NAME

sem_wait, sem_trywait - lock a semaphore (REALTIME)

 SYNOPSIS

#include <semaphore.h>

int sem_wait(sem_t *sem);
int sem_trywait(sem_t *sem);

 DESCRIPTION

The sem_wait() function locks the semaphore referenced by sem by performing a semaphore lock operation on that semaphore. If the semaphore value is currently zero, then the calling thread will not return from the call to sem_wait() until it either locks the semaphore or the call is interrupted by a signal. The sem_trywait() function locks the semaphore referenced by sem only if the semaphore is currently not locked; that is, if the semaphore value is currently positive. Otherwise, it does not lock the semaphore.

Upon successful return, the state of the semaphore is locked and remains locked until the sem_post() function is executed and returns successfully.

The sem_wait() function is interruptible by the delivery of a signal.

 RETURN VALUE

The sem_wait() and sem_trywait() functions return zero 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 unchanged, and the function returns a value of -1 and sets errno to indicate the error.

 ERRORS

The sem_wait() and sem_trywait() functions will fail if:

[EAGAIN]

The semaphore was already locked, so it cannot be immediately locked by the sem_trywait() operation ( sem_trywait only).

[EINVAL]

The sem argument does not refer to a valid semaphore.

[ENOSYS]

The functions sem_wait() and sem_trywait() are not supported by this implementation.

The sem_wait() and sem_trywait() functions may fail if:

[EDEADLK]

A deadlock condition was detected.

[EINTR]

A signal interrupted this function.

 EXAMPLES

None.

 APPLICATION USAGE

Realtime applications may encounter priority inversion when using semaphores. The problem occurs when a high priority thread "locks" (that is, waits on) a semaphore that is about to be "unlocked" (that is, posted) by a low priority thread, but the low priority thread is preempted by a medium priority thread. This scenario leads to priority inversion; a high priority thread is blocked by lower priority threads for an unlimited period of time. During system design, realtime programmers must take into account the possibility of this kind of priority inversion. They can deal with it in a number of ways, such as by having critical sections that are guarded by semaphores execute at a high priority, so that a thread cannot be preempted while executing in its critical section.

 FUTURE DIRECTIONS

None.

 SEE ALSO

semctl(), semget(), semop(), sem_post(), <semaphore.h>.

DERIVATION

Derived from the POSIX Realtime Extension (1003.1b-1993/1003.1i-1995)