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

if (p)
    free(p);

why another explicit check?

If I write something like that, it's to convey the specific knowledge that the pointer may be NULL...to assist in readability and code comprehension. Because it looks a bit weird to make that an assert:

assert(p || !p);
free(p);

(Beyond looking strange, compilers are known to complain about "condition always true" if you turn your warnings up in many such cases.)

So I see it as good practice, if it's not clear from the context.

The converse case, of a pointer being expected to be non null, is usually evident from the previous lines of code:

...
Unhinge_Widgets(p->widgets);
free(p); // why `assert(p)`...you just dereferenced it!
...

But if it's non-obvious, having the assert may be worth the characters typed.