How to check if void pointer points to NULL?

Question

So if you do:

void *ptr = NULL;

What is the best way to check if that void pointer is NULL?

My workaround for now is this:

if (*(void**)ptr == NULL) ...

But this doesn't seem like the best way, as I'm implicitly assuming ptr is of type void** (which it isn't).

Read: [Can I use `if(pointer)` instead of `if(pointer != NULL)`?](http://stackoverflow.com/questions/17772103/can-i-use-ifpointer-instead-of-ifpointer-null) — Grijesh Chauhan, Aug 18 '13 at 06:24
It is not clear what you are trying to do. Your `ptr` does not *point to* a null pointer, it *is* a null pointer. A null pointer does not point to any object, derefrrencing it is UB. — n. m. could be an AI, Aug 19 '13 at 02:51
If `ptr` is NULL, after casting it will still be NULL, and dereferencing it is undefined behavior. — Akib Azmain Turja, Dec 20 '20 at 13:20

score 14 · Accepted Answer · answered Aug 18 '13 at 05:52

14

I'd simply write if (!ptr).

NULL is basically just 0 and !0 is true.

answered Aug 18 '13 at 05:52

nmaier

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Not on all architectures, you will be writing non-portable code. It might have been the 286, when using segment addressing, where NULL != 0, but that was many decades ago. – fadedbee Aug 09 '14 at 11:02
3

I would compare ptr with NULL; like ptr == NULL. There is no guarantee that NULL is equal 0. – jacekmigacz Feb 21 '16 at 13:11
Is that approach advisable in the present world, modern machines? – Himanshu Shekhar Jan 21 '17 at 06:18
1

@jacekmigacz That makes no difference. `ptr == NULL` is same as `ptr == 0` is same as `!ptr`. Even if implementation uses NULL which is not all bits zero, it's still guaranteed that compiler will make all three above behave the same. – user694733 May 08 '17 at 07:04

score 6 · Answer 2 · answered Aug 18 '13 at 06:00

Be sure to include a definition of NULL.

   #include <stddef.h>

   void  *X = NULL;

   // do stuff

   if (X != NULL)  // etc.

If you include <stdio.h> and similar then stddef.h gets pulled in automatically.

Finally, it is interesting to look at the definition of NULL in stddef.h and by doing this you will see why your initial guess at what to do is interesting.

verbose · Answer 3 · 2017-02-03T10:20:58.283

A NULL pointer is a pointer that isn't pointing anywhere. Its value is typically defined in stddef.h as follows:

#define NULL ((void*) 0)

or

#define NULL 0

Since NULL is zero, an if statement to check whether a pointer is NULL is checking whether that pointer is zero. Hence if (ptr) evaluates to 1 when the pointer is not NULL, and conversely, if (!ptr) evaluates to 1 when the pointer is NULL.

Your approach if (*(void**)ptr == NULL) casts the void pointer as a pointer to a pointer, then attempts to dereference it. A dereferenced pointer-to-pointer yields a pointer, so it might seem like a valid approach. However, since ptr is NULL, when you dereference it, you are invoking undefined behavior.

It's a lot simpler to check if (ptr == NULL) or, using terse notation, if (!ptr).

@EricPostpischil you're right of course, and I've edited accordingly. Thanks for the correction. — verbose, Aug 19 '13 at 02:06

score 1 · Answer 4 · answered Aug 19 '13 at 03:39

If your code manages to compile when assigning void *ptr = NULL, then it stands to reason that a simple if statement to compare if it is NULL should suffice, particularly because NULL would have to be defined if the code can compile.

Example of sufficient way to check:

if(ptr==NULL)
{
    rest of code...
}

I wrote a little test program, compiled with gcc on linux, which works:

int main()
{
    void *ptr = NULL;
    if(ptr==NULL)
    {
        return 1;
    }
    return 0;
}

That couldn't compile without a `#include` for one of the several standard headers that define `NULL`. — Keith Thompson, Aug 19 '13 at 03:57

SajithP · Answer 5 · 2017-05-09T01:40:35.360

I know this is a bit old post, but wanted to add something that might be useful.

What I usually do is something like this,

This is my function.

void MyMethod(  const void* l_pData  ///< Source data
              , size_t l_nLen        /**< Number of bytes to convert */) {
    // Return if nothing is provided
    if (l_pData == NULL || ((const char*)(l_pData))[0] == '\0' || 0 == l_nLen) {
        return;
    }

    // Rest of the code
}

You can check for

 - Null data
 - Empty data
 - Invalid length

Following are validated

MyMethod("", 10);
MyMethod(" ", 10);
MyMethod(NULL, 10);
MyMethod("valid", 0);

This is C question. C doesn't have `reinterpret_cast`. – user694733 May 08 '17 at 07:05 — user694733, May 08 '17 at 07:05

How to check if void pointer points to NULL?

5 Answers5