29.5. 例子程序

Example 29-1是一个例子程序, 显示如何使用libpq里面的大对象接口。 程序的一部分是注释掉的,但仍然保留在源码里面供读者参考。 这个程序可以在源码发布的 src/test/examples/testlo.c里找到.

Example 29-1. libpq 的大对象例子程序

/*--------------------------------------------------------------
 *
 * testlo.c--
 *    test using large objects with libpq
 *
 * Copyright (c) 1994, Regents of the University of California
 *
 *--------------------------------------------------------------
 */
#include <stdio.h>
#include "libpq-fe.h"
#include "libpq/libpq-fs.h"
#define BUFSIZE          1024
/*
 * importFile *    把文件 "in_filename" 以大对象 "lob jOid" 的形式输入到u乐平台登录注册库中
 *
 */
Oid
importFile(PGconn *conn, char *filename)
{
    Oid         lobjId;
    int         lobj_fd;
    char        buf[BUFSIZE];
    int         nbytes,
                tmp;
    int         fd;
    /*
     * 打开该文件读
     */
    fd = open(filename, O_RDONLY, 0666);
    if (fd < 0)
    {                           /* 错误 */
        fprintf(stderr, "can't open unix file %s\n", filename);
    }
    /*
     * 创建大对象
     */
    lobjId = lo_creat(conn, INV_READ | INV_WRITE);
    if (lobjId == 0)
        fprintf(stderr, "can't create large object\n");
    lobj_fd = lo_open(conn, lobjId, INV_WRITE);
    /*
     * 从 Unix 文件中读取并写入到转置文件中
     */
    while ((nbytes = read(fd, buf, BUFSIZE)) > 0)
    {
        tmp = lo_write(conn, lobj_fd, buf, nbytes);
        if (tmp < nbytes)
            fprintf(stderr, "error while reading large object\n");
    }
    (void) close(fd);
    (void) lo_close(conn, lobj_fd);
    return lobjId;
}
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, "can't open large object %d\n",
                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] = ' ';
        fprintf(stderr, ">>> %s", buf);
        nread += nbytes;
    }
    free(buf);
    fprintf(stderr, "\n");
    lo_close(conn, lobj_fd);
}
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_READ);
    if (lobj_fd < 0)
    {
        fprintf(stderr, "can't open large object %d\n",
                lobjId);
    }
    lo_lseek(conn, lobj_fd, start, SEEK_SET);
    buf = malloc(len + 1);
    for (i = 0; i < len; i++)
        buf[i] = 'X';
    buf[i] = ' ';
    nwritten = 0;
    while (len - nwritten > 0)
    {
        nbytes = lo_write(conn, lobj_fd, buf + nwritten, len - nwritten);
        nwritten += nbytes;
    }
    free(buf);
    fprintf(stderr, "\n");
    lo_close(conn, lobj_fd);
}
/*
 * exportFile *    把大对象 "lobjOid" 输出到文件 "out_filename"
 *
 */
void
exportFile(PGconn *conn, Oid lobjId, char *filename)
{
    int         lobj_fd;
    char        buf[BUFSIZE];
    int         nbytes,
                tmp;
    int         fd;
    /*
     * 创建一个转置 "object"
     */
    lobj_fd = lo_open(conn, lobjId, INV_READ);
    if (lobj_fd < 0)
    {
        fprintf(stderr, "can't open large object %d\n",
                lobjId);
    }
    /*
     * 打开文件写
     */
    fd = open(filename, O_CREAT | O_WRONLY, 0666);
    if (fd < 0)
    {                           /* 错误 */
        fprintf(stderr, "can't open unix file %s\n",
                filename);
    }
    /*
     * 从大对象中读取并且写入 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\n",
                    filename);
        }
    }
    (void) lo_close(conn, lobj_fd);
    (void) close(fd);
    return;
}
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);
    /* check to see that the backend connection was successfully made */
    if (PQstatus(conn) == CONNECTION_BAD)
    {
        fprintf(stderr, "Connection to database '%s' failed.\n", database);
        fprintf(stderr, "%s", PQerrorMessage(conn));
        exit_nicely(conn);
    }
    res = PQexec(conn, "begin");
    PQclear(res);
    printf("importing file %s\n", in_filename);
/*  lobjOid = importFile(conn, in_filename); */
    lobjOid = lo_import(conn, in_filename);
/*
    printf("as large object %d.\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); */
    lo_export(conn, lobjOid, out_filename);
    res = PQexec(conn, "end");
    PQclear(res);
    PQfinish(conn);
    exit(0);
}