Llibreria libpg: accedir a PostgreSQL

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$ sudo apt-get install libpq5 libpq-dev

Els fiftxes de capçalera estan a /usr/include/postgresql. Concretament, en el codi següent s'utilitza libpq-fe.h. Aquest directori també es pot trobar fent:

$  pg_config --includedir

Per compilar:

$ cc -c -I/usr/include/postgresql testlibpq.c

When linking the final program, specify the option -lpq so that the libpq library gets pulled in, as well as the option -Ldirectory to point the compiler to the directory where the libpq library resides. (Again, the compiler will search some directories by default.) For maximum portability, put the -L option before the -lpq option. For example:

$  cc -o testlibpq testlibpq.o -L/usr/include/postgresql/

Error messages that point to problems in this area could look like the following (el que a mi em passa...).

testlibpq.o: In function `main':
testlibpq.o(.text+0x60): undefined reference to `PQsetdbLogin'
testlibpq.o(.text+0x71): undefined reference to `PQstatus'
testlibpq.o(.text+0xa4): undefined reference to `PQerrorMessage'

This means you forgot -lpq.

/usr/bin/ld: cannot find -lpq

This means you forgot the -L option or did not specify the right directory.

Finalment:

$  cc -o testlibpq testlibpq.o -lpq -L/usr/include/postgresql/

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

testlibpq.c: del codi original, canvio

conninfo = "dbname = postgres";
per
conninfo = "host=localhost dbname = postgres user=postgres password=postgres";
/*
 * 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;
}

i el resultats és:

datname        datdba         encoding       datcollate     datctype       datistemplate  datallowconn   datconnlimit   datlastsysoid  datfrozenxid   dattablespace  datconfig      datacl         

template1      10             6              es_ES.UTF-8    es_ES.UTF-8    t              t              -1             11563          648            1663                          {=c/postgres,postgres=CTc/postgres}
template0      10             6              es_ES.UTF-8    es_ES.UTF-8    t              f              -1             11563          648            1663                          {=c/postgres,postgres=CTc/postgres}
postgres       10             6              es_ES.UTF-8    es_ES.UTF-8    f              t              -1             11563          648            1663                                         
foodmart       10             6              es_ES.UTF-8    es_ES.UTF-8    f              t              -1             11563          648            1663                                         
hibernate      146714         6              es_ES.UTF-8    es_ES.UTF-8    f              t              -1             11563          648            1663                          {=Tc/hibuser,hibuser=CTc/hibuser}
quartz         146763         6              es_ES.UTF-8    es_ES.UTF-8    f              t              -1             11563          648            1663                          {=Tc/pentaho_user,pentaho_user=CTc/pentaho_user}
sampledata     10             6              es_ES.UTF-8    es_ES.UTF-8    f              t              -1             11563          648            1663                                         
openbravo      245020         6              es_ES.UTF-8    es_ES.UTF-8    f              t              -1             11563          648            1663                                         

Per tant, resumint:

$ cc -c -I/usr/include/postgresql testlibpq.c
$  cc -o testlibpq testlibpq.o -lpq -L/usr/include/postgresql/
$ ./testlibpq
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