Introductory concepts

select() IO
Pass a set of FDs to check for read, write and exceptional (OOB) data 
Obtain the set of FDs which are ready for IO

fd_set rfds, wfds, efds;
int readyfds;
struct timeval timeout;

FD_ZERO(&fdset);
FD_SET(0, &rfds);
FD_SET(1, &wfds);
FD_SET(2, &wfds);

readyfds = select(3, &rfds, &wfds, &efds, &timeout);

if (FD_ISSET(0, &rfds)) {
    /* Handle STDIN ready */
}
if (FD_ISSET(1, &wfds)) {
    /* Handle STDOUT ready */
}
if (FD_ISSET(2, &wfds)) {
    /* Handle STDERR ready */
}