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Message-ID: <20220307183831.GL7074@brightrain.aerifal.cx>
Date: Mon, 7 Mar 2022 13:38:32 -0500
From: Rich Felker <dalias@...c.org>
To: James Y Knight <jyknight@...gle.com>
Cc: musl@...ts.openwall.com, "wangjianjian (C)" <wangjianjian3@...wei.com>
Subject: Re: basename with no parameter?
On Mon, Mar 07, 2022 at 11:42:39AM -0500, James Y Knight wrote:
> Note that the C2x standard intends to remove deprecated non-prototype
> declarations, along with K&R-style non-prototype definitions. Empty
> parens in a declaration will be treated as a zero-arg prototype, just as in
> C++. (http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2841.htm).
I was vaguely aware of this, and it's somewhat unfortunate because it
removes some power from the language. There are some magical tricks
you can do with non-prototype declarations and _Generic that will no
longer be possible...
> Additionally, compilers may start emitting some default-on warnings for use
> of these deprecated features even in pre-C2x language modes, although
> that'd likely be suppressed for system headers.
>
> It'd probably be a good idea to remove any such hacks which depend on
> non-prototype declarations, to get ahead of these changes.
Seems like a good idea. I wonder whether we should remove the
declaration entirely here or put back the prototype...
Rich
> On Mon, Mar 7, 2022 at 8:19 AM Rich Felker <dalias@...c.org> wrote:
>
> > On Mon, Mar 07, 2022 at 10:31:41AM +0800, wangjianjian (C) wrote:
> > > Hi all,
> > >
> > > I find that the basename in string.h with _GNU_SOURCE in Musl
> > > libc(Line 119):
> > >
> > > char *basename();
> >
> > This is not a declaration with no parameter. It's a declaration
> > without any prototype.
> >
> > > The man page says that have two different version of basename
> > > however both need one parameter, is this correct?
> >
> > No, that's documenting glibc. There is only one version of basename in
> > musl and it's the standard one.
> >
> > The reason for the non-prototype declaration is explained in commit
> > 37bb3cce4598c19288628e675eaf1cda6e96958f:
> >
> > omit declaration of basename wrongly interpreted as prototype in C++
> >
> > the non-prototype declaration of basename in string.h is an ugly
> > compromise to avoid breaking 2 types of broken software:
> >
> > 1. programs which assume basename is declared in string.h and thus
> > would suffer from dangerous pointer-truncation if an implicit
> > declaration were used.
> >
> > 2. programs which include string.h with _GNU_SOURCE defined but then
> > declare their own prototype for basename using the incorrect GNU
> > signature for the function (which would clash with a correct
> > prototype).
> >
> > however, since C++ does not have non-prototype declarations and
> > interprets them as prototypes for a function with no arguments, we
> > must omit it when compiling C++ code. thankfully, all known broken
> > apps that suffer from the above issues are written in C, not C++.
> >
> > This was from 2012, so it might make sense to do something different
> > now, like putting the correct prototype there and getting any programs
> > it still clashes with fixed.
> >
> > Rich
> >
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