OCEOS/oceos inter-task communication

Synchronization and Inter-task Communication

OCEOS provides two types of inter-job communication, counting semaphores and data queues. Semaphores and data queues are handled by OCEOS in a way different to many other OS.

In most OS a job that waits on a zero semaphore or tries to read an empty data queue may be blocked at that point and wait there indefinitely or with an optional timeout.

In OCEOS an active job may be pre-empted but cannot otherwise be blocked, so three options are provided in case a counting semaphore is zero when waited on or a data queue is empty when read.

One option results in a value always being returned and the job continuing. The returned value will indicate that the semaphore was zero or queue empty, and the job can take this into account.

In the other options, if the semaphore is zero or queue empty the job terminates and will restart from its beginning when the semaphore is signalled or queue written, or after an optional timeout.

When the job becomes active again after a timeout and again encounters the second option the behaviour is as with the first option, the job continues and takes into account that a timeout has occurred.

Because a job restarts when a resource becomes available rather than waiting at the point where it sought the resource any local data developed up to that point will be lost unless stored non-locally for use after the restart. If required this can be done using static variables or structures, or using data queues or counting semaphores.

In OCEOS event communication between tasks or between tasks and interrupt handlers is done typically by using counting semaphores.

• OCEOS Mutexes
• OCEOS Data Queues
• OCEOS Counting Semaphores