Re: about NULL as a parameter to a libc function

From	James Kuyper <jameskuyper@verizon.net>
Newsgroups	comp.lang.c
Subject	Re: about NULL as a parameter to a libc function
Date	2011-10-28 16:45 -0400
Organization	A noiseless patient Spider
Message-ID	<4EAB1459.6030604@verizon.net> (permalink)
References	<14abde31-4573-45e7-bdaf-eb70b22741d7@z28g2000pro.googlegroups.com> <j8f16r$pgt$1@reader1.panix.com> <j8f2fn$vhu$1@dont-email.me>

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On 10/28/2011 04:13 PM, BartC wrote:
...
> When you use a function such as memcpy, you often have to check that each 
> operand is in fact not null:
> 
> if (A && B && N) memcpy(A, B, N);

Often? I just checked the most convenient body of code I had on hand,
consisting of 278 source code files with a total of 107238 lines, and
4017043 bytes of code. A large fraction of that code was written by me.
It contains 85 calls to memcpy(), and not a single instance where the
call was controlled directly by an if() based upon A, B, or N.

There were only two instances where the call was directly controlled by
an if(), but the if tested something other condition entirely - it
determined whether the data needed to be copied, not whether the
pointers were non-null or the count non-zero. Much more common was the
deliberate placement of the memcpy() call in a block that could only be
entered if the relevant pointers were non-null and the count was
non-zero; but that block existed for other reasons than making the
memcpy() safe, so putting the memcpy() call into that block did not
increase the number of conditions that needed to be tested.

> memcpy(A, B, N);
> 
> and know that no copying is done when any argument is null or zero? Having a 
> null source or destination is not necessarily an error (and if it is, you 
> check it conventionally).

Given how rarely I need this feature, I prefer the current situation,
where those who need it can implement it just by writing:

	if (A && B && N) memcpy(A, B, N);

If they need to do it frequently, they can even make it a macro or an
inline function.

