ISO C99 support in <math.h> for C++98

[Yaakov Selkowitz]

On 2016-03-22 09:52, Andre Vieira (lists) wrote:
> On arm and aarch64 the libstdc++ test
> 26_numerics/headers/cmath/c99_classification_macros_c++98.cc has been
> failing with (among others) the following errors:
[snip]
> Since the only code my gcc-5 and trunk builds share is newlib I suspect
> it might be caused by it. I had a look at cmath and found that the
> macros mentioned above are undef'd if the condition
> _GLIBCXX_USE_C99_MATH holds true. This is defined as
> _GLIBCXX98_USE_C99_MATH if -std=gnu++98 is passed and in my latest
> builds _GLIBCXX98_USE_C99_MATH is no longer being defined in
> c++config.h. This define is set by gcc's configure step 'Checking for
> ISO C99 support in <math.h> for C++98' which used to succeed but now fails.
>
> I did get a bit lost after locating the code to check this in
> gcc/libstdc++/acinclude.m4 as it seems it tries to compile a file with
> all the functions (macros in C99), that are undef'd in cmath if
> _GLIBCXX_USE_C99_MATH, which isn't happening... So apparently during
> configuration it seems include math.h does not define this set of
> functions, so _GLIBCXX98_USE_C99_MATH is never defined and thus the
> macros for these functions are never undef'ed in cmath, where all of a
> sudden they seem to be defined.
>
> Does anyone have an idea whats going on here? I wanted to try with
> earlier commits of newlib to maybe identify what change was responsible
> for this new behavior, but I am failing to build due to the
> configuration scripts needing regeneration and I wasn't able to
> regenerate them. I tried a sequence of commands that used to work in the
> past but they no longer do :(

We also had issues enabling C99-in-C++ on Cygwin (which is based on 
newlib), which required a few patches:

https://github.com/cygwinports/gcc/blob/master/0023-glibcxx-use-c99.patch
https://github.com/cygwinports/gcc/blob/master/0026-g%2B%2B-gnu-source.patch

It is on my to-do list to get this working on newlib targets once we 
have a snapshot with the new feature test macros.

-- 
Yaakov