之前一直用fread和fwrite，但是一直没有认真看过manual page。这次看apue才发现，自己之前有个误解。
std io 操作
size_t fwrite(const void *BUF, size_t SIZE,
size_t COUNT, FILE *FP);
If `fwrite' succeeds in writing all the elements you specify, the
result is the same as the argument COUNT. In any event, the
result is the number of complete elements that `fwrite' copied to
the file.
从manual page可以看出，fwrite返回的是写入的个数COUNT，而不是SIZE*COUNT。之前一直误会了fwrite。
现在记下来以免再犯。
size_t fread(void *BUF, size_t SIZE, size_t COUNT,
FILE *FP);
fread返回值与fwrite同理。
The result of `fread' is the number of elements it succeeded in
reading.