31.5. Example Program
Example 31.1 is a sample program which shows how the large object interface inlibpq 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 insrc/test/examples/testlo.c in the source distribution.
Example 31.1. Large Objects withlibpq Example Program
/*------------------------------------------------------------------------- * * testlo.c * test using large objects with libpq * * Portions Copyright (c) 1996-2016, PostgreSQL Global Development Group * Portions Copyright (c) 1994, Regents of the University of California * Portions Copyright (c) 2016-2021, Postgres Professional * * * 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 OidimportFile(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 voidpickout(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 voidoverwrite(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 voidexportFile(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); return;}static voidexit_nicely(PGconn *conn){ PQfinish(conn); exit(1);}intmain(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, "Connection to database failed: %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;}