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
|
.\" Hey Emacs! This file is -*- nroff -*- source.
.\"
.\" This manpage is copyright (C) 1992 Drew Eckhardt,
.\" copyright (C) 1995 Michael Shields.
.\"
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\"
.\" Permission is granted to copy and distribute modified versions of this
.\" manual under the conditions for verbatim copying, provided that the
.\" entire resulting derived work is distributed under the terms of a
.\" permission notice identical to this one
.\"
.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date. The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
.\" the use of the information contained herein. The author(s) may not
.\" have taken the same level of care in the production of this manual,
.\" which is licensed free of charge, as they might when working
.\" professionally.
.\"
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
.\"
.\" Modified 1993-07-24 by Rik Faith <faith@cs.unc.edu>
.\" Modified 1995-05-18 by Jim Van Zandt <jrv@vanzandt.mv.com>
.\" Sun Feb 11 14:07:00 MET 1996 Martin Schulze <joey@linux.de>
.\" * layout slightly modified
.\"
.\" Modified Mon Oct 21 23:05:29 EDT 1996 by Eric S. Raymond <esr@thyrsus.com>
.\" Modified Thu Feb 24 01:41:09 CET 2000 by aeb
.TH SELECT 2 "February 11, 1996" "Linux 1.2" "Linux Programmer's Manual"
.SH NAME
select, FD_CLR, FD_ISSET, FD_SET, FD_ZERO \- synchronous I/O multiplexing
.SH SYNOPSIS
.B #include <sys/time.h>
.br
.B #include <sys/types.h>
.br
.B #include <unistd.h>
.sp
\fBint select(int \fIn\fB, fd_set *\fIreadfds\fB,
fd_set *\fIwritefds\fB, fd_set *\fIexceptfds\fB,
struct timeval *\fItimeout\fB);
.sp
.BI "FD_CLR(int " fd ", fd_set *" set );
.br
.BI "FD_ISSET(int " fd ", fd_set *" set );
.br
.BI "FD_SET(int " fd ", fd_set *" set );
.br
.BI "FD_ZERO(fd_set *" set );
.fi
.SH DESCRIPTION
.B select
waits for a number of file descriptors to change status.
.PP
Three independent sets of descriptors are watched. Those listed in
.I readfds
will be watched to see if characters become
available for reading (more precisely, to see if a read will not
block - in particular, a file descriptor is also ready on end-of-file),
those in
.I writefds
will be watched to see if a write will not block, and
those in
.I exceptfds
will be watched for exceptions. On exit, the sets are modified in place
to indicate which descriptors actually changed status.
.PP
Four macros are provided to manipulate the sets.
.B FD_ZERO
will clear a set.
.B FD_SET
and
.B FD_CLR
add or remove a given descriptor from a set.
.B FD_ISSET
tests to see if a descriptor is part of the set; this is useful after
.B select
returns.
.PP
.I n
is the highest-numbered descriptor in any of the three sets, plus 1.
.PP
.I timeout
is an upper bound on the amount of time elapsed before
.B select
returns. It may be zero, causing
.B select
to return immediately. If
.I timeout
is NULL (no timeout),
.B select
can block indefinitely.
.SH RETURN VALUE
On success,
.B select
returns the number of descriptors contained in the descriptor sets, which
may be zero if the timeout expires before anything interesting happens.
On error, \-1 is returned, and
.I errno
is set appropriately; the sets and
.I timeout
become undefined, so do not
rely on their contents after an error.
.SH ERRORS
.TP
.B EBADF
An invalid file descriptor was given in one of the sets.
.TP
.B EINTR
A non blocked signal was caught.
.TP
.B EINVAL
.I n
is negative.
.TP
.B ENOMEM
.B select
was unable to allocate memory for internal tables.
.SH NOTES
Some code calls
.B select
with all three sets empty,
.B n
zero, and a non-null
.I timeout
as a fairly portable way to sleep with subsecond precision.
.PP
On Linux,
.I timeout
is modified to reflect the amount of time not slept; most other
implementations do not do this. This causes problems both when Linux
code which reads
.I timeout
is ported to other operating systems, and when code is ported to Linux
that reuses a struct timeval for multiple
.BR select s
in a loop without reinitializing it. Consider
.I timeout
to be undefined after
.B select
returns.
.SH EXAMPLE
.nf
#include <stdio.h>
#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h>
int
main(void)
{
fd_set rfds;
struct timeval tv;
int retval;
/* Watch stdin (fd 0) to see when it has input. */
FD_ZERO(&rfds);
FD_SET(0, &rfds);
/* Wait up to five seconds. */
tv.tv_sec = 5;
tv.tv_usec = 0;
retval = select(1, &rfds, NULL, NULL, &tv);
/* Don't rely on the value of tv now! */
if (retval)
printf("Data is available now.\\n");
/* FD_ISSET(0, &rfds) will be true. */
else
printf("No data within five seconds.\\n");
exit(0);
}
.fi
.SH CONFORMING TO
4.4BSD (the
.B select
function first appeared in 4.2BSD). Generally portable to/from
non-BSD systems supporting clones of the BSD socket layer (including
System V variants). However, note that the System V variant typically
sets the timeout variable before exit, but the BSD variant does not.
.SH SEE ALSO
.BR accept (2),
.BR connect (2),
.BR poll (2),
.BR read (2),
.BR recv (2),
.BR send (2),
.BR write (2)
|
