https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113278
--- Comment #1 from Richard Earnshaw <rearnsha at gcc dot gnu.org> ---
newlib certainly implements fileno():
$ nm libc.a|grep fileno
libc_a-fileno.o:
00000000 T fileno
U fileno
libc_a-fileno_u.o:
00000000 T fileno_unlocked
U fileno
So perhaps the issue is that the prototype is missing (or missing with the
default compilation options since it's Posix and I don't think we pass options
to enable that by default). Grepping the source, I suspect the former.
