27.14. Example Programs

These examples and others can be found in the directory src/test/examples in the source code distribution.

Example 27-1. libpq Example Program 1

/*
 * testlibpq.c
 *
 *		Test the C version of LIBPQ, the POSTGRES frontend library.
 */
#include <stdio.h>
#include <stdlib.h>
#include "libpq-fe.h"

static void
exit_nicely(PGconn *conn)
{
	PQfinish(conn);
	exit(1);
}

int
main(int argc, char **argv)
{
	const char *conninfo;
	PGconn	   *conn;
	PGresult   *res;
	int			nFields;
	int			i,
				j;

	/*
	 * If the user supplies a parameter on the command line, use it as
	 * the conninfo string; otherwise default to setting dbname=template1
	 * and using environment variables or defaults for all other connection
	 * parameters.
	 */
	if (argc > 1)
		conninfo = argv[1];
	else
		conninfo = "dbname = template1";

	/* Make a connection to the database */
	conn = PQconnectdb(conninfo);

	/* Check to see that the backend connection was successfully made */
	if (PQstatus(conn) != CONNECTION_OK)
	{
		fprintf(stderr, "Connection to database '%s' failed.\n", PQdb(conn));
		fprintf(stderr, "%s", PQerrorMessage(conn));
		exit_nicely(conn);
	}

	/*
	 * Our test case here involves using a cursor, for which we must be
	 * inside a transaction block.  We could do the whole thing with a
	 * single PQexec() of "select * from pg_database", but that's too
	 * trivial to make a good example.
	 */

	/* Start a transaction block */
	res = PQexec(conn, "BEGIN");
	if (PQresultStatus(res) != PGRES_COMMAND_OK)
	{
		fprintf(stderr, "BEGIN command failed: %s", PQerrorMessage(conn));
		PQclear(res);
		exit_nicely(conn);
	}

	/*
	 * Should PQclear PGresult whenever it is no longer needed to avoid
	 * memory leaks
	 */
	PQclear(res);

	/*
	 * Fetch rows from pg_database, the system catalog of databases
	 */
	res = PQexec(conn, "DECLARE myportal CURSOR FOR select * from pg_database");
	if (PQresultStatus(res) != PGRES_COMMAND_OK)
	{
		fprintf(stderr, "DECLARE CURSOR failed: %s", PQerrorMessage(conn));
		PQclear(res);
		exit_nicely(conn);
	}
	PQclear(res);

	res = PQexec(conn, "FETCH ALL in myportal");
	if (PQresultStatus(res) != PGRES_TUPLES_OK)
	{
		fprintf(stderr, "FETCH ALL failed: %s", PQerrorMessage(conn));
		PQclear(res);
		exit_nicely(conn);
	}

	/* first, print out the attribute names */
	nFields = PQnfields(res);
	for (i = 0; i < nFields; i++)
		printf("%-15s", PQfname(res, i));
	printf("\n\n");

	/* next, print out the rows */
	for (i = 0; i < PQntuples(res); i++)
	{
		for (j = 0; j < nFields; j++)
			printf("%-15s", PQgetvalue(res, i, j));
		printf("\n");
	}

	PQclear(res);

	/* close the portal ... we don't bother to check for errors ... */
	res = PQexec(conn, "CLOSE myportal");
	PQclear(res);

	/* end the transaction */
	res = PQexec(conn, "END");
	PQclear(res);

	/* close the connection to the database and cleanup */
	PQfinish(conn);

	return 0;
}

Example 27-2. libpq Example Program 2

/*
 * testlibpq2.c
 *		Test of the asynchronous notification interface
 *
 * Start this program, then from psql in another window do
 *   NOTIFY TBL2;
 * Repeat four times to get this program to exit.
 *
 * Or, if you want to get fancy, try this:
 * populate a database with the following commands
 * (provided in src/test/examples/testlibpq2.sql):
 *
 *   CREATE 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 this four times:
 *
 *   INSERT INTO TBL1 VALUES (10);
 */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <sys/time.h>
#include "libpq-fe.h"

static void
exit_nicely(PGconn *conn)
{
	PQfinish(conn);
	exit(1);
}

int
main(int argc, char **argv)
{
	const char *conninfo;
	PGconn	   *conn;
	PGresult   *res;
	PGnotify   *notify;
	int			nnotifies;

	/*
	 * If the user supplies a parameter on the command line, use it as
	 * the conninfo string; otherwise default to setting dbname=template1
	 * and using environment variables or defaults for all other connection
	 * parameters.
	 */
	if (argc > 1)
		conninfo = argv[1];
	else
		conninfo = "dbname = template1";

	/* Make a connection to the database */
	conn = PQconnectdb(conninfo);

	/* Check to see that the backend connection was successfully made */
	if (PQstatus(conn) != CONNECTION_OK)
	{
		fprintf(stderr, "Connection to database '%s' failed.\n", PQdb(conn));
		fprintf(stderr, "%s", PQerrorMessage(conn));
		exit_nicely(conn);
	}

	/*
	 * Issue LISTEN command to enable notifications from the rule's NOTIFY.
	 */
	res = PQexec(conn, "LISTEN TBL2");
	if (PQresultStatus(res) != PGRES_COMMAND_OK)
	{
		fprintf(stderr, "LISTEN command failed: %s", PQerrorMessage(conn));
		PQclear(res);
		exit_nicely(conn);
	}

	/*
	 * should PQclear PGresult whenever it is no longer needed to avoid
	 * memory leaks
	 */
	PQclear(res);

