C语言中的空指针、空指针常量、NULL & 0
什么是空指针常量（null pointer constant）?[6.3.2.3-3] An integer constant expression with the value 0, orsuch an expression cast to type void *, is called a null pointerconstant.这里告诉我们：0、0L、'\0'、3 - 3、0 ...
