这个如此诡异的C语言「怪事」是怎么回事？ 第1页

把fopen的参数从"w"换成"wb"，你要写字节流，就要以二进制方式访问。

以文本模式的话，就会给你转换。

原因的话，看这里：

In text mode, CTRL+Z is interpreted as an EOF character on input. In files that are opened for reading/writing by using "a+", fopen checks for a CTRL+Z at the end of the file and removes it, if it is possible. This is done because using fseek and ftell to move within a file that ends with CTRL+Z may cause fseek to behave incorrectly near the end of the file.
In text mode, carriage return-line feed combinations are translated into single line feeds on input, and line feed characters are translated to carriage return-line feed combinations on output. When a Unicode stream-I/O function operates in text mode (the default), the source or destination stream is assumed to be a sequence of multibyte characters. Therefore, the Unicode stream-input functions convert multibyte characters to wide characters (as if by a call to the mbtowc function). For the same reason, the Unicode stream-output functions convert wide characters to multibyte characters (as if by a call to the wctomb function).
If t or b is not given in mode, the default translation mode is defined by the global variable _fmode. If t or b is prefixed to the argument, the function fails and returns NULL.

注意整句加粗的那个。这不是错误，这是你没了解fopen这个函数的参数含义。