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25 .TH SEM_WAIT 3 2020-06-09 "Linux" "Linux Programmer's Manual"
27 sem_wait, sem_timedwait, sem_trywait \- lock a semaphore
30 .B #include <semaphore.h>
32 .BI "int sem_wait(sem_t *" sem );
33 .BI "int sem_trywait(sem_t *" sem );
34 .BI "int sem_timedwait(sem_t *" sem ", const struct timespec *" abs_timeout );
37 Link with \fI\-pthread\fP.
40 Feature Test Macro Requirements for glibc (see
41 .BR feature_test_macros (7)):
46 _POSIX_C_SOURCE >= 200112L
50 decrements (locks) the semaphore pointed to by
52 If the semaphore's value is greater than zero,
53 then the decrement proceeds, and the function returns, immediately.
54 If the semaphore currently has the value zero,
55 then the call blocks until either it becomes possible to perform
56 the decrement (i.e., the semaphore value rises above zero),
57 or a signal handler interrupts the call.
62 except that if the decrement cannot be immediately performed,
63 then call returns an error
74 specifies a limit on the amount of time that the call
75 should block if the decrement cannot be immediately performed.
78 argument points to a structure that specifies an absolute timeout
79 in seconds and nanoseconds since the Epoch, 1970-01-01 00:00:00 +0000 (UTC).
80 This structure is defined as follows:
85 time_t tv_sec; /* Seconds */
86 long tv_nsec; /* Nanoseconds [0 .. 999999999] */
91 If the timeout has already expired by the time of the call,
92 and the semaphore could not be locked immediately,
95 fails with a timeout error
100 If the operation can be performed immediately, then
102 never fails with a timeout error, regardless of the value of
104 Furthermore, the validity of
106 is not checked in this case.
108 All of these functions return 0 on success;
109 on error, the value of the semaphore is left unchanged,
112 is set to indicate the error.
116 The call was interrupted by a signal handler; see
121 is not a valid semaphore.
123 The following additional error can occur for
127 The operation could not be performed without blocking (i.e., the
128 semaphore currently has the value zero).
130 The following additional errors can occur for
131 .BR sem_timedwait ():
135 .I abs_timeout.tv_nsecs
136 is less than 0, or greater than or equal to 1000 million.
139 The call timed out before the semaphore could be locked.
140 .\" POSIX.1-2001 also allows EDEADLK -- "A deadlock condition
141 .\" was detected", but this does not occur on Linux(?).
143 For an explanation of the terms used in this section, see
151 Interface Attribute Value
156 T} Thread safety MT-Safe
162 POSIX.1-2001, POSIX.1-2008.
164 The (somewhat trivial) program shown below operates on an
166 The program expects two command-line arguments.
167 The first argument specifies a seconds value that is used to
168 set an alarm timer to generate a
171 This handler performs a
173 to increment the semaphore that is being waited on in
176 .BR sem_timedwait ().
177 The second command-line argument specifies the length
178 of the timeout, in seconds, for
179 .BR sem_timedwait ().
180 The following shows what happens on two different runs of the program:
184 .RB "$" " ./a.out 2 3"
185 About to call sem_timedwait()
186 sem_post() from handler
187 sem_timedwait() succeeded
188 .RB "$" " ./a.out 2 1"
189 About to call sem_timedwait()
190 sem_timedwait() timed out
199 #include <semaphore.h>
207 #define handle_error(msg) \e
208 do { perror(msg); exit(EXIT_FAILURE); } while (0)
213 write(STDOUT_FILENO, "sem_post() from handler\en", 24);
214 if (sem_post(&sem) == \-1) {
215 write(STDERR_FILENO, "sem_post() failed\en", 18);
221 main(int argc, char *argv[])
228 fprintf(stderr, "Usage: %s <alarm\-secs> <wait\-secs>\en",
233 if (sem_init(&sem, 0, 0) == \-1)
234 handle_error("sem_init");
236 /* Establish SIGALRM handler; set alarm timer using argv[1]. */
238 sa.sa_handler = handler;
239 sigemptyset(&sa.sa_mask);
241 if (sigaction(SIGALRM, &sa, NULL) == \-1)
242 handle_error("sigaction");
244 alarm(atoi(argv[1]));
246 /* Calculate relative interval as current time plus
247 number of seconds given argv[2]. */
249 if (clock_gettime(CLOCK_REALTIME, &ts) == \-1)
250 handle_error("clock_gettime");
252 ts.tv_sec += atoi(argv[2]);
254 printf("main() about to call sem_timedwait()\en");
255 while ((s = sem_timedwait(&sem, &ts)) == \-1 && errno == EINTR)
256 continue; /* Restart if interrupted by handler. */
258 /* Check what happened. */
261 if (errno == ETIMEDOUT)
262 printf("sem_timedwait() timed out\en");
264 perror("sem_timedwait");
266 printf("sem_timedwait() succeeded\en");
268 exit((s == 0) ? EXIT_SUCCESS : EXIT_FAILURE);
272 .BR clock_gettime (2),
273 .BR sem_getvalue (3),
275 .BR sem_overview (7),