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25 .TH SEM_WAIT 3 2021-08-27 "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 .RB ( sem_trywait ())
118 The operation could not be performed without blocking (i.e., the
119 semaphore currently has the value zero).
122 The call was interrupted by a signal handler; see
127 is not a valid semaphore.
130 .RB ( sem_timedwait ())
132 .I abs_timeout.tv_nsecs
133 is less than 0, or greater than or equal to 1000 million.
136 .RB ( sem_timedwait ())
137 The call timed out before the semaphore could be locked.
138 .\" POSIX.1-2001 also allows EDEADLK -- "A deadlock condition
139 .\" was detected", but this does not occur on Linux(?).
141 For an explanation of the terms used in this section, see
149 Interface Attribute Value
154 T} Thread safety MT-Safe
160 POSIX.1-2001, POSIX.1-2008.
162 The (somewhat trivial) program shown below operates on an
164 The program expects two command-line arguments.
165 The first argument specifies a seconds value that is used to
166 set an alarm timer to generate a
169 This handler performs a
171 to increment the semaphore that is being waited on in
174 .BR sem_timedwait ().
175 The second command-line argument specifies the length
176 of the timeout, in seconds, for
177 .BR sem_timedwait ().
178 The following shows what happens on two different runs of the program:
182 .RB "$" " ./a.out 2 3"
183 About to call sem_timedwait()
184 sem_post() from handler
185 sem_timedwait() succeeded
186 .RB "$" " ./a.out 2 1"
187 About to call sem_timedwait()
188 sem_timedwait() timed out
197 #include <semaphore.h>
205 #define handle_error(msg) \e
206 do { perror(msg); exit(EXIT_FAILURE); } while (0)
211 write(STDOUT_FILENO, "sem_post() from handler\en", 24);
212 if (sem_post(&sem) == \-1) {
213 write(STDERR_FILENO, "sem_post() failed\en", 18);
219 main(int argc, char *argv[])
226 fprintf(stderr, "Usage: %s <alarm\-secs> <wait\-secs>\en",
231 if (sem_init(&sem, 0, 0) == \-1)
232 handle_error("sem_init");
234 /* Establish SIGALRM handler; set alarm timer using argv[1]. */
236 sa.sa_handler = handler;
237 sigemptyset(&sa.sa_mask);
239 if (sigaction(SIGALRM, &sa, NULL) == \-1)
240 handle_error("sigaction");
242 alarm(atoi(argv[1]));
244 /* Calculate relative interval as current time plus
245 number of seconds given argv[2]. */
247 if (clock_gettime(CLOCK_REALTIME, &ts) == \-1)
248 handle_error("clock_gettime");
250 ts.tv_sec += atoi(argv[2]);
252 printf("main() about to call sem_timedwait()\en");
253 while ((s = sem_timedwait(&sem, &ts)) == \-1 && errno == EINTR)
254 continue; /* Restart if interrupted by handler. */
256 /* Check what happened. */
259 if (errno == ETIMEDOUT)
260 printf("sem_timedwait() timed out\en");
262 perror("sem_timedwait");
264 printf("sem_timedwait() succeeded\en");
266 exit((s == 0) ? EXIT_SUCCESS : EXIT_FAILURE);
270 .BR clock_gettime (2),
271 .BR sem_getvalue (3),
273 .BR sem_overview (7),