1sem_wait(3) Library Functions Manual sem_wait(3)
2
6 sem_wait, sem_timedwait, sem_trywait - lock a semaphore
7
9 POSIX threads library (libpthread, -lpthread)
10
12 #include <semaphore.h>
13 int sem_wait(sem_t *sem);
15 int sem_trywait(sem_t *sem);
16 int sem_timedwait(sem_t *restrict sem,
17 const struct timespec *restrict abs_timeout);
18 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
20 sem_timedwait():
22 _POSIX_C_SOURCE >= 200112L
23
25 sem_wait() decrements (locks) the semaphore pointed to by sem. If the
26 semaphore's value is greater than zero, then the decrement proceeds,
27 and the function returns, immediately. If the semaphore currently has
28 the value zero, then the call blocks until either it becomes possible
29 to perform the decrement (i.e., the semaphore value rises above zero),
30 or a signal handler interrupts the call.
31 sem_trywait() is the same as sem_wait(), except that if the decrement
33 cannot be immediately performed, then call returns an error (errno set
34 to EAGAIN) instead of blocking.
35 sem_timedwait() is the same as sem_wait(), except that abs_timeout
37 specifies a limit on the amount of time that the call should block if
38 the decrement cannot be immediately performed. The abs_timeout argu‐
39 ment points to a timespec(3) structure that specifies an absolute time‐
40 out in seconds and nanoseconds since the Epoch, 1970-01-01 00:00:00
41 +0000 (UTC).
42 If the timeout has already expired by the time of the call, and the
44 semaphore could not be locked immediately, then sem_timedwait() fails
45 with a timeout error (errno set to ETIMEDOUT).
46 If the operation can be performed immediately, then sem_timedwait()
48 never fails with a timeout error, regardless of the value of abs_time‐
49 out. Furthermore, the validity of abs_timeout is not checked in this
50 case.
51
53 All of these functions return 0 on success; on error, the value of the
54 semaphore is left unchanged, -1 is returned, and errno is set to indi‐
55 cate the error.
56
58 EAGAIN (sem_trywait()) The operation could not be performed without
59 blocking (i.e., the semaphore currently has the value zero).
60 EINTR The call was interrupted by a signal handler; see signal(7).
62 EINVAL sem is not a valid semaphore.
64 EINVAL (sem_timedwait()) The value of abs_timeout.tv_nsecs is less than
66 0, or greater than or equal to 1000 million.
67 ETIMEDOUT
69 (sem_timedwait()) The call timed out before the semaphore could
70 be locked.
71
73 For an explanation of the terms used in this section, see at‐
74 tributes(7).
75 ┌────────────────────────────────────────────┬───────────────┬─────────┐
77 │Interface │ Attribute │ Value │
78 ├────────────────────────────────────────────┼───────────────┼─────────┤
79 │sem_wait(), sem_trywait(), sem_timedwait() │ Thread safety │ MT-Safe │
80 └────────────────────────────────────────────┴───────────────┴─────────┘
81
83 POSIX.1-2008.
84
86 POSIX.1-2001.
87
89 The (somewhat trivial) program shown below operates on an unnamed
90 semaphore. The program expects two command-line arguments. The first
91 argument specifies a seconds value that is used to set an alarm timer
92 to generate a SIGALRM signal. This handler performs a sem_post(3) to
93 increment the semaphore that is being waited on in main() using
94 sem_timedwait(). The second command-line argument specifies the length
95 of the timeout, in seconds, for sem_timedwait(). The following shows
96 what happens on two different runs of the program:
97 $ ./a.out 2 3
99 About to call sem_timedwait()
100 sem_post() from handler
101 sem_timedwait() succeeded
102 $ ./a.out 2 1
103 About to call sem_timedwait()
104 sem_timedwait() timed out
105 Program source
107 #include <errno.h>
109 #include <semaphore.h>
110 #include <signal.h>
111 #include <stdio.h>
112 #include <stdlib.h>
113 #include <time.h>
114 #include <unistd.h>
115 #include <assert.h>
117 sem_t sem;
119 #define handle_error(msg) \
121 do { perror(msg); exit(EXIT_FAILURE); } while (0)
122 static void
124 handler(int sig)
125 {
126 write(STDOUT_FILENO, "sem_post() from handler\n", 24);
127 if (sem_post(&sem) == -1) {
128 write(STDERR_FILENO, "sem_post() failed\n", 18);
129 _exit(EXIT_FAILURE);
130 }
131 }
132 int
134 main(int argc, char *argv[])
135 {
136 struct sigaction sa;
137 struct timespec ts;
138 int s;
139 if (argc != 3) {
141 fprintf(stderr, "Usage: %s <alarm-secs> <wait-secs>\n",
142 argv[0]);
143 exit(EXIT_FAILURE);
144 }
145 if (sem_init(&sem, 0, 0) == -1)
147 handle_error("sem_init");
148 /* Establish SIGALRM handler; set alarm timer using argv[1]. */
150 sa.sa_handler = handler;
152 sigemptyset(&sa.sa_mask);
153 sa.sa_flags = 0;
154 if (sigaction(SIGALRM, &sa, NULL) == -1)
155 handle_error("sigaction");
156 alarm(atoi(argv[1]));
158 /* Calculate relative interval as current time plus
160 number of seconds given argv[2]. */
161 if (clock_gettime(CLOCK_REALTIME, &ts) == -1)
163 handle_error("clock_gettime");
164 ts.tv_sec += atoi(argv[2]);
166 printf("%s() about to call sem_timedwait()\n", __func__);
168 while ((s = sem_timedwait(&sem, &ts)) == -1 && errno == EINTR)
169 continue; /* Restart if interrupted by handler. */
170 /* Check what happened. */
172 if (s == -1) {
174 if (errno == ETIMEDOUT)
175 printf("sem_timedwait() timed out\n");
176 else
177 perror("sem_timedwait");
178 } else
179 printf("sem_timedwait() succeeded\n");
180 exit((s == 0) ? EXIT_SUCCESS : EXIT_FAILURE);
182 }
183
185 clock_gettime(2), sem_getvalue(3), sem_post(3), timespec(3), sem_over‐
186 view(7), time(7)
187Linux man-pages 6.05 2023-07-20 sem_wait(3)