Thread

about NULL as a parameter to a libc function gaoqiang <gaoqiangscut@gmail.com> - 2011-10-28 01:49 -0700
  Re: about NULL as a parameter to a libc function Mark Bluemel <mark_bluemel@pobox.com> - 2011-10-28 10:14 +0100
    Re: about NULL as a parameter to a libc function gaoqiang <gaoqiangscut@gmail.com> - 2011-10-28 03:22 -0700
      Re: about NULL as a parameter to a libc function Mark Bluemel <mark_bluemel@pobox.com> - 2011-10-28 11:29 +0100
        Re: about NULL as a parameter to a libc function Jorgen Grahn <grahn+nntp@snipabacken.se> - 2011-10-29 07:08 +0000
      Re: about NULL as a parameter to a libc function James Kuyper <jameskuyper@verizon.net> - 2011-10-28 07:05 -0400
      Re: about NULL as a parameter to a libc function Keith Thompson <kst-u@mib.org> - 2011-10-28 14:15 -0700
      Re: about NULL as a parameter to a libc function gordonb.4o15x@burditt.org (Gordon Burditt) - 2011-10-28 16:36 -0500
        Re: about NULL as a parameter to a libc function Robert Wessel <robertwessel2@yahoo.com> - 2011-10-28 17:30 -0500
          Re: about NULL as a parameter to a libc function jgharston <jgh@arcade.demon.co.uk> - 2011-10-31 10:40 -0700
      Re: about NULL as a parameter to a libc function Kaz Kylheku <kaz@kylheku.com> - 2011-10-28 22:32 +0000
      Re: about NULL as a parameter to a libc function Nick Keighley <nick_keighley_nospam@hotmail.com> - 2011-10-29 06:20 -0700
        Re: about NULL as a parameter to a libc function "BartC" <bc@freeuk.com> - 2011-10-29 15:40 +0100
          Re: about NULL as a parameter to a libc function Nick Keighley <nick_keighley_nospam@hotmail.com> - 2011-10-30 08:05 -0700
    Re: about NULL as a parameter to a libc function Keith Thompson <kst-u@mib.org> - 2011-10-28 14:13 -0700
      Re: about NULL as a parameter to a libc function Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2011-10-29 15:11 +0300
        Re: about NULL as a parameter to a libc function Nick Keighley <nick_keighley_nospam@hotmail.com> - 2011-10-29 06:41 -0700
        Re: about NULL as a parameter to a libc function Keith Thompson <kst-u@mib.org> - 2011-10-29 11:29 -0700
  Re: about NULL as a parameter to a libc function Malcolm McLean <malcolm.mclean5@btinternet.com> - 2011-10-28 02:36 -0700
    Re: about NULL as a parameter to a libc function Mike Manilone <mike@laptop.mike> - 2011-10-28 12:58 +0000
      Re: about NULL as a parameter to a libc function James Kuyper <jameskuyper@verizon.net> - 2011-10-28 11:00 -0400
        Re: about NULL as a parameter to a libc function Kenneth Brody <kenbrody@spamcop.net> - 2011-11-03 11:13 -0400
      Re: about NULL as a parameter to a libc function Nick Keighley <nick_keighley_nospam@hotmail.com> - 2011-10-29 06:37 -0700
      Re: about NULL as a parameter to a libc function Kenneth Brody <kenbrody@spamcop.net> - 2011-11-03 11:00 -0400
        Re: about NULL as a parameter to a libc function James Kuyper <jameskuyper@verizon.net> - 2011-11-03 12:54 -0400
          Re: about NULL as a parameter to a libc function Kaz Kylheku <kaz@kylheku.com> - 2011-11-03 17:08 +0000
            Re: about NULL as a parameter to a libc function James Kuyper <jameskuyper@verizon.net> - 2011-11-03 13:18 -0400
              Re: about NULL as a parameter to a libc function Kaz Kylheku <kaz@kylheku.com> - 2011-11-03 17:28 +0000
                Re: about NULL as a parameter to a libc function James Kuyper <jameskuyper@verizon.net> - 2011-11-03 13:48 -0400
                Re: about NULL as a parameter to a libc function Kaz Kylheku <kaz@kylheku.com> - 2011-11-03 17:55 +0000
                Re: about NULL as a parameter to a libc function James Kuyper <jameskuyper@verizon.net> - 2011-11-03 14:12 -0400
                Re: about NULL as a parameter to a libc function blp@cs.stanford.edu (Ben Pfaff) - 2011-11-03 11:22 -0700
                Re: about NULL as a parameter to a libc function James Kuyper <jameskuyper@verizon.net> - 2011-11-03 14:28 -0400
                Re: about NULL as a parameter to a libc function Kaz Kylheku <kaz@kylheku.com> - 2011-11-03 19:47 +0000
                Re: about NULL as a parameter to a libc function James Kuyper <jameskuyper@verizon.net> - 2011-11-03 16:19 -0400
