SIMPLE SOLUTIONS

SEM_WAIT(3) - man page online | library functions

Lock a semaphore.

Chapter
2015-08-08
SEM_WAIT(3)                         Linux Programmer's Manual                         SEM_WAIT(3)

NAME sem_wait, sem_timedwait, sem_trywait - lock a semaphore
SYNOPSIS #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 || _XOPEN_SOURCE >= 600
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. sem_trywait() is the same as sem_wait(), except that if the decrement cannot be immedi‐ ately 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 per‐ formed. 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 struc‐ ture 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 ETIMED‐ OUT). 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.
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. The following additional error can occur for sem_trywait(): EAGAIN The operation could not be performed without blocking (i.e., the semaphore cur‐ rently has the value zero). The following additional errors can occur for sem_timedwait(): 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.
ATTRIBUTES For an explanation of the terms used in this section, see attributes(7). ┌───────────────────────────┬───────────────┬─────────┐ │Interface │ Attribute │ Value │ ├───────────────────────────┼───────────────┼─────────┤ │sem_wait(), sem_trywait(), │ Thread safety │ MT-Safe │ │sem_timedwait() │ │ │ └───────────────────────────┴───────────────┴─────────┘
CONFORMING TO POSIX.1-2001, POSIX.1-2008.
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_timed‐ wait(). 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 pro‐ gram: $ ./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 4.04 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at http://www.kernel.org/doc/man-pages/.
Linux 2015-08-08 SEM_WAIT(3)
This manual Reference Other manuals
sem_wait(3) referred by rights(4freebsd) | sem(4freebsd) | sem_close(3) | sem_destroy(3) | sem_getvalue(3) | sem_init(3) | sem_open(3) | sem_overview(7) | sem_post(3) | sem_unlink(3) | signal(7)
refer to attributes(7) | clock_getres(2) | feature_test_macros(7) | sem_getvalue(3) | sem_overview(7) | sem_post(3) | sigaction(2) | signal(7) | time(7)