处理数据时该注意的

signed char 保证至少 8 位，int 保证至少 16 位，long 保证至少 32 位，long long 保证至少 64 位。

The only things guaranteed about integer types are:

1. sizeof(char) == 1
2. sizeof(char) <= sizeof(short)
3. sizeof(short) <= sizeof(int)
4. sizeof(int) <= sizeof(long)
5. sizeof(long) <= sizeof(long long)
6. sizeof(char) * CHAR_BIT >= 8
7. sizeof(short) * CHAR_BIT >= 16
8. sizeof(int) * CHAR_BIT >= 16
9. sizeof(long) * CHAR_BIT >= 32
10. sizeof(long long) * CHAR_BIT >= 64

The other things are implementation defined. Thanks to (4), both long and int can have the same size, but it must be at least 32 bits (thanks to (9)).

references:

http://stackoverflow.com/questions/13398630/why-are-c-int-and-long-types-both-4-bytes