Not getting a NULL pointer error in C

Question

I was purposely trying to get a NULL pointer in C but I failed to do this. Here is what I was coding:

int main(void) {
  int x; //UNINITIALIZED so that it will represent "nothing"
  int *null_test = &x;
  printf("This should be null   %p \n", null_test);
}

But it gave me an address: 0x7fd96c300020 which doesnt seem to be NULL as I was hoping.

Why? I didn't even initializex but still not null?

int x defines an int. An int cannot be NULL by definition. Just a pointer can be NULL. — alk
– alk, Commented Jan 16, 2017 at 7:13
@LudvigRydahl x is not a pointer. How could it point anywhere? It has a specific location, (probably on the stack) which of course can contain garbage. But the value of x is not read anywhere. Neither directly, nor via null_test. Is taking the address without reading from it undefined behaviour? — Gerhardh
– Gerhardh, Commented Jan 16, 2017 at 7:30
The address of a valid variable is always a valid pointer, so null_test points to a valid memory address. — xaxxon
– xaxxon, Commented Jan 16, 2017 at 7:31

Jabberwocky · Accepted Answer · 2017-01-16 07:11:35Z

9

After

int *null_test = &x;

null_test contains the address of x. This address is independent of the content of x even if x has never been initialized.

answered Jan 16, 2017 at 7:11

Jabberwocky

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Comments

tdao · Accepted Answer · 2017-01-16 07:11:18Z

5

Why? I didn't even initializex but still not null?

You need to differentiate between 1) definition and 2) initialisation. A variable is defined when the memory is allocated to it. A variable is initialised when that memory location is filled in with some value.

Here x is uninitialised but already defined - that means being allocated - that means x has a specific location in memory. And so &x is not NULL and so null_test:

int *null_test = &x;

If you want null_test to be NULL, just assign it explicitly

int *null_test = NULL;

edited Jan 16, 2017 at 7:11

answered Jan 16, 2017 at 7:09

tdao

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