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a "null pointer".'' Making good use of the excellent explanations sent to me by Ruslan Ermilov, Garrett Wollman and Bruce Evans, correct the descriptions of null pointers. They are just "null pointers", not nil, not NULL or ".Dv NULL". Suggested by: ru, wollman, bde Reviewed by: ru, wollman Pointy hat: keramida
229 lines
6.0 KiB
Groff
229 lines
6.0 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. 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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.\" $FreeBSD$
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.\"
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.Dd November 17, 2002
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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 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 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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.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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