Re: how to make a macro work as a single line if stmt without braces

From	Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups	comp.lang.c
Subject	Re: how to make a macro work as a single line if stmt without braces
Date	2024-09-21 14:44 -0700
Organization	None to speak of
Message-ID	<87jzf4r8at.fsf@nosuchdomain.example.com> (permalink)
References	<PaWdnZ3R-9zI6nP7nZ2dnZfqn_GdnZ2d@brightview.co.uk> <slrnvet3lr.8b8.ike@iceland.freeshell.org>

Show all headers | View raw

Ike Naar <ike@sdf.org> writes:
> On 2024-09-21, Mark Summerfield <mark@qtrac.eu> wrote:
>> I have this macro:
>>
>> #define WARN(...)                                       \
>>     do {                                                \
>>         fprintf(stderr, "%s#%d: ", __FILE__, __LINE__); \
>>         fprintf(stderr, __VA_ARGS__);                   \
>>     } while (0);
>>
>> [...]
>>
>> I would prefer to be able to write this instead:
>>
>> total++;
>> if (failed)
>>     WARN("failed because...");
>> else
>>     ok++;
>>
>> but doing so results in a compiler error:
>>
>> error: 'else' without a previous 'if'
>
> After expansion of the WARN macro there are two semicolons before the 'else'.
> The first semicolon is from the
>
>     } while (0);
>
> line, the second is from the
>
>     
>     WARN("failed because...");
>
> line. 
> Removing the semicolon from one of these two lines fixes the compiler error.

And removing the semicolon from the macro definition is the only
sensible way to do this.

The do ... while (condition) idiom is the conventional way to write a
macro that can be used in any context that requires a statement.  Adding
the semicolon in the macro definition and expecting users to omit the
semicolon when invoking it is not a good idea.

See question 10.4 of the comp.lang.c FAQ, <https://www.c-faq.com/>.

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
void Void(void) { Void(); } /* The recursive call of the void */

Back to comp.lang.c | Previous | Next — Previous in thread | Next in thread | Find similar

