RTOS conceptsII UnitVI Contents Mailboxes Message queues Pipes

RTOS concepts-II Unit-VI

Contents • • • Mailboxes Message queues Pipes Signals Event registers

Inter-process communication • Tasks must be able to communicate with one another to coordinate their activities to share the data. • IPC means that a process (scheduler, task or ISR) generates some information by setting or resetting a flag or value, or generates an output so that it lets another process take note of it or use it. • IPCs in multiprocessor system are used to generate information about certain sets of computations finishing on one processor and let the other processors take note of it.

Two ways for IPC 1. global data , 2. sending messages Global data: by setting global variables. Usage of global variables create two problems. (i) Shared data problem (ii) It cannot prevent a message from being accessed by other tasks

Sending messages i) iii) iv) Semaphores and mutexes Queues, pipes and mailboxes Signals Event registers etc.

Printer example for IPC • A task that runs a print function. • The scheduler should let the other tasks share this task. • When the printer becomes available, an IPC from the print task is generated and the scheduler takes note of it. • Other tasks take note of it through the scheduler.

Multi-line display example for IPC • It is a task for a multi-line display of outputs and another task for displaying current time on the last line. • When the multi-line display task finishes the display of the last line, an IPC from the display task is generated and the scheduler takes note of it. • Another task continuously updating time-can take and generate an IPC output for the current time. • The need for IPC also arises in a client-server network.

Mailboxes • Mailboxes are also called as message exchange, is typically a pointer size variable • A message mailbox is an IPC message-block that can be used only by a single destined task. • Through a service provided by the kernel, a task or an ISR can deposit a message into the mail box. • Similarly, one or more tasks can receive messages through a service provided by the kernel. • A waiting list is associated with each mailbox in case more than one task wants to receive messages through the mailbox.

Mailboxes • Mailbox object is just like postal mailbox. Post a message Task 1 mailbox Read the message Task 2

Mailboxes types at the different RTOSs Mail Box type permitted by an RTOS Multiple unlimited Messages Queuing Up One message per Mail Box Multiple messages with a priority parameter for each message

Mailbox services provided by the kernel • Initialize the contents of the mailbox. The mailbox may or may not contain a message. • Deposit a message into the mailbox (POST) • Wait for a message to be deposited into the mailbox (PEND) • Get a message from a mailbox if existing (ACCEPT) • A provision for one message-write per box and the next message to be accepted only if the previous one is read. • A mailbox with a provision for only one message per box becomes full with one message.

A mobile phone LCD display example for mailbox • When the time message from a clock process arrives, the time is displayed at the right cornet of the last line. • When the message from another task is to display a phone number, it is displayed at the middle line.

Mailbox management functions Create a mailbox -- RTOS_Box. Create Delete a mailbox -- RTOS_Box. Delete Query a mailbox – RTOS_Box. Query Wait for a mailbox – RTOS_Box. Wait Post a message into the mail box -RTOS_Box. Write • Read a message from the mailbox – RTOS_Box. Accept • • •

Message queue • Message queue is used to send one or more messages to a task. • It is basically an array of mailboxes. • Waiting • Timeout option

Features of message queues 1. Either one process (task) or a set of processes (tasks) can use the queue. 2. A read (deletion) operation from a queue is in a FIFO mode. 3. Generally the queue is a circular queue and the read is like a ring of bytes in a buffer. 4. For reading, one can assign priorities.

Message queues Array of mailboxes Task 1 Task 2 Task 3 Sending Tasks Task 1 Task 2 Task 3 Receiving Tasks

Continued. . • At the time of creating a queue, the queue is given a • Name or ID • Queue length • Sending task waiting list • Receiving task waiting list

Common RTOS features of message queues • Most RTOSs require that you initialize your queues before you use them, by calling a function provided for this purpose. • An identification should be provided to which you want to write or read from the queue. • Each queue either has a user defined size or prefixed size assigned by the scheduler. • When the queue is full, error might occur • When the queue is empty, error might occur • Type and amount of data you can write into

Applications of message queues • • Taking the input from a keyboard To display the output Reading voltages from sensors or transducers Data packet transmission in a network

Types • One way comm. With Ack • Two-way comm. • Broadcasting

One way communication with ack Message queue Task 1 Task 2 semaphore

Two way communication Message queue Task 1 Task 2

Broadcasting Message queue Task

Kernel message queue services • Initialize a queue. It is always assumed to be empty after initialization. • Deposit a message into the queue (POST) • Wait for a message to be deposited into the queue (PEND) • Get a message from a queue (ACCEPT)

Message queue management functions • • • Create a queue -- RTOS_Qcreate Delete a queue Flush a queue --RTOS_Qflush Post a message in queue – RTOS_Qwrite Post a message in front of queue -- RTOS_Qpostfront Read message from queue -- RTOS_Qaccept Broadcast a message Show queue information Show queue waiting list – RTOS_Qwait

Pipes • Pipes are like message-queue functions. • A message pipe or simply pipe in strict sense is an IPC queue between two given interconnected tasks or two sets of tasks. • Unlike queues and mailboxes, pipes have unlimited size. • In a pipe there may be no fixed number of bytes per message. • Example: client-server network.

Pipes Task Write data to pipe Task Pipe Reads data from pipe • Task-to-task and ISR-to-task data transfer can take place using pipes.

Pipes for inter-process communication Task • Pipes can be used for inter-process communication. • One task may send the data packets through one pipe and the other task may send acknowledgements through the other

While using a pipe • 1. 2. 3. 4. You have to define Pipe ID Length Maximum length (not defined in some RTOS) Initial values

Common RTOS features of pipes • Each pipe for a message may need initialization before using the functions in the scheduler for the message pipe. • There may be a provision for multiple connections of the pipes for the multiple destinations of the messages. A number of clients can thus connect to a server.

Pipes • Pipes are used as shell commands in Unix/Linux systems • Symbol is | • Create a pipe • Open a pipe • Close a pipe • Read from a pipe • Write to the pipe

Signals can be passed to indicate an event A ‘signal’ provides the shortest message. It is a one-bit output from a process for IPC. What are signals in IPC? Signals are the flags that are used for the IPC functions for synchronizing. It is the s/w equivalent of the flag at a register that sets on a h/w interrupt. • Many RTOS do not support signals • •

Continued. . • A signal is just a flag that is shared and used by another interrupt servicing process for signal handling function. • A signal raised by one process forces another process to interrupt and catch that signal. • Hence, signals are handled only by very high priority processes as it may disrupt the usual schedule and priority inheritance.

Exception handling by signals • An important use of signal is to handle exceptions. • A signal reports an error during running a task and then lets the scheduler initiate an errorhandling process or function. • The handling is through the use of the ISR handling functions. • Unix and Linux OSs have 31 different types of signals for handling various events.

Signal management functions • • • Install a signal handler Remove an installed signal handler Send a signal to another task Block a signal from being delivered Unblock a blocked signal Ignore a signal

Event Registers • A task can have an event register in which the bits correspond to different events • Each of the bits in the event register is an event flag.

Event Register management function calls • • • Create an event register Delete an event register Query an event register Set an event flag Clear an event flag

Summary • The RTOS handles inter process communication between multiple tasks. • The RTOS provides the following IPCs: Signals, Semaphores, mutexes, queues, mailboxes, pipes, event registers etc. • A mailbox may either provide for only one message or multiple messages in an RTOS. • A queue is a collection or series of mailboxes. • A pipe is a queue or stream of messages that connects the two tasks. • Signal is one-bit output to indicate an event.