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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="sect1" lang="en">
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="lo-examplesect"></a>29.5.�Example Program</h2></div></div></div>
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<p> <a href="lo-examplesect.html#lo-example" title="Example�29.1.�Large Objects with libpq Example Program">Example�29.1, “Large Objects with <span class="application">libpq</span> Example Program”</a> is a sample program which shows how the large object
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in <span class="application">libpq</span> can be used. Parts of the program are
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commented out but are left in the source for the reader's
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benefit. This program can also be found in
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<code class="filename">src/test/examples/testlo.c</code> in the source distribution.</p>
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<a name="lo-example"></a><p class="title"><b>Example�29.1.�Large Objects with <span class="application">libpq</span> Example Program</b></p>
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<div class="example-contents"><pre class="programlisting">/*--------------------------------------------------------------
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* test using large objects with libpq
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* Copyright (c) 1994, Regents of the University of California
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*--------------------------------------------------------------
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#include <stdio.h>
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#include "libpq/libpq-fs.h"
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* import file "in_filename" into database as large object "lobjOid"
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importFile(PGconn *conn, char *filename)
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* open the file to be read in
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fd = open(filename, O_RDONLY, 0666);
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fprintf(stderr, "can't open unix file %s\n", filename);
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* create the large object
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lobjId = lo_creat(conn, INV_READ | INV_WRITE);
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fprintf(stderr, "can't create large object\n");
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lobj_fd = lo_open(conn, lobjId, INV_WRITE);
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* read in from the Unix file and write to the inversion file
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while ((nbytes = read(fd, buf, BUFSIZE)) > 0)
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tmp = lo_write(conn, lobj_fd, buf, nbytes);
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fprintf(stderr, "error while reading large object\n");
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(void) lo_close(conn, lobj_fd);
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pickout(PGconn *conn, Oid lobjId, int start, int len)
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lobj_fd = lo_open(conn, lobjId, INV_READ);
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fprintf(stderr, "can't open large object %d\n",
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lo_lseek(conn, lobj_fd, start, SEEK_SET);
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buf = malloc(len + 1);
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while (len - nread > 0)
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nbytes = lo_read(conn, lobj_fd, buf, len - nread);
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fprintf(stderr, ">>> %s", buf);
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fprintf(stderr, "\n");
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lo_close(conn, lobj_fd);
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overwrite(PGconn *conn, Oid lobjId, int start, int len)
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lobj_fd = lo_open(conn, lobjId, INV_WRITE);
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fprintf(stderr, "can't open large object %d\n",
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lo_lseek(conn, lobj_fd, start, SEEK_SET);
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buf = malloc(len + 1);
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for (i = 0; i < len; i++)
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while (len - nwritten > 0)
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nbytes = lo_write(conn, lobj_fd, buf + nwritten, len - nwritten);
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fprintf(stderr, "\n");
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lo_close(conn, lobj_fd);
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* export large object "lobjOid" to file "out_filename"
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exportFile(PGconn *conn, Oid lobjId, char *filename)
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* open the large object
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lobj_fd = lo_open(conn, lobjId, INV_READ);
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fprintf(stderr, "can't open large object %d\n",
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* open the file to be written to
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fd = open(filename, O_CREAT | O_WRONLY, 0666);
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fprintf(stderr, "can't open unix file %s\n",
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* read in from the inversion file and write to the Unix file
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while ((nbytes = lo_read(conn, lobj_fd, buf, BUFSIZE)) > 0)
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tmp = write(fd, buf, nbytes);
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fprintf(stderr, "error while writing %s\n",
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(void) lo_close(conn, lobj_fd);
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exit_nicely(PGconn *conn)
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main(int argc, char **argv)
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fprintf(stderr, "Usage: %s database_name in_filename out_filename\n",
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in_filename = argv[2];
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out_filename = argv[3];
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* set up the connection
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conn = PQsetdb(NULL, NULL, NULL, NULL, database);
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/* check to see that the backend connection was successfully made */
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if (PQstatus(conn) == CONNECTION_BAD)
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fprintf(stderr, "Connection to database '%s' failed.\n", database);
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fprintf(stderr, "%s", PQerrorMessage(conn));
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res = PQexec(conn, "begin");
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printf("importing file %s\n", in_filename);
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/* lobjOid = importFile(conn, in_filename); */
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lobjOid = lo_import(conn, in_filename);
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printf("as large object %d.\n", lobjOid);
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printf("picking out bytes 1000-2000 of the large object\n");
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pickout(conn, lobjOid, 1000, 1000);
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printf("overwriting bytes 1000-2000 of the large object with X's\n");
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overwrite(conn, lobjOid, 1000, 1000);
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printf("exporting large object to file %s\n", out_filename);
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/* exportFile(conn, lobjOid, out_filename); */
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lo_export(conn, lobjOid, out_filename);
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res = PQexec(conn, "end");
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<br class="example-break">
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