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sem_timedwait(3c) [opensolaris man page]

sem_timedwait(3C)					   Standard C Library Functions 					 sem_timedwait(3C)

NAME
sem_timedwait, sem_reltimedwait_np - lock a semaphore SYNOPSIS
#include <semaphore.h> #include <time.h> int sem_timedwait(sem_t *restrict sem, const struct timespec *restrict abs_timeout); int sem_reltimedwait_np(sem_t *restrict sem, const struct timespec *restrict rel_timeout); DESCRIPTION
The sem_timedwait() function locks the semaphore referenced by sem as in the sem_wait(3C) function. However, if the semaphore cannot be locked without waiting for another process or thread to unlock the semaphore by performing a sem_post(3C) function, this wait is terminated when the specified timeout expires. The sem_reltimedwait_np() function is identical to the sem_timedwait() function, except that the timeout is specified as a relative time interval. For sem_timedwait(), the timeout expires when the absolute time specified by abs_timeout passes, as measured by the CLOCK_REALTIME clock (that is, when the value of that clock equals or exceeds abs_timeout), or if the absolute time specified by abs_timeout has already been passed at the time of the call. For sem_reltimedwait_np(), the timeout expires when the time interval specified by rel_timeout passes, as measured by the CLOCK_REALTIME clock, or if the time interval specified by rel_timeout is negative at the time of the call. The resolution of the timeout is the resolution of the CLOCK_REALTIME clock. The timespec data type is defined as a structure in the <time.h> header. Under no circumstance does the function fail with a timeout if the semaphore can be locked immediately. The validity of the abs_timeout need not be checked if the semaphore can be locked immediately. RETURN VALUES
The sem_timedwait() and sem_reltimedwait_np() functions return 0 if the calling process successfully performed the semaphore lock operation on the semaphore designated by sem. If the call was unsuccessful, the state of the semaphore is be unchanged and the function returns -1 and sets errno to indicate the error. ERRORS
The sem_timedwait() and sem_reltimedwait_np() functions will fail if: EINVAL The sem argument does not refer to a valid semaphore. EINVAL The process or thread would have blocked, and the timeout parameter specified a nanoseconds field value less than zero or greater than or equal to 1,000 million. ETIMEDOUT The semaphore could not be locked before the specified timeout expired. The sem_timedwait() and sem_reltimedwait_np() functions may fail if: EDEADLK A deadlock condition was detected. EINTR A signal interrupted this function. ATTRIBUTES
See attributes(5) for descriptions of the following attributes: +-----------------------------+-----------------------------+ | ATTRIBUTE TYPE | ATTRIBUTE VALUE | +-----------------------------+-----------------------------+ |Interface Stability |Commmitted | +-----------------------------+-----------------------------+ |MT-Level |MT-Safe | +-----------------------------+-----------------------------+ |Standard |See below. | +-----------------------------+-----------------------------+ For sem_timedwait(), see standards(5). SEE ALSO
semctl(2), semget(2), semop(2), time(2), sem_post(3C), sem_trywait(3C)sem_wait(3C), attributes(5), standards(5) SunOS 5.11 5 Feb 2008 sem_timedwait(3C)

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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 possi- ble 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 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 sec- onds 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_time- out. 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 currently 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. 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 ", 24); if (sem_post(&sem) == -1) { write(STDERR_FILENO, "sem_post() failed ", 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> ", 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() "); 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 "); else perror("sem_timedwait"); } else printf("sem_timedwait() succeeded "); 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/. Linux 2012-05-13 SEM_WAIT(3)
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