How to check if void pointer points to NULL?

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.

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;
}

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).

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);