.\" 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	6.3 (Berkeley) 02/03/86
.\"
.TH SELECT 2 ""
.UC 5
.SH NAME
select \- synchronous i/o multiplexing
.SH SYNOPSIS
.nf
.ft B
#include <sys/types.h>
#include <sys/time.h>
.PP
.ft B
nfound = select(nfds, readfds, writefds, exceptfds, timeout)
int nfound, nfds;
fd_set *readfds, *writefds, *exceptfds;
struct timeval *timeout;
.PP
.ft B
int fd;
fd_set fdset;
FD_SET(fd, &fdset)	
FD_CLR(fd, &fdset)	
FD_ISSET(fd, &fdset)	
FD_ZERO(&fdset)	
.fi
.SH DESCRIPTION
.I Select
examines the i/o descriptor sets whose addresses are passed in
.IR readfds ,
.IR writefds ,
and
.I exceptfds
to see if some of their descriptors
are ready for reading, are ready for writing, or have an exceptional
condition pending, respectively.
The first
.I nfds
descriptors are checked in each set;
i.e. the descriptors from 0 through
.IR nfds -1
in the descriptor sets are examined.
On return,
.I select
replaces the given descriptor sets
with subsets consisting of those descriptors that are ready.
The total number of ready descriptors in all the sets is returned in
.IR nfound .
.PP
Some macros are provided for manipulating descriptor sets.
.I "FD_ZERO(&fdset)"
initializes a descriptor set
.I fdset
to the null set.
.I "FD_SET(fd, &fdset)"
includes a particular descriptor
.I fd
in
.IR fdset .
.I "FD_CLR(fd, &fdset)"
removes
.I fd
from
.IR fdset .
.I "FD_ISSET(fd, &fdset)"
is nonzero if
.I fd
is a member of
.IR fdset ,
zero otherwise.
The behavior of these macros is undefined if
a descriptor value is less than zero or greater than or equal to
.IR FD_SETSIZE ,
which is normally at least equal
to the maximum number of descriptors supported by the system.
.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 zero pointers if no descriptors are of interest.
.SH "RETURN VALUE
.I Select
returns the number of ready descriptors that are contained in
the descriptor sets,
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 descriptor sets 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 invalid.  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 sets are always modified on return, even
if the call returns as the result of the timeout.