Does *(NULL) compile successfully by the C99 standard?

Question

I understand this code should never be executed but does *(NULL) compile successfully by the C99 standard ?

#include <stdio.h>
int main(void) {
    *(NULL);
    return 0;
}

PS: Running it on ideone returns 0 but I do not know the compiler version being used :(

Answer 1

*(NULL) is always a constraint violation. *(NULL) as a full expression is "not compilable" in C under any definition of NULL. NULL can be defined an integral zero, which is obviously not dereferencible. NULL can be defined as integral zero cast to void * type, but dereferencing void * pointers is illegal in C.

Note: I'm still not entirely sure whether my statement above about dereferencing void * pointer is correct. Is it a constraint violation or not? The standard does not spell it out explicitly. But at the same time it defines the behavior of unary * for pointers to functions and pointers to objects only. Meanwhile, void is not an object type.

If it is indeed legal to dereference void * pointers, then *(NULL) by itself would produce undefined behavior (assuming (void *) 0 definition of NULL). But there is still one context in which *(NULL) might be valid as a subexpression. It is &*(NULL). The language gives special treatment to &* combination, making it a no-op with well-defined behavior even in situations where * alone would produce undefined behavior.

Answer 2

If NULL is defined as simply 0 (which is permitted), then *(0) is a constraint violation and must be diagnosed. If NULL is defined as ((void *)0) then *((void *)0) invokes undefined behavior (by dereferencing a null pointer) and the compiler is not required to issue any diagnostic, but the program is also not required to behave the way you expect it to.

Answer 3

*(NULL) is undefined behavior.

From the horse mouth:

(C99, 6.5.3.2.p4) "If an invalid value has been assigned to the pointer, the behavior of the unary * operator is undefined.87)"

And (emphasis mine):

87): "Among the invalid values for dereferencing a pointer by the unary * operator are a null pointer, an address inappropriately aligned for the type of object pointed to, and the address of an object after the end of its lifetime."

And of course, NULL is a null pointer:

(C99, 7.17p3) "The macros [..] NULL which expands to an implementation-defined null pointer constant;"

As with all programs that invoke undefined behavior, a compiler has the right to not compile it (see C99, 3.4.3p2).