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From: Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups: comp.lang.c
Subject: Re: how to make a macro work as a single line if stmt without braces
Date: Sat, 21 Sep 2024 06:07:15 -0700
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Mark Summerfield <mark@qtrac.eu> writes:

> I have this macro:
>
> #define WARN(...)                                       \
>     do {                                                \
>         fprintf(stderr, "%s#%d: ", __FILE__, __LINE__); \
>         fprintf(stderr, __VA_ARGS__);                   \
>     } while (0);
>
> which I use like this:
>
> total++;
> if (failed) {
>     WARN("failed because...");
> } else
>     ok++;
>
> 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'

You can define WARN() this way

   #define WARN(...) (                                  \
      fprintf(stderr, "%s#%d: ", __FILE__, __LINE__),   \
      fprintf(stderr, __VA_ARGS__)                      \
   )

and it will work as you want it to.