- lock a semaphore
cc [ flag... ] file... -lrt [ library... ] #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);
The sem_timedwait() function locks the semaphore referenced by sem as in the sem_wait(3RT) function. However, if the semaphore cannot be locked without waiting for another process or thread to unlock the semaphore by performing a sem_post(3RT) 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.
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.
The sem_timedwait() and sem_reltimedwait_np() functions will fail if:
The sem argument does not refer to a valid semaphore.
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.
The semaphore could not be locked before the specified timeout expired.
The sem_timedwait() and sem_reltimedwait_np() functions may fail if:
A deadlock condition was detected.
A signal interrupted this function.
See attributes(5) for descriptions of the following attributes:
The sem_timedwait() is function Standard. The sem_reltimedwait_np() function is Stable.