Squid internal overview

Initiating IO
Network IO is initiated by requesting for an FD to become ready for reading or writing

commSetSelect(fd, COMM_SELECT_{READ,WRITE}, callback, callbackdata, timeout)

The callback is called when the FD becomes ready for the requested IO to be performed


..
/* Need to read from file descriptor, register interest for this */
commSetSelect(fd, COMM_SELECT_READ, doread, rdata, 0);
..
/* in comm_select() each FD is checked for read/write readiness, and the
   callback is called */
..
void doread(int fd, void *data)
{
    /* This is called when fd is ready to be read from */
    /* Do some IO */
    /* Now we want to write .. */
    commSetSelect(fd, COMM_SELECT_WRITE, dowrite, data, 0);
}