[Python-Dev] use of #pragma GCC system_header to suppress _XOPEN_SOURCE warnings

[Skip Montanaro]

I really hate to disrupt this scintillating function decorator discussion...

At work the goal for our C++ build environment is "no warnings with gcc
-Wall".  We are not there yet (and thankfully I am not a participant in that
particular holy grail), but those involved in that effort encountered a
warning the other day generated because _XOPEN_SOURCE is defined in
pyconfig.h.  Unfortunately GCC also defines it, so a "macro redefined"
warning is emitted when compiling with GCC and including Python.h.

I'm not about to suggest that the _XOPEN_SOURCE definition be removed from
pyconfig.h.in.  I trust that Martin v. Löwis and others involved have good
reasons to leave it in.  Perhaps adding

    #pragma GCC system_header

to pyconfig.h.in would be acceptable though.  This tells GCC that the file
it appears in is a system header file and to not bitch about trivial stuff
like macro redefinitions.  This suggestion comes from one of the C++ experts
here.  I would never have figured it out myself.  The scope of the pragma is
only the file in which it appears, and it doesn't take effect for even that
file until it's first encountered.  This suggests that perhaps a slight
restructuring of pyconfig.h.in would help to reduce its effect even more:

    <all macros which don't start with "_">

    #pragms GCC system_header

    <all macros which do start with "_">

(Pyconfig.h.in is almost there now.)

Some exceptions to the above structure might be necessary if the definition
of a macro in the first group is conditioned on the value of a macro from
the second group.  I don't believe this is the case, at least not at the
moment.  It's unlikely to ever be the case since any smarts for such tests
more appropriately belong in the configure script (or would be hand-coded on
non-configure-type platforms).

I've adjusted my local copy of pyconfig.h.in and am building CVS HEAD and
the 2.3 maintenance branch on both Solaris and MacOSX to see if any problems
arise.  I'd appreciate some feedback on whether or not this is deemed a
reasonable idea though.

Thx,

Skip