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Re: standard error codes

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From Keith Thompson <kst-u@mib.org>
Newsgroups comp.lang.c
Subject Re: standard error codes
Date Tue, 10 Jan 2017 12:12:02 -0800
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BartC <bc@freeuk.com> writes:
> On 10/01/2017 17:07, Keith Thompson wrote:
>> BartC <bc@freeuk.com> writes:
>> [...]
>>> (I just tested one of my intepreters counting the number of string
>>> equal/not equal ops, and the number of relative compare ops. The task
>>> was running a compiler. All ops were on counted strings. Results:
>>>
>>>    string == !=      140221 instances
>>>    string < <= >= >  0      instances
>>>
>>> So memeq is needed!)
>>
>> memeq or streq, or both?
>
> These are counted strings so memeq. streq might be useful for strings 
> for lengths are not known. If there was a choice of only one, then I 
> think memeq would be more useful.

It would be odd to add memeq() but not streq() to the standard.

>> Either one would be needed only if replacing calls to memcmp/strcmp
>> by call to an optimized memeq/streq saves enough time to be
>> significant.  If you can demonstrate that (memeq(x, y)) can be
>> significantly faster than (memcmp(x, y) == 0), then you'll have a
>> valid argument.
>
> If memcmp didn't exist and you wrote your own routine to compare two 
> blocks of N bytes for equality, would you bother to also find out which 
> of two unequal blocks was 'greater than' the other (once you've defined 
> what that means), and set up a -1, 0 or 1 output code?
>
> I'm guessing you would take the simple approach!

But memcmp() does exist.  Given that, the simple approach is to call
memcmp() and compare the result to 0 if all I care about is
(in)equality.

There's a sample implementation of strcmp() (but not memcmp() in K&R1.
It compares a byte at a time (reasonable in that context), and
distinguishing between less-than and greater-than (likely more useful
for strings than for byge buffers) is essentially free.  It's the
difference between a subtraction and a comparison.

>> An experiment I did the other day shows that, at least for some
>> implementations, memcmp() returns the difference between the first
>> corresponding bytes that differ.  Would returning a zero/nonzero
>> result be faster than returning the result of a subtraction?
>
> OK, so the result isn't even -1, 0 or 1!

Right.  You could have found that out by reading the standard,
or a man page.  You really should understand the existing standard
library functions before suggesting new ones.

> In that case it would be difficult to call, if getting a 1/0 result 
> means an extra instruction instead of just returning the result of a 
> subtraction.

No, it's not difficult to call.  C programmers have been doing it for
decades.  Is "> 0" harder to type than "== 1"?

