Bug 58016 - stdatomic.h missing in 4.8.1
Summary: stdatomic.h missing in 4.8.1
Alias: None
Product: gcc
Classification: Unclassified
Component: c (show other bugs)
Version: 4.8.1
: P3 normal
Target Milestone: ---
Assignee: Not yet assigned to anyone
Depends on:
Reported: 2013-07-28 18:18 UTC by Jeff Hammond
Modified: 2013-07-29 18:45 UTC (History)
1 user (show)

See Also:
Known to work:
Known to fail:
Last reconfirmed:

patch to define macros indicating missing C11 support (316 bytes, patch)
2013-07-29 16:33 UTC, Jeff Hammond
Details | Diff

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Description Jeff Hammond 2013-07-28 18:18:34 UTC
GCC 4.8.1 provides a -std=c11 option that defines __STDC_VERSION__ >= 201112L and does not define __STDC_NO_ATOMICS__, hence is required to provide stdatomic.h.  This requirement is not met.

This ticket is closely related to 53769 but I am not reporting the fact that the macros aren't defined, but rather the missing header.

> cat test-c11-atomics.c 
#if __STDC_VERSION__ >= 201112L
# ifdef __STDC_NO_ATOMICS__
#  error Your C11 compiler is not required to provide stdatomic.h
# else
#  include <stdatomic.h>
# endif
# error Your C compiler isn't providing C11.

int main(int argc, char * argv[])
    return 0;

> gcc-mp-4.8 -g -Wall -std=c11 test-c11-atomics.c 
test-c11-atomics.c:4:23: fatal error: stdatomic.h: No such file or directory
 #include <stdatomic.h>
compilation terminated.

> gcc-mp-4.8 -v
Using built-in specs.
Target: x86_64-apple-darwin11
Configured with: ../gcc-4.8.1/configure --prefix=/opt/local --build=x86_64-apple-darwin11 --enable-languages=c,c++,objc,obj-c++,lto,fortran,java --libdir=/opt/local/lib/gcc48 --includedir=/opt/local/include/gcc48 --infodir=/opt/local/share/info --mandir=/opt/local/share/man --datarootdir=/opt/local/share/gcc-4.8 --with-local-prefix=/opt/local --with-system-zlib --disable-nls --program-suffix=-mp-4.8 --with-gxx-include-dir=/opt/local/include/gcc48/c++/ --with-gmp=/opt/local --with-mpfr=/opt/local --with-mpc=/opt/local --with-ppl=/opt/local --with-cloog=/opt/local --enable-cloog-backend=isl --disable-cloog-version-check --enable-stage1-checking --disable-multilib --enable-lto --enable-libstdcxx-time --with-as=/opt/local/bin/as --with-ld=/opt/local/bin/ld --with-ar=/opt/local/bin/ar --with-bugurl=https://trac.macports.org/newticket --with-pkgversion='MacPorts gcc48 4.8.1_0'
Thread model: posix
gcc version 4.8.1 (MacPorts gcc48 4.8.1_0) 

> uname -a
Darwin Jeffs-MacBook-Pro.local 11.4.2 Darwin Kernel Version 11.4.2: Thu Aug 23 16:25:48 PDT 2012; root:xnu-1699.32.7~1/RELEASE_X86_64 x86_64
Comment 1 joseph@codesourcery.com 2013-07-28 19:27:20 UTC
I don't know whether Andrew intends stdatomic.h to go in GCC or glibc, but 
in any case I consider this a duplicate of bug 53769, which in turn I 
don't really consider a useful bug report at all (incompleteness of the 
implementation of an option documented in the manual as incomplete should 
not be considered a bug).
Comment 2 Jeff Hammond 2013-07-28 20:42:50 UTC
If GCC doesn't support C11, it should not claim to support C11 via __STDC_VERSION__.  The C11 standard definition isn't a recommendation from which implementers can pick and choose based upon their priorities.  Documentation an implementations failure to comply with a standard does not absolve an implementation from lying about its features with ISO standard macros.  The macro is part of the standard; the documentation is not.

In any case, there is an absolutely trivial way for GCC to satisfy the C11 standard with respect to stdatomic.h, and it involves __STDC_NO_ATOMICS__.  The failure to define this macro or to provide stdatomic.h make GCC non-compliant with C11, in which case __STDC_VERSION__ is defined improperly.
Comment 3 Jonathan Wakely 2013-07-29 10:27:35 UTC
Noone disputes it's not conforming. The point is the support is incomplete. It's known to be incomplete. It's documented as incomplete.  Reporting a bug to say it's incomplete doesn't serve any useful purpose, it will be complete when it's completed.
Comment 4 joseph@codesourcery.com 2013-07-29 13:43:27 UTC
__STDC_VERSION__ describes *intent* of command-line options (as regards 
differences between standard versions, to the extent that those are 
implemented).  This is the same principle that has been documented for 
__STDC__ since at least GCC 2.0.

  "Sometimes people say that defining @code{__STDC__} in a compiler that
  does not completely conform to the ANSI C standard somehow violates the
  standard.  This is illogical.  The standard is a standard for compilers
  that are supposed to conform.  It says nothing about what any other
  compilers should do.  Whatever the ANSI C standard says is relevant to
  the design of plain @samp{gcc} without @samp{-ansi} only for pragmatic
  reasons, not as a requirement."

(quoted from the GCC 2.0 manual).

As a pragmatic matter, it's useful for users of standards modes that are 
incomplete to be able to tell which of those modes is in use, and 
__STDC_VERSION__ is the natural macro to define to distinguish between 
them.  gcc -std=c11 is a compiler explicitly claimed not to conform.
Comment 5 Jeff Hammond 2013-07-29 16:15:18 UTC
Can someone tell me where the appropriate place to define __STDC_NO_ATOMICS__ and __STDC_NO_THREADS__ in GCC so I can submit a patch?  I'd rather solve the problem and take 1-2 steps forward towards C11 compliance rather than debate the philosophical aspects of the problem.
Comment 6 Jeff Hammond 2013-07-29 16:33:58 UTC
Created attachment 30568 [details]
patch to define macros indicating missing C11 support

If GCC defines __STDC_NO_ATOMICS__ and __STDC_NO_THREADS__, it is no longer non-compliant w.r.t. C11 to not provide stdatomic.h and threads.h.  This would resolve bugs 53769 and 58016, albeit in a trivial way.

I will not be surprised at all if this patch is rejected, if for no other reason than my institution has not signed a contributor agreement with FSF (they almost certainly will if I ask).  My goal is to inspire someone else to do it properly since it seems trivial and arguably necessary.
Comment 7 joseph@codesourcery.com 2013-07-29 16:39:32 UTC
__STDC_NO_THREADS__ is defined in glibc's stdc-predef.h because it 
describes combination compiler and library properties.

The correct fix for atomics for 4.9 will be to implement them - see Andrew 
MacLeod's patches and recent discussion on gcc-patches - and the state of 
C11 in 4.8 is what it is and 4.8 is subject to normal release branch rules 
(regression and documentation fixes only, generally).
Comment 8 Jeff Hammond 2013-07-29 18:45:55 UTC
My patch was w.r.t. the trunk as of earlier today, not 4.8 but that's fine.  I just subscribed to various GCC lists to track these developments.

I'll apply my patch locally so that I can use 4.8.1 and get the desired behavior.