4.2BSD/usr/man/man2/select.2

.TH SELECT 2 "2 July 1983"
.UC 4
.SH NAME
select \- synchronous i/o multiplexing
.SH SYNOPSIS
.nf
.ft B
#include <sys/time.h>
.PP
.ft B
nfound = select(nfds, readfds, writefds, execptfds, timeout)
int nfound, nfds, *readfds, *writefds, *execptfds;
struct timeval *timeout;
.fi
.SH DESCRIPTION
.I Select
examines the i/o descriptors specified by the bit masks
.IR readfds ,
.IR writefds ,
and
.I execptfds
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 which are ready.
The total number of ready descriptors 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 execptfds
may be given as 0 if no descriptors are of interest.
.SH "RETURN VALUE
.I Select
returns the number of descriptors which 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]
An signal was delivered before any of the selected for
events occurred or the time limit expired.
.SH SEE ALSO
accept(2), connect(2), read(2), write(2), recv(2), send(2)
.SH BUGS
The descriptor masks are always modified on return, even
if the call returns as the result of the timeout.