Re: Does ANSI C allow free() to always be empty?

From	Nick Bowler <nbowler@draconx.ca>
Newsgroups	comp.lang.c
Subject	Re: Does ANSI C allow free() to always be empty?
Date	2014-03-10 15:56 +0000
Organization	A noiseless patient Spider
Message-ID	<lfknbn$uft$1@dont-email.me> (permalink)
References	<401aa474-05e6-4cf4-a3c2-5e2c2343157e@googlegroups.com> <20140308224204.214@kylheku.com>

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On Sun, 09 Mar 2014 06:55:27 +0000, Kaz Kylheku wrote:
> On 2014-03-09, partremmaps@gmail.com <partremmaps@gmail.com> wrote:
>> According to the ANSI C Standard, may free() be an empty function?
> 
> Of course, this is an empty function:
> 
>   void free(void *ptr) { }
> 
> But just to be sure, that is what you mean, right?
> 
> I believe the answer is yes.
> 
>> A couple of people who are in the daily habit of giving C advice on
>> IRC told me that even in these implementations and situations, free()
>> may be an empty function and do nothing and still be standards
>> compliant.
> 
> This is correct. To require free to actually liberate memory would
> mean that an implementation cannot turn free into a no-op and provide
> garbage collection.

Unfortunately, at any point during the execution of a C program it
is, in general, undecidable whether or not any region of memory is
"unreferenced"[1].  This implies that no conforming C implementation
can provide any sort of useful garbage collection for malloc.  The
garbage collectors for C necessarily require the programmer to not
do various silly things with pointers (such as encrypting them, or
receiving them as input from the user) that are technically allowed
by the language.

[1] "Unreferenced" here means the program cannot construct a valid
    pointer to that memory by any sequence of well-defined operations.

