例 34.1是一个展示libpq中大对象接口如何使用的例子程序。部分程序被注释但仍保留在代码中，用户可以利用之。该程序可以在源代码的src/test/examples/testlo.c中找到。
例 34.1. 用libpq操作大对象的例子程序
/*-----------------------------------------------------------------
*
* testlo.c
* 测试通过libpq实用大对象
*
* Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
*
* 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 -
* 把文件 "in_filename" 作为一个大对象 "lobjOid" 载入到数据库
*
*/
static Oid
importFile(PGconn *conn, char *filename)
{
Oid lobjId;
int lobj_fd;
char buf[BUFSIZE];
int nbytes,
tmp;
int fd;
/*
* 打开要读入的文件
*/
fd = open(filename, O_RDONLY, 0666);
if (fd < 0)
{ /* error */
fprintf(stderr, "cannot open unix file\"%s\"\n", filename);
}
/*
* 创建大对象
*/
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);
/*
* 从该 Unix 文件读取并写入到大对象
*/
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 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, "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 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_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 -
* 把大对象 "lobjOid" 导出成文件 "out_filename"
*
*/
static void
exportFile(PGconn *conn, Oid lobjId, char *filename)
{
int lobj_fd;
char buf[BUFSIZE];
int nbytes,
tmp;
int fd;
/*
* 打开大对象
*/
lobj_fd = lo_open(conn, lobjId, INV_READ);
if (lobj_fd < 0)
fprintf(stderr, "cannot open large object %u", lobjId);
/*
* 打开要写入的文件
*/
fd = open(filename, O_CREAT | O_WRONLY | O_TRUNC, 0666);
if (fd < 0)
{ /* 错误 */
fprintf(stderr, "cannot open unix file\"%s\"",
filename);
}
/*
* 从大对象读入并写出到 Unix 文件
*/
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 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_filename\n",
argv[0]);
exit(1);
}
database = argv[1];
in_filename = argv[2];
out_filename = argv[3];
/*
* 设置连接
*/
conn = PQsetdb(NULL, NULL, NULL, NULL, database);
/* 检查看看后端连接是否成功建立 */
if (PQstatus(conn) != CONNECTION_OK)
{
fprintf(stderr, "Connection to database failed: %s",
PQerrorMessage(conn));
exit_nicely(conn);
}
/* 设置总是安全的搜索路径，这样恶意用户就无法取得控制权。 */
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;
}
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