How would you implement a basic event-loop?

I used to wonder a lot about the same!

A GUI main loop looks like this, in pseudo-code:

void App::exec() {
    for(;;) {
        vector<Waitable> waitables;
        waitables.push_back(m_networkSocket);
        waitables.push_back(m_xConnection);
        waitables.push_back(m_globalTimer);
        Waitable* whatHappened = System::waitOnAll(waitables);
        switch(whatHappened) {
            case &m_networkSocket: readAndDispatchNetworkEvent(); break;
            case &m_xConnection: readAndDispatchGuiEvent(); break;
            case &m_globalTimer: readAndDispatchTimerEvent(); break;
        }
    }
}

What is a "Waitable"? Well, it's system dependant. On UNIX it's called a "file descriptor" and "waitOnAll" is the ::select system call. The so-called vector<Waitable> is a ::fd_set on UNIX, and "whatHappened" is actually queried via FD_ISSET. The actual waitable-handles are acquired in various ways, for example m_xConnection can be taken from ::XConnectionNumber(). X11 also provides a high-level, portable API for this -- ::XNextEvent() -- but if you were to use that, you wouldn't be able to wait on several event sources simultaneously.

How does the blocking work? "waitOnAll" is a syscall that tells the OS to put your process on a "sleep list". This means you are not given any CPU time until an event occurs on one of the waitables. This, then, means your process is idle, consuming 0% CPU. When an event occurs, your process will briefly react to it and then return to idle state. GUI apps spend almost all their time idling.

What happens to all the CPU cycles while you're sleeping? Depends. Sometimes another process will have a use for them. If not, your OS will busy-loop the CPU, or put it into temporary low-power mode, etc.

Please ask for further details!

Python:

You can look at the implementation of the Twisted reactor which is probably the best implementation for an event loop in python. Reactors in Twisted are implementations of an interface and you can specify a type reactor to run: select, epoll, kqueue (all based on a c api using those system calls), there are also reactors based on the QT and GTK toolkits.

A simple implementation would be to use select:

#echo server that accepts multiple client connections without forking threads

import select
import socket
import sys

host = ''
port = 50000
backlog = 5
size = 1024
server = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
server.bind((host,port))
server.listen(backlog)
input = [server,sys.stdin]
running = 1

#the eventloop running
while running:
    inputready,outputready,exceptready = select.select(input,[],[])

    for s in inputready:

        if s == server:
            # handle the server socket
            client, address = server.accept()
            input.append(client)

        elif s == sys.stdin:
            # handle standard input
            junk = sys.stdin.readline()
            running = 0

        else:
            # handle all other sockets
            data = s.recv(size)
            if data:
                s.send(data)
            else:
                s.close()
                input.remove(s)
server.close()