effect of free()

[Alan J Rosenthal]

Perhaps I can shed a little light on this discussion (not that it's been
completely void of light so far, far from it).

The original code looked something like this:

    char *a, *b, *malloc();
    a = malloc(...);
    b = a;
    free(a);
    ... b ...

The poster asked if free(a) freed b.  Seemed to be thinking of a lisp-like
language where objects don't go away until the last pointer to them does.

I think the following is a good way to think of it, given that pointers are
quite abstract objects.  Suppose you tell me your work phone number, and I
write it down.  I go home and copy it into my address book.  I use the copy in
my address book when I telephone you, but for some reason leave the piece of
paper you wrote your number on in my wallet.  A month later I see you and you
say "the number's been changed, here give me that piece of paper" and you
correct it on the piece of paper.  But I go home and the old number's written
down in my address book, not crossed off or altered.  Obviously I can't phone
you using the number you've crossed out, but can I phone you using my previous
copy of the number, which was copied when the number was still valid?

Copying a pointer doesn't copy what it points to, any more than copying the
phone number duplicates the phone it's the number of.

ajr

p.s. of course, as well, since C is call-by-value, "free(a)" can't change the
value of `a' either, but I think the original poster understood that.