Chapter 10. Thread synchronization mechanisms in RHEL for Real Time

Mutex derives from the terms mutual exclusion. The mutual exclusion object synchronizes access to a resource. It is a mechanism that ensures only one thread can acquire a mutex at a time.

The mutex algorithm creates a serial access to each section of code, so that only one thread executes the code at any one time. Mutexes are created using an attribute object known as the mutex attribute object. It is an abstract object, which contains several attributes that depends on the POSIX options you choose to implement. The attribute object is defined with the pthread_mutex_t variable. The object stores the attributes defined for the mutex. The pthread_mutex_init(&my_mutex, &my_mutex_attr), pthread_mutexattr_setrobust() and pthread_mutexattr_getrobust() functions return 0, when successful. On error, they return the error number.

In real-time, you can either retain the attribute object to initialize more mutexes of the same type or you can clean up (destroy) the attribute object. The mutex is not affected in either case. Mutexes include the standard and advanced type of mutexes.

A non-robust mutex does not release automatically and stays locked until you manually release it.

Barriers operate in a very different way when compared to other thread synchronization methods. The barriers define a point in the code where all active threads stop until all threads and processes reach this barrier. Barriers are used in situations when a running application needs to ensure that all threads have completed specific tasks before execution can continue.

The barrier mutex in real-time, take following two variables:

The barrier sets the state to pass only when the specified number of threads reach the defined barrier. When the barrier state is set to pass, the threads and processes proceed further. The pthread_barrier_init() function allocates the required resources to use the defined barrier and initializes it with the attributes referenced by the attr attribute object.

The pthread_barrier_init() and pthread_barrier_destroy() functions return the zero value, when successful. On error, they return an error number.

In real-time, condition variables (condvar) is a POSIX thread construct that waits for a particular condition to be achieved before proceeding. In general, the signaled condition relates to the state of data that the thread shares with another thread. For example, a condvar can be used to signal a data entry into a processing queue and a thread waiting to process that data from the queue. Using the pthread_cond_init() function, you can initialize a condition variable.

The pthread_cond_init(), pthread_cond_wait(), and pthread_cond_signal() functions return the zero value, when successful. On error, it returns the error number.

The mentioned mutex options provides guidance on the mutex classes to consider when writing or porting an application.

The mentioned list of function types and the description provides information on the functions to use for thread synchronization mechanisms for the real-time kernel.