What is the correct usage of realloc() when it fails and returns NULL?

Question

Can anyone summarize what is the correct usage of realloc()?

What do you do when realloc() fails?

From what I have seen so far, it seems that if realloc() fails, you have to free() old pointer. Is that true?

Here is an example:

   1.  char *ptr = malloc(sizeof(*ptr) * 50);
   2.  ...
   3.  char *new_ptr = realloc(ptr, sizeof(*new_ptr) * 60);
   4.  if (!new_ptr) {
   5.      free(ptr);
   6.      return NULL;
   7.  }

Suppose realloc() fails on line 3. Am I doing the right thing on line 5 by free()ing ptr?

Answer 1

From http://www.c-faq.com/malloc/realloc.html

If realloc cannot find enough space at all, it returns a null pointer, and leaves the previous region allocated.

Therefore you would indeed need to free the previously allocated memory still.

Answer 2

It depends on what you want to do. When realloc fails, what is it you want to do: free the old block or keep it alive and unchanged? If you want to free it, then free it.

Keep in mind also, that in C89/90 if you make a realloc request with zero target size, realloc function may return a null pointer even though the original memory was successfully deallocated. This was a defect in C89/90, since there was no way to tell the success from failure on null return.

In C99 this defect was fixed and the strict relationship between null return and success/failure of reallocation was guaranteed. In C99 null return always means total failure of realloc.