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Q&A

Are static pointers implicitly initialized to NULL?

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Consider this code:

#include <stdio.h>

int main(void) {
    static void *ptr;
    if(ptr == NULL) puts("It's NULL!");
}

I wonder if this is guaranteed to print "It's NULL!"

I know that

  1. Initializing or assigning a pointer to 0 is equivalent to initializing or assigning it to NULL

  2. Static variables are initialized to zero unless explicitly initialized

  3. NULL is not guaranteed to be a zero bit pattern, even though it's often is

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1 answer

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Yes, it is guaranteed to evaluate to true. All variables with static storage duration are set to zero in case of arithmetic types or set to null in case they are pointers. The relevant part is C17 6.7.9/10:

If an object that has automatic storage duration is not initialized explicitly, its value is indeterminate. If an object that has static or thread storage duration is not initialized explicitly, then:

  • if it has pointer type, it is initialized to a null pointer;
  • if it has arithmetic type, it is initialized to (positive or unsigned) zero;
  • if it is an aggregate, every member is initialized (recursively) according to these rules, and any padding is initialized to zero bits;
  • if it is a union, the first named member is initialized (recursively) according to these rules, and any padding is initialized to zero bits;

To clarify NULL vs null pointers:

  • NULL is a macro guaranteed to be a null pointer constant. It can only be 0 or (void*)0. The binary representation of this macro is very likely all zeroes.
  • The NULL macro is not to be confused with a null pointer which is any pointer variable which has been assigned a null pointer constant. When such assignment happens, the pointer becomes a null pointer. This null pointer may in theory have an exotic, implementation-defined representation.
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