1 .\" Copyright (c) 1983, 1991, 1993
2 .\" The Regents of the University of California. All rights reserved.
4 .\" Redistribution and use in source and binary forms, with or without
5 .\" modification, are permitted provided that the following conditions
7 .\" 1. Redistributions of source code must retain the above copyright
8 .\" notice, this list of conditions and the following disclaimer.
9 .\" 2. Redistributions in binary form must reproduce the above copyright
10 .\" notice, this list of conditions and the following disclaimer in the
11 .\" documentation and/or other materials provided with the distribution.
12 .\" 3. Neither the name of the University nor the names of its contributors
13 .\" may be used to endorse or promote products derived from this software
14 .\" without specific prior written permission.
16 .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
17 .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
18 .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
19 .\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
20 .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
21 .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
22 .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
23 .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
24 .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
25 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
28 .\" @(#)select.2 8.2 (Berkeley) 3/25/94
29 .\" $FreeBSD: src/lib/libc/sys/select.2,v 1.14.2.5 2001/12/14 18:34:01 ru Exp $
30 .\" $DragonFly: src/lib/libc/sys/select.2,v 1.8 2007/08/02 00:21:54 swildner Exp $
37 .Nd synchronous I/O multiplexing
43 .Fn select "int nfds" "fd_set *readfds" "fd_set *writefds" "fd_set *exceptfds" "struct timeval *timeout"
46 .Fn FD_ISSET fd &fdset
50 examines the I/O descriptor sets whose addresses are passed in
55 to see if some of their descriptors
56 are ready for reading, are ready for writing, or have an exceptional
57 condition pending, respectively.
58 The only exceptional condition detectable is out-of-band
59 data received on a socket.
62 descriptors are checked in each set;
63 i.e., the descriptors from 0 through
65 in the descriptor sets are examined.
68 replaces the given descriptor sets
69 with subsets consisting of those descriptors that are ready
70 for the requested operation.
72 returns the total number of ready descriptors in all the sets.
74 The descriptor sets are stored as bit fields in arrays of integers.
75 The following macros are provided for manipulating such descriptor sets:
77 initializes a descriptor set
81 includes a particular descriptor
90 .Fn FD_ISSET fd &fdset
96 The behavior of these macros is undefined if
97 a descriptor value is less than zero or greater than or equal to
99 which is normally at least equal
100 to the maximum number of descriptors supported by the system.
104 is a non-nil pointer, it specifies the maximum interval to wait for the
105 selection to complete. System activity can lengthen the interval by
106 an indeterminate amount.
110 is a nil pointer, the select blocks indefinitely.
112 To effect a poll, the
114 argument should be non-nil, pointing to a zero-valued timeval structure.
121 may be given as nil pointers if no descriptors are of interest.
123 Even if no descriptors are of interest, the timeout works as described,
124 effectively putting the process into an interruptible sleep for the
128 is nil, the process will block until a signal is received.
131 returns the number of ready descriptors that are contained in
133 or -1 if an error occurred.
134 If the time limit expires,
139 returns with an error,
140 including one due to an interrupted call,
141 the descriptor sets will be unmodified.
148 One of the descriptor sets specified an invalid descriptor.
150 A signal was delivered before the time limit expired and
151 before any of the selected events occurred.
153 The specified time limit is invalid. One of its components is
154 negative or too large.
162 .Xr getdtablesize 2 ,
173 In order to accommodate programs which might potentially
174 use a larger number of open files with
177 to increase this size by having the program define
179 before the inclusion of any header which includes
184 is greater than the number of open files,
186 is not guaranteed to examine the unused file descriptors. For historical
189 will always examine the first 256 descriptors.
193 function call appeared in
197 allows systems to modify the original timeout in place.
198 Thus, it is unwise to assume that the timeout value will be unmodified