NULL, NUL, EOF, '\0'区别

NULL is a macro defined in several standard headers, 0 is an integer constant, '\0' is a character constant, and nul is the name of the character constant. All of these are not interchangeable:
NULL is to be used for pointers only since it may be defined as ((void *)0), this would cause problems with anything but pointers.
0 can be used anywhere, it is the generic symbol for each type's zero value and the compiler will sort things out.
'\0' should be used only in a character context.
nul is not defined in C or C++, it shouldn't be used unless you define it yourself in a suitable manner, like:
#define nul '\0'

翻译一下就是：
NULL是一个宏，它在几个标准头文件中定义，0是一个整型常量，'\0'是一个字符常量，而NUL是一个字符常量的名字。这几个术语都不可互换。
1、NULL用于表示什么也不指向，也就是空指针((void *)0)
2、0可以被用于任何地方，它是表示各种类型零值的符号并且编译器会挑出它
3、'\0'应该只被用于结束字符串，其值等于0
4、NUL并没有在C和C++中定义，如果你要使用的话，需要自己定义，如：#define NUL '\0'
5、EOF：文件结束符，通常定义为-1（ASCII代码值的范围是0~255，不可能出现-1，因此可以用EOF作为文件结束标志）。EOF的表示方式：shell中为Ctrl+D，MS-DOS中为Ctrl+Z