@itemize
@item
This function is missing on some platforms:
-FreeBSD 6.0, NetBSD 3.0, Cygwin, mingw.
+FreeBSD 6.0, NetBSD 5.0, Cygwin, mingw, MSVC 9.
+@item
+This function is not declared in @code{<unistd.h>}
+(without @code{-D_GNU_SOURCE}) on some platforms:
+glibc (at least 2.11--2.13).
@end itemize