1SEM_WAIT(3) Linux Programmer's Manual SEM_WAIT(3)
2
6 sem_wait, sem_timedwait, sem_trywait - lock a semaphore
7
9 #include <semaphore.h>
10 int sem_wait(sem_t *sem);
12 int sem_trywait(sem_t *sem);
13 int sem_timedwait(sem_t *restrict sem,
14 const struct timespec *restrict abs_timeout);
15 Link with -pthread.
17 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
19 sem_timedwait():
21 _POSIX_C_SOURCE >= 200112L
22
24 sem_wait() decrements (locks) the semaphore pointed to by sem. If the
25 semaphore's value is greater than zero, then the decrement proceeds,
26 and the function returns, immediately. If the semaphore currently has
27 the value zero, then the call blocks until either it becomes possible
28 to perform the decrement (i.e., the semaphore value rises above zero),
29 or a signal handler interrupts the call.
30 sem_trywait() is the same as sem_wait(), except that if the decrement
32 cannot be immediately performed, then call returns an error (errno set
33 to EAGAIN) instead of blocking.
34 sem_timedwait() is the same as sem_wait(), except that abs_timeout
36 specifies a limit on the amount of time that the call should block if
37 the decrement cannot be immediately performed. The abs_timeout argu‐
38 ment points to a structure that specifies an absolute timeout in sec‐
39 onds and nanoseconds since the Epoch, 1970-01-01 00:00:00 +0000 (UTC).
40 This structure is defined as follows:
41 struct timespec {
43 time_t tv_sec; /* Seconds */
44 long tv_nsec; /* Nanoseconds [0 .. 999999999] */
45 };
46 If the timeout has already expired by the time of the call, and the
48 semaphore could not be locked immediately, then sem_timedwait() fails
49 with a timeout error (errno set to ETIMEDOUT).
50 If the operation can be performed immediately, then sem_timedwait()
52 never fails with a timeout error, regardless of the value of abs_time‐
53 out. Furthermore, the validity of abs_timeout is not checked in this
54 case.
55
57 All of these functions return 0 on success; on error, the value of the
58 semaphore is left unchanged, -1 is returned, and errno is set to indi‐
59 cate the error.
60
62 EAGAIN (sem_trywait()) The operation could not be performed without
63 blocking (i.e., the semaphore currently has the value zero).
64 EINTR The call was interrupted by a signal handler; see signal(7).
66 EINVAL sem is not a valid semaphore.
68 EINVAL (sem_timedwait()) The value of abs_timeout.tv_nsecs is less than
70 0, or greater than or equal to 1000 million.
71 ETIMEDOUT
73 (sem_timedwait()) The call timed out before the semaphore could
74 be locked.
75
77 For an explanation of the terms used in this section, see at‐
78 tributes(7).
79 ┌────────────────────────────────────────────┬───────────────┬─────────┐
81 │Interface │ Attribute │ Value │
82 ├────────────────────────────────────────────┼───────────────┼─────────┤
83 │sem_wait(), sem_trywait(), sem_timedwait() │ Thread safety │ MT-Safe │
84 └────────────────────────────────────────────┴───────────────┴─────────┘
85
87 POSIX.1-2001, POSIX.1-2008.
88
90 The (somewhat trivial) program shown below operates on an unnamed sema‐
91 phore. The program expects two command-line arguments. The first ar‐
92 gument specifies a seconds value that is used to set an alarm timer to
93 generate a SIGALRM signal. This handler performs a sem_post(3) to in‐
94 crement the semaphore that is being waited on in main() using
95 sem_timedwait(). The second command-line argument specifies the length
96 of the timeout, in seconds, for sem_timedwait(). The following shows
97 what happens on two different runs of the program:
98 $ ./a.out 2 3
100 About to call sem_timedwait()
101 sem_post() from handler
102 sem_timedwait() succeeded
103 $ ./a.out 2 1
104 About to call sem_timedwait()
105 sem_timedwait() timed out
106 Program source
108 #include <unistd.h>
110 #include <stdio.h>
111 #include <stdlib.h>
112 #include <semaphore.h>
113 #include <time.h>
114 #include <assert.h>
115 #include <errno.h>
116 #include <signal.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("main() about to call sem_timedwait()\n");
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), sem_overview(7),
186 time(7)
187
189 This page is part of release 5.13 of the Linux man-pages project. A
190 description of the project, information about reporting bugs, and the
191 latest version of this page, can be found at
192 https://www.kernel.org/doc/man-pages/.
193Linux 2021-08-27 SEM_WAIT(3)