Erlang-C port not working on some unix machines

Question

I am using the standard erlang-c port code as described in the tutorial http://www.erlang.org/doc/tutorial/c_port.html#id63121 and in Joe Armstrong's book.

On my macbook, the erlang code and c-code communicate happily using write_exact and read_exact. (I have not touched any of that standard code).

But when running exactly the same code on a unix pdp machine, the communication from c to erlang breaks down. Specifically, in the server's loop:

loop(Port) ->
    receive
        {call, Caller, Msg} ->
           Port ! {self(), {command, encode(Msg)}},
           receive
               {Port, {data, Data}} ->  <---- WE NEVER GET HERE !!!
                   Caller ! {complex, decode(Data)}
           end,
           loop(Port);
       stop ->
           Port ! {self(), close},
           receive
               {Port, closed} ->
                   exit(normal)
           end;
       {'EXIT', Port, Reason} ->
           exit(port_terminated)
   end.

The c-code receives the encoded-message successfully from the erlang code. The c-code then writes back a response using write_exact (in this case, its an extremely simple OK response with a two byte header). But the erlang code never receives this message. The loop function above never receives {Port, {data, Data}}...

The c-code for writing the response is the standard code (write_cmd and write_exact). I have not touched any of this standard code:

int write_exact(byte* buff, int len)
{
   assert(len < BUFF_SIZE);
   int i, wrote = 0;
   do
   {
       if ((i = (int)write(1, buff+wrote, len-wrote)) <= 0)
       {
           return (i);
       }
       wrote += i;
   } while (wrote < len);
   return len;
}

Debugging the c-side, the write() function gets called - but somehow the erlang side never receives the message back from the port.

Do you guys know have any ideas what might be causing this message to fail to get back from c to erlang in this case on this machine? (It works on macbooks and used to work on the very same pdp machine - and I haven't touched the erlang code since it last worked). Could it be something to do with permissions or something, preventing the write in write_exact from actually writing anything? I am at a loss.