Re: bug: "gcc -std=gnu99" passes AC_PROG_CC_C11

[Adrian Bunk]

On Wed, Sep 26, 2012 at 08:09:48PM -0700, Paul Eggert wrote:
> >> C11 is not incompatible with C99, the way that C99 was incompatible with 
> >> C89.
>...
> > The CC="gcc -std=gnu99" is needed in my example, it is not a part of the 
> > scenario when the compiler defaults to C99 - which is neither for gcc 
> > nor for other compilers that implausible.
> 
> Yes, that's plausible.

And now I also found a case where the difference between gnu99 and gnu11 
matters today:

glibc 2.16 has the following protected by what boils down to
  #if defined _GNU_SOURCE || defined _ISOC11_SOURCE || \
      (defined __STDC_VERSION__ && __STDC_VERSION__ >= 201112L)

- timespec_get() and TIME_UTC
- aligned_alloc()
- at_quick_exit()
- quick_exit()
- CMPLX/CMPLXF/CMPLXL
- static_assert


> But having '-std=gnu11' break things doesn't
> sound plausible.

Sorry, I shouldn't have started talking about a separate different thing 
in one email with my "does -std=gnu11 break anything?" remark.

>...
> > So autoconf is now trying an option that will fail the test with the 
> > current compiler, and will likely be deprecated
> 
> Perhaps we should back off on how many C11 features we're checking, so
> that IBM XL C V12.1 passes too?  That might be more useful.

On a downside, that requires users to check for more features.

It's a policy decision which features should be guaranteed to be 
available when ac_cv_prog_cc_c11 is not "no", and for which features 
autoconf should provide separate tests and autoconf users then have
to check separately.

cu
Adrian

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