Question about the definition of _XOPEN_SOURCE

[Heinz Junkes]

Please excuse me if this question has already been answered. But unfortunately I have not been able to find a clear statement.

Why is RTEMS _XOPEN_SOURCE not set with a value when building a BSP with --enable-posix?

in the rtems-libbsd you can find a definition in some places.
z.B. in dhcpd/configure -D_XOPEN_SOURCE=700 and
in glibc-bugs.h #define _XOPEN_SOURCE 500
…

That seems a little unstructured to me. I'm sure there's an explanation why this is so complicated. But which value is the correct one e.g. for PowerPC and RTEMS 5.1?

Danke Heinz