> From: Jakob Bohm [mailto:jb-open...@wisemo.com]
> Sent: Tuesday, May 29, 2012 8:34 AM
> On 5/27/2012 2:29 AM, Jeremy Farrell wrote:
> >> From: Jakob Bohm [mailto:jb-open...@wisemo.com]
> >> On 5/25/2012 5:30 PM, Ken Goldman wrote:
> >>> On 5/25/2012 3:33 AM, Jakob Bohm wrote:
> >>>
> >>>> ANSI C and POSIX free() is NOT required to handle free(NULL)
> >>>> as a NOP.
> >>> I checked reputable sources (Plauger, Harbison and Steele, the ANSI
> >>> spec, and the IEEE POSIX spec).
> >>>
> >>> All agree that (e.g. ANSI)
> >>>
> >>> "If ptr is a null pointer, no action occurs."
> >>>
> >>>
> >> Which version of the ANSI Spec, and where did you get a copy?
> > I quoted from C99 in a recent message; can't remember where I got it
> (12 years ago ...), perhaps Techstreet. There are 'final drafts' of C99
> and C11 legally available for free on the web I believe.
> >
> > Various revisions of POSIX are available free for reference online;
> the current POSIX.1-2008 at
> http://pubs.opengroup.org/onlinepubs/9699919799/ and the older POSIX.1-
> 2004 at http://pubs.opengroup.org/onlinepubs/009695399/
> >
>
> Note that when considering portability, C99 is not yet
> fully implemented everywhere, so when I say "ANSI C"
> without qualification, I generally refer to C89 plus
> the two TR update documents that mostly focused on
> things many compilers were already doing.
>
> Having no current access to the C89 standard or its
> drafts, I am relying on the experience that many "C89"
> real world systems I have encountered did not tolerate
> free(NULL). In contrast all recent C++ specifications
> (starting with The C++ Programming Language 2nd Ed.
> which preceded all published standards), insist that
> delete (char*)NULL must be a safe NOP.
>
> As C99 adopted many other features from C++ (such as
> declarations between statements), the free(NULL)
> behavior might also be new in C99.
C99 was what I had most conveniently to hand, but as I mentioned originally the
requirement has been the same in all versions of Standard C from C89 through to
the current C11. C89 has:
4.10.3.2 The free function
Synopsis
#include <stdlib.h>
void free(void *ptr);
Description
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. Otherwise, if the argument does
not match a pointer earlier returned by the calloc , malloc , or
realloc function, or if the space has been deallocated by a call to
free or realloc , the behavior is undefined.
Returns
The free function returns no value.
The Rationale for C89 states "The null pointer is specified as a valid argument
to this function to reduce the need for special-case coding".
Other standards documents of the time (XPG3, for example) mention adding the
sentence on NULL for conformance with the draft C Standard, so it looks like
the ANSI C committee was the driver for explicitly requiring this behaviour; I
don't know if any earlier implementations worked that way, K&R 1 didn't mention
the free library function. I don't know C++, but Stroustrup's site says C++ 2
was written in '91, so he'd be assuming the C89 library as the basis.
I'm a bit surprised you've come across systems at all recently which don't
tolerate free of NULL. I've tended to assume they do without explicitly testing
it (I'm fool enough to rely blindly on standards for some things), but I would
usually only make such a call in unusual error paths; I guess it may come back
to bite me sometime.
Regards,
jjf
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