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1 | .\" Copyright (c) 1983, 1991, 1993 |
2 | .\" The Regents of the University of California. All rights reserved. | |
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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. | |
dc71b7ab | 12 | .\" 3. Neither the name of the University nor the names of its contributors |
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13 | .\" may be used to endorse or promote products derived from this software |
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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 | .\" | |
7a09efda | 31 | .Dd February 22, 2018 |
984263bc MD |
32 | .Dt SELECT 2 |
33 | .Os | |
34 | .Sh NAME | |
35 | .Nm select | |
36 | .Nd synchronous I/O multiplexing | |
37 | .Sh LIBRARY | |
38 | .Lb libc | |
39 | .Sh SYNOPSIS | |
8c6b45fc | 40 | .In sys/select.h |
984263bc | 41 | .Ft int |
7a09efda | 42 | .Fn select "int nfds" "fd_set * restrict readfds" "fd_set * restrict writefds" "fd_set * restrict exceptfds" "struct timeval * restrict timeout" |
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43 | .Fn FD_SET fd &fdset |
44 | .Fn FD_CLR fd &fdset | |
45 | .Fn FD_ISSET fd &fdset | |
46 | .Fn FD_ZERO &fdset | |
47 | .Sh DESCRIPTION | |
48 | .Fn Select | |
49 | examines the I/O descriptor sets whose addresses are passed in | |
50 | .Fa readfds , | |
51 | .Fa writefds , | |
52 | and | |
53 | .Fa exceptfds | |
54 | to see if some of their descriptors | |
55 | are ready for reading, are ready for writing, or have an exceptional | |
56 | condition pending, respectively. | |
57 | The only exceptional condition detectable is out-of-band | |
58 | data received on a socket. | |
59 | The first | |
60 | .Fa nfds | |
61 | descriptors are checked in each set; | |
62 | i.e., the descriptors from 0 through | |
63 | .Fa nfds Ns No -1 | |
64 | in the descriptor sets are examined. | |
65 | On return, | |
66 | .Fn select | |
67 | replaces the given descriptor sets | |
68 | with subsets consisting of those descriptors that are ready | |
69 | for the requested operation. | |
70 | .Fn Select | |
71 | returns the total number of ready descriptors in all the sets. | |
72 | .Pp | |
73 | The descriptor sets are stored as bit fields in arrays of integers. | |
74 | The following macros are provided for manipulating such descriptor sets: | |
75 | .Fn FD_ZERO &fdset | |
76 | initializes a descriptor set | |
77 | .Fa fdset | |
78 | to the null set. | |
79 | .Fn FD_SET fd &fdset | |
80 | includes a particular descriptor | |
81 | .Fa fd | |
82 | in | |
83 | .Fa fdset . | |
84 | .Fn FD_CLR fd &fdset | |
85 | removes | |
86 | .Fa fd | |
87 | from | |
88 | .Fa fdset . | |
89 | .Fn FD_ISSET fd &fdset | |
90 | is non-zero if | |
91 | .Fa fd | |
92 | is a member of | |
93 | .Fa fdset , | |
94 | zero otherwise. | |
95 | The behavior of these macros is undefined if | |
96 | a descriptor value is less than zero or greater than or equal to | |
97 | .Dv FD_SETSIZE , | |
98 | which is normally at least equal | |
99 | to the maximum number of descriptors supported by the system. | |
100 | .Pp | |
101 | If | |
102 | .Fa timeout | |
8095c904 | 103 | is non-NULL, it specifies the maximum interval to wait for the |
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104 | selection to complete. System activity can lengthen the interval by |
105 | an indeterminate amount. | |
106 | .Pp | |
107 | If | |
108 | .Fa timeout | |
8095c904 | 109 | is a null pointer, the select blocks indefinitely. |
984263bc MD |
110 | .Pp |
111 | To effect a poll, the | |
112 | .Fa timeout | |
8095c904 | 113 | argument should be non-NULL, pointing to a zero-valued timeval structure. |
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114 | .Pp |
115 | Any of | |
116 | .Fa readfds , | |
117 | .Fa writefds , | |
118 | and | |
119 | .Fa exceptfds | |
8095c904 | 120 | may be given as null pointers if no descriptors are of interest. |
56c95343 SS |
121 | .Pp |
122 | Even if no descriptors are of interest, the timeout works as described, | |
89ea1e59 SS |
123 | effectively putting the process into an interruptible sleep for the |
124 | specified timeout. | |
125 | If | |
126 | .Fa timeout | |
8095c904 | 127 | is NULL, the process will block until a signal is received. |
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128 | .Sh RETURN VALUES |
129 | .Fn Select | |
130 | returns the number of ready descriptors that are contained in | |
131 | the descriptor sets, | |
132 | or -1 if an error occurred. | |
133 | If the time limit expires, | |
134 | .Fn select | |
135 | returns 0. | |
136 | If | |
137 | .Fn select | |
138 | returns with an error, | |
139 | including one due to an interrupted call, | |
140 | the descriptor sets will be unmodified. | |
141 | .Sh ERRORS | |
142 | An error return from | |
143 | .Fn select | |
144 | indicates: | |
145 | .Bl -tag -width Er | |
146 | .It Bq Er EBADF | |
147 | One of the descriptor sets specified an invalid descriptor. | |
148 | .It Bq Er EINTR | |
149 | A signal was delivered before the time limit expired and | |
150 | before any of the selected events occurred. | |
151 | .It Bq Er EINVAL | |
152 | The specified time limit is invalid. One of its components is | |
153 | negative or too large. | |
154 | .It Bq Er EINVAL | |
155 | .Fa nfds | |
156 | was invalid. | |
157 | .El | |
158 | .Sh SEE ALSO | |
159 | .Xr accept 2 , | |
160 | .Xr connect 2 , | |
161 | .Xr getdtablesize 2 , | |
162 | .Xr gettimeofday 2 , | |
163 | .Xr read 2 , | |
164 | .Xr recv 2 , | |
165 | .Xr send 2 , | |
166 | .Xr write 2 , | |
167 | .Xr clocks 7 | |
168 | .Sh NOTES | |
169 | The default size of | |
170 | .Dv FD_SETSIZE | |
171 | is currently 1024. | |
172 | In order to accommodate programs which might potentially | |
173 | use a larger number of open files with | |
174 | .Fn select , | |
175 | it is possible | |
176 | to increase this size by having the program define | |
177 | .Dv FD_SETSIZE | |
178 | before the inclusion of any header which includes | |
44cb301e | 179 | .In sys/types.h . |
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180 | .Pp |
181 | If | |
182 | .Fa nfds | |
183 | is greater than the number of open files, | |
184 | .Fn select | |
185 | is not guaranteed to examine the unused file descriptors. For historical | |
186 | reasons, | |
187 | .Fn select | |
188 | will always examine the first 256 descriptors. | |
0b84df5c SW |
189 | .Sh HISTORY |
190 | The | |
191 | .Fn select | |
192 | function call appeared in | |
193 | .Bx 4.2 . | |
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194 | .Sh BUGS |
195 | .St -susv2 | |
196 | allows systems to modify the original timeout in place. | |
197 | Thus, it is unwise to assume that the timeout value will be unmodified | |
198 | by the | |
199 | .Fn select | |
200 | call. |