Processes communicate with each other and with the kernel to coordinate their activities. Linux supports a number of Inter-Process Communication (IPC) mechanisms.
Signals:
Signals are one of the oldest inter-process communication methods used by Unix TM systems. They are used to signal asynchronous events to one or more processes. A signal could be generated by a keyboard interrupt or an error condition such as the process attempting to access a non-existent location in its virtual memory. Signals are also used by the shells to signal job control commands to their child processes.
Pipes and FIFOS(Nmaed Pipes):
A Pipe is a method of connecting the standard output of one process to the
standard input of another.
This feature is widely used, even on the UNIX command line (in the shell).
ls sort lp
Pipes and FIFOs (also known as named pipes) provide a unidirectional interprocess communication channel. A pipe has a read end and a write end. Data written to the write end of a pipe can be read from the read end of the pipe.
A pipe is created using pipe(2), which creates a new pipe and returns two file descriptors, one referring to the read end of the pipe, the other referring to the write end. Pipes can be used to create a communication channel between related processes; see pipe(2) for an example.
A FIFO (short for First In First Out) has a name within the file system (created using mkfifo(3)), and is opened using open(2). Any process may open a FIFO, assuming the file permissions allow it. The read end is opened using the O_RDONLY flag; the write end is opened using the O_WRONLY flag.
Shared Memory:
To transfer large amount of data between kernel and user process, shared memory is provided.
mbuff driver used for shared memory. Any real-time or kernel task or user process can access this memory at any time.
Semaphores:
A semaphore is like a key that allows a task to carry out some operation or to access a resource. If the task can acquire the semaphore, it can carry out the intended operation or access the resource.
A kernel can support many different types of semaphores, including 1.Binary and 2.Counting semaphores
Binary Semaphores :
A binary semaphore can have a value of either 0 or 1. When a binary semaphore’s value is 0, the semaphore is considered unavailable (or empty); when the value is 1, the binary semaphore is considered available (or full ).
Counting Semaphores :
A counting semaphore uses a count to allow it to be acquired or released multiple times. When creating a counting semaphore, assign the semaphore a count that denotes the number of semaphore tokens it has initially.
One or more tasks can continue to acquire a token from the counting semaphore until no tokens are left. When all the tokens are gone, the count equals 0, and the counting semaphore moves from the available state to the unavailable state. To move from the unavailable state back to the available state, a semaphore token must be released by any task.
Note that, as with binary semaphores, counting semaphores are global resources that can be shared by all tasks that need them. This feature allows any task to release a counting semaphore token. Each release operation increments the count by one, even if the task making this call did not acquire a token in the first place.
Message Queues:
A message queue is a buffer-like object through which tasks and ISRs send and receive messages to communicate and synchornize with data. A message queue is like a pipeline. It temporarily holds messages from a sender until the intended receiver is ready to read them. This temporary buffering decouples a sending and receiving task; that is, it frees the tasks from having to send and receive messages simultaneously.
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