linux-kernel - Re: [PATCH 1/2] tools/nolibc: add stdarg.h header

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Message-ID: <b2c2ca69-b9bb-40b1-a05a-6d2f66e01034@t-8ch.de>
Date:   Tue, 29 Aug 2023 12:16:23 +0200
From:   Thomas Weißschuh <linux@...ssschuh.net>
To:     Willy Tarreau <w@....eu>
Cc:     Shuah Khan <shuah@...nel.org>, Zhangjin Wu <falcon@...ylab.org>,
        Yuan Tan <tanyuan@...ylab.org>, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org
Subject: Re: [PATCH 1/2] tools/nolibc: add stdarg.h header

On 2023-08-29 11:26:19+0200, Willy Tarreau wrote:
> On Tue, Aug 29, 2023 at 11:14:09AM +0200, Thomas Weißschuh wrote:
> > Hi Willy!
> > 
> > On 2023-08-29 08:28:27+0200, Willy Tarreau wrote:
> > > On Sun, Aug 27, 2023 at 10:00:15AM +0200, Thomas Weißschuh wrote:
> > > > This allows nolic to work with `-nostdinc` avoiding any reliance on
> > > > system headers.
> > > > 
> > > > The implementation has been lifted from musl libc 1.2.4.
> > > > There is already an implementation of stdarg.h in include/linux/stdarg.h
> > > > but that is GPL licensed and therefore not suitable for nolibc.
> > > 
> > > I'm a bit confused because for me, stdarg was normally provided by the
> > > compiler, but I could be mistaken. It's just that it reminds me not so
> > > old memories. Therefore maybe we just need to include or define
> > > "something" to use it.
> > 
> > It is indeed provided by the compiler.
> 
> OK. But then, doesn't it mean that if we don't provide our stdarg.h,
> the compilers' will be used ? I'm asking because we're already using
> va_list and va_args, for example in vfprintf() in stdio.h, which
> precisely includes <stdarg.h> so it must indeed come from the compiler.

It will be used *iff* -nostdinc is *not* passed.

I think we need to clarify the definition of the word "provided".
For me it means that the compiler ships an implementation of this header
file in the compiler-specific include directory.

If -nostdinc is passed this include directory is not actually usable.

If a user wants to avoid the implicit usage of any system-provided
headers they need to pass -nostdinc, as far as I know there is no flag
to keep only the compiler-specific include directories.

One usecase is in nolibc-test itself, where Zhangjin ran into weird
and inconsistent behavior of system includes being pulled in.
By using -nostdinc we avoid this.

I can also see this being useful for normal users.

> > I could not find anybody doing this differently.
> > Using builtins seems to me to be the normal way to expose compiler
> > implementation specifics.
> 
> OK but it's already what the compiler does itself in its own stdarg that
> is provided. That's why I don't understand what specific case we're trying
> to cover here, I feel like we're providing an alternate stdarg in case the
> compiler doesn't provide one except that I've not seen a compiler not
> provide it (even tcc comes with it), it's like stddef.

It's all about supporting -nostdinc.


FYI stdint.h is also provided by nolibc, gcc and glibc.