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From: diamond@csl.sony.co.jp (Norman Diamond)
Newsgroups: comp.lang.c
Subject: Re: effect of free()
Message-ID: <10817@riks.csl.sony.co.jp>
Date: 8 Sep 89 05:42:06 GMT
References: <319@cubmol.BIO.COLUMBIA.EDU> <3756@buengc.BU.EDU> <1989Aug17.005548.745@twwells.com> <16022@vail.ICO.ISC.COM> <248@seti.inria.fr>
Reply-To: diamond@riks. (Norman Diamond)
Organization: Sony Computer Science Laboratory Inc., Tokyo, Japan
Lines: 45

bill@twwells.com (T. William Wells) writes:

>>> ...For example, the following code fragment is nonportable:
>>> 	char    *ptr;
>>> 	ptr = malloc(1);
>>> 	...
>>> 	free(ptr);
>>> 	...
>>> 	if (ptr == 0) ...
>>> The if might cause a trap when the value of ptr is accessed.

In article <16022@vail.ICO.ISC.COM> rcd@ico.ISC.COM (Dick Dunn) writes:

>>Not true.  The "if" only examines the value of the pointer, not what it
>>points to.

In article <248@seti.inria.fr> jourdan@minos.inria.fr (Martin Jourdan) writes:

>You're wrong, Dick....  Imagine a segmented memory architecture with
>protections, and imagine that the call to "free" above frees the last
>used block in the segment ...
>The morale of the whole story is: DO NOT DO ANYTHING WITH A FREED
>POINTER....

In fact the block cannot be freed by "free."  It can be freed by the
next malloc/calloc, and might be freed by the next realloc.  However,
if there are no *alloc calls between the free and the if, it has to
work.

Why?

Well, because a freed pointer is still valid in a call to realloc
(as long as there are no intervening *alloc calls), and the data still
have to be sitting in limbo, not really quite freed yet.  (Much to the
chagrin of efficiency experts everywhere.)

To cover all cases though:  IT IS MADNESS TO DO ANYTHING WITH A FREED
POINTER.

--
-- 
Norman Diamond, Sony Corporation (diamond@ws.sony.junet)
  The above opinions are inherited by your machine's init process (pid 1),
  after being disowned and orphaned.  However, if you see this at Waterloo or
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