PostgreSQL 8.2.4 Documentation
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# 30.5. Example Program

Example 30-1 is a sample program which shows how the large object interface in libpq can be used. Parts of the program are commented out but are left in the source for the reader's benefit. This program can also be found in `src/test/examples/testlo.c` in the source distribution.

Example 30-1. Large Objects with libpq Example Program

    /*--------------------------------------------------------------
     *
     * testlo.c--
     *    test using large objects with libpq
     *
     * Copyright (c) 1994, Regents of the University of California
     *
     *--------------------------------------------------------------
     */
    #include <stdio.h>
    #include "libpq-fe.h"
    #include "libpq/libpq-fs.h"

    #define BUFSIZE          1024

    /*
     * importFile
     *    import file "in_filename" into database as large object "lobjOid"
     *
     */
    Oid
    importFile(PGconn *conn, char *filename)
    {
        Oid         lobjId;
        int         lobj_fd;
        char        buf[BUFSIZE];
        int         nbytes,
                    tmp;
        int         fd;

        /*
         * open the file to be read in
         */
        fd = open(filename, O_RDONLY, 0666);
        if (fd < 0)
        {                           /* error */
            fprintf(stderr, "can't open unix file %s\n", filename);
        }

        /*
         * create the large object
         */
        lobjId = lo_creat(conn, INV_READ | INV_WRITE);
        if (lobjId == 0)
            fprintf(stderr, "can't create large object\n");

        lobj_fd = lo_open(conn, lobjId, INV_WRITE);

        /*
         * read in from the Unix file and write to the inversion file
         */
        while ((nbytes = read(fd, buf, BUFSIZE)) > 0)
        {
            tmp = lo_write(conn, lobj_fd, buf, nbytes);
            if (tmp < nbytes)
                fprintf(stderr, "error while reading large object\n");
        }

        (void) close(fd);
        (void) lo_close(conn, lobj_fd);

        return lobjId;
    }

    void
    pickout(PGconn *conn, Oid lobjId, int start, int len)
    {
        int         lobj_fd;
        char       *buf;
        int         nbytes;
        int         nread;

        lobj_fd = lo_open(conn, lobjId, INV_READ);
        if (lobj_fd < 0)
        {
            fprintf(stderr, "can't open large object %d\n",
                    lobjId);
        }

        lo_lseek(conn, lobj_fd, start, SEEK_SET);
        buf = malloc(len + 1);

        nread = 0;
        while (len - nread > 0)
        {
            nbytes = lo_read(conn, lobj_fd, buf, len - nread);
            buf[nbytes] = ' ';
            fprintf(stderr, ">>> %s", buf);
            nread += nbytes;
        }
        free(buf);
        fprintf(stderr, "\n");
        lo_close(conn, lobj_fd);
    }

    void
    overwrite(PGconn *conn, Oid lobjId, int start, int len)
    {
        int         lobj_fd;
        char       *buf;
        int         nbytes;
        int         nwritten;
        int         i;

        lobj_fd = lo_open(conn, lobjId, INV_WRITE);
        if (lobj_fd < 0)
        {
            fprintf(stderr, "can't open large object %d\n",
                    lobjId);
        }

        lo_lseek(conn, lobj_fd, start, SEEK_SET);
        buf = malloc(len + 1);

        for (i = 0; i < len; i++)
            buf[i] = 'X';
        buf[i] = ' ';

        nwritten = 0;
        while (len - nwritten > 0)
        {
            nbytes = lo_write(conn, lobj_fd, buf + nwritten, len - nwritten);
            nwritten += nbytes;
        }
        free(buf);
        fprintf(stderr, "\n");
        lo_close(conn, lobj_fd);
    }

    /*
     * exportFile
     *    export large object "lobjOid" to file "out_filename"
     *
     */
    void
    exportFile(PGconn *conn, Oid lobjId, char *filename)
    {
        int         lobj_fd;
        char        buf[BUFSIZE];
        int         nbytes,
                    tmp;
        int         fd;

        /*
         * open the large object
         */
        lobj_fd = lo_open(conn, lobjId, INV_READ);
        if (lobj_fd < 0)
        {
            fprintf(stderr, "can't open large object %d\n",
                    lobjId);
        }

        /*
         * open the file to be written to
         */
        fd = open(filename, O_CREAT | O_WRONLY, 0666);
        if (fd < 0)
        {                           /* error */
            fprintf(stderr, "can't open unix file %s\n",
                    filename);
        }

        /*
         * read in from the inversion file and write to the Unix file
         */
        while ((nbytes = lo_read(conn, lobj_fd, buf, BUFSIZE)) > 0)
        {
            tmp = write(fd, buf, nbytes);
            if (tmp < nbytes)
            {
                fprintf(stderr, "error while writing %s\n",
                        filename);
            }
        }

        (void) lo_close(conn, lobj_fd);
        (void) close(fd);

        return;
    }

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

    int
    main(int argc, char **argv)
    {
        char       *in_filename,
                   *out_filename;
        char       *database;
        Oid         lobjOid;
        PGconn     *conn;
        PGresult   *res;

        if (argc != 4)
        {
            fprintf(stderr, "Usage: %s database_name in_filename out_filename\n",
                    argv[0]);
            exit(1);
        }

        database = argv[1];
        in_filename = argv[2];
        out_filename = argv[3];

        /*
         * set up the connection
         */
        conn = PQsetdb(NULL, NULL, NULL, NULL, database);

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

        res = PQexec(conn, "begin");
        PQclear(res);

        printf("importing file %s\n", in_filename);
    /*  lobjOid = importFile(conn, in_filename); */
        lobjOid = lo_import(conn, in_filename);
    /*
        printf("as large object %d.\n", lobjOid);

        printf("picking out bytes 1000-2000 of the large object\n");
        pickout(conn, lobjOid, 1000, 1000);

        printf("overwriting bytes 1000-2000 of the large object with X's\n");
        overwrite(conn, lobjOid, 1000, 1000);
    */

        printf("exporting large object to file %s\n", out_filename);
    /*    exportFile(conn, lobjOid, out_filename); */
        lo_export(conn, lobjOid, out_filename);

        res = PQexec(conn, "end");
        PQclear(res);
        PQfinish(conn);
        exit(0);
    }

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