Re: [PATCH v2 03/11] tools/nolibc/stdio: remove perror if NOLIBC_IGNORE_ERRNO is set

Thomas Weißschuh
2025-09-21T16:37:39Z

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@intel.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@intel.com>
> > Acked-by: Thomas Weißschuh <linux@weissschuh.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