#include <semaphore.h> int sem_wait(sem_t *sem); int sem_trywait(sem_t *sem); int sem_timedwait(sem_t *sem, const struct timespec *abs_timeout);
Link with -pthread.
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
sem_timedwait(): _POSIX_C_SOURCE >= 200112L
sem_trywait() is the same as sem_wait(), except that if the decrement cannot be immediately performed, then call returns an error (errno set to EAGAIN) instead of blocking.
sem_timedwait() is the same as sem_wait(), except that abs_timeout specifies a limit on the amount of time that the call should block if the decrement cannot be immediately performed. The abs_timeout argument points to a structure that specifies an absolute timeout in seconds and nanoseconds since the Epoch, 1970-01-01 00:00:00 +0000 (UTC). This structure is defined as follows:
struct timespec {
time_t tv_sec; /* Seconds */
long tv_nsec; /* Nanoseconds [0 .. 999999999] */
};
If the timeout has already expired by the time of the call, and the semaphore could not be locked immediately, then sem_timedwait() fails with a timeout error (errno set to ETIMEDOUT).
If the operation can be performed immediately, then sem_timedwait() never fails with a timeout error, regardless of the value of abs_timeout. Furthermore, the validity of abs_timeout is not checked in this case.
The following additional error can occur for sem_trywait():
The following additional errors can occur for sem_timedwait():
| Interface | Attribute | Value |
| sem_wait(), sem_trywait(), sem_timedwait() | Thread safety | MT-Safe |
The (somewhat trivial) program shown below operates on an unnamed semaphore. The program expects two command-line arguments. The first argument specifies a seconds value that is used to set an alarm timer to generate a SIGALRM signal. This handler performs a sem_post(3) to increment the semaphore that is being waited on in main() using sem_timedwait(). The second command-line argument specifies the length of the timeout, in seconds, for sem_timedwait(). The following shows what happens on two different runs of the program:
$ ./a.out 2 3 About to call sem_timedwait() sem_post() from handler sem_timedwait() succeeded $ ./a.out 2 1 About to call sem_timedwait() sem_timedwait() timed out
#include <unistd.h> #include <stdio.h> #include <stdlib.h> #include <semaphore.h> #include <time.h> #include <assert.h> #include <errno.h> #include <signal.h> sem_t sem; #define handle_error(msg) \ do { perror(msg); exit(EXIT_FAILURE); } while (0) static void handler(int sig) { write(STDOUT_FILENO, "sem_post() from handler\n", 24); if (sem_post(&sem) == -1) { write(STDERR_FILENO, "sem_post() failed\n", 18); _exit(EXIT_FAILURE); } } int main(int argc, char *argv[]) { struct sigaction sa; struct timespec ts; int s; if (argc != 3) { fprintf(stderr, "Usage: %s <alarm-secs> <wait-secs>\n", argv[0]); exit(EXIT_FAILURE); } if (sem_init(&sem, 0, 0) == -1) handle_error("sem_init"); /* Establish SIGALRM handler; set alarm timer using argv[1] */ sa.sa_handler = handler; sigemptyset(&sa.sa_mask); sa.sa_flags = 0; if (sigaction(SIGALRM, &sa, NULL) == -1) handle_error("sigaction"); alarm(atoi(argv[1])); /* Calculate relative interval as current time plus number of seconds given argv[2] */ if (clock_gettime(CLOCK_REALTIME, &ts) == -1) handle_error("clock_gettime"); ts.tv_sec += atoi(argv[2]); printf("main() about to call sem_timedwait()\n"); while ((s = sem_timedwait(&sem, &ts)) == -1 && errno == EINTR) continue; /* Restart if interrupted by handler */ /* Check what happened */ if (s == -1) { if (errno == ETIMEDOUT) printf("sem_timedwait() timed out\n"); else perror("sem_timedwait"); } else printf("sem_timedwait() succeeded\n"); exit((s == 0) ? EXIT_SUCCESS : EXIT_FAILURE); }