Re: [PATCH] add _DIRENT_HAVE_D_* constants to dirent.h

[Rich Felker <dalias@libc.org>]

On Thu, Jan 14, 2016 at 05:44:49PM -0500, Rich Felker wrote:
> On Wed, Jan 13, 2016 at 07:09:19PM +0100, Felix Janda wrote:
> > they can be used for checking existence of non-standard fields of
> > struct dirent, are used by various programs to avoid configure
> > checks and are at least present on glibc, uclibc, dietlibc and
> > newlib.
> > ---
> > The motivation for this patch is
> > 
> > http://oss.sgi.com/archives/xfs/2016-01/msg00388.html
> > ---
> >  include/dirent.h | 3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/include/dirent.h b/include/dirent.h
> > index 5aa8510..b2ffe8a 100644
> > --- a/include/dirent.h
> > +++ b/include/dirent.h
> > @@ -57,6 +57,9 @@ int getdents(int, struct dirent *, size_t);
> >  #endif
> >  
> >  #ifdef _GNU_SOURCE
> > +#define _DIRENT_HAVE_D_OFF
> > +#define _DIRENT_HAVE_D_RECLEN
> > +#define _DIRENT_HAVE_D_TYPE
> >  int versionsort(const struct dirent **, const struct dirent **);
> >  #endif
> 
> I like the concept of defining macros to publish the availability of
> extensions like this, but I don't think they should be dependent on
> _GNU_SOURCE. At the very least they should also be available under
> _DEFAULT_SOURCE (_BSD_SOURCE), but since they're in the reserved
> namespace (underscore followed by capital) I'd prefer to have them
> exposed unconditionally.
> 
> I'm also skeptical of whether d_off and d_reclen should be promoted as
> public APIs. They don't seem to be useful (unlike d_type which is
> highly useful) and may lock down some assumptions about
> implementation.

Perhaps to make a discussion of this more concrete: is there other
software that's using the d_reclen or d_off fields conditionally on
having the above macros defined? What is is using them for?

I read the above-linked mailing list discussion and I think the
autoconf test is a better approach than depending on the macros (since
they're not present on older musl or on other non-glibc libcs/systems)
but we should probably add at least _DIRENT_HAVE_D_TYPE and perhaps
the others too.

Rich