228 lines
5.9 KiB
Groff
228 lines
5.9 KiB
Groff
.\" 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. 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 June 25, 2020
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.Dt SELECT 2
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.Os
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.Sh NAME
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.Nm select
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.Nd synchronous I/O multiplexing
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In sys/select.h
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.Ft int
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.Fn select "int nfds" "fd_set *readfds" "fd_set *writefds" "fd_set *exceptfds" "struct timeval *timeout"
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.Fn FD_SET fd &fdset
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.Fn FD_CLR fd &fdset
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.Fn FD_ISSET fd &fdset
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.Fn FD_ZERO &fdset
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.Sh DESCRIPTION
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The
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.Fn select
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system call
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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 exceptfds
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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 only exceptional condition detectable is out-of-band
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data received on a socket.
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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.
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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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The
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.Fn select
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system call
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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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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 the maximum interval to wait for the
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selection to complete.
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System activity can lengthen the interval by
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an indeterminate amount.
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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 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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Any of
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.Fa readfds ,
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.Fa writefds ,
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and
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.Fa exceptfds
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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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The
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.Fn select
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system call
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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 system call,
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the descriptor sets will be unmodified.
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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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.It Bq Er EBADF
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One of the descriptor sets specified an invalid descriptor.
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.It Bq Er EFAULT
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One of the arguments
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.Fa readfds , writefds , exceptfds ,
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or
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.Fa timeout
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points to an invalid address.
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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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.It Bq Er EINVAL
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The specified time limit is invalid.
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One of its components is
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negative or too large.
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.It Bq Er EINVAL
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The
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.Fa nfds
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argument
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was invalid.
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.El
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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 getdtablesize 2 ,
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.Xr gettimeofday 2 ,
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.Xr kqueue 2 ,
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.Xr poll 2 ,
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.Xr pselect 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 clocks 7
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.Sh NOTES
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The default size of
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.Dv FD_SETSIZE
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is currently 1024.
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In order to accommodate programs which might potentially
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use a larger number of open files with
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.Fn select ,
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it is possible
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to increase this size by having the program define
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.Dv FD_SETSIZE
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before the inclusion of any header which includes
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.In sys/types.h .
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.Pp
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If
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.Fa nfds
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is greater than the number of open files,
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.Fn select
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is not guaranteed to examine the unused file descriptors.
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For historical
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reasons,
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.Fn select
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will always examine the first 256 descriptors.
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.Sh STANDARDS
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The
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.Fn select
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system call and
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.Fn FD_CLR ,
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.Fn FD_ISSET ,
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.Fn FD_SET ,
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and
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.Fn FD_ZERO
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macros conform with
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.St -p1003.1-2001 .
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.Sh HISTORY
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The
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.Fn select
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system call appeared in
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.Bx 4.2 .
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.Sh BUGS
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.St -susv2
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allows systems to modify the original timeout in place.
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Thus, it is unwise to assume that the timeout value will be unmodified
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by the
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.Fn select
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system call.
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.Fx
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does not modify the return value, which can cause problems for applications
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ported from other systems.
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