the tuple
inserted if the last query is an INSERT command.
Otherwise, returns an empty string.
char* PQoidStatus(PGresult *res);
PQprint
+ Prints out all the tuples in an intelligent man-
ner. The psql + program uses this function for its
output.
void PQprint(
FILE* fout, /* output stream */
PGresult* res, /* query results */
PQprintOpt *ps /* option structure */
);
PQprintOpt is a typedef'ed structure as defined
below. typedef struct _PQprintOpt {
bool header; /* print table headings
and row count */
bool align; /* fill align the fields
*/
bool standard; /* old brain dead format
(needs align) */
bool html3; /* output html3+ tables
*/
bool expanded; /* expand tables */
bool pager; /* use pager if needed
*/
char *fieldSep; /* field separator */
char *caption; /* html table caption
(or NULL) */
char **fieldName; /* null terminated array
of field names (or NULL) */ } PQprintOpt;
PQclear
Frees the storage associated with the PGresult.
Every query result should be properly freed when it
is no longer used. Failure to do this will result
in memory leaks in the frontend application. The
PQresult* passed in should be a value which is
returned from PQexec(). Calling PQclear() on an
uninitialized PQresult pointer will very likely
result in a core dump.
void PQclear(PQresult *res);
Fast Path
Postgres 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.
PGresult* PQfn(PGconn* conn,
int fnid,
int *result_buf,
int *result_len,
int result_is_int,
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 load
the return value. The caller must have allocated suffi-
cient space to store the return value. The result length
will be returned in the storage pointed to by result_len.
If the result is to be an integer value, than
result_is_int should be set to 1; otherwise it should be
set to 0. args and nargs specify the arguments 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 not longer needed.
Asynchronous Notification
Postgres supports asynchronous notification via the LISTEN
and NOTIFY commands. A backend registers its interest in
a particular relation with the LISTEN command. All back-
ends listening on a particular relation will be notified
asynchronously when a NOTIFY of that relation name is exe-
cuted by another backend. No additional information is
passed from the notifier to the listener. Thus, typi-
cally, any actual data that needs to be communicated is
transferred through the relation.
Libpq applications are notified whenever a connected back-
end has received an asynchronous notification. However,
the communication from the backend to the frontend is not
asynchronous. Notification comes piggy-backed on other
query results. Thus, an application must submit queries,
even empty ones, in order to receive notice of backend
notification. In effect, the Libpq application must poll
the backend to see if there is any pending notification
information. After the execution of a query, a frontend
may call PQNotifies to see if any notification data is
available from the backend.
PQNotifies
returns the notification from a list of unhandled
notifications from the backend. Returns NULL if
there are no pending notifications from the back-
end. PQNotifies behaves like the popping of a
stack. Once a notification is returned from PQno-
tifies, it is considered handled and will be
removed from the list of notifications.
PGnotify* PQNotifies(PGconn *conn);
The second sample program gives an example of the use of
asynchronous notification.
Functions Associated with the COPY Command
The copy command in Postgres 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 full advantage of this
capability.
PQgetline
Reads a newline-terminated line of characters
(transmitted by the backend server) into a buffer
string of size length . Like fgets (3), this rou-
tine copies up to length -1 characters into string
. It is like gets (3), however, in that it con-
verts the terminating newline into a null charac-
ter.
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 characters "\.", which
indicates that the backend server has finished
sending the results of the copy command. There-
fore, if the application ever expects to receive
lines that are more than length -1 characters long,
the application must be sure to check the return
value of PQgetline very carefully.
The code in
../src/bin/psql/psql.c
contains routines that correctly handle the copy
protocol.
int PQgetline(PGconn *conn,
char *string,
int length)
PQputline
Sends a null-terminated string to the backend
server.
The application must explicitly send the characters
"\." to indicate to the backend that it has fin-
ished sending its data.
void PQputline(PGconn *conn,
char *string);
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 char16, d float8)");
PQexec(conn, "copy foo from stdin");
PQputline(conn, "3<TAB>hello world<TAB>4.5\n");
PQputline(conn,"4<TAB>goodbye world<TAB>7.11\n");
...
PQputline(conn,"\.\n");
PQendcopy(conn);
LIBPQ Tracing Functions
PQtrace
Enable tracing of the frontend/backend communica-
tion to a debugging file stream.
void PQtrace(PGconn *conn
FILE *debug_port)
PQuntrace
Disable tracing started by PQtrace
void PQuntrace(PGconn *conn)
User Authentication Functions
If the user has generated the appropriate authentication
credentials (e.g., obtaining Kerberos tickets), the fron-
tend/backend authentication process is handled by PQexec
without any further intervention. The authentication
method is now determined entirely by the DBA (see
pga_hba.conf(5)). The following routines no longer have
any effect and should not be used.
fe_getauthname
Returns a pointer to static space containing what-
ever name the user has authenticated. Use of this
routine in place of calls to getenv (3) or getpwuid
(3) by applications is highly recommended, as it is
entirely possible that the authenticated user name
is not the same as value of the USER environment
variable or the user's entry in /etc/passwd .
char *fe_getauthname(char* errorMessage)
fe_setauthsvc
Specifies that Libpq should use authentication ser-
vice name rather than its compiled-in default.
This value is typically taken from a command-line
switch.
void fe_setauthsvc(char *name,
char* errorMessage)
Any error messages from the authentication attempts
are returned in the errorMessage argument.
BUGS
The query buffer is 8192 bytes long, and queries over that
length will be silently truncated.
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 (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 (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 (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
*
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];
* Then start up this program
* After the program has begun, 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 (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) {
/* async notification only come back as a result of a query*/
/* we can send empty queries */
res = PQexec(conn, "");
/* printf("res->status = %s\n", pgresStatus[PQresultStatus(res)]); */
/* check for asynchronous returns */
notify = PQnotifies(conn);
if (notify) {
fprintf(stderr,
"ASYNC NOTIFY of '%s' from backend pid '%d' received\n",
notify->relname, notify->be_pid);
free(notify);
break;
}
PQclear(res);
}
/* 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 (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 (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 (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);
}