在C中为指针分配0 vs NULL

Question

I wondered today about that it is possible to assign an integer 0 to a pointer, ie doing the same like assigning NULL. Until today I assumed, this was new with C++. Has this been a feature of C from the beginning or was it relaxed with a more recent language-standard?

Answer 1

Per The C Programming Language by Kernighan and Ritchie, 1978, page 192:

However, it is guaranteed that assignment of the constant 0 to a pointer will produce a null pointer distinguishable from a pointer to any object.

I cannot speak to the history before that.

Answer 2

According to the C Standard (6.3.2.3 Pointers)

3 An integer constant expression with the value 0 , or such an expression cast to type void *, is called a null pointer constant .66) If a null pointer constant is converted to a pointer type, the resulting pointer, called a null pointer, is guaranteed to compare unequal to a pointer to any object or function.

Answer 3

In K&R first edition (pre-ANSI), page 97-98 we have some example code with:

#define NULL 0 /* pointer value for error report */

"C guarantees that no pointer that validly points at data will contain zero, so a return value of zero can be used to signal an abnormal event... We write NULL instead of zero, however, to indicate more clearly that this is a special value for a pointer. In general, integers cannot meaningfully be assigned to pointers."

Clearly here 0 , and therefore NULL , is not typed as a pointer but an int . It's worth noting that void * was not introduced until edition 2 (ANSI).