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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;
}

Comments


Nov. 14, 2005, 11:19 p.m.

To compile this examples, you must try:

gcc -I/usr/local/include -L/usr/local/lib -o testlibpq -lpq testlibpq.c

Regards,
Clay

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