Squid internals

The comm loop (src/comm_select.c)
Outline
All non-disk IO goes through here
Simple outline of comm_select()
Handle any async disk IO callbacks
Check FD timeouts once a second - checkTimeouts()
Create a list of file descriptors which are waiting for IO (from their status in fd_table[])
Call poll() or select() to determine which FDs are ready
Call the callback handlers registered on the ready FDs

How IO happens
..

/* Some part in code */
commSetSelect(fd, COMM_SELECT_READ, handlerfunc, handlerdata, timeout);
..

/* Back to IO loop - comm_select() */
if (FD_ISSET(fd, &readfds))		/* If we're set for read */
    F = &fd_table[fd];			/* Get a pointer to the fd state */
    hdl = F->read_handler;		/* Get the read handler callback ptr */
    F->read_handler == NULL;		/* Clear the read handler callback ptr */
    hdl(fd, F->read_data);		/* Call the read handler callback */
}