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SEM_WAIT(3) | Linux Programmer's Manual | SEM_WAIT(3) |
NAME¶
sem_wait, sem_timedwait, sem_trywait - lock a semaphoreSYNOPSIS¶
#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);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
DESCRIPTION¶
sem_wait() decrements (locks) the semaphore pointed to by sem. If the semaphore's value is greater than zero, then the decrement proceeds, and the function returns, immediately. If the semaphore currently has the value zero, then the call blocks until either it becomes possible to perform the decrement (i.e., the semaphore value rises above zero), or a signal handler interrupts the call.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).
struct timespec { time_t tv_sec; /* Seconds */ long tv_nsec; /* Nanoseconds [0 .. 999999999] */ };
RETURN VALUE¶
All of these functions return 0 on success; on error, the value of the semaphore is left unchanged, -1 is returned, and errno is set to indicate the error.ERRORS¶
- EINTR
- The call was interrupted by a signal handler; see signal(7).
- EINVAL
- sem is not a valid semaphore.
- EAGAIN
- The operation could not be performed without blocking (i.e., the semaphore currently has the value zero).
- EINVAL
- The value of abs_timeout.tv_nsecs is less than 0, or greater than or equal to 1000 million.
- ETIMEDOUT
- The call timed out before the semaphore could be locked.
CONFORMING TO¶
POSIX.1-2001.NOTES¶
A signal handler always interrupts a blocked call to one of these functions, regardless of the use of the sigaction(2) SA_RESTART flag.EXAMPLE¶
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
Program source¶
#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); }
SEE ALSO¶
clock_gettime(2), sem_getvalue(3), sem_post(3), sem_overview(7), time(7)COLOPHON¶
This page is part of release 3.44 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.2012-05-13 | Linux |