NULL: 定义为0或0L或(void *)0，用于指示一个指针值是空，即什么都不指；
' ': 用且只用字符串结束符;
NUL : 0x00,0值字符，可以用于结束ASCII字符串，和' '类似，但是在c/c++中没有定义，如果要使用的话，需要自定义为 #define NUL ' ';
EOF ：通常定义为-1, 文件结束符标志，一般是ctrl+z.
1.0 can be used anywhere, it is the generic symbol for each type's zero value and the compiler will sort things out.
2.' ' should be used only in a character context.
3.NULL is to be used for pointers only since it may be defined as ((void *)0), this would cause problems with anything but pointers.
4.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 ' '