Re: Puzzling "implicit declaration" Warning

From	Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups	comp.lang.c
Subject	Re: Puzzling "implicit declaration" Warning
Date	2020-02-07 02:12 -0800
Organization	None to speak of
Message-ID	<87imki1zae.fsf@nosuchdomain.example.com> (permalink)
References	<WsmdnVL9ioPMhaDDnZ2dnUU7_83NnZ2d@giganews.com> <slrnr3q9cl.km3.grahn+nntp@frailea.sa.invalid>

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Jorgen Grahn <grahn+nntp@snipabacken.se> writes:
> On Fri, 2020-02-07, Robbie Hatley wrote:
[...]
>> And yet, the program works, so obviously Gnu extensions are being
>> properly compiled.
>
> I'm not sure it really works; the warning means the compiler will
> assume the prototype is 'int exp10()'.  Then at the linking stage
> there is no additional type checking and no protection against GNU
> extensions, so it will link ok.
>
> I don't know offhand what happens when you get the prototype wrong,
> but it's a bad thing and I make sure it never happens.  (Others will
> be able to tell you.)

What happens is undefined behavior.

If you examine your math.h header, I think you'll find that the
declaration of exp10 isn't made visible unless _GNU_SOURCE has been
defined.

Under C90 rules, the behavior would be well defined *if* the function
returned an int result and expected an argument of the type you actually
gave it.  Under C99 and later, calling a function with no visible
declaration is a constraint violation (and if the required diagnostic is
a non-fatal warning, the behavior is undefined).

With a simpler example, compiling with gcc yields (after a couple of
warnings) a program that behaves as if the declaration were visible, but
clang doesn't.  One of the warnings from gcc indicates that it treats
exp10 as a built-in function, so it's not too surprising that it's able
to generate a "correct" call.

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
Working, but not speaking, for Philips Healthcare
void Void(void) { Void(); } /* The recursive call of the void */

Thread

Puzzling "implicit declaration" Warning Robbie Hatley <see.my.signature@for.my.contact.info> - 2020-02-06 23:57 -0800
  Re: Puzzling "implicit declaration" Warning Jorgen Grahn <grahn+nntp@snipabacken.se> - 2020-02-07 08:53 +0000
    Re: Puzzling "implicit declaration" Warning Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2020-02-07 02:12 -0800
      Re: Puzzling "implicit declaration" Warning Robbie Hatley <see.my.signature@for.my.contact.info> - 2020-02-08 01:53 -0800
        Re: Puzzling "implicit declaration" Warning Ben Bacarisse <ben.usenet@bsb.me.uk> - 2020-02-08 12:59 +0000
          Re: Puzzling "implicit declaration" Warning Manfred <noname@invalid.add> - 2020-02-08 17:21 +0100
            Re: Puzzling "implicit declaration" Warning Manfred <noname@invalid.add> - 2020-02-08 17:34 +0100
            Re: Puzzling "implicit declaration" Warning Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2020-02-08 14:37 -0800
        Re: Puzzling "implicit declaration" Warning scott@slp53.sl.home (Scott Lurndal) - 2020-02-08 18:39 +0000
    Re: Puzzling "implicit declaration" Warning Robbie Hatley <see.my.signature@for.my.contact.info> - 2020-02-08 01:15 -0800
      Re: Puzzling "implicit declaration" Warning chad <cdalten@gmail.com> - 2020-02-08 05:05 -0800
      Re: Puzzling "implicit declaration" Warning Bart <bc@freeuk.com> - 2020-02-08 14:05 +0000
      Re: Puzzling "implicit declaration" Warning Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2020-02-08 14:35 -0800
        Re: Puzzling "implicit declaration" Warning Bart <bc@freeuk.com> - 2020-02-08 22:39 +0000
          Re: Puzzling "implicit declaration" Warning Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2020-02-08 15:30 -0800
          Re: Puzzling "implicit declaration" Warning David Brown <david.brown@hesbynett.no> - 2020-02-09 12:24 +0100
        Re: Puzzling "implicit declaration" Warning Robbie Hatley <see.my.signature@for.my.contact.info> - 2020-02-09 23:42 -0800
          Re: Puzzling "implicit declaration" Warning chad <cdalten@gmail.com> - 2020-02-10 05:34 -0800
          Re: Puzzling "implicit declaration" Warning Ben Bacarisse <ben.usenet@bsb.me.uk> - 2020-02-10 16:12 +0000
          Re: Puzzling "implicit declaration" Warning Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2020-02-10 11:14 -0800
            Re: Puzzling "implicit declaration" Warning Jorgen Grahn <grahn+nntp@snipabacken.se> - 2020-02-11 07:06 +0000
            Re: puzzling enragement by innocuous variable names Robbie Hatley <see.my.signature@for.my.contact.info> - 2020-09-10 19:47 -0700
              Re: puzzling enragement by innocuous variable names Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2020-09-10 20:28 -0700
              Re: puzzling enragement by innocuous variable names Peter 'Shaggy' Haywood <phaywood@alphalink.com.au> - 2020-09-13 14:57 +1000
                Re: puzzling enragement by innocuous variable names gazelle@shell.xmission.com (Kenny McCormack) - 2020-09-15 17:58 +0000
              Re: puzzling enragement by innocuous variable names Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2020-09-14 09:38 -0600
          Re: Puzzling "implicit declaration" Warning James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-02-11 00:21 -0500
  Re: Puzzling "implicit declaration" Warning Les Cargill <lcargill99@comcast.com> - 2020-02-08 17:00 -0600
    Re: Puzzling "implicit declaration" Warning Robbie Hatley <see.my.signature@for.my.contact.info> - 2020-02-09 22:50 -0800
      Re: Puzzling "implicit declaration" Warning Ben Bacarisse <ben.usenet@bsb.me.uk> - 2020-02-10 16:00 +0000
        Re: Puzzling "implicit declaration" Warning scott@slp53.sl.home (Scott Lurndal) - 2020-02-10 16:28 +0000
          Re: Puzzling "implicit declaration" Warning Ben Bacarisse <ben.usenet@bsb.me.uk> - 2020-02-10 16:42 +0000

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