`fwrite` 在 `fread` 之后不能直接工作吗? [英] `fwrite` doesn't work directly after `fread`?

问题描述

我有一个程序，它使用 stdio 来读取和写入二进制文件.它缓存当前流位置，并且不会查找读/写偏移量是否已经在所需位置.

I have a program which uses stdio for reading and writing a binary file. It caches the current stream position and will not seek if the read/write offset is already at the desired position.

然而，一个有趣的问题出现了，当读取一个字节并写入下一个字节时，它实际上并没有被写入！

However, an interesting problem appears, that when a byte is read and the following byte is written, it doesn't actually get written!

这是一个重现问题的程序:

Here is a program to reproduce the problem:

#include <cstdio>

int main() {
    FILE *f = fopen("test.bin", "wb");
    unsigned char d[1024] = { 0 };
    fwrite(d, 1, 1024, f);
    fclose(f);
    f = fopen("test.bin", "rb+");
    for (size_t i = 0; i < 1024; i++) {
        unsigned char a[1] = { 255 - (unsigned char)(i) };
        fflush(f);
        fwrite(a, 1, 1, f);
        fflush(f);
        fseek(f, i, SEEK_SET);
        fread(a, 1, 1, f);
        printf("%02X ", a[0]);
    }
    fclose(f);
    return 0;
}

你应该看到它把字节FF写到00，但是只写了第一个字节，因为它没有跟随fread立即.

You are supposed to see it write the bytes FF down to 00, however only the first byte is written because it does not follow a fread immediately.

如果它在 fwrite 之前寻找，它的行为是正确的.

If it seeks before fwrite, it acts correctly.

该问题发生在 Visual Studio 2010/2012 和 TDM-GCC 4.7.1 (Windows) 上，但是它适用于 codepad 我猜是因为它是在 Linux 上执行的.

The problem happens on Visual Studio 2010/2012 and TDM-GCC 4.7.1 (Windows), however it works on codepad which I guess is due to it being executed on Linux.

知道为什么会这样吗?

推荐答案

C99 §7.18.5.3/6(引自 N869 最终草案):

C99 §7.18.5.3/6 (quoted from N869 final draft):

“当文件以更新模式打开时('+'作为第二个或第三个字符上面的模式参数值列表)[…] 输入不应直接跟随输出，而没有对文件定位函数的干预调用，除非输入操作遇到结束文件.”

“When a file is opened with update mode (’+’ as the second or third character in the above list of mode argument values) […] input shall not be directly followed by output without an intervening call to a file positioning function, unless the input operation encounters end- of-file.”

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