[Toybox] FYI musl's support horizon.

[enh]

On Fri, Aug 27, 2021 at 3:21 PM Rich Felker <dalias at libc.org> wrote:

> On Fri, Aug 27, 2021 at 10:21:51AM -0500, Rob Landley wrote:
> > On 8/27/21 8:56 AM, Rich Felker wrote:
> > > On Thu, Aug 26, 2021 at 03:56:47PM -0700, enh wrote:
> > >> On Thu, Aug 26, 2021 at 2:34 PM Rob Landley <rob at landley.net> wrote:
> > >
> > > Rob asked me for some input on this:
> > >
> > >> > In my private emails somebody is trying to make the last aboriginal
> linux
> > >> > release work and the old busybox isn't building anymore because
> makedev()
> > >> > used
> > >> > to be in #include <sys/types.h> and now it's moved to
> <sys/sysmacros.h>.
> > >> > (Why? I
> > >> > dunno. Third base.)
> > >
> > > This was glibc bug #19239, and clearly needed to be fixed. Generic
> > > lowercase macro names like "major" and "minor" are *extreme* namespace
> > > pollution for a standard header to be doing.
> >
> > I stared at that a bit before figuring out you were objecting because
> they're
> > MACROS. (Generic words being FUNCTION names from headers is common:
> stdio.h
> > defines remove(), rename(), rewind()... stdlib.h has random(), free(),
> abort(),
> > exit()...)
>
> Because those names are defined as being there by the C language and
> expected to be there by anyone writing programs in C. Indeed, it makes
> it far *worse* that major and minor were macros, since they break
> things in more contexts, but if they were functions it would still be
> inappropriate for sys/types.h to be exposing them.
>
> > > sys/sysmacros.h was
> > > always the correct way to get these macros on glibc, but for whatever
> > > reason (compatibility with some ancient historic unices?) glibc was
> > > making sys/types.h include sys/sysmacros.h implicitly, at least under
> > > some profiles.
> >
> > The problem is the man page said to #include <sys/types.h> to get it, so
> that's
> > what people did.
>
> That's unfortunate. There's been a lot of wrong stuff in man pages
> but Michael is very good about taking reports and getting it
> corrected.
>

i think the problem is that it _said_ that, not that it says that now:

SYNOPSIS
       #include <sys/sysmacros.h>

https://man7.org/linux/man-pages/man3/major.3.html


> > And even today the man page doesn't particularly call them out
> > as macros, it gives them function prototypes and even calls them
> functions:
> >
> >   The  major()  and  minor() functions perform the converse task: given a
> >   device ID, they return, respectively, the major and  minor  components.
>
> Yes. Historically they were always macros, but glibc made the macros
> wrap functions because they did something more elaborate with munging
> the bits to fit major and minor numbers larger than 8 bits, splitting
> the extra ones across the high 48 bits...
>

he's said he's not against pointing out differences between glibc and other
C libraries, but i haven't actually tried to send him any such changes yet
(that i recall).


> > I thought it was like getc() being officially undecided about whether or
> not
> > it's a macro. (Could be either way.)
>
> Generally it's assumed that the standard headers may (at least in C,
> and for standard functions) additionally define a macro anywhere a
> function is specified as long as it doesn't evaluate arguments the
> wrong number of times. Where the spec explicitly mentions a macro,
> that means there's no obligation to avoid multiple evaluation. However
> there are good reasons not to do this (breaking C++ callers, and when
> the name is not a standard function, just breaking programs that don't
> expect it to be there and which use the name for something else
> locally.
>
> > > What made this even worse was that, under BSD or GNU profile
> > > (including by default), glibc's stdlib.h included sys/types.h. This
> > > meant anything using stdlib.h got the namespace breakage.
> >
> > Is it "breakage" if it's the standard for Linux and has been for years?
>
> Yes.
>
> > I had a function called throw() in a turbo C++ program I wrote for DOS
> back in
> > college and a compiler upgrade turned that into a keyword and broke my
> stuff
> > because they'd decided to gratuitously add stuff to the language AFTER I
> wrote
> > my code. If it had been "compiling it on Linux behaves differently" I
> wouldn't
> > have minded so much, it was a version upgrade of the same THING breaking
> stuff
> > that bothered me.
>
> That's why we're careful to honor the implementation's obligations on
> namespace (fully in plain C or POSIX profile, and modulo well-known
> extension functions in default/BSD profile). Avoiding this kind of
> breakage requires both sides (implementation and application) to honor
> their obligations though. If either doesn't, upgrades can break stuff.
>
> > > In commit f552c792c7ce5a560f214e1104d93ee5b0833967 (2011) musl
> > > emulated this glibc behavior in _GNU_SOURCE profile, but commit
> > > a31a30a0076c284133c0f4dfa32b8b37883ac930 (2019) removed it since glibc
> > > had already fixed the problem and pushed applications still depending
> > > on the wrong behavior to fix their stuff.
> >
> > So sys/types.h couldn't have prototyped the functions (inside #ifndef if
> > necessary) and sys/macros.h couldn't have defined faster macro vesions
> then, and
> > you get whichever one you #include first?
> >
> > Eh, water over the bridge. Glibc decided to break userspace and musl
> fell in
> > line a few years later. I'm just not convinced glibc made a good call.
>
> No, we did something awful as a glibc compat hack back in 2011, only
> in _GNU_SOURCE mode because it was so awful, and removed it after it
> was no longer providing any glibc-compat. That type of hack would not
> be approved to begin with nowadays.
>
> Rich
>
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