This is the mail archive of the
libc-alpha@sourceware.org
mailing list for the glibc project.
Re: [PATCH 05/10] Remove __need macros from errno.h (__need_Emath, __need_error_t).
On Wed, 10 May 2017, Florian Weimer wrote:
> On 05/09/2017 11:38 PM, Joseph Myers wrote:
> > On Tue, 9 May 2017, Zack Weinberg wrote:
> >
> > > diff --git a/misc/sys/cdefs.h b/misc/sys/cdefs.h
> > > index 74f9a49b05..075c996f28 100644
> > > --- a/misc/sys/cdefs.h
> > > +++ b/misc/sys/cdefs.h
> > > @@ -326,6 +326,13 @@
> > > # define __attribute_artificial__ /* Ignore */
> > > #endif
> > > +/* Force use of the initial-exec TLS model. */
> > > +#if __GNUC_PREREQ (3,3)
> > > +# define __attribute_tls_model_ie__ __attribute__ ((tls_model
> > > ("initial-exec")))
> >
> > This is not namespace-clean for an installed header (needs to be
> > __tls_model__).
>
> The larger problem is that we need to error out if __GLIBC_USE (TLS_ERRNO) and
> the compiler does not support the initial-exec TLS model because it cannot
> produce the required ABI for the errno symbol.
You don't need initial-exec TLS for errno. If a port wishes to export TLS
errno directly, it's perfectly valid for the headers to do
extern _Thread_local int errno;
#define errno errno
with any C11 compiler, and if the compiler only supports GD model TLS,
that's fine for accessing an IE variable (it may or may not get optimized
to IE by the linker). Declaring the TLS model is simply an optimization.
(This is not an assertion about the relative efficiency of a GD access to
errno versus calling __errno_location which in turn does an IE access.)
--
Joseph S. Myers
joseph@codesourcery.com