C语言文件操作问题,gcc与VC 6.0编译结果不同

khb_gl

1 /*
* =====================================================================================
3 *
4 *       Filename:  test.c
5 *
6 *    Description:  对同一文件进行边写边读.
7 *                  反复写1个字节,然后往回读取出1个字节.
8 *
9 *        Version:  1.0
10 *        Created:  2009-05-27 16:16:07
11 *       Revision:  none
12 *       Compiler:  gcc
13 *
14 *         Author:  KuangHaiBin (khb.hnu(AT)gmail.com)
15 *        Company:  Hunan University
16 *
17 * =====================================================================================
18 */
19
20 #include "stdio.h"
21 #include "stdlib.h"
22 #include "errno.h"
23
24 int
25 main()
26 {
27   char data_writed = '0';
28   char data_readed = 0;
29   int length_bytes_readed = 0;
30
31   int   file_pos = 0; /*  文件指针偏移值 >0  */
32   char    *fp_file_name = "1.txt";
33   FILE    *fp = NULL;
34
35   fp    = fopen( fp_file_name, "wb+" );
36
37   if ( fp == NULL )
38     {
39       fprintf ( stderr, "couldn't open file '%s'; %s\n",
40                 fp_file_name, strerror(errno) );
41       exit (EXIT_FAILURE);
42     }
43
44   for (data_writed = '0' ; data_writed <= '9'; data_writed += 1)
45     {
46       /*fseek(fp, length_bytes_readed, SEEK_SET);*/
47
48       file_pos = ftell(fp); /*  获取当前文件指针偏移  */
49       printf("The file pointer is at byte %ld\n", file_pos);
50
51       fwrite(&data_writed, sizeof(char), 1, fp);
52
53       fseek(fp, length_bytes_readed, SEEK_SET); /*  回退文件指针  */
54       fread(&data_readed, sizeof(char), 1, fp);
55
56       length_bytes_readed += 1;
57       printf("data_readed = %c\n", data_readed);
58     }/*  end of for (data_writed = 'A'...)  */
59
60   if ( fclose(fp) == EOF )
61     {
62       fprintf ( stderr, "couldn't close file '%s'; %s\n",
63                 fp_file_name, strerror(errno) );
64       exit (EXIT_FAILURE);
65     }
66
67   return(0);
68 }

gcc结果: The file pointer is at byte 0 data_readed = 0 The file pointer is at byte 1 data_readed = 1 The file pointer is at byte 2 data_readed = 2 The file pointer is at byte 3 data_readed = 3 The file pointer is at byte 4 data_readed = 4 The file pointer is at byte 5 data_readed = 5 The file pointer is at byte 6 data_readed = 6 The file pointer is at byte 7 data_readed = 7 The file pointer is at byte 8 data_readed = 8 The file pointer is at byte 9 data_readed = 9

Vc 6结果: The file pointer is at byte 0 data_readed = 0 The file pointer is at byte 1 data_readed = 0 The file pointer is at byte 1 data_readed = 0 The file pointer is at byte 2 data_readed = 0 The file pointer is at byte 3 data_readed = 0 The file pointer is at byte 4 data_readed = 0 The file pointer is at byte 5 data_readed = 0 The file pointer is at byte 6 data_readed = 0 The file pointer is at byte 7 data_readed = 0 The file pointer is at byte 8 data_readed = 0

上面的代码中gcc的结果是正确的,vc的结果错误.
但是把代码中第46行代码注释去除,得到的结果是一样的.都正确.
第46行代码的作用是显示设置写文件指针位置.根据推算,是不需要设置的.
原因是什么呢?一直不解.

[ 本帖最后由 khb_gl 于 2009-5-27 17:29 编辑 ]

anthony1983

楼主想说明什么

khb_gl

原帖由 anthony1983 于 2009-5-27 17:06 发表
楼主想说明什么

第一次发帖,还不怎么熟悉

langue

首先挑个刺：
readed => read
writed => written

调试发现是 fwrite() 没有更新文件当前位置指针。

khb_gl

原帖由 langue 于 2009-5-27 17:49 发表
首先挑个刺：
readed => read
writed => written

调试发现是 fwrite() 没有更新文件当前位置指针。

这个刺挑得好

langue 一针见血.

问题就是:
在VC 6中为什么第二次调用fwrite()失败,返回0.而不是1.
而gcc却成功.

changsha

vc 2010没问题

皇家救星

不知道是bug还是规定　反正vc6.0里面的fwrite就是这样　fread后要fseek才能再fwrite

皇家救星

When the "r+", "w+", or "a+" access type is specified, both reading and writing are allowed (the file is said to be open for "update"). However, when you switch between reading and writing, there must be an intervening fflush, fsetpos, fseek, or rewind operation. The current position can be specified for the fsetpos or fseek operation, if desired.


这是msdn上的原文　

khb_gl

原帖由 皇家救星 于 2009-5-28 13:52 发表
When the "r+", "w+", or "a+" access type is specified, both reading and writing are allowed (the file is said to be open for "update". However, when you switch between reading and writing, there ...

非常感谢,明白罗.去实验一下.看来找答案还得从MSDN等官方文档下手