+@item
+This function is missing on some platforms:
+glibc 2.3.6, MacOS X 10.3, FreeBSD 6.0, NetBSD 3.0, OpenBSD 3.8, AIX
+5.1, HP-UX 11, IRIX 6.5, OSF/1 5.1, Cygwin 1.5.x, mingw, Interix 3.5, BeOS.
+But the replacement function is not safe to be used in libraries and
+is not multithread-safe. Also, the replacement does not guarantee
+that @samp{dirfd(fdopendir(n))==n} (dirfd might fail, or return a
+different file descriptor than n).
+@item
+This function exists but is not declared on some platforms:
+FreeBSD 7.3.
+@item
+This function does not reject non-directory file descriptors on some
+platforms:
+GNU/Hurd.