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

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.

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 even might be a pessimization. For example, if 99% of the time your pointers aren't NULL, then there would be a redundant NULL check 99% of the time to avoid an extra function call 1% of the time.

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

If ptr is NULL, no operation is performed.