Introductory concepts

An IO-driven loop
Register callbacks on IO events
The callback is called when the registered IO event completes

void init(void)
{
    registeriocallback(fd, READ, handleread, handlereaddata);
    enter_io_loop();
}

void enter_io_loop(void)
{
    /* Construct list of read/write fds */
    /* Perform select() to find which are ready */
    numready = select(numfds, &rfds, &wfds, &exfds, NULL);

    /* Go through each that are ready, and call their callback */
    for (i = 0; i < numfds; i++) {
        if (FD_ISSET(i, &rfds)) { rhandle(i, rdata); }
        if (FD_ISSET(i, &wfds)) { whandle(i, rdata); }
        if (FD_ISSET(i, &exfds)) { exhandle(i, rdata); }
    }
}

void handleread(int fd, void *data)
{
    /* Handle reading data from fd */
}