RTOS Explained: Understanding Interprocess Communications

A key feature of any kind of operating system is to be able to pass data between processes (tasks, threads, and interrupt service routines). The best RTOSes give the application developer as much flexibility as possible in how to do this. A single messaging option could be good for one situation but not for another.

The RTXC Quadros RTOS provides three different object classes for passing data. Each class has unique properties that support interprocess communication according to the needs of the application. All three allow both synchronous and asynchronous communication between execution entities through the use of kernel services. And all three support a design that allows multiple producers and consumers for maximum application flexibility.

The Queue class allows user-defined, fixed-size data to be passed in First-In-First-Out (FIFO) order between tasks, preserving its chronological nature. Queues are circular. The kernel copies data from a source buffer into the queue (enqueue) and from the queue into a destination buffer (dequeue) in three variants: simple enqueue/dequeue; enqueue/dequeue waiting on full/empty conditions; and enqueue/dequeue with limited duration waiting on full/empty conditions. The RTXC kernel maintains the status and ensures proper operation on EMPTY and FULL conditions.

Under normal conditions the services associated with the Queue class provide the necessary synchronization. However, in the case of certain transitional conditions when special synchronization is required, queue semaphores are also supported. These queue semaphores are used in connection with RTXC services that operate on a set of semaphores associated with multiple queues or other types of events in order to eliminate polling for available data.

message passing RTOS

Mailboxes, messages and envelopes
are elements of the Mailbox/Message class

The Mailbox/Message class allows variable length data to be passed from a sending task to a receiving task with or without a priority. Mailboxes are globally available to all tasks and are exchange points for messages sent by a sending task to a receiving task. The sender and the receiver must agree on the mailbox they will use for passing Messages between them. Messages are prepared by the sending task and consist of a message envelope and a message body. As a matter of system safety, the Message envelope is allocated by the sender but maintained by the kernel.The kernel has no interest in the message body other than to see to its delivery at the designated mailbox. Only the sender and the receiver tasks know the form and content of the message. Due to the fact that messages sizes can vary, message services do not copy the data into a mailbox. Instead, they manipulate pointers to the message. The result is a clean, efficient means for passing data of any size.

The developer can choose to pass data in a FIFO manner or in one of two priorities, Urgent or Normal. The receiver task gets Urgent priority messages before those of Normal priority.

A task can send Messages synchronously or asynchronously. For synchronous messages the sending task sends the message and waits for the receiver to acknowledge its receipt before proceeding. (NOTE: it is possible to associate an alarm with the receipt of the acknowledgement so that the duration of any waiting condition can be limited.) For asynchronous message transfers, the sending task sends the message but continues without waiting for an acknowledgement. The task can choose to wait at a later time for an acknowledgement.

Normal send/receive operations have the same three variants as Queues: simple send/receive; send/receive waiting for successful completion; and send/receive with limited duration waiting for success. Like Queues, Mailboxes have related semaphores intended for use with transitional events to eliminate the necessity of polling for available data.

The Pipe class, allows data to be passed from a producer task, thread or Interrupt Service Routine (ISR) to a consumer task, thread or ISR. Pipes are data passing mechanisms in the RTXC Quadros RTOS that permit rapid, unidirectional transfers from one or more producers to one or more consumers. The producer or the consumer may be an ISR, a thread or a task. Pipes are ideally suited to communications applications where data is acquired and buffered at a high frequency and then must be processed rapidly while the next block is being acquired.

Functionally, pipes are buffers of data that are filled by the producer and processed by the consumer. The RTXC Quadros pipe services perform the necessary functions of managing the buffers in the pipe while the actual reading and writing of the data in the pipe buffers is the responsibility of the consumer and producer, respectively. In operation, the producer allocates a free buffer from the pipe, fills it with data, puts it back into the pipe. The consumer gets the full buffer, processes the data in it, and then frees the empty buffer back to the pipe where it can be re-used.

Normally, pipe buffers are allocated sequentially and processed in the same order. However, RTXC also permits the producer to put a full buffer at the front of the pipe instead of at the tail, causing the consumer to process it next.

Pipes can also be associated with thread gates (in the RTXC/ss configuration) so that certain pipe operations can result in operations on associated thread gates. Such a capability permits close synchronization between pipe producers and consumers and minimizes the amount of RAM required for pipe buffers.


The three flexible interprocess communications object classes in the RTXC Quadros RTOS eliminates the necessity of trying to force-fit one method of moving data into all situations a design might require. The developer can choose the one that best fits the application’s needs and have the appropriate kernel services to implement efficient data transfers for optimal time and memory considerations.

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