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From: Ike Naar <ike@sdf.org>
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 09:15:40 -0000 (UTC)
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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.