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
the section called “Library Functions”. For information about the large
object function interface, see Chapter 32, Large Objects.
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, “ECPG Program Accessing Large Objects” 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; }