Re: about NULL as a parameter to a libc function

From	"BartC" <bc@freeuk.com>
Newsgroups	comp.lang.c
Subject	Re: about NULL as a parameter to a libc function
Date	2011-10-29 13:00 +0100
Organization	A noiseless patient Spider
Message-ID	<j8gpuk$p58$1@dont-email.me> (permalink)
References	<14abde31-4573-45e7-bdaf-eb70b22741d7@z28g2000pro.googlegroups.com><j8f16r$pgt$1@reader1.panix.com> <j8f2fn$vhu$1@dont-email.me> <lnhb2slp7p.fsf@nuthaus.mib.org>

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"Keith Thompson" <kst-u@mib.org> wrote in message 
news:lnhb2slp7p.fsf@nuthaus.mib.org...
> "BartC" <bc@freeuk.com> writes:

>>Wouldn't it be a lot easier to just write:
>>
>> 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).
>
> One could make exactly the same argument for a simple assignment
> (without the N, of course).  Perhaps it would be simpler to be able to
> write:
>
>    *A = *B;
>
> and have it quietly do nothing if either A or B is a null pointer.

With a basic language statement like assignment, you wouldn't expect it to 
be backed up with logic to make sure operands are not null. And it can't be 
called remotely, where there would be no opportunity for the writer of the 
*A=*B to put in extra checks.

A function (any function really, rather than just memcpy) is different; it 
can be called remotely, and it is easy to add checks, and to define specific 
behaviours.

> as others have said in parallel followups, when you're writing a
> statement that copies data, you should generally have enough awareness
> of the context to *know* that there's actually something to be copied.
>
> Do you actually find yourself making such checks in your own code?

Yes. Both when calling C functions (and often these are called from 
higher-level scripting languages, which use null for empty strings, and can 
crash when directly calling a C function), and when writing my own functions 
where the caller cannot quite be trusted 100%, and even then I might put 
checks in anyway as I don't know what other callers might do in future.

Of course these are not always as performance-critical as memcpy() where 
there is less room for manoeuvre. I can understand that requiring extra 
checks means the compiler cannot, for example, optimise a 4-byte memcpy into 
a couple of inline instructions.

-- 
Bartc

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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