Occasionally, a thread running within a mutex needs to wait for an event, in which case it blocks or sleeps. When a thread is waiting for another thread to communicate its disposition, it uses a condition variable in conjunction with a mutex. Although a mutex is exclusive and the code it protects is sharable (at certain moments), condition variables enable the synchronization of differing events that share a mutex, but not necessarily data. Several condition variables may be used by threads to signal each other when a task is complete, which then allows the next waiting thread to take ownership of the mutex.

A condition variable enables threads to atomically block and test the condition under the protection of a mutual exclusion lock (mutex) until the condition is satisfied. If the condition is false, a thread blocks on a condition variable and atomically releases the mutex that is waiting for the condition to change. If another thread changes the condition, it may wake up waiting threads by signaling the associated condition variable. The waiting threads, upon awakening, reacquire the mutex and re-evaluate the condition.

Condition variables and mutexes should be global. Condition variables that are allocated in writable memory can synchronize threads among processes if they are shared by the cooperating processes (see mmap(2)) and are initialized for this purpose.

The scope of a condition variable is either intra-process or inter-process. This is dependent upon whether the argument is passed implicitly or explicitly to the initialization of that condition variable. A condition variable does not need to be explicitly initialized. A condition variable is initialized with all zeros, by default, and its scope is set to within the calling process. For inter-process synchronization, a condition variable must be initialized once, and only once, before use.

A condition variable must not be simultaneously initialized by multiple threads or re-initialized while in use by other threads.

Condition variables attributes may be set to the default or customized at initialization. POSIX threads even allow the default values to be customized. Establishing these attributes varies depending upon whether POSIX or Solaris threads are used. Similar to the distinctions between POSIX and Solaris thread creation, POSIX condition variables implement the default, intra-process, unless an attribute object is modified for inter-process prior to the initialization of the condition variable. Solaris condition variables also implement as the default, intra-process; however, they set this attribute according to the argument, type, passed to their initialization function.

The condition wait interface allows a thread to wait for a condition and atomically release the associated mutex that it needs to hold to check the condition. The thread waits for another thread to make the condition true and that thread's resulting call to signal and wakeup the waiting thread.

A condition signal allows a thread to unblock the next thread waiting on the condition variable, whereas, a condition broadcast allows a thread to unblock all threads waiting on the condition variable.

The condition destroy functions destroy any state, but not the space, associated with the condition variable.

See attributes(5) for descriptions of the following attributes:

fork(2), mmap(2), setitimer(2), shmop(2), cond_broadcast(3C), cond_destroy(3C), cond_init(3C), cond_signal(3C), cond_timedwait(3C), cond_wait(3C), pthread_cond_broadcast(3C), pthread_cond_destroy(3C), pthread_cond_init(3C), pthread_cond_signal(3C), pthread_cond_timedwait(3C), pthread_cond_wait(3C), pthread_condattr_init(3C), signal(3C), attributes(5), mutex(5), standards(5)

If more than one thread is blocked on a condition variable, the order in which threads are unblocked is determined by the scheduling policy.

USYNC_THREAD does not support multiple mapplings to the same logical synch object. If you need to mmap() a synch object to different locations within the same address space, then the synch object should be initialized as a shared object USYNC_PROCESS for Solaris, and PTHREAD_PROCESS_PRIVATE for POSIX.