	/* Quit after four notifies are received. */
	nnotifies = 0;
	while (nnotifies < 4)
	{
		/*
		 * Sleep until something happens on the connection.  We use select(2)
		 * to wait for input, but you could also use poll() or similar
		 * facilities.
		 */
		int			sock;
		fd_set		input_mask;

		sock = PQsocket(conn);

		if (sock < 0)
			break;				/* shouldn't happen */

		FD_ZERO(&input_mask);
		FD_SET(sock, &input_mask);

		if (select(sock + 1, &input_mask, NULL, NULL, NULL) < 0)
		{
			fprintf(stderr, "select() failed: %s\n", strerror(errno));
			exit_nicely(conn);
		}

		/* Now check for input */
		PQconsumeInput(conn);
		while ((notify = PQnotifies(conn)) != NULL)
		{
			fprintf(stderr,
					"ASYNC NOTIFY of '%s' received from backend pid %d\n",
					notify->relname, notify->be_pid);
			PQfreemem(notify);
			nnotifies++;
		}
	}

	fprintf(stderr, "Done.\n");

	/* close the connection to the database and cleanup */
	PQfinish(conn);

	return 0;
}

Example 27-3. libpq Example Program 3

/*
 * testlibpq3.c
 *		Test out-of-line parameters and binary I/O.
 *
 * Before running this, populate a database with the following commands
 * (provided in src/test/examples/testlibpq3.sql):
 *
 * CREATE TABLE test1 (i int4, t text, b bytea);
 *
 * INSERT INTO test1 values (1, 'joe''s place', '\\000\\001\\002\\003\\004');
 * INSERT INTO test1 values (2, 'ho there', '\\004\\003\\002\\001\\000');
 *
 * The expected output is:
 *
 * tuple 0: got
 *  i = (4 bytes) 1
 *  t = (11 bytes) 'joe's place'
 *  b = (5 bytes) \000\001\002\003\004
 *
 */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include "libpq-fe.h"

/* for ntohl/htonl */
#include <netinet/in.h>
#include <arpa/inet.h>


static void
exit_nicely(PGconn *conn)
{
	PQfinish(conn);
	exit(1);
}

int
main(int argc, char **argv)
{
	const char *conninfo;
	PGconn	   *conn;
	PGresult   *res;
	const char *paramValues[1];
	int			i,
				j;
	int			i_fnum,
				t_fnum,
				b_fnum;

	/*
	 * If the user supplies a parameter on the command line, use it as
	 * the conninfo string; otherwise default to setting dbname=template1
	 * and using environment variables or defaults for all other connection
	 * parameters.
	 */
	if (argc > 1)
		conninfo = argv[1];
	else
		conninfo = "dbname = template1";

	/* Make a connection to the database */
	conn = PQconnectdb(conninfo);

	/* Check to see that the backend connection was successfully made */
	if (PQstatus(conn) != CONNECTION_OK)
	{
		fprintf(stderr, "Connection to database '%s' failed.\n", PQdb(conn));
		fprintf(stderr, "%s", PQerrorMessage(conn));
		exit_nicely(conn);
	}

	/*
	 * The point of this program is to illustrate use of PQexecParams()
	 * with out-of-line parameters, as well as binary transmission of
	 * results.  By using out-of-line parameters we can avoid a lot of
	 * tedious mucking about with quoting and escaping.  Notice how we
	 * don't have to do anything special with the quote mark in the
	 * parameter value.
	 */

	/* Here is our out-of-line parameter value */
	paramValues[0] = "joe's place";

	res = PQexecParams(conn,
					   "SELECT * FROM test1 WHERE t = $1",
					   1,		/* one param */
					   NULL,	/* let the backend deduce param type */
					   paramValues,
					   NULL,	/* don't need param lengths since text */
					   NULL,	/* default to all text params */
					   1);		/* ask for binary results */

	if (PQresultStatus(res) != PGRES_TUPLES_OK)
	{
		fprintf(stderr, "SELECT failed: %s", PQerrorMessage(conn));
		PQclear(res);
		exit_nicely(conn);
	}

	/* Use PQfnumber to avoid assumptions about field order in result */
	i_fnum = PQfnumber(res, "i");
	t_fnum = PQfnumber(res, "t");
	b_fnum = PQfnumber(res, "b");

	for (i = 0; i < PQntuples(res); i++)
	{
		char	   *iptr;
		char	   *tptr;
		char	   *bptr;
		int			blen;
		int			ival;

		/* Get the field values (we ignore possibility they are null!) */
		iptr = PQgetvalue(res, i, i_fnum);
		tptr = PQgetvalue(res, i, t_fnum);
		bptr = PQgetvalue(res, i, b_fnum);

		/*
		 * The binary representation of INT4 is in network byte order,
		 * which we'd better coerce to the local byte order.
		 */
		ival = ntohl(*((uint32_t *) iptr));

		/*
		 * The binary representation of TEXT is, well, text, and since
		 * libpq was nice enough to append a zero byte to it, it'll work
		 * just fine as a C string.
		 *
		 * The binary representation of BYTEA is a bunch of bytes, which
		 * could include embedded nulls so we have to pay attention to
		 * field length.
		 */
		blen = PQgetlength(res, i, b_fnum);

		printf("tuple %d: got\n", i);
		printf(" i = (%d bytes) %d\n",
			   PQgetlength(res, i, i_fnum), ival);
		printf(" t = (%d bytes) '%s'\n",
			   PQgetlength(res, i, t_fnum), tptr);
		printf(" b = (%d bytes) ", blen);
		for (j = 0; j < blen; j++)
			printf("\\%03o", bptr[j]);
		printf("\n\n");
	}

	PQclear(res);

	/* close the connection to the database and cleanup */
	PQfinish(conn);

	return 0;
}