4 POSIX specification:@* @url{http://www.opengroup.org/susv3xbd/time.h.html}
8 Portability problems fixed by Gnulib:
11 @samp{struct timespec} is not defined on some platforms.
14 Some platforms provide a @code{NULL} macro that cannot be used in arbitrary
19 Portability problems not fixed by Gnulib: