.\" Copyright (c) 1983 Regents of the University of California.
.\" All rights reserved.  The Berkeley software License Agreement
.\" specifies the terms and conditions for redistribution.
.\"
.\"	@(#)select.2	6.2 (Berkeley) %G%
.\"
.TH SELECT 2 ""
.UC 5
.SH NAME
select \- synchronous i/o multiplexing
.SH SYNOPSIS
.nf
.ft B
#include <sys/time.h>
.PP
.ft B
nfound = select(nfds, readfds, writefds, exceptfds, timeout)
int nfound, nfds, *readfds, *writefds, *exceptfds;
struct timeval *timeout;
.fi
.SH DESCRIPTION
.I Select
examines the i/o descriptors specified by the bit masks
.IR readfds ,
.IR writefds ,
and
.I exceptfds
to see if they are ready for reading, writing, or have an exceptional
condition pending, respectively.
File descriptor 
.I f
is represented by the bit \*(lq1<<f\*(rq in
the mask.
.I Nfds
desciptors are checked,
i.e. the bits from 0 through
.IR nfds -1
in the masks are examined.
.I Select
returns, in place, a mask of those descriptors that are ready.
The total number of bits set in all the masks is returned in
.IR nfound .
.PP
If
.I timeout
is a non-zero pointer, it specifies a maximum interval to wait for the
selection to complete.  If 
.I timeout
is a zero pointer, the select blocks indefinitely.  To affect a poll, the
.I timeout
argument should be non-zero, pointing to a zero valued timeval structure.
.PP
Any of
.IR readfds ,
.IR writefds ,
and
.I exceptfds
may be given as 0 if no descriptors are of interest.
.SH "RETURN VALUE
.I Select
returns the number of 'one' bits that are contained in
the bit masks,
or \-1 if an error occurred.
If the time limit expires then
.I select
returns 0.
.SH "ERRORS
An error return from \fIselect\fP indicates:
.TP 15
[EBADF]
One of the bit masks specified an invalid descriptor.
.TP 15
[EINTR]
A signal was delivered before the time limit expired and
before any of the selected events occurred.
.TP 15
[EINVAL]
The specified time limit is unacceptable.  One of its components is
negative or too large.
.SH SEE ALSO
accept(2), connect(2), read(2), write(2), recv(2), send(2), getdtablesize(2)
.SH BUGS
The descriptor masks are always modified on return, even
if the call returns as the result of the timeout.