Large Objects

PostgreSQL 9.6.3 Documentation
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# 34.12. Large Objects

Large objects are not directly supported by ECPG, but ECPG application can manipulate large objects through the libpq large object functions, obtaining the necessary `PGconn` object by calling the `ECPGget_PGconn()` function. (However, use of the `ECPGget_PGconn()` function and touching `PGconn` objects directly should be done very carefully and ideally not mixed with other ECPG database access calls.)

For more details about the `ECPGget_PGconn()`, see Section 34.11. For information about the large object function interface, see Chapter 33.

Large object functions have to be called in a transaction block, so when autocommit is off, `BEGIN` commands have to be issued explicitly.

Example 34-2 shows an example program that illustrates how to create, write, and read a large object in an ECPG application.

Example 34-2. ECPG Program Accessing Large Objects

    #include <stdio.h>
    #include <stdlib.h>
    #include <libpq-fe.h>
    #include <libpq/libpq-fs.h>

    EXEC SQL WHENEVER SQLERROR STOP;

    int
    main(void)
    {
        PGconn     *conn;
        Oid         loid;
        int         fd;
        char        buf[256];
        int         buflen = 256;
        char        buf2[256];
        int         rc;

        memset(buf, 1, buflen);

        EXEC SQL CONNECT TO testdb AS con1;

        conn = ECPGget_PGconn("con1");
        printf("conn = %p\n", conn);

        /* create */
        loid = lo_create(conn, 0);
        if (loid &lt; 0)
            printf("lo_create() failed: %s", PQerrorMessage(conn));

        printf("loid = %d\n", loid);

        /* write test */
        fd = lo_open(conn, loid, INV_READ|INV_WRITE);
        if (fd &lt; 0)
            printf("lo_open() failed: %s", PQerrorMessage(conn));

        printf("fd = %d\n", fd);

        rc = lo_write(conn, fd, buf, buflen);
        if (rc &lt; 0)
            printf("lo_write() failed\n");

        rc = lo_close(conn, fd);
        if (rc &lt; 0)
            printf("lo_close() failed: %s", PQerrorMessage(conn));

        /* read test */
        fd = lo_open(conn, loid, INV_READ);
        if (fd &lt; 0)
            printf("lo_open() failed: %s", PQerrorMessage(conn));

        printf("fd = %d\n", fd);

        rc = lo_read(conn, fd, buf2, buflen);
        if (rc &lt; 0)
            printf("lo_read() failed\n");

        rc = lo_close(conn, fd);
        if (rc &lt; 0)
            printf("lo_close() failed: %s", PQerrorMessage(conn));

        /* check */
        rc = memcmp(buf, buf2, buflen);
        printf("memcmp() = %d\n", rc);

        /* cleanup */
        rc = lo_unlink(conn, loid);
        if (rc &lt; 0)
            printf("lo_unlink() failed: %s", PQerrorMessage(conn));

        EXEC SQL COMMIT;
        EXEC SQL DISCONNECT ALL;
        return 0;
    }

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