Re: is there anything unstandard in the c part here

From	James Kuyper <jameskuyper@verizon.net>
Newsgroups	comp.lang.c
Subject	Re: is there anything unstandard in the c part here
Date	2011-08-13 08:26 -0400
Organization	A noiseless patient Spider
Message-ID	<j25qgr$6d5$1@dont-email.me> (permalink)
References	(4 earlier) <87y5zce4mc.fsf@gmail.com> <lnlivccn6a.fsf@nuthaus.mib.org> <87sjpg77q8.fsf@gmail.com> <j1gklf$ldh$1@dont-email.me> <87r54pbxl9.fsf@gmail.com>

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On 08/13/2011 03:05 AM, Gareth Owen wrote:
> James Kuyper <jameskuyper@verizon.net> writes:
> 
>> The only examples you've given so far are ones where the bug would
>> have remained even a cast had been inserted to matching the
>> destination pointer type.
> 
> But that's the *point*.  
> 
> If you change the destination pointer at some distant code point (in a
> header file say) the cast does not change.  So the code won't compile
> and the runtime disaster is avoided.  

Let me review what we've been discussing, since the context has been
dropped along the way. The original claim was that leaving out the cast
hides an error. A simple example would be:

	char *p;

	// somewhere else, possibly not even in the same file:
	p = malloc(count);

At some later time, the declaration is changed:

	wchar *p;

Now there is certainly an error in the malloc() statement. However, the
absence of the cast doesn't hide the error; the error is in the argument
of malloc(), and it remains hidden even after inserting an unnecessary cast:

	p = (wchar*)malloc(count);

You could use the following kludge to "fix" the real problem:

	p = (wchar*)malloc(count * sizeof(wchar));

I believe that the point you're trying to raise is that the cast
automatically creates a diagnosable error if the type of *p changes
again. That will serve to remind you to fix the real error, which is in
the argument of malloc(), and which the compiler cannot diagnose.
However, why use such a kludge in the first place? With that approach
the line needs to be edited any time the type changes. The right way to
fix it is:

	p = malloc(count * sizeof *p);

Then if we were to change the type of p once again, say to allow storage
of UTF-32 values:

	unsigned long *p;

then the "missing" cast would not hide any errors, because there would
be no error to be hidden. The argument of malloc(), which was the real
problem in previous versions of this code, will automatically adjust to
match changes in the type of *p. The statement does not need to edited,
because it remains correct. The fact that it is correct can be verified
simply by looking only at the statement itself, without even knowing the
type of *p, a fact which is directly connected to the fact that the
statement doesn't need to be changed.

> Now, you can assert "that can't happen", but it has happened.  I've seen
> it happen. Someone codes something a malloc on the assumption the
> pointer type is a char*, and omits the sizeof() because sizeof(char) is
> 1, and then the pointer type changes at some later date - to a wchar*,
> say - and the malloc() is wrong.

I never claimed that something like what you describe couldn't happen;
we merely disagree on what the problem is, and therefore, on what the
right solution is. The problem not a missing cast, and it is not
properly fixed by using sizeof(char). The right fix is sizeof *p, which
completely eliminates any need for a cast.

Let me redefine the challenge somewhat: can you demonstrate a case where
the right idiom ( count * sizeof *p ) is used, where leaving out the
cast hides an error, an error that would  no longer be hidden if the
appropriate cast were inserted?
-- 
James Kuyper

