35.5. Example Program

35.5. Example Program
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+ Example 35.1 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 35.1. Large Objects with libpq Example Program
+/*-----------------------------------------------------------------
+ *
+ * testlo.c
+ *    test using large objects with libpq
+ *
+ * Portions Copyright (c) 1996-2022, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ *
+ * IDENTIFICATION
+ *    src/test/examples/testlo.c
+ *
+ *-----------------------------------------------------------------
+ */
+#include <stdio.h>
+#include <stdlib.h>
+
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <unistd.h>
+
+#include "libpq-fe.h"
+#include "libpq/libpq-fs.h"
+
+#define BUFSIZE         1024
+
+/*
+ * importFile -
+ *    import file "in_filename" into database as large object "lobjOid"
+ *
+ */
+static 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, "cannot open unix file\"%s\"\n", filename);
+    }
+
+    /*
+     * create the large object
+     */
+    lobjId = lo_creat(conn, INV_READ | INV_WRITE);
+    if (lobjId == 0)
+        fprintf(stderr, "cannot create large object");
+
+    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 \"%s\"", filename);
+    }
+
+    close(fd);
+    lo_close(conn, lobj_fd);
+
+    return lobjId;
+}
+
+static 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, "cannot open large object %u", 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] = '\0';
+        fprintf(stderr, ">>> %s", buf);
+        nread += nbytes;
+        if (nbytes <= 0)
+            break;              /* no more data? */
+    }
+    free(buf);
+    fprintf(stderr, "\n");
+    lo_close(conn, lobj_fd);
+}
+
+static 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, "cannot open large object %u", lobjId);
+
+    lo_lseek(conn, lobj_fd, start, SEEK_SET);
+    buf = malloc(len + 1);
+
+    for (i = 0; i < len; i++)
+        buf[i] = 'X';
+    buf[i] = '\0';
+
+    nwritten = 0;
+    while (len - nwritten > 0)
+    {
+        nbytes = lo_write(conn, lobj_fd, buf + nwritten, len - nwritten);
+        nwritten += nbytes;
+        if (nbytes <= 0)
+        {
+            fprintf(stderr, "\nWRITE FAILED!\n");
+            break;
+        }
+    }
+    free(buf);
+    fprintf(stderr, "\n");
+    lo_close(conn, lobj_fd);
+}
+
+
+/*
+ * exportFile -
+ *    export large object "lobjOid" to file "out_filename"
+ *
+ */
+static 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, "cannot open large object %u", lobjId);
+
+    /*
+     * open the file to be written to
+     */
+    fd = open(filename, O_CREAT | O_WRONLY | O_TRUNC, 0666);
+    if (fd < 0)
+    {                           /* error */
+        fprintf(stderr, "cannot open unix file\"%s\"",
+                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\"",
+                    filename);
+        }
+    }
+
+    lo_close(conn, lobj_fd);
+    close(fd);
+}
+
+static 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_OK)
+    {
+        fprintf(stderr, "%s", PQerrorMessage(conn));
+        exit_nicely(conn);
+    }
+
+    /* Set always-secure search path, so malicious users can't take control. */
+    res = PQexec(conn,
+                 "SELECT pg_catalog.set_config('search_path', '', false)");
+    if (PQresultStatus(res) != PGRES_TUPLES_OK)
+    {
+        fprintf(stderr, "SET failed: %s", PQerrorMessage(conn));
+        PQclear(res);
+        exit_nicely(conn);
+    }
+    PQclear(res);
+
+    res = PQexec(conn, "begin");
+    PQclear(res);
+    printf("importing file \"%s\" ...\n", in_filename);
+/*  lobjOid = importFile(conn, in_filename); */
+    lobjOid = lo_import(conn, in_filename);
+    if (lobjOid == 0)
+        fprintf(stderr, "%s\n", PQerrorMessage(conn));
+    else
+    {
+        printf("\tas large object %u.\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); */
+        if (lo_export(conn, lobjOid, out_filename) < 0)
+            fprintf(stderr, "%s\n", PQerrorMessage(conn));
+    }
+
+    res = PQexec(conn, "end");
+    PQclear(res);
+    PQfinish(conn);
+    return 0;
+}
+
+
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