DCE for the HP e3000 (B3821-90003)

44 Chapter5

Programming with Kernel Threads

Threads Synchronization and Communication

All threads in a process execute within a single address space and share

resources. When threads share resources in an unsynchronized way,

incorrect output can result from race conditions or thread scheduling

anomalies. The Kernal Threads provide the following facilities and

routines to synchronize thread access to shared resources.

Mutexes (Mutual Exclusion Objects)

Mutexes are used to synchronize access by multiple threads to a shared

resource, allowing access by only one thread at a time. Routines for

creating and managing mutexes are:

• pthread_mutex_init(mutex,attr)

• pthread_mutex_destroy(mutex)

• pthread_mutex_lock(mutex)

• pthread_mutex_trylock(mutex)

• pthread_mutex_unlock(mutex)

Condition Variables

Condition variables provide an explicit communication vehicle between

threads. A condition variable is a shared resource, and requires a

mutex to protect it. A condition variable is used to block one or more

threads until a condition becomes true, then any or all of the blocked

threads can be unblocked. Routines for creating and managing

condition variables are:

• pthread_cond_init(cond,attr)

• pthread_cond_broadcast(cond)

• pthread_cond_signal(cond)

• pthread_cond_wait(cond,mutex)

• pthread_cond_destroy(cond)

Join Facility

The join facility is the simplest means of synchronizing threads, and

uses neither shared resources or mutexes. The join facility causes the

calling thread to wait until the speciﬁed thread ﬁnishes and returns a

status value to the calling thread. Routines for joining and detaching

threads are:

• pthread_join(thread,status)

• pthread_detach(thread)