The timeoutcat program

Interface

     timeoutcat timeout fdcontrol

• timeoutcat reads stdin and writes on stdout. It exits 0 when stdin closes, or 111 on a hard error.
• timeoutcat normally never exits as long as it has unflushed data in its buffers.
• fdcontrol must be either 0 or a valid open file descriptor.
• If timeout seconds elapse without data being available on stdin, timeoutcat exits 99, or 111 if there is still unflushed data in its buffers. A 0 value for timeout means infinite.
• If one of timeoutcat's ends is a socket, timeoutcat shuts the relevant part down before exiting. This transmits a close request to the other end of the socket, even when another process may still be reading or writing the file. timeoutcat should be the only process reading from a socket, or writing to it. Other processes will be unable to use the socket once timeoutcat has died.
• If fdcontrol gets closed, and one of timeoutcat's ends is a socket, timeoutcat shuts down the end it is not using. The point of this is to allow a socket to be properly handled by a set of two mutually-controlling timeoutcat processes: if one of them dies unexpectedly, the other one will shutdown() the relevant end of the socket.
  A 0 value for fdcontrol means no such trigger.