Thread

is there anything unstandard in the c part here Uno <Uno@example.invalid> - 2011-07-25 17:45 -0600
  Re: is there anything unstandard in the c part here John Gordon <gordon@panix.com> - 2011-07-25 23:53 +0000
    Re: is there anything unstandard in the c part here gwowen <gwowen@gmail.com> - 2011-07-26 01:50 -0700
      Re: is there anything unstandard in the c part here tom st denis <tom@iahu.ca> - 2011-07-26 06:17 -0700
        Re: is there anything unstandard in the c part here Kenneth Brody <kenbrody@spamcop.net> - 2011-07-26 11:08 -0400
          Re: is there anything unstandard in the c part here tom st denis <tom@iahu.ca> - 2011-07-26 10:51 -0700
            Re: is there anything unstandard in the c part here Nick Keighley <nick_keighley_nospam@hotmail.com> - 2011-07-27 00:08 -0700
              Re: is there anything unstandard in the c part here Kenneth Brody <kenbrody@spamcop.net> - 2011-07-27 13:41 -0400
                Re: is there anything unstandard in the c part here Angel <angel+news@spamcop.net> - 2011-07-28 10:58 +0000
                Re: is there anything unstandard in the c part here Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-07-28 13:58 +0100
            Re: is there anything unstandard in the c part here Kenneth Brody <kenbrody@spamcop.net> - 2011-07-27 13:24 -0400
              Re: is there anything unstandard in the c part here Angel <angel+news@spamcop.net> - 2011-07-28 11:07 +0000
                Re: is there anything unstandard in the c part here Kenneth Brody <kenbrody@spamcop.net> - 2011-07-28 13:02 -0400
                Re: is there anything unstandard in the c part here Roberto Waltman <usenet@rwaltman.com> - 2011-07-28 17:16 -0400
      Re: is there anything unstandard in the c part here James Kuyper <jameskuyper@verizon.net> - 2011-07-26 09:39 -0400
        Re: is there anything unstandard in the c part here Gareth Owen <gwowen@gmail.com> - 2011-07-27 07:24 +0100
          Re: is there anything unstandard in the c part here James Kuyper <jameskuyper@verizon.net> - 2011-07-27 07:11 -0400
          Re: is there anything unstandard in the c part here Noob <root@127.0.0.1> - 2011-07-27 15:20 +0200
          Re: is there anything unstandard in the c part here John Gordon <gordon@panix.com> - 2011-07-27 14:25 +0000
            Re: is there anything unstandard in the c part here Keith Thompson <kst-u@mib.org> - 2011-07-27 08:10 -0700
              Re: is there anything unstandard in the c part here John Gordon <gordon@panix.com> - 2011-07-27 17:42 +0000
                Re: is there anything unstandard in the c part here Keith Thompson <kst-u@mib.org> - 2011-07-27 12:02 -0700
                Re: is there anything unstandard in the c part here Tim Rentsch <txr@alumni.caltech.edu> - 2011-07-28 16:04 -0700
                Re: is there anything unstandard in the c part here James Kuyper <jameskuyper@verizon.net> - 2011-07-27 22:35 -0400
          Re: is there anything unstandard in the c part here Kenneth Brody <kenbrody@spamcop.net> - 2011-07-27 13:34 -0400
            Re: is there anything unstandard in the c part here Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-07-28 02:13 +0100
              Re: is there anything unstandard in the c part here Ike Naar <ike@sverige.freeshell.org> - 2011-07-28 07:33 +0000
                Re: is there anything unstandard in the c part here Willem <willem@toad.stack.nl> - 2011-07-28 07:58 +0000
                Re: is there anything unstandard in the c part here pete <pfiland@mindspring.com> - 2011-07-28 04:25 -0400
                Re: is there anything unstandard in the c part here Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-07-28 12:26 +0100
            Re: is there anything unstandard in the c part here gwowen <gwowen@gmail.com> - 2011-07-28 04:24 -0700
              Re: is there anything unstandard in the c part here Angel <angel+news@spamcop.net> - 2011-07-28 11:36 +0000
                Re: is there anything unstandard in the c part here Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-07-28 15:11 +0100
                Re: is there anything unstandard in the c part here Angel <angel+news@spamcop.net> - 2011-07-28 14:50 +0000
                Re: is there anything unstandard in the c part here Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-07-28 17:28 +0100
                Re: is there anything unstandard in the c part here Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2011-07-28 10:38 -0600
                Re: is there anything unstandard in the c part here Michael Press <rubrum@pacbell.net> - 2011-08-02 17:20 -0700
                Re: is there anything unstandard in the c part here Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2011-08-02 23:26 -0600
                Re: is there anything unstandard in the c part here Michael Press <rubrum@pacbell.net> - 2011-08-03 16:09 -0700
                Re: is there anything unstandard in the c part here Kenneth Brody <kenbrody@spamcop.net> - 2011-07-28 13:22 -0400
                Re: is there anything unstandard in the c part here James Kuyper <jameskuyper@verizon.net> - 2011-07-28 22:22 -0400
                Re: is there anything unstandard in the c part here Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-07-29 04:17 +0100
