checking for NULL before calling free

Question

Many C code freeing pointers calls:

if (p)
  free(p);

But why? I thought C standard say the free function doesn\'t do anything

Answer 1

Compilers, even when inlining are not smart enough to know the function will return immediately. Pushing parameters etc on stack and setting the call up up is obviously more expensive than testing a pointer. I think it is always good practice to avoid execution of anything, even when that anything is a no-op. Testing for null is a good practice. An even better practice is to ensure your code does not reach this state and therefore eliminate the need for the test altogether.