Questions on use of malloc() and free()

Question

Thanks for notice my question.

In C Primer Plus, it first writes

The argument to free() should be a pointer to a block of memory allocated by malloc(); you can’t use free() to free memory allocated by other means

which means ONE malloc(), ONE and ONLY one free().

But later it goes

It’s okay to use a different pointer variable with free() than with malloc(); what must agree are the addresses stored in the pointers.

which seems contradict to the first statement.

So my understanding is that as long as a pair of malloc() and free() share the same address there is no error, and the name of pointers doesn't matter. Am I right?

Answer 1

For example:

void* p = malloc (100);
void* q = p;
free (q);

...is fine. The argument for free () is the value that was returned by malloc. The sentence

"It’s okay to use a different pointer variable with free() than with malloc()"

is actually pointless and just creates confusion - of course it is fine to use different variables as long as the value is the same.