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From: Bart <bc@freeuk.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 10:13:00 +0100
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On 21/09/2024 08:35, Mark Summerfield wrote:
> 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'

Remove the semicolon at the end of the macro.

As it is, it compiles to:

  if (failed) do {...} while 0;; else ok++:

There are two semicolons together, the second terminates the whole if 
statement, leaving an open 'else'.