GCC支持void指针和函数指针的加减运算。void和函数的size为1。允许对void和函数类型执行sizeof，其返回1.
选项-Wpointer-arith产生一个警告，如果这些扩展被使用的话。
In GNU C, addition and subtraction operations are supported on pointers to void and on
pointers to functions. This is done by treating the size of a void or of a function as 1.
A consequence of this is that sizeof is also allowed on void and on function types, and
returns 1.
The option ‘-Wpointer-arith’ requests a warning if these extensions are used