fd_set
The Linux manual documents fd_set in 8 places. Pick the section you want.
_newselect (2)synchronous I/O multiplexingpselect (2)synchronous I/O multiplexingpselect6 (2)synchronous I/O multiplexingselect (2)synchronous I/O multiplexingFD_CLR (3)synchronous I/O multiplexingFD_ISSET (3)synchronous I/O multiplexingFD_SET (3)synchronous I/O multiplexingFD_ZERO (3)synchronous I/O multiplexing