[Bug libc/29132] New: If <stdc-predef.h> is not preincluded, then #define __STDC_xxx violate C11 6.10.8

[Bug libc/29132] New: If <stdc-predef.h> is not preincluded, then #define __STDC_xxx violate C11 6.10.8

[pavel.morozkin at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=29132

            Bug ID: 29132
           Summary: If <stdc-predef.h> is not preincluded, then #define
                    __STDC_xxx violate C11 6.10.8
           Product: glibc
           Version: unspecified
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: libc
          Assignee: unassigned at sourceware dot org
          Reporter: pavel.morozkin at gmail dot com
                CC: drepper.fsp at gmail dot com
  Target Milestone: ---

If <stdc-predef.h> is not preincluded, then #define __STDC_xxx violate C11
6.10.8.

In GCC the <stdc-predef.h> is preincluded => OK.
In Clang the <stdc-predef.h> is not preincluded => not OK (see
https://github.com/llvm/llvm-project/issues/53132).

Relevant quotes:
C11, 6.10.8 Predefined macro names:
> None of these macro names, nor the identifier defined, shall be the subject of a
> #define or a #undef preprocessing directive.
> The values of the predefined macros listed in the following subclauses176) (except for
> _ _FILE_ _ and _ _LINE_ _) remain constant throughout the translation unit.

Consider fixing.

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[Bug libc/29132] If <stdc-predef.h> is not preincluded, then #define __STDC_xxx violate C11 6.10.8

[fweimer at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=29132

Florian Weimer <fweimer at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |WAITING
                 CC|                            |fweimer at redhat dot com
   Last reconfirmed|                            |2022-05-09

--- Comment #1 from Florian Weimer <fweimer at redhat dot com> ---
I'm not sure what you expect glibc to do here. I think these constants must
become available without including anything, so this needs compiler support.
GCC provides this with the pre-include mechanism.

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[Bug libc/29132] If <stdc-predef.h> is not preincluded, then #define __STDC_xxx violate C11 6.10.8

[schwab@linux-m68k.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=29132

--- Comment #2 from Andreas Schwab <schwab@linux-m68k.org> ---
<stdc-predef.h> is part of the implementation, so it is not subject to 6.10.8.

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[Bug libc/29132] If <stdc-predef.h> is not preincluded, then #define __STDC_xxx violate C11 6.10.8

[pavel.morozkin at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=29132

--- Comment #3 from Pavel M <pavel.morozkin at gmail dot com> ---
>From glibc-2.31 (the latest one probably is the same):
stdio.h:
#include <libio/stdio.h>

libio/stdio.h:
#include <bits/libc-header-start.h>

bits/libc-header-start.h:
#include <features.h>

features.h:
#include <stdc-predef.h>

stdc-predef.h:
#define __STDC_IEC_559__                1
#define __STDC_IEC_559_COMPLEX__        1
#define __STDC_ISO_10646__              201706L

Here we see that an implementation, which does not preinclude <stdc-predef.h>,
defines __STDC_xxx, which violates C11 6.10.8.

Example:
//__STDC_xxx is not defined
#include <stdio.h>
//__STDC_xxx is defined

Here we see that the values of the predefined macros do not remain constant
throughout the translation unit.

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[Bug libc/29132] If <stdc-predef.h> is not preincluded, then #define __STDC_xxx violate C11 6.10.8

[joseph at codesourcery dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=29132

--- Comment #4 from joseph at codesourcery dot com <joseph at codesourcery dot com> ---
The purpose of the <stdc-predef.h> feature, added in coordination between 
GCC and glibc, is to support that standard requirement in the combined 
implementation.  If you use a compiler without support for that feature, 
you're getting exactly what should be expected.

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[Bug libc/29132] If <stdc-predef.h> is not preincluded, then #define __STDC_xxx violate C11 6.10.8

[pavel.morozkin at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=29132

--- Comment #5 from Pavel M <pavel.morozkin at gmail dot com> ---
The users expect the absence of C standard violations. The "non-GCC with glibc"
developers very probably don't know that <stdc-predef.h> needs to be
preincluded. Consider at least documenting it in "using glibc with non-GCC" (or
other similar) guide.

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