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25 .TH SEM_WAIT 3 2021-03-22 "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 *restrict " sem ,
35 .BI " const struct timespec *restrict " abs_timeout );
38 Link with \fI\-pthread\fP.
41 Feature Test Macro Requirements for glibc (see
42 .BR feature_test_macros (7)):
47 _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
152 Interface Attribute Value
157 T} Thread safety MT-Safe
163 POSIX.1-2001, POSIX.1-2008.
165 The (somewhat trivial) program shown below operates on an
167 The program expects two command-line arguments.
168 The first argument specifies a seconds value that is used to
169 set an alarm timer to generate a
172 This handler performs a
174 to increment the semaphore that is being waited on in
177 .BR sem_timedwait ().
178 The second command-line argument specifies the length
179 of the timeout, in seconds, for
180 .BR sem_timedwait ().
181 The following shows what happens on two different runs of the program:
185 .RB "$" " ./a.out 2 3"
186 About to call sem_timedwait()
187 sem_post() from handler
188 sem_timedwait() succeeded
189 .RB "$" " ./a.out 2 1"
190 About to call sem_timedwait()
191 sem_timedwait() timed out
200 #include <semaphore.h>
208 #define handle_error(msg) \e
209 do { perror(msg); exit(EXIT_FAILURE); } while (0)
214 write(STDOUT_FILENO, "sem_post() from handler\en", 24);
215 if (sem_post(&sem) == \-1) {
216 write(STDERR_FILENO, "sem_post() failed\en", 18);
222 main(int argc, char *argv[])
229 fprintf(stderr, "Usage: %s <alarm\-secs> <wait\-secs>\en",
234 if (sem_init(&sem, 0, 0) == \-1)
235 handle_error("sem_init");
237 /* Establish SIGALRM handler; set alarm timer using argv[1]. */
239 sa.sa_handler = handler;
240 sigemptyset(&sa.sa_mask);
242 if (sigaction(SIGALRM, &sa, NULL) == \-1)
243 handle_error("sigaction");
245 alarm(atoi(argv[1]));
247 /* Calculate relative interval as current time plus
248 number of seconds given argv[2]. */
250 if (clock_gettime(CLOCK_REALTIME, &ts) == \-1)
251 handle_error("clock_gettime");
253 ts.tv_sec += atoi(argv[2]);
255 printf("main() about to call sem_timedwait()\en");
256 while ((s = sem_timedwait(&sem, &ts)) == \-1 && errno == EINTR)
257 continue; /* Restart if interrupted by handler. */
259 /* Check what happened. */
262 if (errno == ETIMEDOUT)
263 printf("sem_timedwait() timed out\en");
265 perror("sem_timedwait");
267 printf("sem_timedwait() succeeded\en");
269 exit((s == 0) ? EXIT_SUCCESS : EXIT_FAILURE);
273 .BR clock_gettime (2),
274 .BR sem_getvalue (3),
276 .BR sem_overview (7),