                Re: about NULL as a parameter to a libc function Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-24 22:13 -0800
                Re: about NULL as a parameter to a libc function Ike Naar <ike@sverige.freeshell.org> - 2011-11-03 18:20 +0000
                Re: about NULL as a parameter to a libc function Ike Naar <ike@sverige.freeshell.org> - 2011-11-03 18:34 +0000
                Re: about NULL as a parameter to a libc function Kaz Kylheku <kaz@kylheku.com> - 2011-11-03 19:49 +0000
                Re: about NULL as a parameter to a libc function James Kuyper <jameskuyper@verizon.net> - 2011-11-03 16:21 -0400
                Re: about NULL as a parameter to a libc function Ike Naar <ike@iceland.freeshell.org> - 2011-11-03 22:19 +0000
                Re: about NULL as a parameter to a libc function Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-24 22:17 -0800
          Re: about NULL as a parameter to a libc function Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-24 21:19 -0800
    Re: about NULL as a parameter to a libc function Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-10-28 14:26 +0100
      Re: about NULL as a parameter to a libc function Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2011-10-28 12:46 -0600
      Re: about NULL as a parameter to a libc function Keith Thompson <kst-u@mib.org> - 2011-10-28 14:19 -0700
        Re: about NULL as a parameter to a libc function Keith Thompson <kst-u@mib.org> - 2011-10-28 16:37 -0700
  Re: about NULL as a parameter to a libc function James Kuyper <jameskuyper@verizon.net> - 2011-10-28 07:22 -0400
    Re: about NULL as a parameter to a libc function Kenneth Brody <kenbrody@spamcop.net> - 2011-11-03 10:40 -0400
      Re: about NULL as a parameter to a libc function Kaz Kylheku <kaz@kylheku.com> - 2011-11-03 15:10 +0000
      Re: about NULL as a parameter to a libc function Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-11-04 03:03 +0000
  Re: about NULL as a parameter to a libc function Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-10-28 08:42 -0400
  Re: about NULL as a parameter to a libc function "io_x" <a@b.c.invalid> - 2011-10-28 16:31 +0200
  Re: about NULL as a parameter to a libc function "BartC" <bc@freeuk.com> - 2011-10-28 20:29 +0100
    Re: about NULL as a parameter to a libc function Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2011-10-29 14:49 +0300
      Re: about NULL as a parameter to a libc function Ike Naar <ike@sverige.freeshell.org> - 2011-10-29 13:41 +0000
  Re: about NULL as a parameter to a libc function John Gordon <gordon@panix.com> - 2011-10-28 19:53 +0000
    Re: about NULL as a parameter to a libc function "BartC" <bc@freeuk.com> - 2011-10-28 21:13 +0100
      Re: about NULL as a parameter to a libc function John Gordon <gordon@panix.com> - 2011-10-28 20:33 +0000
        Re: about NULL as a parameter to a libc function James Kuyper <jameskuyper@verizon.net> - 2011-10-28 16:56 -0400
          Re: about NULL as a parameter to a libc function blp@cs.stanford.edu (Ben Pfaff) - 2011-10-28 14:30 -0700
      Re: about NULL as a parameter to a libc function James Kuyper <jameskuyper@verizon.net> - 2011-10-28 16:45 -0400
      Re: about NULL as a parameter to a libc function Keith Thompson <kst-u@mib.org> - 2011-10-28 15:35 -0700
        Re: about NULL as a parameter to a libc function "BartC" <bc@freeuk.com> - 2011-10-29 13:00 +0100
          Re: about NULL as a parameter to a libc function Keith Thompson <kst-u@mib.org> - 2011-10-29 11:26 -0700
            Re: about NULL as a parameter to a libc function "BartC" <bc@freeuk.com> - 2011-10-29 20:24 +0100
              Re: about NULL as a parameter to a libc function Keith Thompson <kst-u@mib.org> - 2011-10-29 14:03 -0700
      Re: about NULL as a parameter to a libc function Nick Keighley <nick_keighley_nospam@hotmail.com> - 2011-10-29 07:30 -0700
      Re: about NULL as a parameter to a libc function Kenneth Brody <kenbrody@spamcop.net> - 2011-11-03 11:26 -0400
        Re: about NULL as a parameter to a libc function James Kuyper <jameskuyper@verizon.net> - 2011-11-03 12:59 -0400
        Re: about NULL as a parameter to a libc function Keith Thompson <kst-u@mib.org> - 2011-11-03 13:42 -0700
  Re: about NULL as a parameter to a libc function Kaz Kylheku <kaz@kylheku.com> - 2011-10-28 20:17 +0000
