Free a null pointer anyway or check first?

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?

Don't check for nullness yourself. `free` already has to do that anyway and is guaranteed to do nothing when called with a null pointer. Simple as that.

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.

[The Open Group specification for `free()`](

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) states that:

> If `ptr` is a null pointer, no action shall occur.

This implies that the implementation of `free()` will probably begin with something to the effect of:

if (ptr == NULL)
return;

which will have very little overhead. No guarantees, but there isn't much work the function could do *before* it makes that check, so checking on your own is unnecessary.