290 lines
7.5 KiB
Groff
290 lines
7.5 KiB
Groff
.\" $NetBSD: select.2,v 1.5 1995/06/27 22:32:28 cgd Exp $
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.\"
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.\" Copyright (c) 1983, 1991, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\" This product includes software developed by the University of
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.\" California, Berkeley and its contributors.
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.\" 4. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)select.2 8.2 (Berkeley) 3/25/94
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.\"
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.Dd March 18, 2015
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.Dt SELECT 2
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.Os BSD 4.2
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.Sh NAME
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.Nm FD_CLR ,
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.Nm FD_COPY ,
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.Nm FD_ISSET ,
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.Nm FD_SET ,
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.Nm FD_ZERO ,
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.Nm select
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.Nd synchronous I/O multiplexing
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.Sh SYNOPSIS
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.Fd #include <sys/select.h>
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.\"
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.Ft void
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.Fo FD_CLR
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.Fa fd
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.Fa "fd_set *fdset"
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.Fc
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.Ft void
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.Fo FD_COPY
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.Fa "fd_set *fdset_orig"
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.Fa "fd_set *fdset_copy"
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.Fc
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.Ft int
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.Fo FD_ISSET
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.Fa fd
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.Fa "fd_set *fdset"
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.Fc
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.Ft void
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.Fo FD_SET
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.Fa fd
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.Fa "fd_set *fdset"
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.Fc
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.Ft void
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.Fo FD_ZERO
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.Fa "fd_set *fdset"
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.Fc
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.Ft int
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.Fo select
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.Fa "int nfds"
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.Fa "fd_set *restrict readfds"
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.Fa "fd_set *restrict writefds"
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.Fa "fd_set *restrict errorfds"
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.Fa "struct timeval *restrict timeout"
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.Fc
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.Sh DESCRIPTION
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.Fn select
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examines the I/O descriptor sets whose addresses are passed in
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.Fa readfds ,
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.Fa writefds ,
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and
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.Fa errorfds
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to see if some of their descriptors
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are ready for reading, are ready for writing, or have an exceptional
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condition pending, respectively.
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The first
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.Fa nfds
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descriptors are checked in each set;
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i.e., the descriptors from 0 through
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.Fa nfds Ns No -1
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in the descriptor sets are examined. (Example: If you have set two file descriptors "4" and "17",
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.Fa nfds
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should not be "2", but rather "17 + 1" or "18".)
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On return,
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.Fn select
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replaces the given descriptor sets
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with subsets consisting of those descriptors that are ready
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for the requested operation.
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.Fn select
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returns the total number of ready descriptors in all the sets.
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.Pp
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The descriptor sets are stored as bit fields in arrays of integers.
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The following macros are provided for manipulating such descriptor sets:
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.Fn FD_ZERO &fdset
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initializes a descriptor set
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.Fa fdset
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to the null set.
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.Fn FD_SET fd &fdset
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includes a particular descriptor
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.Fa fd
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in
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.Fa fdset .
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.Fn FD_CLR fd &fdset
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removes
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.Fa fd
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from
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.Fa fdset .
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.Fn FD_ISSET fd &fdset
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is non-zero if
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.Fa fd
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is a member of
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.Fa fdset ,
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zero otherwise.
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.Fn FD_COPY &fdset_orig &fdset_copy
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replaces an already allocated
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.Fa &fdset_copy
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file descriptor set with a copy of
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.Fa &fdset_orig .
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The behavior of these macros is undefined if
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a descriptor value is less than zero or greater than or equal to
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.Dv FD_SETSIZE ,
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which is normally at least equal
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to the maximum number of descriptors supported by the system.
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.Pp
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If
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.Fa timeout
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is not a null pointer, it specifies a maximum interval to wait for the
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selection to complete.
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.Pp
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If
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.Fa timeout
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is a null pointer, the select blocks indefinitely.
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.Pp
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To effect a poll, the
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.Fa timeout
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argument should be not be a null pointer,
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but it should point to a zero-valued timeval structure.
