关于libpq各个函数接口的说明，参考：http://www.postgres.cn/docs/9.3/libpq.html

例子1：

/*
 * testlibpq.c
 *
 *      Test the C version of libpq,the Postgresql 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=postgres and using
     * environment variables or defaults for all other connection parameters.
     */
    if (argc > 1)
        conninfo = argv[1];
    else
        conninfo = "dbname = postgres";

    /* 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 Failed: %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;
}

/*
 * 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,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) < 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;
}

例子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
 *
 * tuple 0: got
 *  i = (4 bytes) 2
 *  t = (8 bytes) 'ho there'
 *  b = (5 bytes) \004\003\002\001\000
 */
#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);
}

/*
 * This function prints a query result that is a binary-format fetch from
 * a table defined as in the comment above.  We split it out because the
 * main() function uses it twice.
 */
static void
show_binary_results(PGresult *res)
{
    int         i,j;
    int         i_fnum,t_fnum,b_fnum;

    /* 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_fnum);
        tptr = PQgetvalue(res,t_fnum);
        bptr = PQgetvalue(res,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,b_fnum);

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

int
main(int argc,char **argv)
{
    const char *conninfo;
    PGconn     *conn;
    PGresult   *res;
    const char *paramValues[1];
    int         paramLengths[1];
    int         paramFormats[1];
    uint32_t    binaryIntVal;

    /*
     * If the user supplies a parameter on the command line,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 data.
     *
     * This first example transmits the parameters as text,but receives the
     * results in binary format.  By using out-of-line parameters we can
     * avoid a lot of tedious mucking about with quoting and escaping,even
     * though the data is text.  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,/* 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);
    }

    show_binary_results(res);

    PQclear(res);

    /*
     * In this second example we transmit an integer parameter in binary
     * form,and again retrieve the results in binary form.
     *
     * Although we tell PQexecParams we are letting the backend deduce
     * parameter type,we really force the decision by casting the parameter
     * symbol in the query text.  This is a good safety measure when sending
     * binary parameters.
     */

    /* Convert integer value "2" to network byte order */
    binaryIntVal = htonl((uint32_t) 2);

    /* Set up parameter arrays for PQexecParams */
    paramValues[0] = (char *) &binaryIntVal;
    paramLengths[0] = sizeof(binaryIntVal);
    paramFormats[0] = 1;        /* binary */

    res = PQexecParams(conn,"SELECT * FROM test1 WHERE i = $1::int4",paramLengths,paramFormats,1);      /* ask for binary results */

    if (PQresultStatus(res) != PGRES_TUPLES_OK)
    {
        fprintf(stderr,PQerrorMessage(conn));
        PQclear(res);
        exit_nicely(conn);
    }

    show_binary_results(res);

    PQclear(res);

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

    return 0;
}