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

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Message-ID: <c10503a9-5c63-44a8-9ea7-a7bf6c4ed3fb@t-8ch.de>
Date: Sun, 21 Sep 2025 18:37:30 +0200
From: Thomas Weißschuh <linux@...ssschuh.net>
To: Willy Tarreau <w@....eu>
Cc: Benjamin Berg <benjamin@...solutions.net>, 
	linux-um@...ts.infradead.org, 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

On 2025-09-21 09:55:11+0200, Willy Tarreau wrote:
> 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
>   }

For the plain `errno` variable and printf(%m) we don't have such
fallbacks. With NOLIBC_IGNORE_ERRNO the compilation either fails or the
results are undefined. Personally I prefer not defining perror() here.


Thomas