Free a null pointer anyway or check first?

Question

Suppose I came across an instance in a program where I would either free a NULL pointer, or first check whether it was NULL and skip the free() function call.

Would it be more efficient to simply free a NULL pointer? I searched around a bit and apparently, for implementations post C89, it is harmless to free a NULL pointer - so the decision comes down to efficiency.

My presumption is that there may potentially entail quite a bit of overhead when calling free(). As such, perhaps the simple logical check before calling the free() function is quite necessary.

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tl;dr version,

What is happening internally when a call to free() is made that may make it more or less efficient to first check whether or pointer is NULL before freeing?

Answer 1

The C standard guarantees that calling free(NULL) is harmless and has no effect. So, unless you believe that calling free() on a NULL pointer indicates that you've got a logic error elsewhere in your program, there's no reason to double-check that.