summaryrefslogtreecommitdiff
path: root/lib/libc/sys/select.2
blob: 662e23e2d5f2fd0a193acd9c8ed4163f6601c813 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
.\"	$OpenBSD: select.2,v 1.13 1999/05/23 14:10:55 aaron Exp $
.\"	$NetBSD: select.2,v 1.5 1995/06/27 22:32:28 cgd Exp $
.\"
.\" Copyright (c) 1983, 1991, 1993
.\"	The Regents of the University of California.  All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\" 3. All advertising materials mentioning features or use of this software
.\"    must display the following acknowledgement:
.\"	This product includes software developed by the University of
.\"	California, Berkeley and its contributors.
.\" 4. Neither the name of the University nor the names of its contributors
.\"    may be used to endorse or promote products derived from this software
.\"    without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.\"     @(#)select.2	8.2 (Berkeley) 3/25/94
.\"
.Dd March 25, 1994
.Dt SELECT 2
.Os
.Sh NAME
.Nm select
.Nd synchronous I/O multiplexing
.Sh SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <sys/time.h>
.Fd #include <unistd.h>
.Ft int
.Fn select "int nfds" "fd_set *readfds" "fd_set *writefds" "fd_set *exceptfds" "struct timeval *timeout"
.Fn FD_SET fd &fdset
.Fn FD_CLR fd &fdset
.Fn FD_ISSET fd &fdset
.Fn FD_ZERO &fdset
.Sh DESCRIPTION
.Fn select
examines the I/O descriptor sets whose addresses are passed in
.Fa readfds ,
.Fa writefds ,
and
.Fa exceptfds
to see if some of their descriptors
are ready for reading, are ready for writing, or have an exceptional
condition pending, respectively.
The first
.Fa nfds
descriptors are checked in each set;
i.e., the descriptors from 0 through
.Fa nfds Ns No -1
in the descriptor sets are examined.
On return,
.Fn select
replaces the given descriptor sets
with subsets consisting of those descriptors that are ready
for the requested operation.
.Fn select
returns the total number of ready descriptors in all the sets.
.Pp
The descriptor sets are stored as bit fields in arrays of integers.
The following macros are provided for manipulating such descriptor sets:
.Fn FD_ZERO &fdset
initializes a descriptor set
.Fa fdset
to the null set.
.Fn FD_SET fd &fdset
includes a particular descriptor
.Fa fd
in
.Fa fdset .
.Fn FD_CLR fd &fdset
removes
.Fa fd
from
.Fa fdset .
.Fn FD_ISSET fd &fdset
is non-zero if
.Fa fd
is a member of
.Fa fdset ,
zero otherwise.
The behavior of these macros is undefined if
a descriptor value is less than zero or greater than or equal to
.Dv FD_SETSIZE ,
which is normally at least equal
to the maximum number of descriptors supported by the system.
.Pp
If
.Fa timeout
is a non-nil pointer, it specifies a maximum interval to wait for the
selection to complete.  If 
.Fa timeout
is a nil pointer, the select blocks indefinitely.  To effect a poll, the
.Fa timeout
argument should be non-nil, pointing to a zero-valued timeval structure.
.Fa timeout
is not changed by
.Fn select ,
and may be reused on subsequent calls; however, it is good style to
re-initialize it before each invocation of
.Fn select .
.Pp
Any of
.Fa readfds ,
.Fa writefds ,
and
.Fa exceptfds
may be given as nil pointers if no descriptors are of interest.
.Sh RETURN VALUES
.Fn select
returns the number of ready descriptors that are contained in
the descriptor sets,
or -1 if an error occurred.
If the time limit expires,
.Fn select
returns 0.
If
.Fn select
returns with an error,
including one due to an interrupted call,
the descriptor sets will be unmodified.
.Sh ERRORS
An error return from
.Fn select
indicates:
.Bl -tag -width Er
.It Bq Er EFAULT
One or more of
.Fa readfds ,
.Fa writefds ,
or
.Fa exceptfds
points outside the process's allocated address space.
.It Bq Er EBADF
One of the descriptor sets specified an invalid descriptor.
.It Bq Er EINTR
A signal was delivered before the time limit expired and
before any of the selected events occurred.
.It Bq Er EINVAL
The specified time limit is invalid.  One of its components is
negative or too large.
.El
.Sh SEE ALSO
.Xr accept 2 ,
.Xr connect 2 ,
.Xr gettimeofday 2 ,
.Xr poll 2 ,
.Xr read 2 ,
.Xr recv 2 ,
.Xr send 2 ,
.Xr write 2 ,
.Xr getdtablesize 3
.Sh BUGS
Although the provision of
.Xr getdtablesize 3
was intended to allow user programs to be written independent
of the kernel limit on the number of open files, the dimension
of a sufficiently large bit field for select remains a problem.
The default size
.Dv FD_SETSIZE
(currently 256) is somewhat smaller than
the current kernel limit to the number of open files.
However, in order to accommodate programs which might potentially
use a larger number of open files with select, it is possible
to increase this size within a program by providing
a larger definition of
.Dv FD_SETSIZE
before the inclusion of
.Aq Pa sys/types.h .
The kernel will cope, and the userland libraries provided with the
system are also ready for large numbers of file descriptors.
.Pp
Alternatively, to be really safe, it is possible to allocate
.Ft fd_set
bit-arrays dynamically.  The idea is to permit a program to
work properly even if it is
.Xr execve 2 Ns 'd
with 4000 file descriptors pre-allocated.
The following illustrates the technique which is used by
userland libraries:
.Pp
.Bd -literal -offset indent -compact
	fd_set *fdsr;
	int max = fd;

	fsdr = (fd_set *)calloc(howmany(max+1, NFDBITS),
	    sizeof(fd_mask));
	if (fdsr == NULL) {
		...
		return (-1);
	}
	FD_SET(fd, fdsr);
	n = select(max+1, fdsr, NULL, NULL, &tv);
	...
	free(fdsr);
.Ed
.Pp
Alternatively, it is possible to use the
.Xr poll 2
interface.
.Xr poll 2
is more efficient when the size of
.Fn select Ns 's
.Ft fd_set
bit-arrays are very large, and for fixed numbers of
file descriptors one need not size and dynamically allocate a
memory object.
.Pp
.Fn select
should probably have been designed to return the time remaining from the
original timeout, if any, by modifying the time value in place.
Even though some systems stupidly act in this different way, it is
unlikely this semantic will ever be commonly implemented, as the
change causes massive source code compatibility problems.
Furthermore, recent new standards have dictated the current behaviour.
In general, due to the existance of those brain-damaged
non-conforming systems, it is unwise to assume that the timeout
value will be unmodified by the
.Fn select
call, and the caller should reinitialize it on each invocation.
Calculating the delta is easily done by calling
.Xr gettimeofday 2
before and after the call to
.Fn select Ns ,
and using
.Fn timersub
(as described in
.Xr getitimer 2 Ns ).
.Pp
Internally to the kernel,
.Fn select
works poorly if multiple processes wait on the same file descriptor.
Given that, it is rather surprising to see that many daemons are
written that way (ie.
.Xr httpd 8 ).
.Sh HISTORY
The
.Fn select
function call appeared in
.Bx 4.2 .