    Re: about NULL as a parameter to a libc function Richard Damon <news.x.richarddamon@xoxy.net> - 2011-10-28 18:51 -0400
  Re: about NULL as a parameter to a libc function Nobody <nobody@nowhere.com> - 2011-10-28 21:40 +0100
    Re: about NULL as a parameter to a libc function James Kuyper <jameskuyper@verizon.net> - 2011-10-28 17:09 -0400
      Re: about NULL as a parameter to a libc function Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2011-10-29 15:03 +0300
        Re: about NULL as a parameter to a libc function James Kuyper <jameskuyper@verizon.net> - 2011-10-29 09:42 -0400
    Re: about NULL as a parameter to a libc function Kaz Kylheku <kaz@kylheku.com> - 2011-10-28 21:18 +0000
      Re: about NULL as a parameter to a libc function Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-10-28 23:30 +0100
      Re: about NULL as a parameter to a libc function Keith Thompson <kst-u@mib.org> - 2011-10-28 15:37 -0700
        Re: about NULL as a parameter to a libc function Kaz Kylheku <kaz@kylheku.com> - 2011-10-28 23:47 +0000
          Re: about NULL as a parameter to a libc function Keith Thompson <kst-u@mib.org> - 2011-10-28 17:13 -0700
            Re: about NULL as a parameter to a libc function Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-10-29 04:31 +0100
              Re: about NULL as a parameter to a libc function Kaz Kylheku <kaz@kylheku.com> - 2011-10-29 05:42 +0000
                Re: about NULL as a parameter to a libc function Seebs <usenet-nospam@seebs.net> - 2011-10-30 17:22 +0000
                Re: about NULL as a parameter to a libc function Patrick Scheible <kkt@zipcon.net> - 2011-10-30 23:43 -0700
                Re: about NULL as a parameter to a libc function Robert Wessel <robertwessel2@yahoo.com> - 2011-10-31 05:31 -0500
                Re: about NULL as a parameter to a libc function Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2011-10-31 12:42 +0200
                Re: about NULL as a parameter to a libc function Robert Wessel <robertwessel2@yahoo.com> - 2011-10-31 13:11 -0500
                Re: about NULL as a parameter to a libc function Seebs <usenet-nospam@seebs.net> - 2011-10-31 17:18 +0000
      Re: about NULL as a parameter to a libc function Nick Keighley <nick_keighley_nospam@hotmail.com> - 2011-10-29 07:32 -0700
    Re: about NULL as a parameter to a libc function jacob navia <jacob@spamsink.net> - 2011-10-28 23:41 +0200
      Re: about NULL as a parameter to a libc function Kaz Kylheku <kaz@kylheku.com> - 2011-10-28 22:42 +0000
        Re: about NULL as a parameter to a libc function jacob navia <jacob@spamsink.net> - 2011-10-29 10:09 +0200
          Re: about NULL as a parameter to a libc function Nobody <nobody@nowhere.com> - 2011-10-29 10:11 +0100
            Re: about NULL as a parameter to a libc function Angel <angel+news@spamcop.net> - 2011-10-29 09:42 +0000
            Re: about NULL as a parameter to a libc function jacob navia <jacob@spamsink.net> - 2011-10-29 16:17 +0200
          Re: about NULL as a parameter to a libc function Nick Keighley <nick_keighley_nospam@hotmail.com> - 2011-10-29 07:39 -0700
            Re: about NULL as a parameter to a libc function jacob navia <jacob@spamsink.net> - 2011-10-29 19:29 +0200
              Re: about NULL as a parameter to a libc function "io_x" <a@b.c.invalid> - 2011-10-29 20:25 +0200
              Re: about NULL as a parameter to a libc function Nick Keighley <nick_keighley_nospam@hotmail.com> - 2011-10-30 08:14 -0700
    Re: about NULL as a parameter to a libc function ImpalerCore <jadill33@gmail.com> - 2011-10-31 18:30 -0700
      Re: about NULL as a parameter to a libc function Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-24 18:21 -0800
    Re: about NULL as a parameter to a libc function Kenneth Brody <kenbrody@spamcop.net> - 2011-11-03 11:31 -0400
      Re: about NULL as a parameter to a libc function Keith Thompson <kst-u@mib.org> - 2011-11-03 13:45 -0700
  Re: about NULL as a parameter to a libc function Jonathan Leffler <jleffler@earthlink.net> - 2011-10-29 23:41 -0700
    Re: about NULL as a parameter to a libc function James Kuyper <jameskuyper@verizon.net> - 2011-10-30 07:13 -0400

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