FILE* 指针结尾不等于写入数据的大小

发布于 2024-07-06 03:07:05 字数 436 浏览 6 评论 0原文

非常简单地说，我有以下代码片段：

FILE* test = fopen("C:\\core.u", "w");
printf("Filepointer at: %d\n", ftell(test));
fwrite(data, size, 1, test);
printf("Written: %d bytes.\n", size);
fseek(test, 0, SEEK_END);
printf("Filepointer is now at %d.\n", ftell(test));
fclose(test);

它输出：

Filepointer at: 0
Written: 73105 bytes.
Filepointer is now at 74160.

这是为什么？为什么写入的字节数与文件指针不匹配？

原文

Very simply put, I have the following code snippet:

FILE* test = fopen("C:\\core.u", "w");
printf("Filepointer at: %d\n", ftell(test));
fwrite(data, size, 1, test);
printf("Written: %d bytes.\n", size);
fseek(test, 0, SEEK_END);
printf("Filepointer is now at %d.\n", ftell(test));
fclose(test);

and it outputs:

Filepointer at: 0
Written: 73105 bytes.
Filepointer is now at 74160.

Why is that? Why does the number of bytes written not match the file pointer?

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醉生梦死 2024-07-13 03:07:05

由于您以文本模式打开文件，因此它会将行尾标记（例如 LF）转换为 CR/LF。

如果您在 Windows 上运行，则很可能会出现这种情况（考虑到您的文件名以 "c:\" 开头）。

如果您以“wb”模式打开文件，我怀疑您会发现数字是相同的：

FILE* test = fopen("C:\\core.u", "wb");

C99 标准在 7.19.5.3 The fopen function 中有这样的说法：

参数模式指向一个字符串。如果字符串是以下之一，则文件是
以指定模式打开。否则，行为是未定义的。
r 打开文本文件进行读取
w 截断为零长度或创建用于写入的文本文件
a 附加；打开或创建文本文件以在文件末尾写入
rb 打开二进制文件进行读取
wb 截断为零长度或创建用于写入的二进制文件
ab 追加；打开或创建二进制文件以在文件末尾写入
r+ 打开文本文件进行更新（读取和写入）
w+ 截断至零长度或创建用于更新的文本文件
a+ 追加；打开或创建文本文件进行更新，在文件末尾写入
r+b 或 rb+ 打开二进制文件进行更新（读取和写入）
w+b 或 wb+ 截断至零长度或创建用于更新的二进制文件
a+b 或 ab+ 追加；打开或创建二进制文件进行更新，在文件末尾写入

您可以看到它们区分 w 和 wb。我不认为需要实现来区别对待这两者，但对二进制数据使用二进制模式通常更安全。

Since you're opening the file in text mode, it will convert end-of-line markers, such as LF, into CR/LF.

This is likely if you're running on Windows (and you probably are, given that your file name starts with "c:\").

If you open the file in "wb" mode, I suspect you'll find the numbers are identical:

FILE* test = fopen("C:\\core.u", "wb");

The C99 standard has this to say in 7.19.5.3 The fopen function:

The argument mode points to a string. If the string is one of the following, the file is
open in the indicated mode. Otherwise, the behaviour is undefined.
r open text file for reading
w truncate to zero length or create text file for writing
a append; open or create text file for writing at end-of-file
rb open binary file for reading
wb truncate to zero length or create binary file for writing
ab append; open or create binary file for writing at end-of-file
r+ open text file for update (reading and writing)
w+ truncate to zero length or create text file for update
a+ append; open or create text file for update, writing at end-of-file
r+b or rb+ open binary file for update (reading and writing)
w+b or wb+ truncate to zero length or create binary file for update
a+b or ab+ append; open or create binary file for update, writing at end-of-file

You can see they distinguish between w and wb. I don't believe an implementation is required to treat the two differently but it's usually safer to use binary mode for binary data.

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