What does “#define _GNU_SOURCE” imply?

Question

Today I had to use the basename() function, and the man 3 basename (here) gave me some strange message:

Notes

Answer 1

From some mailing list via google:

Look at glibc's include/features.h:

_GNU_SOURCE All of the above, plus GNU extensions.

Which means it enables all this:

STRICT_ANSI, _ISOC99_SOURCE, _POSIX_SOURCE, _POSIX_C_SOURCE, _XOPEN_SOURCE, _XOPEN_SOURCE_EXTENDED, _LARGEFILE_SOURCE, _LARGEFILE64_SOURCE, _FILE_OFFSET_BITS=N, _BSD_SOURCE, _SVID_SOURCE

So it enables a whole lot of compiling flags for gcc