0 is an integer constant,
' ' is a character constant,
nul is the name of the character constant.
NULL is a macro defined in several standard headers,
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.
' ' 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 ' '
EOF ：通常定义为-1, 文件结束符标志，一般是ctrl+z.
关于EOF的详细介绍可以参考http://www.ruanyifeng.com/blog/2011/11/eof.html