Question

I’m implementing a system that runs game servers. I have a process (the “game controller”) that creates two pipe pairs and forks a child. The child dups its STDIN to one pipe, and dups its STDOUT and STDERR to the other, then runs execlp() to run the game code.

The game controller will have two threads. The first will block in accept() on a named UNIX socket receiving input from another application, and the second thread is blocking read()ing the out and error pipe from the game server.

Occasionally, the first thread will receive a command to send a string to the stdin pipe of the game server. At this point, somehow I need to stop the second thread from read()ing so that the first thread can read the reply from the out and error pipe.

(It is worth noting that I will know how many characters/lines long the reply is, so I will know when to stop reading and let the second thread resume reading, resetting the process.)

How can I temporarily switch the read control to another thread, as above?

Answer 1

There are a couple of options. One would be to have the second thread handle all of the reading, and give the first thread a way to signal it to tell it to pass the input back. But this will be somewhat complicated; you will need to set up a method for signalling between the threads, and make sure that the first thread tells the second thread that it wants the input before the second thread reads it and handles it itself. There will be potential for various kinds of race conditions that could make your code unpredictable.

Another option is to avoid using threads at all. Just use select(2) (or poll(2)) to allow one thread to wait for activity on several file descriptors at once. select allows you to indicate the set of file descriptors that you are interested in. As soon as any activity happens on one of them (a connection is available to accept, data is available to read), select will return, indicating the set of file descriptors that are ready. You can then accept or read on the appropriate descriptors, and when you are done, loop and call select again to wait for the next I/O event.