linux-kernel - Re: [PATCH v2 03/11] tools/nolibc/stdio: remove perror if NOLIBC_IGNORE

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Message-ID: <20250921075511.GA16684@1wt.eu>
Date: Sun, 21 Sep 2025 09:55:11 +0200
From: Willy Tarreau <w@....eu>
To: Benjamin Berg <benjamin@...solutions.net>
Cc: linux-um@...ts.infradead.org,
        Thomas Weißschuh <linux@...ssschuh.net>,
        linux-kselftest@...r.kernel.org,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        linux-kernel@...r.kernel.org, Benjamin Berg <benjamin.berg@...el.com>
Subject: Re: [PATCH v2 03/11] tools/nolibc/stdio: remove perror if
 NOLIBC_IGNORE_ERRNO is set

Hi Benjamin,

On Fri, Sep 19, 2025 at 05:34:12PM +0200, Benjamin Berg wrote:
> From: Benjamin Berg <benjamin.berg@...el.com>
> 
> There is no errno variable when NOLIBC_IGNORE_ERRNO is defined. As such,
> the perror function does not make any sense then and cannot compile.
> 
> Fixes: acab7bcdb1bc ("tools/nolibc/stdio: add perror() to report the errno value")
> Signed-off-by: Benjamin Berg <benjamin.berg@...el.com>
> Acked-by: Thomas Weißschuh <linux@...ssschuh.net>
> ---
>  tools/include/nolibc/stdio.h | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/tools/include/nolibc/stdio.h b/tools/include/nolibc/stdio.h
> index 7630234408c5..c512159b8374 100644
> --- a/tools/include/nolibc/stdio.h
> +++ b/tools/include/nolibc/stdio.h
> @@ -597,11 +597,13 @@ int sscanf(const char *str, const char *format, ...)
>  	return ret;
>  }
>  
> +#ifndef NOLIBC_IGNORE_ERRNO
>  static __attribute__((unused))
>  void perror(const char *msg)
>  {
>  	fprintf(stderr, "%s%serrno=%d\n", (msg && *msg) ? msg : "", (msg && *msg) ? ": " : "", errno);
>  }
> +#endif

Please instead place the ifndef inside the function so that code calling
perror() continues to build. The original goal of that macro was to
further shrink programs at the expense of losing error details. But we
should be able to continue to build working programs with that macro
defined. There's nothing hard set in stone regarding this but here it's
easy to preserve a working behavior by having something like this for
example:

  static __attribute__((unused))
  void perror(const char *msg)
  {
 +#ifdef NOLIBC_IGNORE_ERRNO
 + 	fprintf(stderr, "%s\n", (msg && *msg) ? msg : "unknown error");
 +#else
  	fprintf(stderr, "%s%serrno=%d\n", (msg && *msg) ? msg : "", (msg && *msg) ? ": " : "", errno);
 +#endif
  }

thanks!
Willy