Is it good practice to free a NULL pointer in C? [duplicate]

Answer 1

Quoting the C standard, 7.20.3.2/2 from ISO-IEC 9899:

void free(void *ptr);

If ptr is a null pointer, no action occurs.

Don't check for NULL, it only adds more dummy code to read and is thus a bad practice.

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However, you must always check for NULL pointers when using malloc & co. In that case NULL mean that something went wrong, most likely that no memory was available.

Answer 2

In my opinion, no, at least not in your case.

If you couldn't allocate memory, you should have checked that WAY before the call of free.

Answer 3

See http://linux.die.net/man/3/free which states:

If ptr is NULL, no operation is performed.

Answer 4

It is good practice to not bother checking for NULL before calling free. Checking just adds unnecessary clutter to your code, and free(NULL) is guaranteed to be safe. From section 7.20.3.2/2 of the C99 standard:

The free function causes the space pointed to by ptr to be deallocated, that is, made available for further allocation. If ptr is a null pointer, no action occurs.

As noted in the comments, some people sometimes wonder if checking for NULL is more efficient than making a possibly unnecessary function call. However, this:

• Is a premature micro-optimization.
• Shouldn't matter. Checking for NULL first might be a pessimization. For example, if 99% of the time your pointers aren't NULL, then there would be an extra NULL check 99% of the time to avoid an extra function call 1% of the time.