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# Synchronization [POSIX Section 11]

## Functions Implemented

    int sem_init(sem_t *sem, int pshared, unsigned int value);
    int sem_destroy(sem_t *sem);
    int sem_wait(sem_t *sem);
    int sem_trywait(sem_t *sem);
    int sem_post(sem_t *sem);
    int sem_getvalue(sem_t *sem, int *sval);
    int pthread_mutexattr_init( pthread_mutexattr_t *attr);
    int pthread_mutexattr_destroy( pthread_mutexattr_t *attr);
    int pthread_mutex_init(pthread_mutex_t *mutex,
    	               const pthread_mutexattr_t *mutex_attr);
    int pthread_mutex_destroy(pthread_mutex_t *mutex);
    int pthread_mutex_lock(pthread_mutex_t *mutex);
    int pthread_mutex_trylock(pthread_mutex_t *mutex);
    int pthread_mutex_unlock(pthread_mutex_t *mutex);
    int pthread_condattr_init(pthread_condattr_t *attr);
    int pthread_condattr_destroy(pthread_condattr_t *attr);
    int pthread_cond_init(pthread_cond_t *cond,
    	               const pthread_condattr_t *attr);
    int pthread_cond_destroy(pthread_cond_t *cond);
    int pthread_cond_signal(pthread_cond_t *cond);
    int pthread_cond_broadcast(pthread_cond_t *cond);
    int pthread_cond_wait(pthread_cond_t *cond,
    	               pthread_mutex_t *mutex);
    int pthread_cond_timedwait(pthread_cond_t *cond,
    	                   pthread_mutex_t *mutex,
    	                   const struct timespec *abstime);

## Functions Omitted

    sem_t *sem_open(const char *name, int oflag, ...);		// TBA
    int sem_close(sem_t *sem);					// TBA
    int sem_unlink(const char *name);				// TBA
    int pthread_mutexattr_getpshared( const pthread_mutexattr_t *attr,
    	                          int *pshared );
    int pthread_mutexattr_setpshared( const pthread_mutexattr_t *attr,
    	                          int pshared );
    int  pthread_condattr_getpshared( const pthread_condattr_t *attr,
    	                          int *pshared);
    int  pthread_condattr_setpshared( const pthread_condattr_t *attr,
    	                          int pshared);

## Notes

- The presence of semaphores is controlled by the CYGPKG_POSIX_SEMAPHORES option. This in turn causes the _POSIX_SEMAPHORES feature test macro to be defined and the semaphore API to be made available.

- The pshared argument to sem_init() is not implemented, its value is ignored.

- Functions sem_open(), sem_close() and sem_unlink() are present but always return an error (ENOSYS).

- The exact priority inversion protocols supported may be controlled with the _POSIX_THREAD_PRIO_INHERIT and _POSIX_THREAD_PRIO_PROTECT configuration options.

- {_POSIX_THREAD_PROCESS_SHARED} is not defined, so the process-shared mutex and condition variable attributes are not supported, and neither are the functions pthread_mutexattr_getpshared(), pthread_mutexattr_setpshared(), pthread_condattr_getpshared() and pthread_condattr_setpshared().

- Condition variables do not become bound to a particular mutex when pthread_cond_wait() is called. Hence different threads may wait on a condition variable with different mutexes. This is at variance with the standard, which requires a condition variable to become (dynamically) bound by the first waiter, and unbound when the last finishes. However, this difference is largely benign, and the cost of policing this feature is non-trivial.

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