In C, why is NULL and 0 triggering an if statement

Question

I have a function that is called by

myStruct *structName = myFunction(0);

The function looks like

myStruct *myfunction(int x)
{

    if ( x == NULL)
    {
        return NULL;
    }

/*rest of code*/
}

Passing NULL or 0 BOTH trigger the if (x== NULL) statement and I can't figure out why. I need to return from the function on NULL but continue if the value is 0. Any change made can only be made within the function itself.

Answer 1

In C, NULL is a macro that expands either to 0 or (void*)0 (or something that has a similar effect).

In the first case, you can not differentiate between NULL and 0, because they are literally the same.
In the second case, your code will cause a compile error, because you can't compare an integer variable with a pointer.

Answer 2

First some background ...

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The macros are NULL which expands to an implementation-defined null pointer constant; C11 §7.19 3

NULL typically is an integer constant 0 or (void*)0 or the like. It may have a different implementation or type - It could be ((int*) 0xDEADBEEF) as strange as that may be.

NULL might be type int. It might be type void * or something else. The type of NULL is not defined.

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When the null pointer constant NULL is cast to any pointer, is is a null pointer. An integer 0 cast to a pointer is also a null pointer. A system could have many different (bit-wise) null pointers. They all compare equally to each other. They all compare unequally to any valid object/function. Recall this compare is done as pointers, not integers.

An integer constant expression with the value 0, or such an expression cast to type void *, is called a null pointer constant. If a null pointer constant is converted to a pointer type, the resulting pointer, called a null pointer, is guaranteed to compare unequal to a pointer to any object or function. C11 §6.3.2.3 3

int x;
if (&x == NULL) ... // this is false

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So after all that chapter and verse how to distinguish NULL from 0?

If the macro NULL is defined as an int 0 - it is game over - there is no difference between 0 and NULL.

If NULL is not an int, then code can use _Generic() to differentiate NULL and 0. This does not help OP's "Any change made can only be made within the function itself." requirement as that function accepts an int augment.

If NULL is an int that has a different bit-pattern than 0, then a simple memcmp() can differentiate.

I suspect the whole reason for this exercise is to realize there is no portable method to distinguish NULL from 0.

Answer 3

Chapter and verse:

6.3.2.3 Pointers
...
3     An integer constant expression with the value 0, or such an expression cast to type void *, is called a null pointer constant. ⁶⁶⁾ If a null pointer constant is converted to a pointer type, the resulting pointer, called a null pointer, is guaranteed to compare unequal to a pointer to any object or function.

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^{66) The macro NULL is defined in <stddef.h> (and other headers) as a null pointer constant; see 7.19}

In short, any 0-valued expression that appears in a pointer context is interpreted as NULL.