int nfds;
fd_set readfds;
char buf;

while(1) {
FD_ZERO(&readfds);

FD_SET(outfd, &readfds);
FD_SET(stdin, &readfds);

// nfds = MAX(outfd, stdin) + 1
nfds = outfd + 1;

select(nfds, &readfds, NULL, NULL, NULL);

if(FD_ISSET(stdin, &readfds)) {
    read(stdin, &buf, 1);
    write(infd, &buf, 1, 0);
}
if(FD_ISSET(outfd, &readfds)) {
    read(outfd, &buf, 1);
    write(infd, &buf, 1, 0);
}

} // while