[Bug libc/10110] New: Separate __STDC_* predefines from features.h

[Bug libc/10110] New: Separate __STDC_* predefines from features.h

[jsm28 at gcc dot gnu dot org]

To support implementing the C99 requirement that the __STDC_* predefined
macros are constant throughout the translation unit (not undefined before
system headers are included and defined afterwards), glibc should put those
such macros it defines in a standalone header included from <features.h>
that is also suitable for the compiler to preinclude by default.

A glibc patch is at <http://sourceware.org/ml/libc-alpha/2009-04/txt00001.txt>
and I'll also attach it to this PR.  Note that this patch is very simple and
it should be clear that it is safe; it's the GCC patch included with the same
message <http://sourceware.org/ml/libc-alpha/2009-04/msg00005.html> that's
the more complicated side of implementing this C99 requirement.

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           Summary: Separate __STDC_* predefines from features.h
           Product: glibc
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: libc
        AssignedTo: drepper at redhat dot com
        ReportedBy: jsm28 at gcc dot gnu dot org
                CC: glibc-bugs at sources dot redhat dot com


http://sourceware.org/bugzilla/show_bug.cgi?id=10110

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[Bug libc/10110] Separate __STDC_* predefines from features.h

[jsm28 at gcc dot gnu dot org]

------- Additional Comments From jsm28 at gcc dot gnu dot org  2009-04-28 15:17 -------
Created an attachment (id=3913)
 --> (http://sourceware.org/bugzilla/attachment.cgi?id=3913&action=view)
glibc patch


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[Bug libc/10110] Separate __STDC_* predefines from features.h

[drepper at redhat dot com]

------- Additional Comments From drepper at redhat dot com  2009-06-16 15:31 -------
I don't like this super-complicated mechanism.  Just hardcode the values in gcc
(in the spec file or where else).  They must not change anywhere.  Then glibc
can be changed to not redefine them if they are already defined.

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           What    |Removed                     |Added
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             Status|NEW                         |WAITING


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[Bug libc/10110] Separate __STDC_* predefines from features.h

[joseph at codesourcery dot com]

------- Additional Comments From joseph at codesourcery dot com  2009-06-16 16:20 -------
Subject: Re:  Separate __STDC_* predefines from features.h

On Tue, 16 Jun 2009, drepper at redhat dot com wrote:

> I don't like this super-complicated mechanism.  Just hardcode the values in gcc
> (in the spec file or where else).  They must not change anywhere.  Then glibc
> can be changed to not redefine them if they are already defined.

GCC could reasonably take responsibility for __STDC_IEC_559__ and 
__STDC_IEC_559_COMPLEX__, knowing that glibc implements the library parts 
of the requirements, and define a macro to indicate that it has taken 
responsibility for these macros.  (That would best be a separate macro, 
__IEC_559_MACROS_PREDEFINED__ or similar, since options such as 
-ffast-math mean the IEEE arithmetic requirements are not being followed 
and so these macros shouldn't actually be predefined when certain options 
are being used.)  But as I noted in the original patch submission this 
doesn't work so well for __STDC_ISO_10646__; that *should* change when 
glibc is updated for new versions of Unicode; I said:

    I strongly suspect that the value of __STDC_ISO_10646__ in glibc is
    out of date (the comment says Unicode 3.1 and the value is 200009L,
    but there have been subsequent changes updating glibc to Unicode 5).
    Updating this (in features.h before this patch or stdc-predef.h after
    it) is clearly independent of this patch.

I can implement a fixincludes-based approach, where the GCC build process 
extracts the value of __STDC_ISO_10646__ from the installed system headers 
(while determining __STDC_IEC_559__ and __STDC_IEC_559_COMPLEX__ directly 
based on the options passed to GCC); this would avoid glibc changes being 
needed, but make it more important to run the mkheaders script GCC 
installs to update the fixed headers when glibc is updated and an existing 
GCC installation is not rebuilt.  But the preinclusion mechanism in GCC is 
still needed for the use the DFP people wish to make of the GCC patch (I 
have confirmed they are interested in it), predefining __STDC_DEC_FP__ 
only when building with -I/usr/include/dfp to get DFP versions of headers 
(which include core glibc ones with #include_next); all the complexity 
goes in GCC (with some additional complexity to extract values at build 
time) and all you avoid is a small glibc patch to put some definitions in 
their own header.



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[Bug libc/10110] Separate __STDC_* predefines from features.h

[pasky at suse dot cz]

------- Additional Comments From pasky at suse dot cz  2010-06-01 03:58 -------
So this should be now marked as SUSPENDED, waiting for the gcc changes, right?

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           What    |Removed                     |Added
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             Status|WAITING                     |SUSPENDED


http://sourceware.org/bugzilla/show_bug.cgi?id=10110

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