Thread

how to make a macro work as a single line if stmt without braces Mark Summerfield <mark@qtrac.eu> - 2024-09-21 07:35 +0000
  Re: how to make a macro work as a single line if stmt without braces Lawrence D'Oliveiro <ldo@nz.invalid> - 2024-09-21 08:10 +0000
  Re: how to make a macro work as a single line if stmt without braces David Brown <david.brown@hesbynett.no> - 2024-09-21 10:47 +0200
    Re: how to make a macro work as a single line if stmt without braces Andrey Tarasevich <andreytarasevich@hotmail.com> - 2024-09-21 12:27 -0700
      Re: how to make a macro work as a single line if stmt without braces Bart <bc@freeuk.com> - 2024-09-21 21:11 +0100
      Re: how to make a macro work as a single line if stmt without braces Opus <ifonly@youknew.org> - 2024-09-21 22:39 +0200
      Re: how to make a macro work as a single line if stmt without braces Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-09-21 14:54 -0700
        Re: how to make a macro work as a single line if stmt without braces Andrey Tarasevich <andreytarasevich@hotmail.com> - 2024-09-21 20:05 -0700
          Re: how to make a macro work as a single line if stmt without braces Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-09-21 21:33 -0700
          Re: how to make a macro work as a single line if stmt without braces David Brown <david.brown@hesbynett.no> - 2024-09-22 14:15 +0200
          Re: how to make a macro work as a single line if stmt without braces Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-09-27 03:38 -0700
      Re: how to make a macro work as a single line if stmt without braces David Brown <david.brown@hesbynett.no> - 2024-09-22 13:59 +0200
        Re: how to make a macro work as a single line if stmt without braces Kaz Kylheku <643-408-1753@kylheku.com> - 2024-09-22 15:11 +0000
          Re: how to make a macro work as a single line if stmt without braces Bart <bc@freeuk.com> - 2024-09-22 16:56 +0100
            Re: how to make a macro work as a single line if stmt without braces Michael S <already5chosen@yahoo.com> - 2024-09-22 19:27 +0300
              Re: how to make a macro work as a single line if stmt without braces Bart <bc@freeuk.com> - 2024-09-22 17:47 +0100
              Re: how to make a macro work as a single line if stmt without braces Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-09-24 07:36 -0700
                Re: how to make a macro work as a single line if stmt without braces Andrey Tarasevich <andreytarasevich@hotmail.com> - 2024-09-24 07:52 -0700
                Re: how to make a macro work as a single line if stmt without braces David Brown <david.brown@hesbynett.no> - 2024-09-24 17:24 +0200
                Re: how to make a macro work as a single line if stmt without braces Bart <bc@freeuk.com> - 2024-09-24 17:22 +0100
                Re: how to make a macro work as a single line if stmt without braces Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-09-24 12:28 -0700
                Re: how to make a macro work as a single line if stmt without braces Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-09-27 01:43 -0700
                Re: how to make a macro work as a single line if stmt without braces Bart <bc@freeuk.com> - 2024-09-24 17:35 +0100
                Re: how to make a macro work as a single line if stmt without braces Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-09-24 12:30 -0700
                Re: how to make a macro work as a single line if stmt without braces Bart <bc@freeuk.com> - 2024-09-24 20:42 +0100
                Re: how to make a macro work as a single line if stmt without braces Kaz Kylheku <643-408-1753@kylheku.com> - 2024-09-24 19:59 +0000
                Re: how to make a macro work as a single line if stmt without braces Bart <bc@freeuk.com> - 2024-09-24 21:12 +0100
                Re: how to make a macro work as a single line if stmt without braces Kaz Kylheku <643-408-1753@kylheku.com> - 2024-09-24 22:36 +0000
            Re: how to make a macro work as a single line if stmt without braces Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-09-22 13:39 -0700
              Re: how to make a macro work as a single line if stmt without braces David Brown <david.brown@hesbynett.no> - 2024-09-23 08:16 +0200
                Re: how to make a macro work as a single line if stmt without braces Richard Harnden <richard.nospam@gmail.invalid> - 2024-09-23 08:06 +0100
                Re: how to make a macro work as a single line if stmt without braces David Brown <david.brown@hesbynett.no> - 2024-09-23 11:14 +0200
                Re: how to make a macro work as a single line if stmt without braces Richard Harnden <richard.nospam@gmail.invalid> - 2024-09-23 11:37 +0100
                Re: how to make a macro work as a single line if stmt without braces David Brown <david.brown@hesbynett.no> - 2024-09-23 14:22 +0200
          Modern text editor - 'bout time someone paid attention to keeping the thread title relevant!!! (Was: how to make a macro work as a single line if stmt without braces) gazelle@shell.xmission.com (Kenny McCormack) - 2024-09-22 16:03 +0000
          Re: how to make a macro work as a single line if stmt without braces David Brown <david.brown@hesbynett.no> - 2024-09-22 18:03 +0200
      Re: how to make a macro work as a single line if stmt without braces Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-09-28 05:02 -0700
        Re: how to make a macro work as a single line if stmt without braces Michael S <already5chosen@yahoo.com> - 2024-09-28 20:30 +0300
          Re: how to make a macro work as a single line if stmt without braces Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-09-28 21:53 -0700
            Re: how to make a macro work as a single line if stmt without braces Michael S <already5chosen@yahoo.com> - 2024-09-29 12:48 +0300
              Re: how to make a macro work as a single line if stmt without braces Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-09-29 19:18 -0700
              Re: how to make a macro work as a single line if stmt without braces Alan Mackenzie <acm@muc.de> - 2024-09-30 15:05 +0000
        Re: how to make a macro work as a single line if stmt without braces Andrey Tarasevich <andreytarasevich@hotmail.com> - 2024-09-28 21:02 -0700
          Re: how to make a macro work as a single line if stmt without braces Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-09-28 22:47 -0700
            Re: how to make a macro work as a single line if stmt without braces Andrey Tarasevich <andreytarasevich@hotmail.com> - 2024-09-29 07:41 -0700
              Re: how to make a macro work as a single line if stmt without braces Michael S <already5chosen@yahoo.com> - 2024-09-29 18:04 +0300
              Re: how to make a macro work as a single line if stmt without braces scott@slp53.sl.home (Scott Lurndal) - 2024-09-29 18:30 +0000
              Re: how to make a macro work as a single line if stmt without braces Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-09-29 16:39 -0700
                Re: how to make a macro work as a single line if stmt without braces Kaz Kylheku <643-408-1753@kylheku.com> - 2024-09-30 02:29 +0000
                Re: how to make a macro work as a single line if stmt without braces Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2024-09-30 11:26 +0200
              Re: how to make a macro work as a single line if stmt without braces Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-09-29 18:39 -0700
              Re: how to make a macro work as a single line if stmt without braces Kaz Kylheku <643-408-1753@kylheku.com> - 2024-09-30 02:03 +0000
              Re: how to make a macro work as a single line if stmt without braces David Brown <david.brown@hesbynett.no> - 2024-09-30 13:17 +0200
  Re: how to make a macro work as a single line if stmt without braces Bart <bc@freeuk.com> - 2024-09-21 10:13 +0100
    Re: how to make a macro work as a single line if stmt without braces Mark Summerfield <mark@qtrac.eu> - 2024-09-21 09:53 +0000
  Re: how to make a macro work as a single line if stmt without braces Ike Naar <ike@sdf.org> - 2024-09-21 09:15 +0000
    Re: how to make a macro work as a single line if stmt without braces Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-09-21 14:44 -0700
  Re: how to make a macro work as a single line if stmt without braces Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-09-21 06:07 -0700
    Re: how to make a macro work as a single line if stmt without braces Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-09-21 15:55 -0700
  Re: how to make a macro work as a single line if stmt without braces Andrey Tarasevich <andreytarasevich@hotmail.com> - 2024-09-21 12:17 -0700
  Re: how to make a macro work as a single line if stmt without braces Blue-Maned_Hawk <bluemanedhawk@invalid.invalid> - 2024-09-21 21:18 +0000
    Re: how to make a macro work as a single line if stmt without braces Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-09-24 06:18 -0700

csiph-web