It is not just funopen(3) that is a BSDism.

lib/libc/stdio/funopen.3

@@ -1,4 +1,4 @@
-.\"	$NetBSD: funopen.3,v 1.15 2010/03/22 19:30:54 joerg Exp $
+.\"	$NetBSD: funopen.3,v 1.16 2011/09/11 07:37:06 jruoho Exp $
 .\"
 .\" Copyright (c) 1990, 1991, 1993
 .\"	The Regents of the University of California.  All rights reserved.
@@ -31,7 +31,7 @@
 .\"
 .\"     @(#)funopen.3	8.1 (Berkeley) 6/9/93
 .\"
-.Dd June 9, 1993
+.Dd September 9, 2011
 .Dt FUNOPEN 3
 .Os
 .Sh NAME
@@ -167,9 +167,7 @@ The
 .Fn funopen
 functions first appeared in
 .Bx 4.4 .
-.Sh BUGS
-The
-.Fn funopen
-function
-may not be portable to systems other than
-.Bx .
+.Sh CAVEATS
+All three functions are specific to
+.Nx
+and thus unportable.