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
|
'\" t
.\" Copyright (C) 2006 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH sem_wait 3 2023-07-20 "Linux man-pages 6.05.01"
.SH NAME
sem_wait, sem_timedwait, sem_trywait \- lock a semaphore
.SH LIBRARY
POSIX threads library
.RI ( libpthread ", " \-lpthread )
.SH SYNOPSIS
.nf
.B #include <semaphore.h>
.PP
.BI "int sem_wait(sem_t *" sem );
.BI "int sem_trywait(sem_t *" sem );
.BI "int sem_timedwait(sem_t *restrict " sem ,
.BI " const struct timespec *restrict " abs_timeout );
.fi
.PP
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.BR sem_timedwait ():
.nf
_POSIX_C_SOURCE >= 200112L
.fi
.SH DESCRIPTION
.BR sem_wait ()
decrements (locks) the semaphore pointed to by
.IR sem .
If the semaphore's value is greater than zero,
then the decrement proceeds, and the function returns, immediately.
If the semaphore currently has the value zero,
then the call blocks until either it becomes possible to perform
the decrement (i.e., the semaphore value rises above zero),
or a signal handler interrupts the call.
.PP
.BR sem_trywait ()
is the same as
.BR sem_wait (),
except that if the decrement cannot be immediately performed,
then call returns an error
.RI ( errno
set to
.BR EAGAIN )
instead of blocking.
.PP
.BR sem_timedwait ()
is the same as
.BR sem_wait (),
except that
.I abs_timeout
specifies a limit on the amount of time that the call
should block if the decrement cannot be immediately performed.
The
.I abs_timeout
argument points to a
.BR timespec (3)
structure that specifies an absolute timeout
in seconds and nanoseconds since the Epoch, 1970-01-01 00:00:00 +0000 (UTC).
.PP
If the timeout has already expired by the time of the call,
and the semaphore could not be locked immediately,
then
.BR sem_timedwait ()
fails with a timeout error
.RI ( errno
set to
.BR ETIMEDOUT ).
.PP
If the operation can be performed immediately, then
.BR sem_timedwait ()
never fails with a timeout error, regardless of the value of
.IR abs_timeout .
Furthermore, the validity of
.I abs_timeout
is not checked in this case.
.SH RETURN VALUE
All of these functions return 0 on success;
on error, the value of the semaphore is left unchanged,
\-1 is returned, and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EAGAIN
.RB ( sem_trywait ())
The operation could not be performed without blocking (i.e., the
semaphore currently has the value zero).
.TP
.B EINTR
The call was interrupted by a signal handler; see
.BR signal (7).
.TP
.B EINVAL
.I sem
is not a valid semaphore.
.TP
.B EINVAL
.RB ( sem_timedwait ())
The value of
.I abs_timeout.tv_nsecs
is less than 0, or greater than or equal to 1000 million.
.TP
.B ETIMEDOUT
.RB ( sem_timedwait ())
The call timed out before the semaphore could be locked.
.\" POSIX.1-2001 also allows EDEADLK -- "A deadlock condition
.\" was detected", but this does not occur on Linux(?).
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lbx lb lb
l l l.
Interface Attribute Value
T{
.na
.nh
.BR sem_wait (),
.BR sem_trywait (),
.BR sem_timedwait ()
T} Thread safety MT-Safe
.TE
.sp 1
.SH STANDARDS
POSIX.1-2008.
.SH HISTORY
POSIX.1-2001.
.SH EXAMPLES
The (somewhat trivial) program shown below operates on an
unnamed semaphore.
The program expects two command-line arguments.
The first argument specifies a seconds value that is used to
set an alarm timer to generate a
.B SIGALRM
signal.
This handler performs a
.BR sem_post (3)
to increment the semaphore that is being waited on in
.I main()
using
.BR sem_timedwait ().
The second command-line argument specifies the length
of the timeout, in seconds, for
.BR sem_timedwait ().
The following shows what happens on two different runs of the program:
.PP
.in +4n
.EX
.RB "$" " ./a.out 2 3"
About to call sem_timedwait()
sem_post() from handler
sem_timedwait() succeeded
.RB "$" " ./a.out 2 1"
About to call sem_timedwait()
sem_timedwait() timed out
.EE
.in
.SS Program source
\&
.\" SRC BEGIN (sem_wait.c)
.EX
#include <errno.h>
#include <semaphore.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <unistd.h>
\&
#include <assert.h>
\&
sem_t sem;
\&
#define handle_error(msg) \e
do { perror(msg); exit(EXIT_FAILURE); } while (0)
\&
static void
handler(int sig)
{
write(STDOUT_FILENO, "sem_post() from handler\en", 24);
if (sem_post(&sem) == \-1) {
write(STDERR_FILENO, "sem_post() failed\en", 18);
_exit(EXIT_FAILURE);
}
}
\&
int
main(int argc, char *argv[])
{
struct sigaction sa;
struct timespec ts;
int s;
\&
if (argc != 3) {
fprintf(stderr, "Usage: %s <alarm\-secs> <wait\-secs>\en",
argv[0]);
exit(EXIT_FAILURE);
}
\&
if (sem_init(&sem, 0, 0) == \-1)
handle_error("sem_init");
\&
/* Establish SIGALRM handler; set alarm timer using argv[1]. */
\&
sa.sa_handler = handler;
sigemptyset(&sa.sa_mask);
sa.sa_flags = 0;
if (sigaction(SIGALRM, &sa, NULL) == \-1)
handle_error("sigaction");
\&
alarm(atoi(argv[1]));
\&
/* Calculate relative interval as current time plus
number of seconds given argv[2]. */
\&
if (clock_gettime(CLOCK_REALTIME, &ts) == \-1)
handle_error("clock_gettime");
\&
ts.tv_sec += atoi(argv[2]);
\&
printf("%s() about to call sem_timedwait()\en", __func__);
while ((s = sem_timedwait(&sem, &ts)) == \-1 && errno == EINTR)
continue; /* Restart if interrupted by handler. */
\&
/* Check what happened. */
\&
if (s == \-1) {
if (errno == ETIMEDOUT)
printf("sem_timedwait() timed out\en");
else
perror("sem_timedwait");
} else
printf("sem_timedwait() succeeded\en");
\&
exit((s == 0) ? EXIT_SUCCESS : EXIT_FAILURE);
}
.EE
.\" SRC END
.SH SEE ALSO
.BR clock_gettime (2),
.BR sem_getvalue (3),
.BR sem_post (3),
.BR timespec (3),
.BR sem_overview (7),
.BR time (7)
|
