PostgreSQL 7.1.3 Documentation
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# 2.6. Sample Program

    /*--------------------------------------------------------------
     *
     * testlo.c--
     *    test using large objects with libpq
     *
     * Copyright (c) 1994, Regents of the University of California
     *
     *
     * IDENTIFICATION
     *    /usr/local/devel/pglite/cvs/src/doc/manual.me,v 1.16 1995/09/01 23:55:00 jolly Exp
     *
     *--------------------------------------------------------------
     */
    #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 "lob
    jOid"
     *
     */
    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_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);

        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;

        /*
         * create an inversion "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 Unix file and write to the inversion 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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