V8/usr/man/man2/select.2

.TH SELECT 2
.SH NAME
select \- synchronous I/O multiplexing
.SH SYNOPSIS
.ft B
#include <sys/types.h>
.br
int select(nfds, readfds, writefds, milli);
.br
fd_set *readfds, *writefds;
.ft R
.SH DESCRIPTION
.I Select
examines the IO descriptors specified by the bit masks
.I readfds
and
.I writefds
to see if they are ready for reading and/or writing respectively
and returns, in place, a mask of those descriptors which are ready.
Only descriptors 0 through
.IR nfds "\-1"
are examined.
The total number of ready descriptors is returned.
.PP
File descriptors referring to ordinary files and non-stream
special files always appear ready.
.PP
.I Milli
is the maximum number of milliseconds to wait before giving
up if no descriptors come active.
If no maximum wait is desired a very large integer can be given.
If
.I milli
is 0,
.I select
returns whatever information is available without blocking.
.PP
Either
.I readfds
or
.I writefds
may be null if no descriptors are interesting.
.I Readfds
and
.I writefds
point to a structure containing a simple bit mask:
file descriptor
.I f
is represented by the bit
.RI 1<<( f % N )
in
.IR readfds\->fds_bits [ f/N ],
where 
.I N
is the number of bits in an int
(given by the constant expression
.IR NBBY*NBPW ).
These macros manipulate the bits of mask
.IR s :
.TP "\w'FD_ISSET(n,s)  'u"
.RI FD_ZERO( s )
clear all bits
.TP
.RI FD_SET( n,s )
set bit
.I n
.TP
.RI FD_CLR( n,s )
clear bit
.I n
.TP
.RI FD_ISSET( n,s )
test bit
.I n
.SH BUGS
\fIMilli\fP is rounded up to the nearest second.
.br
.I Select
is intended for use with streams;
it does not work with non-stream
special files that can block.