C and pointer Chapter 3 semantic "trap" 3.5 null pointer is not a string

A null pointer is not an empty string
With one important exception , stay C Language converts an integer to a pointer , The final result depends on the specific C Compiler implementation . This exception is constant 0, The compiler guarantees that 0 The converted pointer is not equal to any valid pointer . For the sake of code documentation , constant 0 This value is often replaced by a symbol ： 
#define NULL 0
Of course, whether directly using constants 0, Still use symbols NULL, The effect is the same . The important thing to remember is , When constant 0 When converted to pointer , This pointer must not be dereferenced （dereference）. let me put it another way , Will be 0 When assigned to a pointer variable , We must not attempt to use the contents stored in the memory pointed to by this pointer .
The following writing is completely legal ： 
if (p == (char*)0) {
    ...
}
But if you want to write like this ： 
if (strcmp(p, (char*)0) == 0) { //illegal behavior
    ...
}
It's illegal , The reason is that library functions strcmp The implementation of will include an operation , Used to view the contents of memory pointed to by its pointer parameters . If p It's a null pointer , even to the extent that  
printf(p); //undefined behavior
and  
printf("%s", p); //undefined behavior
The behavior of is undefined . and , Similar statements will have different effects on different computers .