34.12. 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 necessaryPGconn object by calling theECPGget_PGconn() function. (However, use of theECPGget_PGconn() function and touchingPGconn objects directly should be done very carefully and ideally not mixed with other ECPG database access calls.)
For more details about theECPGget_PGconn(), seeSection 34.11. For information about the large object function interface, seeChapter 33.
Large object functions have to be called in a transaction block, so when autocommit is off,BEGIN commands have to be issued explicitly.
Example 34.2 shows an example program that illustrates how to create, write, and read a large object in an ECPG application.
Example 34.2. 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;intmain(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; EXEC SQL SELECT pg_catalog.set_config('search_path', '', false); EXEC SQL COMMIT; 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;}| Prev | Up | Next |
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