Welcome to Software Development on Codidact!
Will you help us build our independent community of developers helping developers? We're small and trying to grow. We welcome questions about all aspects of software development, from design to code to QA and more. Got questions? Got answers? Got code you'd like someone to review? Please join us.
Are static pointers implicitly initialized to NULL?
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
-
Initializing or assigning a pointer to
0is equivalent to initializing or assigning it toNULL -
Static variables are initialized to zero unless explicitly initialized
-
NULLis not guaranteed to be a zero bit pattern, even though it's often is
1 answer
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:
-
NULLis a macro guaranteed to be a null pointer constant. It can only be0or(void*)0. The binary representation of this macro is very likely all zeroes. - The
NULLmacro 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.
0 comment threads
0 comment threads