33.12. Large Objects
Large objects are not directly supported by ECPG, but ECPG
application can manipulate large objects through the libpq large
object functions, obtaining the necessary PGconn object by calling the ECPGget_PGconn() function. (However, use of the
ECPGget_PGconn() function and
touching PGconn objects directly should be
done very carefully and ideally not mixed with other ECPG
database access calls.)
For more details about the ECPGget_PGconn(), see Section 33.11. For information about the
large object function interface, see Глава 32.
Large object functions have to be called in a transaction block, so when autocommit is off, BEGIN commands have to be issued explicitly.
Example 33-2 shows an example program that illustrates how to create, write, and read a large object in an ECPG application.
Example 33-2. ECPG Program Accessing Large Objects
#include <stdio.h>
#include <stdlib.h>
#include <libpq-fe.h>
#include <libpq/libpq-fs.h>
EXEC SQL WHENEVER SQLERROR STOP;
int
main(void)
{
PGconn *conn;
Oid loid;
int fd;
char buf[256];
int buflen = 256;
char buf2[256];
int rc;
memset(buf, 1, buflen);
EXEC SQL CONNECT TO testdb AS con1;
conn = ECPGget_PGconn("con1");
printf("conn = %p\n", conn);
/* create */
loid = lo_create(conn, 0);
if (loid < 0)
printf("lo_create() failed: %s", PQerrorMessage(conn));
printf("loid = %d\n", loid);
/* write test */
fd = lo_open(conn, loid, INV_READ|INV_WRITE);
if (fd < 0)
printf("lo_open() failed: %s", PQerrorMessage(conn));
printf("fd = %d\n", fd);
rc = lo_write(conn, fd, buf, buflen);
if (rc < 0)
printf("lo_write() failed\n");
rc = lo_close(conn, fd);
if (rc < 0)
printf("lo_close() failed: %s", PQerrorMessage(conn));
/* read test */
fd = lo_open(conn, loid, INV_READ);
if (fd < 0)
printf("lo_open() failed: %s", PQerrorMessage(conn));
printf("fd = %d\n", fd);
rc = lo_read(conn, fd, buf2, buflen);
if (rc < 0)
printf("lo_read() failed\n");
rc = lo_close(conn, fd);
if (rc < 0)
printf("lo_close() failed: %s", PQerrorMessage(conn));
/* check */
rc = memcmp(buf, buf2, buflen);
printf("memcmp() = %d\n", rc);
/* cleanup */
rc = lo_unlink(conn, loid);
if (rc < 0)
printf("lo_unlink() failed: %s", PQerrorMessage(conn));
EXEC SQL COMMIT;
EXEC SQL DISCONNECT ALL;
return 0;
}