Feature Conditional for M_PI

[Corinna Vinschen]

On Aug 31 10:50, Joel Sherrill wrote:
> On Mon, Aug 31, 2020 at 10:30 AM Corinna Vinschen via Newlib <
> newlib@sourceware.org> wrote:
> 
> > On Aug 31 17:27, Corinna Vinschen via Newlib wrote:
> > > On Aug 31 10:10, Joel Sherrill wrote:
> > > > Hi
> > > >
> > > > I was porting some code from Linux to Cygwin and came across this.
> > M_PI in
> > > > math.h is defined by POSIX as part of XSI. It does not appear to be
> > part of
> > > > C99 or C++03.  I have this cut down to show the problem:
> > > >
> > > > ==========================
> > > > #include <math.h>
> > > >
> > > > double pi = M_PI;
> > > > ==========================
> > > >
> > > > And this script to try various feature defines and compilers:
> > > >
> > > > ===========================
> > > > GCC=${GCC:-g++}
> > > >
> > > > ${GCC} -c m.c
> > > > ${GCC} -D_XOPEN_SOURCE=700 -c m.c
> > > > ${GCC} -D_POSIX_C_SOURCE=200809L -c m.c
> > > > ${GCC} -D_XOPEN_SOURCE=700 -c -D_POSIX_C_SOURCE=200809L -c m.c
> > > > ===========================
> > > >
> > > > All of those compiler invocations work on Linux but the third one does
> > not
> > > > work on Cygwin or RTEMS which use newlib.
> > > >
> > > > Is the proper thing to do to add  -D_XOPEN_SOURCE=700 when compiling
> > this
> > > > program?
> > > >
> > > > Just curious if Linux is defining _XOPEN_SOURCE by default and newlib
> > > > doesn't.

Actually, Yaakov pointed out on IRC that this is a C++ problem on Linux.
You're running the above test with g++.  If you perform the third test
with gcc on Linux:

  gcc -D_POSIX_C_SOURCE=200809L -c m.c

you'll get the same error as on newlib and FreeBSD:

m.c: In function ‘main’:
m.c:8:15: error: ‘M_PI’ undeclared (first use in this function)
    8 |   double pi = M_PI;
          |               ^~~~
	  m.c:8:15: note: each undeclared identifier is reported only once for each function it appears in

The problem in glibc and/or g++ is that this does *not* occur when
building with g++.


Corinna