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

[Skip Montanaro]

    >> Unfortunately GCC also defines it, so a "macro redefined" warning is
    >> emitted when compiling with GCC and including Python.h.

    Martin> That is not true. GCC does not define that macro, on any
    Martin> platform.

I was shown today that g++ defines that macro internally (I misspoke when I
referred to gcc yesterday - I tend to lump them together and forget that C++
is a different language than C), at least 3.3.2 on Solaris/Intel does.

Given this small source file:

    #include "Python.h"

    int main() {
            printf("%x\n", PyList_GetItem);
            return 0;
    }

compilation with gcc (we're using 3.3.2, but I also tried with 3.4.0)
produces no warnings, but compilation with g++ it produces this output:

    g++ -I/opt/lang/python/include/python2.3 -c example.c
    In file included from /opt/lang/python/include/python2.3/Python.h:8,
                     from example.c:2:
    /opt/lang/python/include/python2.3/pyconfig.h:862:1: warning: "_XOPEN_SOURCE" redefined
    <built-in>:73:1: warning: this is the location of the previous definition

Sure enough, grepping through the gcc/g++ installation directory doesn't
turn up an actual header file containing "#define _XOPEN_SOURCE".  I did
find a file (.../lib/gcc/i386-pc-solaris2.8/3.4.0/install-tools/
mkheaders.conf) that contains this line:

    FIXPROTO_DEFINES="-D_XOPEN_SOURCE"

    Martin> Some system header may define it, though. I believe this is a
    Martin> bug in the system - the macro *should* be defined in the
    Martin> application. What system are you referring to, and where does
    Martin> that define _XOPEN_SOURCE? Is that definition conditional?

Does the fact that we're talking g++ and a built-in macro instead of gcc and
a macro defined in a .h file suggest a different way to suppress this
warning?

A little more searching located the standards(5) man page on the system.  In
part it says:

  X/Open CAE
     To build or compile an application that conforms to  one  of
     the X/Open CAE specifications, use the following guidelines.
     Applications need not set the POSIX feature test  macros  if
     they require both CAE and POSIX functionality.

     ...

     SUSv2 The application must define _XOPEN_SOURCE=500.

This suggests that Sun expects the application or its build tools to define
_XOPEN_SOURCE, not the compiler or its header files.  That suggests a bug in
g++.

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