UDP sockets raise exception on MIPS platform

I am running ruby-1.8.6 under OpenWrt (*), which is a small MIPS platform
(4MB flash, 16MB RAM) running linux-2.4.34 and uClibc.

Everything seems to work fine apart from UDP sockets. As soon as you try to
read using 'recvfrom', an exception is raised:

root@OpenWrt:~# irb
irb(main):001:0> require 'socket'
=> true
irb(main):002:0> s = UDPSocket.new
=> #<UDPSocket:0x444000>
irb(main):003:0> s.bind("0.0.0.0",1234)
=> 0
irb(main):004:0> d = s.recvfrom(512)
IOError: recv for buffered IO
        from (irb):4:in `recvfrom'
        from (irb):4
irb(main):005:0>

But strangely, TCP sockets work just fine.

Now, the only place I can see this exception raised is here in
etc/socket/socket.c:

    if (rb_read_pending(fptr->f)) {
        rb_raise(rb_eIOError, "recv for buffered IO");
    }

If I modify the READ_DATA_PENDING macro in io.c so it is always zero, then
this problem goes away. However, TCP sockets are then broken :-(

In any case, I don't understand at all what this rb_read_pending call is
doing, so I don't feel comfortable simply removing this check for recvfrom.

Does anyone have any suggestions as to the problem, or where I should look
further?

Thanks,

Brian.

(*) http://www.openwrt.org/
I am using the brcm-2.4 flavour of OpenWrt Kamikaze 7.07

Almost no patching is required to make Ruby cross-compile under OpenWrt. See
https://dev.openwrt.org/browser/packages/lang/ruby