what is _XOPEN_SOURCE=500 needed for?

[Soeren Sandmann]

Daniel Stone wrote:

>So, at a guess, we do something like this:
>x11proto.pc.in: Cflags: $(FDBITSFLAGS)
>  
>
I basically agree that this is right.

The other possibility is to add a new installed header file, called 
something like
Xproto-config.h, then conditionally include that in all public headers. 
The advantage
of that would be fewer -D's on the compile lines. The big disadvantage 
is that we would
add weird

#ifdef HAVE_XPROTO_CONFIG
#include <xproto-config.h>
#endif

lines in the public headers, and we would still require all users of 
xproto to define HAVE_XPROTO_CONFIG. For this reason, adding -D's to the 
command line (which
is automatable with pkg-config) is better.

> configure.ac:
> case "$platform" in
>	*-gnu-*)
>	    FDBITSFLAGS="-D_XOPEN_SOURCE=500"
>	    ;;
> esac
> AC_SUBST([FDBITSFLAGS])

One of the advantages of auto* over imake is that it has the ability to 
detect at compile-time
what flags are needed. If possible using that feature is better than 
just magically
asserting that "_XOPEN_SOURCE is needed on Linux". So I prefer something 
like
the following instead:

# Find out what defines are necessary to make struct fd_set contain
# fds_bits
#

fds_bits_found=false;

# No defines necessary?
if test x$fds_bits_found = xfalse ; then
   AC_CHECK_MEMBER(fd_set.fds_bits,
               [
                   fds_bits_found=true
               ],,
               [
                   #include <sys/select.h>
               ])
fi

# With _XOPEN_SOURCE?

unset ac_cv_member_fd_set_fds_bits

if test x$fds_bits_found = xfalse ; then
   AC_CHECK_MEMBER(fd_set.fds_bits,
            [
                fds_bits_found=true
                PROTO_DEFINES="$PROTO_DEFINES -D_XOPEN_SOURCE"
            ],,
            [
                #define _XOPEN_SOURCE
                #include <sys/select.h>
            ])
fi



Søren