Re: Feature Conditional for M_PI

[Joel Sherrill <joel.sherrill@gmail.com>]

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.
> >
> > In glibc's math.h, M_PI is guarded with
> >
> >   #if defined __USE_MISC || defined __USE_XOPEN
> >
> > In newlib, it's guarded with
> >
> >   #if __BSD_VISIBLE || __XSI_VISIBLE
> >
> > Note that this is identical to the guards on at least FreeBSD.
> >
> > In both cases, newlib as well as glibc, "MISC" is defined by default,
> > but "BSD" isn't.  That's why your 3rd invocation fails on BSDs and
> > newlib/Cygwin, but not on Linux.
>

Thanks for poking at the BSDs. I went and tried that also.

>
> Huh, wait!  I bet this does *not* fail on BSD because BSD very likely
> defines __BSD_VISIBLE by default.
>

The third case fails on FreeBSD 12 as well.

>
> We could move the math.h constants to __MISC_VISIBLE || __XSI_VISIBLE,
> perhaps that makes more sense for us?
>

https://github.com/freebsd/freebsd/blob/master/lib/msun/src/math.h#L143 has
this:

 #if __BSD_VISIBLE || __XSI_VISIBLE

which matches your expectations on their wrapper but not the outcome.
I guess they don't define __BSD_VISIBLE by default.

If we want newlib to follow glibc, then this should change to MISC but
I certainly would want to hear from some of the standards folks who
are on this list. :)

--joel

>
>
> Corinna
>
>