                Re: is there anything unstandard in the c part here James Kuyper <jameskuyper@verizon.net> - 2011-07-28 23:40 -0400
                Re: is there anything unstandard in the c part here Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-07-29 13:52 +0100
                Re: is there anything unstandard in the c part here Gareth Owen <gwowen@gmail.com> - 2011-07-31 09:19 +0100
                Re: is there anything unstandard in the c part here James Kuyper <jameskuyper@verizon.net> - 2011-07-31 09:49 -0400
                Re: is there anything unstandard in the c part here Gareth Owen <gwowen@gmail.com> - 2011-08-02 06:56 +0100
                Re: is there anything unstandard in the c part here James Kuyper <jameskuyper@verizon.net> - 2011-08-02 06:53 -0400
              Re: is there anything unstandard in the c part here James Kuyper <jameskuyper@verizon.net> - 2011-07-28 07:36 -0400
              Re: is there anything unstandard in the c part here Kenneth Brody <kenbrody@spamcop.net> - 2011-07-28 13:16 -0400
      Re: is there anything unstandard in the c part here John Gordon <gordon@panix.com> - 2011-07-26 14:58 +0000
        Re: is there anything unstandard in the c part here Paul N <gw7rib@aol.com> - 2011-07-26 14:56 -0700
          Re: is there anything unstandard in the c part here Nick Keighley <nick_keighley_nospam@hotmail.com> - 2011-07-27 00:13 -0700
          Re: is there anything unstandard in the c part here James Kuyper <jameskuyper@verizon.net> - 2011-07-27 07:28 -0400
            Re: is there anything unstandard in the c part here Nick Keighley <nick_keighley_nospam@hotmail.com> - 2011-07-27 04:44 -0700
            Re: is there anything unstandard in the c part here Keith Thompson <kst-u@mib.org> - 2011-07-27 08:24 -0700
              Re: is there anything unstandard in the c part here James Kuyper <jameskuyper@verizon.net> - 2011-07-27 22:41 -0400
                Re: is there anything unstandard in the c part here Tim Rentsch <txr@alumni.caltech.edu> - 2011-07-28 16:19 -0700
              Re: is there anything unstandard in the c part here Tim Rentsch <txr@alumni.caltech.edu> - 2011-07-28 16:06 -0700
              Re: is there anything unstandard in the c part here Uno <Uno@example.invalid> - 2011-07-30 01:54 -0600
      Re: is there anything unstandard in the c part here Kenneth Brody <kenbrody@spamcop.net> - 2011-07-26 11:01 -0400
        Re: is there anything unstandard in the c part here Chad <cdalten@gmail.com> - 2011-07-26 09:49 -0700
          Re: is there anything unstandard in the c part here Kenneth Brody <kenbrody@spamcop.net> - 2011-07-26 13:18 -0400
          Re: is there anything unstandard in the c part here John Gordon <gordon@panix.com> - 2011-07-26 17:44 +0000
          Re: is there anything unstandard in the c part here Angel <angel+news@spamcop.net> - 2011-07-26 17:45 +0000
          Re: is there anything unstandard in the c part here Seebs <usenet-nospam@seebs.net> - 2011-07-26 18:42 +0000
            Re: is there anything unstandard in the c part here Bill Waddington <william.waddington@beezmo.com> - 2011-07-26 11:55 -0700
              Re: is there anything unstandard in the c part here lawrence.jones@siemens.com - 2011-07-26 18:11 -0400
                Re: is there anything unstandard in the c part here Uno <Uno@example.invalid> - 2011-07-27 20:58 -0600
            Re: is there anything unstandard in the c part here arnuld <sunrise@invalid.address> - 2011-08-02 05:24 +0000
              Re: is there anything unstandard in the c part here Ian Collins <ian-news@hotmail.com> - 2011-08-02 18:10 +1200
              Re: is there anything unstandard in the c part here Keith Thompson <kst-u@mib.org> - 2011-08-02 00:03 -0700
              Re: is there anything unstandard in the c part here James Kuyper <jameskuyper@verizon.net> - 2011-08-02 06:59 -0400
              Re: is there anything unstandard in the c part here Seebs <usenet-nospam@seebs.net> - 2011-08-02 16:06 +0000
      Re: is there anything unstandard in the c part here Seebs <usenet-nospam@seebs.net> - 2011-07-26 18:42 +0000
      Re: is there anything unstandard in the c part here Nick Keighley <nick_keighley_nospam@hotmail.com> - 2011-07-27 00:06 -0700
        Re: is there anything unstandard in the c part here Gareth Owen <gwowen@gmail.com> - 2011-08-02 06:58 +0100
          Re: is there anything unstandard in the c part here Keith Thompson <kst-u@mib.org> - 2011-08-02 00:01 -0700
            Re: is there anything unstandard in the c part here Gareth Owen <gwowen@gmail.com> - 2011-08-05 06:22 +0100
              Re: is there anything unstandard in the c part here Keith Thompson <kst-u@mib.org> - 2011-08-04 23:54 -0700
              Re: is there anything unstandard in the c part here James Kuyper <jameskuyper@verizon.net> - 2011-08-05 07:37 -0400
                Re: is there anything unstandard in the c part here Gareth Owen <gwowen@gmail.com> - 2011-08-13 08:05 +0100
                Re: is there anything unstandard in the c part here James Kuyper <jameskuyper@verizon.net> - 2011-08-13 08:26 -0400
