Question

So, I have some function returning a pointer, or NULL on error. Now I’d like to add another possible value for a different error/condition. I heard this mentioned in some other answer before – using malloc() to create a unique pointer that will serve as a possible value for such functions to return (so now the can return a proper pointer, NULL or 0xWhatever and you can be sure 0xWhatever won’t be used for anything else). So, naturally malloc(1) is probably a safe bet, but I was wondering if malloc(0) is also safe for this. Will a malloc(0) address possibly be used for something else? Can someone clarify on how this technique should work in general, and maybe what it is called?

Answer 1

From the C99 standard:

If the size of the space requested is zero, the behavior is implementation-defined: either a null pointer is returned, or the behavior is as if the size were some nonzero value, except that the returned pointer shall not be used to access an object.

You’re better off using malloc(1). That’s guaranteed to return non-NULL if the memory is available.