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.Pp
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.Fa timeout
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is not changed by
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.Fn select ,
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and may be reused on subsequent calls, however it is good style to re-initialize
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it before each invocation of
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.Fn select .
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.Pp
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Any of
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.Fa readfds ,
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.Fa writefds ,
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and
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.Fa errorfds
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may be given as null pointers if no descriptors are of interest.
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.Sh RETURN VALUES
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.Fn select
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returns the number of ready descriptors that are contained in
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the descriptor sets,
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or -1 if an error occurred.
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If the time limit expires,
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.Fn select
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returns 0.
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If
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.Fn select
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returns with an error,
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including one due to an interrupted call,
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the descriptor sets will be unmodified and the global variable
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.Va errno
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will be set to indicate the error.
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.Sh ERRORS
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An error return from
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.Fn select
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indicates:
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.Bl -tag -width Er
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.\" ===========
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.It Bq Er EAGAIN
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The kernel was (perhaps temporarily) unable
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to allocate the requested number of file descriptors.
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.\" ===========
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.It Bq Er EBADF
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One of the descriptor sets specified an invalid descriptor.
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.\" ===========
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.It Bq Er EINTR
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A signal was delivered before the time limit expired and
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before any of the selected events occurred.
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.\" ===========
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.It Bq Er EINVAL
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The specified time limit is invalid. One of its components is
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negative or too large.
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.\" ===========
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.It Bq Er EINVAL
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.Fa ndfs
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is greater than FD_SETSIZE and _DARWIN_UNLIMITED_SELECT is not defined.
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.El
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.Sh LEGACY SYNOPSIS
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.Fd #include <sys/select.h>
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.D1 "- or -"
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.Fd #include <sys/types.h>
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.Fd #include <sys/time.h>
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.Fd #include <unistd.h>
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.Pp
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.Fo FD_SET
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.Fa fd
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.Fa &fdset
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.Fc ;
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.Pp
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.Fo FD_CLR
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.Fa fd
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.Fa &fdset
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.Fc ;
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.Pp
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.Fo FD_ISSET
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.Fa fd
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.Fa &fdset
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.Fc ;
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.Pp
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.Fo FD_COPY
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.Fa &fdset_orig
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.Fa &fdset_copy
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.Fc ;
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.Pp
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.Fo FD_ZERO
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.Fa &fdset
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.Fc ;
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.Sh COMPATIBILITY
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.Fn select
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now returns with
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.Va errno
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set to EINVAL when
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.Fa nfds
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is greater than FD_SETSIZE.
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Use a smaller value for
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.Fa nfds
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or compile with -D_DARWIN_UNLIMITED_SELECT.
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.Sh SEE ALSO
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.Xr accept 2 ,
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.Xr connect 2 ,
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.Xr connectx 2 ,
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.Xr getdtablesize 2 ,
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.Xr gettimeofday 2 ,
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.Xr read 2 ,
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.Xr recv 2 ,
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.Xr send 2 ,
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.Xr write 2 ,
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.Xr compat 5
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.Sh BUGS
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Although the provision of
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.Xr getdtablesize 2
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was intended to allow user programs to be written independent
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of the kernel limit on the number of open files, the dimension
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of a sufficiently large bit field for select remains a problem.
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The default size
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.Dv FD_SETSIZE
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(currently 1024) is somewhat smaller than
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the current kernel limit to the number of open files.
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However, in order to accommodate programs which might potentially
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use a larger number of open files with select, it is possible
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to increase this size within a program by providing
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a larger definition of
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.Dv FD_SETSIZE
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before the inclusion of
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.Aq Pa sys/types.h .
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.Pp
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.Fn select
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should probably have been designed to return the time remaining from the
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original timeout, if any, by modifying the time value in place.
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However, it is unlikely this semantic will ever be implemented, as the
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change would cause source code compatibility problems.
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In general it is unwise to assume that the timeout value will be
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unmodified by the
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.Fn select
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call, and the caller should reinitialize it on each invocation.
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.Sh HISTORY
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The
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.Fn select
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function call appeared in
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.Bx 4.2 .
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