Why doesn't free(p) set p to NULL?

Question

Any reasons why this can not be standard behavior of free()?

multiple pointers pointing to the same object:

#include 
#i         


        

Answer 1

C function parameters are always passed by value, so in order to modify the pointer passed to free() you would need to pass a pointer to the pointer being deallocated, which can lead bugs caused by forgotten & operators.

Secondly, if that pointer had any aliases, the programmer would still be responsible for nulling them out. I could see problems caused by programmers assuming that all references were set to NULL.

Finally, it's not always necessary to set the pointer to NULL. Imagine a function which allocates some memory, does some work and frees it before returning. I could see how setting the pointer to NULL might not seem optimal.

Answer 2

If it did, you would have to pass a pointer to a pointer to the function:

int * p = malloc( sizeof( int ));
free( & p );

which I'm sure many people would get wrong.

Answer 3

With reference to the quote from Stroustrup about delete, Peter Norvig also makes a similar remark. He writes (not about C++!):

"Nothing is destroyed until it is replaced"
 - Auguste Comte (1798-1857) (on the need for revolutionary new
   theories (or on the need to do x.f = null in garbage-collected
   languages with destructors))

In my C code, I find the following macro very useful:

#define free(p) free((void *)(p)),(p)=NULL /* zero p */

This, as written, uses its argument twice. But this isn't a problem, as any usage such as free(p++) or free(find_named_object("foo")) will give a compile-time error (lvalue required). And you can hide the macro by using (free)(p++), or by calling it something else e.g. FREE.