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26 .TH SEM_WAIT 3 2012-05-13 "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 || _XOPEN_SOURCE\ >=\ 600
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(?).
146 A signal handler always interrupts a blocked call to
147 one of these functions, regardless of the use of the
151 .\" sem_wait() is always interrupted on most other implementations,
152 .\" but on FreeBSD 5.4 SA_RESTART does cause restarting.
155 The (somewhat trivial) program shown below operates on an
157 The program expects two command-line arguments.
158 The first argument specifies a seconds value that is used to
159 set an alarm timer to generate a
162 This handler performs a
164 to increment the semaphore that is being waited on in
167 .BR sem_timedwait ().
168 The second command-line argument specifies the length
169 of the timeout, in seconds, for
170 .BR sem_timedwait ().
171 The following shows what happens on two different runs of the program:
175 .RB "$" " ./a.out 2 3"
176 About to call sem_timedwait()
177 sem_post() from handler
178 sem_timedwait() succeeded
179 .RB "$" " ./a.out 2 1"
180 About to call sem_timedwait()
181 sem_timedwait() timed out
190 #include <semaphore.h>
198 #define handle_error(msg) \\
199 do { perror(msg); exit(EXIT_FAILURE); } while (0)
204 write(STDOUT_FILENO, "sem_post() from handler\\n", 24);
205 if (sem_post(&sem) == \-1) {
206 write(STDERR_FILENO, "sem_post() failed\\n", 18);
212 main(int argc, char *argv[])
219 fprintf(stderr, "Usage: %s <alarm\-secs> <wait\-secs>\\n",
224 if (sem_init(&sem, 0, 0) == \-1)
225 handle_error("sem_init");
227 /* Establish SIGALRM handler; set alarm timer using argv[1] */
229 sa.sa_handler = handler;
230 sigemptyset(&sa.sa_mask);
232 if (sigaction(SIGALRM, &sa, NULL) == \-1)
233 handle_error("sigaction");
235 alarm(atoi(argv[1]));
237 /* Calculate relative interval as current time plus
238 number of seconds given argv[2] */
240 if (clock_gettime(CLOCK_REALTIME, &ts) == \-1)
241 handle_error("clock_gettime");
243 ts.tv_sec += atoi(argv[2]);
245 printf("main() about to call sem_timedwait()\\n");
246 while ((s = sem_timedwait(&sem, &ts)) == \-1 && errno == EINTR)
247 continue; /* Restart if interrupted by handler */
249 /* Check what happened */
252 if (errno == ETIMEDOUT)
253 printf("sem_timedwait() timed out\\n");
255 perror("sem_timedwait");
257 printf("sem_timedwait() succeeded\\n");
259 exit((s == 0) ? EXIT_SUCCESS : EXIT_FAILURE);
263 .BR clock_gettime (2),
264 .BR sem_getvalue (3),
266 .BR sem_overview (7),