Oid lo_creat(PGconn *conn, int mode)creates a new large object. The mode is a bitmask describing several different attributes of the new object. The symbolic constants listed here are defined in
/usr/local/postgres95/src/backend/libpq/libpq-fs.hThe access type (read, write, or both) is controlled by OR ing together the bits INV_READ and INV_WRITE. If the large object should be archived -- that is, if historical versions of it should be moved periodically to a special archive relation -- then the INV_ARCHIVE bit should be set. The low-order sixteen bits of mask are the storage manager number on which the large object should reside. For sites other than Berkeley, these bits should always be zero. The commands below create an (Inversion) large object:
inv_oid = lo_creat(INV_READ|INV_WRITE|INV_ARCHIVE);
Oid
lo_import(PGconn *conn, text *filename)
The filename argument specifies the UNIX pathname of
the file to be imported as a large object.
int
lo_export(PGconn *conn, Oid lobjId, text *filename)
The lobjId argument specifies the Oid of the large
object to export and the filename argument specifies
the UNIX pathname of the file.
int
lo_open(PGconn *conn, Oid lobjId, int mode, ...)
The lobjId argument specifies the Oid of the large
object to open. The mode bits control whether the
object is opened for reading INV_READ), writing or
both.
A large object cannot be opened before it is created.
lo_open returns a large object descriptor for later use
in lo_read, lo_write, lo_lseek, lo_tell, and lo_close.
int
lo_write(PGconn *conn, int fd, char *buf, int len)
writes len bytes from buf to large object fd. The fd
argument must have been returned by a previous lo_open.
The number of bytes actually written is returned. In
the event of an error, the return value is negative.
int
lo_lseek(PGconn *conn, int fd, int offset, int whence)
This routine moves the current location pointer for the
large object described by fd to the new location specified
by offset. The valid values for .i whence are
SEEK_SET SEEK_CUR and SEEK_END.
int
lo_close(PGconn *conn, int fd)
where fd is a large object descriptor returned by
lo_open. On success, lo_close returns zero. On error,
the return value is negative.
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';
../src/test/examplesFrontend applications which use the large object interface in LIBPQ should include the header file libpq/libpq-fs.h and link with the libpq library.
/*--------------------------------------------------------------
*
* 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 "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
}
/*
* create the large object
*/
lobjId = lo_creat(conn, INV_READ|INV_WRITE);
if (lobjId == 0) {
fprintf(stderr, "can't 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
}
}
(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",
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;
}
fprintf(stderr,"0);
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",
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;
}
fprintf(stderr,"0);
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",
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
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
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_filename0,
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.0, database);
fprintf(stderr,"%s",PQerrorMessage(conn));
exit_nicely(conn);
}
res = PQexec(conn, "begin");
PQclear(res);
printf("importing file
/* lobjOid = importFile(conn, in_filename); */
lobjOid = lo_import(conn, in_filename);
/*
printf("as large object %d.0, lobjOid);
printf("picking out bytes 1000-2000 of the large object0);
pickout(conn, lobjOid, 1000, 1000);
printf("overwriting bytes 1000-2000 of the large object with X's0);
overwrite(conn, lobjOid, 1000, 1000);
*/
printf("exporting large object to file
/* exportFile(conn, lobjOid, out_filename); */
lo_export(conn, lobjOid,out_filename);
res = PQexec(conn, "end");
PQclear(res);
PQfinish(conn);
exit(0);
}