lobj.sgml

PostgreSQL7.4.6 for Linux

     use:
<programlisting>
CREATE TABLE image (
    name            text,
    raster          oid
);

INSERT INTO image (name, raster)
    VALUES ('beautiful image', lo_import('/etc/motd'));

SELECT lo_export(image.raster, '/tmp/motd') FROM image
    WHERE name = 'beautiful image';
</programlisting>
</para>

<para>
These functions read and write files in the server's file system, using the
permissions of the database's owning user.  Therefore, their use is restricted
to superusers.  (In contrast, the client-side import and export functions
read and write files in the client's file system, using the permissions of
the client program.  Their use is not restricted.)
</para>
</sect1>

<sect1 id="lo-examplesect">
<title>Example Program</title>

<para>
     <xref linkend="lo-example"> is a sample program which shows how the large object  
     interface
     in  <application>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
     <filename>src/test/examples/testlo.c</filename> in the source distribution.
</para>

  <example id="lo-example">
   <title>Large Objects with <application>libpq</application> Example Program</title>
<programlisting>
/*--------------------------------------------------------------
 *
 * testlo.c--
 *    test using large objects with libpq
 *
 * Copyright (c) 1994, Regents of the University of California
 *
 *--------------------------------------------------------------
 */
#include &lt;stdio.h&gt;
#include &quot;libpq-fe.h&quot;
#include &quot;libpq/libpq-fs.h&quot;

#define BUFSIZE          1024

/*
 * importFile
 *    import file &quot;in_filename&quot; into database as large object &quot;lobjOid&quot;
 *
 */
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 &lt; 0)
    {                           /* error */
        fprintf(stderr, &quot;can't open unix file %s\n&quot;, filename);
    }

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

    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)) &gt; 0)
    {
        tmp = lo_write(conn, lobj_fd, buf, nbytes);
        if (tmp &lt; nbytes)
            fprintf(stderr, &quot;error while reading large object\n&quot;);
    }

    (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 &lt; 0)
    {
        fprintf(stderr, &quot;can't open large object %d\n&quot;,
                lobjId);
    }

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

    nread = 0;
    while (len - nread &gt; 0)
    {
        nbytes = lo_read(conn, lobj_fd, buf, len - nread);
        buf[nbytes] = ' ';
        fprintf(stderr, &quot;&gt;&gt;&gt; %s&quot;, buf);
        nread += nbytes;
    }
    free(buf);
    fprintf(stderr, &quot;\n&quot;);
    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 &lt; 0)
    {
        fprintf(stderr, &quot;can't open large object %d\n&quot;,
                lobjId);
    }

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

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

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

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

    /*
     * create an inversion &quot;object&quot;
     */
    lobj_fd = lo_open(conn, lobjId, INV_READ);
    if (lobj_fd &lt; 0)
    {
        fprintf(stderr, &quot;can't open large object %d\n&quot;,
                lobjId);
    }

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

    /*
     * read in from the Unix file and write to the inversion file
     */
    while ((nbytes = lo_read(conn, lobj_fd, buf, BUFSIZE)) &gt; 0)
    {
        tmp = write(fd, buf, nbytes);
        if (tmp &lt; nbytes)
        {
            fprintf(stderr, &quot;error while writing %s\n&quot;,
                    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, &quot;Usage: %s database_name in_filename out_filename\n&quot;,
                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, &quot;Connection to database '%s' failed.\n&quot;, database);
        fprintf(stderr, &quot;%s&quot;, PQerrorMessage(conn));
        exit_nicely(conn);
    }

    res = PQexec(conn, &quot;begin&quot;);
    PQclear(res);

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

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

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

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

    res = PQexec(conn, &quot;end&quot;);
    PQclear(res);
    PQfinish(conn);
    exit(0);
}
</programlisting>
</example>

</sect1>
</chapter>

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