                Re: is there anything unstandard in the c part here Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-08-13 14:09 +0100
          Re: is there anything unstandard in the c part here Kenneth Brody <kenbrody@spamcop.net> - 2011-08-02 12:58 -0400
            Re: is there anything unstandard in the c part here Nick Keighley <nick_keighley_nospam@hotmail.com> - 2011-08-10 01:31 -0700
              Re: is there anything unstandard in the c part here James Kuyper <jameskuyper@verizon.net> - 2011-08-10 08:28 -0400
                Re: is there anything unstandard in the c part here Gareth Owen <gwowen@gmail.com> - 2011-08-13 08:14 +0100
                Re: is there anything unstandard in the c part here "io_x" <a@b.c.invalid> - 2011-08-13 10:08 +0200
                Re: is there anything unstandard in the c part here James Kuyper <jameskuyper@verizon.net> - 2011-08-13 08:35 -0400
                Re: is there anything unstandard in the c part here "J. J. Farrell" <jjf@bcs.org.uk> - 2011-08-14 04:00 +0100
                Re: is there anything unstandard in the c part here Kenneth Brody <kenbrody@spamcop.net> - 2011-08-15 14:43 -0400
          Re: is there anything unstandard in the c part here Nick Keighley <nick_keighley_nospam@hotmail.com> - 2011-08-10 01:34 -0700
            Re: is there anything unstandard in the c part here Gareth Owen <gwowen@gmail.com> - 2011-08-13 08:16 +0100
  Re: is there anything unstandard in the c part here Barry Schwarz <schwarzb@dqel.com> - 2011-07-25 17:52 -0700
    Re: is there anything unstandard in the c part here "J. J. Farrell" <jjf@bcs.org.uk> - 2011-07-26 02:10 +0100
    Re: is there anything unstandard in the c part here Seebs <usenet-nospam@seebs.net> - 2011-07-26 06:17 +0000
      Re: is there anything unstandard in the c part here Nick Keighley <nick_keighley_nospam@hotmail.com> - 2011-07-26 00:03 -0700
        Re: is there anything unstandard in the c part here "Kleuskes & Moos" <kleuske@xs4all.nl> - 2011-07-26 00:08 -0700
        Re: is there anything unstandard in the c part here Ian Collins <ian-news@hotmail.com> - 2011-07-26 19:43 +1200
        Re: is there anything unstandard in the c part here Seebs <usenet-nospam@seebs.net> - 2011-07-26 07:56 +0000
          Re: is there anything unstandard in the c part here Nick Keighley <nick_keighley_nospam@hotmail.com> - 2011-07-26 04:50 -0700
            Re: is there anything unstandard in the c part here Angel <angel+news@spamcop.net> - 2011-07-26 12:03 +0000
            Re: is there anything unstandard in the c part here Seebs <usenet-nospam@seebs.net> - 2011-07-26 18:42 +0000
            Re: is there anything unstandard in the c part here Nobody <nobody@nowhere.com> - 2011-07-27 19:10 +0100
        Re: is there anything unstandard in the c part here Angel <angel+news@spamcop.net> - 2011-07-26 11:53 +0000
  Re: is there anything unstandard in the c part here Nick Keighley <nick_keighley_nospam@hotmail.com> - 2011-07-26 00:07 -0700
    Re: is there anything unstandard in the c part here James Kuyper <jameskuyper@verizon.net> - 2011-07-26 06:29 -0400
      Re: is there anything unstandard in the c part here Uno <Uno@example.invalid> - 2011-07-27 21:32 -0600
        Re: is there anything unstandard in the c part here Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-07-28 12:19 +0100
          Re: is there anything unstandard in the c part here pete <pfiland@mindspring.com> - 2011-07-28 20:25 -0400
            Re: is there anything unstandard in the c part here Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2011-07-28 18:38 -0600
              Re: is there anything unstandard in the c part here pete <pfiland@mindspring.com> - 2011-07-28 21:05 -0400
          Re: is there anything unstandard in the c part here "Morris Keesan" <mkeesan@post.harvard.edu> - 2011-07-29 14:16 -0400
            Re: is there anything unstandard in the c part here Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-07-29 20:27 +0100
            Re: is there anything unstandard in the c part here Uno <Uno@example.invalid> - 2011-07-30 01:10 -0600
          Re: is there anything unstandard in the c part here Uno <Uno@example.invalid> - 2011-07-30 00:56 -0600
            Re: is there anything unstandard in the c part here Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-07-30 12:23 +0100
              Re: is there anything unstandard in the c part here Uno <Uno@example.invalid> - 2011-08-01 00:01 -0600
                Re: is there anything unstandard in the c part here Ike Naar <ike@sverige.freeshell.org> - 2011-08-01 07:50 +0000
                Re: is there anything unstandard in the c part here Uno <Uno@example.invalid> - 2011-08-01 07:43 -0600
                Re: is there anything unstandard in the c part here Ike Naar <ike@sdf.lonestar.org> - 2011-08-01 15:20 +0000
                Re: is there anything unstandard in the c part here Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-08-01 14:27 +0100
                Re: is there anything unstandard in the c part here Uno <Uno@example.invalid> - 2011-08-01 08:02 -0600
                Re: is there anything unstandard in the c part here Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-08-01 15:42 +0100

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