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

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Thread

standard  error codes Thiago Adams <thiago.adams@gmail.com> - 2017-01-08 06:34 -0800
  Re: standard  error codes Malcolm McLean <malcolm.mclean5@btinternet.com> - 2017-01-08 11:05 -0800
    Re: standard  error codes Thiago Adams <thiago.adams@gmail.com> - 2017-01-09 03:33 -0800
      Re: standard  error codes Malcolm McLean <malcolm.mclean5@btinternet.com> - 2017-01-09 04:55 -0800
        Re: standard  error codes Keith Thompson <kst-u@mib.org> - 2017-01-09 09:06 -0800
          Re: standard  error codes Thiago Adams <thiago.adams@gmail.com> - 2017-01-09 09:52 -0800
            Re: standard error codes BartC <bc@freeuk.com> - 2017-01-09 21:50 +0000
              Re: standard error codes BartC <bc@freeuk.com> - 2017-01-09 22:36 +0000
                Re: standard error codes supercat@casperkitty.com - 2017-01-09 14:51 -0800
                Re: standard error codes BartC <bc@freeuk.com> - 2017-01-09 23:39 +0000
                Re: standard error codes supercat@casperkitty.com - 2017-01-09 15:51 -0800
                Re: standard error codes Malcolm McLean <malcolm.mclean5@btinternet.com> - 2017-01-09 16:08 -0800
                Re: standard error codes supercat@casperkitty.com - 2017-01-09 18:51 -0800
                Re: standard error codes Malcolm McLean <malcolm.mclean5@btinternet.com> - 2017-01-10 02:02 -0800
                Re: standard error codes supercat@casperkitty.com - 2017-01-10 07:34 -0800
                Re: standard error codes BartC <bc@freeuk.com> - 2017-01-10 01:39 +0000
                Re: standard error codes Keith Thompson <kst-u@mib.org> - 2017-01-10 09:07 -0800
                Re: standard error codes supercat@casperkitty.com - 2017-01-10 09:59 -0800
                Re: standard error codes Keith Thompson <kst-u@mib.org> - 2017-01-10 12:02 -0800
                Re: standard error codes Malcolm McLean <malcolm.mclean5@btinternet.com> - 2017-01-10 12:53 -0800
                Re: standard error codes Keith Thompson <kst-u@mib.org> - 2017-01-10 13:03 -0800
                Re: standard error codes Malcolm McLean <malcolm.mclean5@btinternet.com> - 2017-01-10 15:41 -0800
                Re: standard error codes Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-01-11 02:56 +0000
                Re: standard error codes supercat@casperkitty.com - 2017-01-11 07:42 -0800
                Re: standard error codes supercat@casperkitty.com - 2017-01-10 12:59 -0800
                Re: standard error codes BartC <bc@freeuk.com> - 2017-01-10 18:14 +0000
                Re: standard error codes Keith Thompson <kst-u@mib.org> - 2017-01-10 12:12 -0800
                Re: standard error codes BartC <bc@freeuk.com> - 2017-01-10 20:35 +0000
                Re: standard error codes Keith Thompson <kst-u@mib.org> - 2017-01-10 13:00 -0800
                Re: standard error codes supercat@casperkitty.com - 2017-01-11 08:16 -0800
                Re: standard error codes Richard Heathfield <rjh@cpax.org.uk> - 2017-01-11 18:25 +0000
                Re: standard error codes BartC <bc@freeuk.com> - 2017-01-11 19:09 +0000
                Re: standard error codes Keith Thompson <kst-u@mib.org> - 2017-01-11 12:03 -0800
                Re: standard error codes Richard Heathfield <rjh@cpax.org.uk> - 2017-01-11 20:10 +0000
                Re: standard error codes Keith Thompson <kst-u@mib.org> - 2017-01-11 12:19 -0800
                Re: standard error codes BartC <bc@freeuk.com> - 2017-01-11 20:31 +0000
                Re: standard error codes Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2017-01-11 15:32 -0500
                Re: standard error codes Richard Heathfield <rjh@cpax.org.uk> - 2017-01-11 20:48 +0000
                Re: standard error codes Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2017-01-11 15:58 -0500
                Re: standard error codes Keith Thompson <kst-u@mib.org> - 2017-01-11 13:01 -0800
                Re: standard error codes Richard Heathfield <rjh@cpax.org.uk> - 2017-01-11 21:05 +0000
                Re: standard error codes BartC <bc@freeuk.com> - 2017-01-11 21:08 +0000
                Re: standard error codes Gareth Owen <gwowen@gmail.com> - 2017-01-11 21:38 +0000
                Re: standard error codes BartC <bc@freeuk.com> - 2017-01-11 21:58 +0000
                Re: standard error codes David Brown <david.brown@hesbynett.no> - 2017-01-12 12:11 +0100
                Re: standard error codes Keith Thompson <kst-u@mib.org> - 2017-01-12 09:28 -0800
                Re: standard error codes Richard Heathfield <rjh@cpax.org.uk> - 2017-01-12 18:06 +0000
                Re: standard error codes Keith Thompson <kst-u@mib.org> - 2017-01-12 11:39 -0800
                Re: standard error codes Richard Heathfield <rjh@cpax.org.uk> - 2017-01-12 19:50 +0000
                Re: standard error codes Keith Thompson <kst-u@mib.org> - 2017-01-12 13:03 -0800
                Re: standard error codes Richard Heathfield <rjh@cpax.org.uk> - 2017-01-12 21:13 +0000
                Re: standard error codes <william@wilbur.25thandClement.com> - 2017-01-12 20:00 -0800
                Re: standard error codes Malcolm McLean <malcolm.mclean5@btinternet.com> - 2017-01-13 01:56 -0800
                Re: standard error codes Richard Heathfield <rjh@cpax.org.uk> - 2017-01-13 12:54 +0000
                Re: standard error codes Keith Thompson <kst-u@mib.org> - 2017-01-13 09:35 -0800
                Re: standard error codes Richard Heathfield <rjh@cpax.org.uk> - 2017-01-13 17:38 +0000
                Re: standard error codes James Kuyper <jameskuyper@verizon.net> - 2017-01-13 13:06 -0500
                Re: standard error codes raltbos@xs4all.nl (Richard Bos) - 2017-01-18 12:00 +0000
                Re: standard error codes Richard Heathfield <rjh@cpax.org.uk> - 2017-01-18 12:13 +0000
                Re: standard error codes Malcolm McLean <malcolm.mclean5@btinternet.com> - 2017-01-12 16:43 -0800
                Re: standard error codes <william@wilbur.25thandClement.com> - 2017-01-12 19:34 -0800
                Re: standard error codes Keith Thompson <kst-u@mib.org> - 2017-01-13 09:27 -0800
                Re: standard error codes raltbos@xs4all.nl (Richard Bos) - 2017-01-12 19:33 +0000
                Re: standard error codes Keith Thompson <kst-u@mib.org> - 2017-01-12 12:10 -0800
                Re: standard error codes supercat@casperkitty.com - 2017-01-11 11:50 -0800
                Re: standard error codes Richard Heathfield <rjh@cpax.org.uk> - 2017-01-11 20:38 +0000
                Re: standard error codes jadill33@gmail.com - 2017-01-11 12:26 -0800
                Re: standard error codes Richard Heathfield <rjh@cpax.org.uk> - 2017-01-11 20:44 +0000
      Re: standard  error codes Keith Thompson <kst-u@mib.org> - 2017-01-09 09:03 -0800
  Re: standard  error codes supercat@casperkitty.com - 2017-01-08 11:40 -0800

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