Re: Updating <sys/cdefs.h> in glibc and gnulib

[Bruno Haible]

Florian Weimer wrote:
> Does gnulib still override <sys/cdefs.h> unconditionally?

Gnulib does not override <sys/cdefs.h>, and never did.

That is, when a package that uses Gnulib does
  #include <sys/cdefs.h>
it will get the <sys/cdefs.h> of the system (from glibc, *BSD, Cygwin,
etc.).

Only when a package uses the compiler option "-I /usr/include/sys"
and
  #include <cdefs.h>
there would be a conflict between what Gnulib ships and the installed
file in /usr/include/sys. But nobody does that, because there would
already be a conflict between <time.h> and <sys/time.h>.

What Gnulib does is to ship a copy of glibc's <sys/cdefs.h> as <cdefs.h>,
not <sys/cdefs.h>. And it is used for a few compilation units only
(essentially regex, fnmatch, glob, and a few others).

> Why does gnulib bundle <sys/cdefs.h>?

Gnulib takes the source code of regex, fnmatch, glob, and a few other
functions from glibc and makes them portable to other platforms. Since
this source code contains references to __glibc_unlikely,
__attribute_warn_unused_result__, etc., Gnulib uses the copy of cdefs.h
to define these macros in the way the source code shared with glibc
expects it.

Some of these symbols are also defined on other platforms, sometimes
differently. Therefore Gnulib has to be careful to not override essential
symbols of these other platforms. It's not trivial, but there appears
to be enough room to navigate through the two constraints. [1]

> In the past, some gnulib-using programs supplied their own copy
> of <sys/cdefs.h> instead, even when building against glibc.  This caused
> build failures in the glibc headers because they (quite reasonably)
> assumed that <sys/cdefs.h> defines the macros for that glibc version.

You need to take this up with the respective packages. As I said above,
Gnulib overrides <cdefs.h>, not <sys/cdefs.h>.

There were some problems around the 'glob' code, but they were resolved
more than 1.5 years ago. This area is still a bit fragile, though.

> We could move glibc's internal definitions to a new header, reducing
> <sys/cdefs.h> in scope, but presumably that means gnulib would just
> starting bundling that other header, and we would have the same issue
> once more.

Yes, such a move would be pointless.

Bruno

[1] https://lists.gnu.org/archive/html/bug-gnulib/2023-01/msg00238.html