Thread

Does ANSI C allow free() to always be empty? partremmaps@gmail.com - 2014-03-08 22:08 -0800
  Re: Does ANSI C allow free() to always be empty? Kaz Kylheku <kaz@kylheku.com> - 2014-03-09 06:55 +0000
    Re: Does ANSI C allow free() to always be empty? David Brown <david.brown@hesbynett.no> - 2014-03-10 00:47 +0100
      Re: Does ANSI C allow free() to always be empty? partremmaps@gmail.com - 2014-03-09 22:37 -0700
        Re: Does ANSI C allow free() to always be empty? David Brown <david.brown@hesbynett.no> - 2014-03-10 10:20 +0100
          Re: Does ANSI C allow free() to always be empty? Keith Thompson <kst-u@mib.org> - 2014-03-10 08:52 -0700
          Re: Does ANSI C allow free() to always be empty? partremmaps@gmail.com - 2014-03-10 20:51 -0700
            Re: Does ANSI C allow free() to always be empty? Robert Wessel <robertwessel2@yahoo.com> - 2014-03-11 01:08 -0500
            Re: Does ANSI C allow free() to always be empty? David Brown <david.brown@hesbynett.no> - 2014-03-11 12:52 +0100
              Re: Does ANSI C allow free() to always be empty? partremmaps@gmail.com - 2014-03-12 00:09 -0700
                Re: Does ANSI C allow free() to always be empty? David Brown <david.brown@hesbynett.no> - 2014-03-12 12:59 +0100
      Re: Does ANSI C allow free() to always be empty? Kaz Kylheku <kaz@kylheku.com> - 2014-03-10 09:01 +0000
    Re: Does ANSI C allow free() to always be empty? Nick Bowler <nbowler@draconx.ca> - 2014-03-10 15:56 +0000
  Re: Does ANSI C allow free() to always be empty? partremmaps@gmail.com - 2014-03-09 01:11 -0800
    Re: Does ANSI C allow free() to always be empty? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2014-03-09 12:39 +0000
      Re: Does ANSI C allow free() to always be empty? gazelle@shell.xmission.com (Kenny McCormack) - 2014-03-09 13:05 +0000
      Re: Does ANSI C allow free() to always be empty? partremmaps@gmail.com - 2014-03-09 22:28 -0700
        Re: Does ANSI C allow free() to always be empty? Martin Shobe <martin.shobe@yahoo.com> - 2014-03-10 07:27 -0500
          Re: Does ANSI C allow free() to always be empty? partremmaps@gmail.com - 2014-03-10 20:55 -0700
            Re: Does ANSI C allow free() to always be empty? Martin Shobe <martin.shobe@yahoo.com> - 2014-03-11 08:19 -0500
              Re: Does ANSI C allow free() to always be empty? partremmaps@gmail.com - 2014-03-11 23:22 -0700
                Re: Does ANSI C allow free() to always be empty? Martin Shobe <martin.shobe@yahoo.com> - 2014-03-12 08:09 -0500
                Re: Does ANSI C allow free() to always be empty? James Kuyper <jameskuyper@verizon.net> - 2014-03-12 11:29 -0400
      Re: Does ANSI C allow free() to always be empty? Martin Shobe <martin.shobe@yahoo.com> - 2014-03-10 07:29 -0500
  Re: Does ANSI C allow free() to always be empty? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-03-09 09:14 +0000
    Re: Does ANSI C allow free() to always be empty? Robert Wessel <robertwessel2@yahoo.com> - 2014-03-10 15:33 -0500
  Re: Does ANSI C allow free() to always be empty? partremmaps@gmail.com - 2014-03-09 23:05 -0700
    Re: Does ANSI C allow free() to always be empty? Martin Shobe <martin.shobe@yahoo.com> - 2014-03-10 07:43 -0500
      Re: Does ANSI C allow free() to always be empty? gordonb.3qcze@burditt.org (Gordon Burditt) - 2014-03-10 12:05 -0500
        Re: Does ANSI C allow free() to always be empty? partremmaps@gmail.com - 2014-03-10 20:16 -0700
        Re: Does ANSI C allow free() to always be empty? Tim Rentsch <txr@alumni.caltech.edu> - 2014-03-29 16:27 -0700
      Re: Does ANSI C allow free() to always be empty? partremmaps@gmail.com - 2014-03-10 21:48 -0700
        Re: Does ANSI C allow free() to always be empty? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-03-11 05:52 +0000
  Re: Does ANSI C allow free() to always be empty? partremmaps@gmail.com - 2014-03-20 19:14 -0700
    Re: Does ANSI C allow free() to always be empty? Kaz Kylheku <kaz@kylheku.com> - 2014-03-21 03:25 +0000
      Re: Does ANSI C allow free() to always be empty? Richard Damon <Richard@Damon-Family.org> - 2014-03-21 20:58 -0400
        Re: Does ANSI C allow free() to always be empty? Tim Rentsch <txr@alumni.caltech.edu> - 2014-03-29 16:11 -0700
          Re: Does ANSI C allow free() to always be empty? Richard Damon <Richard@Damon-Family.org> - 2014-03-29 22:27 -0400
            Re: Does ANSI C allow free() to always be empty? Tim Rentsch <txr@alumni.caltech.edu> - 2014-03-30 13:18 -0700
              Re: Does ANSI C allow free() to always be empty? Richard Damon <Richard@Damon-Family.org> - 2014-03-30 20:49 -0400
                Re: Does ANSI C allow free() to always be empty? Tim Rentsch <txr@alumni.caltech.edu> - 2014-04-17 10:52 -0700
            Re: Does ANSI C allow free() to always be empty? James Kuyper <jameskuyper@verizon.net> - 2014-03-31 11:18 -0400
    Re: Does ANSI C allow free() to always be empty? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-03-21 03:31 +0000
    Re: Does ANSI C allow free() to always be empty? Tim Rentsch <txr@alumni.caltech.edu> - 2014-03-29 15:39 -0700
      Re: Does ANSI C allow free() to always be empty? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-03-30 04:22 +0000

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