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 07:05 -0400
Organization	A noiseless patient Spider
Message-ID	<j8e2a1$916$1@dont-email.me> (permalink)
References	<14abde31-4573-45e7-bdaf-eb70b22741d7@z28g2000pro.googlegroups.com> <j8drvi$41k$1@dont-email.me> <3d06e0dd-7918-4d19-a0a8-ff5ec1db2bcd@z28g2000pro.googlegroups.com>

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On 10/28/2011 06:22 AM, gaoqiang wrote:
> On Oct 28, 5:14�pm, Mark Bluemel <mark_blue...@pobox.com> wrote:
>> On 10/28/2011 09:49 AM, gaoqiang wrote:
>>
>>> a good function should deal with unwanted parameters.
>>
>> That's one point of view. Another point of view is that code should
>> abide by the contract of the api and pass the appropriate arguments to
>> to functions.
>>
>>> why memcpy,strcmp .etc ,these libc functions, do not
>>> accept NULL pointer ? called with NULL, just get �segment
>>> fault. �for efficiency ?
>>
>> For example, my man page for strcmp states "The strcmp() function
>> compares the two strings s1 and s2" - a NULL pointer isn't a string, so
>> why should the function deal with it?
> 
> I don't think it explains. NULL is not a string,then what about
> memcmp ?
> NULL is a pointer now ...
> 
> you remind me .
> 
> I just got an idea that NULL pointer is nothing different with other
> invalid
> pointer. If a function checks the NULL pointer ,then why not other
> invalid
> pointers? to say void*p=0x01 ?

First, some nit picking. "NULL" is a macro, which is required to expand
into an expression which is a null pointer constant. "null" is an
adjective which can be applied to, among other things, a C pointer
value. Please try to clearly distinguish between "NULL" and "null".

There are many operations that can be performed on pointer values; some
pointer values are valid for use with some operations, but not with
others. A null pointer is a valid pointer value, but it's the least
valid type of pointer value - there's only two things that can be done
with it: it can be assigned to a pointer variable, and it can be
compared for equality with other pointer values. What this means is that
in C, a null pointer value is used for the very special purpose of not
pointing at anything. It is in fact prohibited for a null pointer to
compare equal to any pointer value that actually points at any object or
function, while it is required that null pointers compare equal to each
other (6.5.9p6).

The definition

	void *p = 0x01;

is a constraint violation. Initialization of a scalar object like p is
handled according to the rules of simple assignment (6.7.8p11), and
initializing a pointer object with an integer doesn't fit any of the
allowed combinations of left and right operands for a simple assignment
given in 6.5.16.1p1. There would be special handling in this context if
the initializer were an integer constant expression with a value of
zero; such an expression would qualify as a null pointer constant, and
that is one of the cases that would be permitted by 6.5.16.1p1. It would
result in p being initialized with a null pointer value of type void*.
No such special handling applies to 0x01. What you meant is probably
better expressed by

	void *p = (void*)0x01;

However, there's still a problem - the expression (void*)0x01 is NOT
guaranteed to be a valid pointer value for ANY purpose: "the result is
implementation-defined, might not be correctly aligned, might not point
to an entity of the referenced type, and might be a trap
representation.56)" (6.3.2.3p5).
-- 
James Kuyper

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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