2 .\" Copyright (C) 2006 Michael Kerrisk <mtk.manpages@gmail.com>
4 .\" %%%LICENSE_START(VERBATIM)
5 .\" Permission is granted to make and distribute verbatim copies of this
6 .\" manual provided the copyright notice and this permission notice are
7 .\" preserved on all copies.
9 .\" Permission is granted to copy and distribute modified versions of this
10 .\" manual under the conditions for verbatim copying, provided that the
11 .\" entire resulting derived work is distributed under the terms of a
12 .\" permission notice identical to this one.
14 .\" Since the Linux kernel and libraries are constantly changing, this
15 .\" manual page may be incorrect or out-of-date. The author(s) assume no
16 .\" responsibility for errors or omissions, or for damages resulting from
17 .\" the use of the information contained herein. The author(s) may not
18 .\" have taken the same level of care in the production of this manual,
19 .\" which is licensed free of charge, as they might when working
22 .\" Formatted or processed versions of this manual, if unaccompanied by
23 .\" the source, must acknowledge the copyright and authors of this work.
26 .TH SEM_WAIT 3 2017-03-13 "Linux" "Linux Programmer's Manual"
28 sem_wait, sem_timedwait, sem_trywait \- lock a semaphore
31 .B #include <semaphore.h>
33 .BI "int sem_wait(sem_t *" sem );
35 .BI "int sem_trywait(sem_t *" sem );
37 .BI "int sem_timedwait(sem_t *" sem ", const struct timespec *" abs_timeout );
40 Link with \fI\-pthread\fP.
43 Feature Test Macro Requirements for glibc (see
44 .BR feature_test_macros (7)):
48 _POSIX_C_SOURCE\ >=\ 200112L
51 decrements (locks) the semaphore pointed to by
53 If the semaphore's value is greater than zero,
54 then the decrement proceeds, and the function returns, immediately.
55 If the semaphore currently has the value zero,
56 then the call blocks until either it becomes possible to perform
57 the decrement (i.e., the semaphore value rises above zero),
58 or a signal handler interrupts the call.
63 except that if the decrement cannot be immediately performed,
64 then call returns an error
75 specifies a limit on the amount of time that the call
76 should block if the decrement cannot be immediately performed.
79 argument points to a structure that specifies an absolute timeout
80 in seconds and nanoseconds since the Epoch, 1970-01-01 00:00:00 +0000 (UTC).
81 This structure is defined as follows:
86 time_t tv_sec; /* Seconds */
87 long tv_nsec; /* Nanoseconds [0 .. 999999999] */
92 If the timeout has already expired by the time of the call,
93 and the semaphore could not be locked immediately,
96 fails with a timeout error
101 If the operation can be performed immediately, then
103 never fails with a timeout error, regardless of the value of
105 Furthermore, the validity of
107 is not checked in this case.
109 All of these functions return 0 on success;
110 on error, the value of the semaphore is left unchanged,
113 is set to indicate the error.
117 The call was interrupted by a signal handler; see
122 is not a valid semaphore.
124 The following additional error can occur for
128 The operation could not be performed without blocking (i.e., the
129 semaphore currently has the value zero).
131 The following additional errors can occur for
132 .BR sem_timedwait ():
136 .I abs_timeout.tv_nsecs
137 is less than 0, or greater than or equal to 1000 million.
140 The call timed out before the semaphore could be locked.
141 .\" POSIX.1-2001 also allows EDEADLK -- "A deadlock condition
142 .\" was detected", but this does not occur on Linux(?).
144 For an explanation of the terms used in this section, see
150 Interface Attribute Value
155 T} Thread safety MT-Safe
158 POSIX.1-2001, POSIX.1-2008.
161 The (somewhat trivial) program shown below operates on an
163 The program expects two command-line arguments.
164 The first argument specifies a seconds value that is used to
165 set an alarm timer to generate a
168 This handler performs a
170 to increment the semaphore that is being waited on in
173 .BR sem_timedwait ().
174 The second command-line argument specifies the length
175 of the timeout, in seconds, for
176 .BR sem_timedwait ().
177 The following shows what happens on two different runs of the program:
181 .RB "$" " ./a.out 2 3"
182 About to call sem_timedwait()
183 sem_post() from handler
184 sem_timedwait() succeeded
185 .RB "$" " ./a.out 2 1"
186 About to call sem_timedwait()
187 sem_timedwait() timed out
196 #include <semaphore.h>
204 #define handle_error(msg) \\
205 do { perror(msg); exit(EXIT_FAILURE); } while (0)
210 write(STDOUT_FILENO, "sem_post() from handler\\n", 24);
211 if (sem_post(&sem) == \-1) {
212 write(STDERR_FILENO, "sem_post() failed\\n", 18);
218 main(int argc, char *argv[])
225 fprintf(stderr, "Usage: %s <alarm\-secs> <wait\-secs>\\n",
230 if (sem_init(&sem, 0, 0) == \-1)
231 handle_error("sem_init");
233 /* Establish SIGALRM handler; set alarm timer using argv[1] */
235 sa.sa_handler = handler;
236 sigemptyset(&sa.sa_mask);
238 if (sigaction(SIGALRM, &sa, NULL) == \-1)
239 handle_error("sigaction");
241 alarm(atoi(argv[1]));
243 /* Calculate relative interval as current time plus
244 number of seconds given argv[2] */
246 if (clock_gettime(CLOCK_REALTIME, &ts) == \-1)
247 handle_error("clock_gettime");
249 ts.tv_sec += atoi(argv[2]);
251 printf("main() about to call sem_timedwait()\\n");
252 while ((s = sem_timedwait(&sem, &ts)) == \-1 && errno == EINTR)
253 continue; /* Restart if interrupted by handler */
255 /* Check what happened */
258 if (errno == ETIMEDOUT)
259 printf("sem_timedwait() timed out\\n");
261 perror("sem_timedwait");
263 printf("sem_timedwait() succeeded\\n");
265 exit((s == 0) ? EXIT_SUCCESS : EXIT_FAILURE);
269 .BR clock_gettime (2),
270 .BR sem_getvalue (3),
272 .BR sem_overview (7),