Re: better atoi

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From	Keith Thompson <kst-u@mib.org>
Newsgroups	comp.lang.c
Subject	Re: better atoi
Date	Sat, 11 Feb 2017 01:30:49 -0800
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Joe Pfeiffer <pfeiffer@cs.nmsu.edu> writes:
> Keith Thompson <kst-u@mib.org> writes:
[...]
>> No, if you set errno to 0 before calling strtol() you can rely in it not
>> being set to ERANGE unless there's an error.
>>
>> N1570 7.5p3:
>>
>>     The value of errno may be set to nonzero by a library function
>>     call whether or not there is an error, provided the use of
>>     errno is not documented in the description of the function in
>>     this International Standard.
>>
>> Since the use of errno *is* documented for strtol(), it can't set it to
>> nonzero unless there's an error.  (I suppose you could interpret that
>> wording in a way that allows to set errno even if it's successful, but
>> that's not a reasonable reading.)
>
> My recollection is that there are functions that do that (though I can't
> provide an example that I remember clearly does it -- I want to say
> open(), but would be very happy to be wrong).

open() is not defined by the C standard (it's defined by POSIX), so it's
not 100% relevant.  But POSIX says:

    Upon successful completion, these functions shall open the
    file and return a non-negative integer representing the file
    descriptor. Otherwise, these functions shall return -1 and set
    errno to indicate the error. If -1 is returned, no files shall
    be created or modified.

The value returned by open() is sufficient to tell you whether it
succeeded or failed.  If it failed, errno tells you why.

In general, functions *can* set errno to some non-zero value even if
they succeed, as long as they still indicate that they succeeded.
That's because functions can call other functions, and the caller can
succeed even if the callee fails.

I think strtol() is a bit of a special case, since there are cases where
errno is the only direct way to tell whether it succeeded or failed.

-- 
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"

Thread

better atoi fir <profesor.fir@gmail.com> - 2017-02-10 12:24 -0800
  Re: better atoi fir <profesor.fir@gmail.com> - 2017-02-10 12:35 -0800
  Re: better atoi Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2017-02-10 14:48 -0700
    Re: better atoi fir <profesor.fir@gmail.com> - 2017-02-10 14:12 -0800
      Re: better atoi Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2017-02-10 22:36 -0700
        Re: better atoi fir <profesor.fir@gmail.com> - 2017-02-11 01:07 -0800
          Re: better atoi Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2017-02-11 10:10 -0700
            Re: better atoi fir <profesor.fir@gmail.com> - 2017-02-11 09:26 -0800
              Re: better atoi Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2017-02-11 15:36 -0700
                Re: better atoi fir <profesor.fir@gmail.com> - 2017-02-12 00:18 -0800
                Re: better atoi fir <profesor.fir@gmail.com> - 2017-02-12 00:31 -0800
                Re: better atoi fir <profesor.fir@gmail.com> - 2017-02-12 03:47 -0800
                Re: better atoi fir <profesor.fir@gmail.com> - 2017-02-12 04:22 -0800
                Re: better atoi fir <profesor.fir@gmail.com> - 2017-02-12 06:08 -0800
                Re: better atoi fir <profesor.fir@gmail.com> - 2017-02-12 06:13 -0800
                Re: better atoi fir <profesor.fir@gmail.com> - 2017-02-12 07:31 -0800
                Re: better atoi fir <profesor.fir@gmail.com> - 2017-02-12 08:02 -0800
                Re: better atoi fir <profesor.fir@gmail.com> - 2017-02-12 08:08 -0800
                Re: better atoi fir <profesor.fir@gmail.com> - 2017-02-12 08:17 -0800
                Re: better atoi fir <profesor.fir@gmail.com> - 2017-02-12 08:23 -0800
                Re: better atoi Robert Latest <boblatest@yahoo.com> - 2017-02-19 11:11 +0000
            Re: better atoi Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-02-11 17:33 +0000
              Re: better atoi Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2017-02-11 15:35 -0700
                Re: better atoi "James R. Kuyper" <jameskuyper@verizon.net> - 2017-02-11 17:52 -0500
                Re: better atoi Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2017-02-11 18:53 -0700
    Re: better atoi Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-02-10 22:50 +0000
      Re: better atoi Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2017-02-10 21:59 -0700
        Re: better atoi Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-02-11 12:08 +0000
          Re: better atoi Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2017-02-11 09:39 -0700
            Re: better atoi Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-02-11 17:26 +0000
              Re: better atoi Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2017-02-11 15:30 -0700
                Re: better atoi Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-02-11 22:58 +0000
                Re: better atoi Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2017-02-11 18:48 -0700
                Re: better atoi Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-02-12 02:16 +0000
                Re: better atoi "James R. Kuyper" <jameskuyper@verizon.net> - 2017-02-12 09:57 -0500
                Re: better atoi fir <profesor.fir@gmail.com> - 2017-02-12 07:27 -0800
                Re: better atoi jameskuyper@verizon.net - 2017-02-12 08:44 -0800
                Re: better atoi fir <profesor.fir@gmail.com> - 2017-02-12 09:15 -0800
                Re: better atoi Malcolm McLean <malcolm.mclean5@btinternet.com> - 2017-02-12 09:44 -0800
                Re: better atoi BartC <bc@freeuk.com> - 2017-02-12 18:32 +0000
                Re: better atoi Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-02-12 18:56 +0000
                Re: better atoi BartC <bc@freeuk.com> - 2017-02-12 19:18 +0000
                Re: better atoi Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-02-13 00:16 +0000
                Re: better atoi BartC <bc@freeuk.com> - 2017-02-13 01:56 +0000
                Re: better atoi Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-02-13 10:44 +0000
                Re: better atoi BartC <bc@freeuk.com> - 2017-02-13 11:38 +0000
                Re: better atoi Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-02-13 14:37 +0000
                Re: better atoi BartC <bc@freeuk.com> - 2017-02-13 15:00 +0000
                Re: better atoi Keith Thompson <kst-u@mib.org> - 2017-02-12 11:15 -0800
                Re: better atoi BartC <bc@freeuk.com> - 2017-02-12 19:25 +0000
                Re: better atoi Keith Thompson <kst-u@mib.org> - 2017-02-12 11:54 -0800
                Re: better atoi supercat@casperkitty.com - 2017-02-12 13:13 -0800
    Re: better atoi Keith Thompson <kst-u@mib.org> - 2017-02-10 15:16 -0800
      Re: better atoi Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2017-02-10 22:01 -0700
        Re: better atoi Keith Thompson <kst-u@mib.org> - 2017-02-11 01:30 -0800
      Re: better atoi Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2017-02-10 22:26 -0700
    Re: better atoi Nick Bowler <nbowler@draconx.ca> - 2017-02-11 00:26 +0000
      Re: better atoi Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2017-02-10 22:32 -0700
    Re: better atoi "James R. Kuyper" <jameskuyper@verizon.net> - 2017-02-19 11:10 -0500

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