Linux IPC in 3 minutes

Feb 19, 2021

This is a quick summary of Linux IPC to help us in recalling the most important concepts.

Linux IPC (Inter Process Communication)

IPC mechanisms are used to synchronize (sync) the processes and threads in linux.

IPC mechanisms in linux are:

  • Memory based: Shared variables, memory and regular files.
  • Channel based: Pipes and message queues
  • Stream based: Sockets

Memory based IPC

Shared files and variables

The regular files on linux can be used to communicate between processes. A race condition might arise when multiple processes tries to access at the file at exact same time. This is prevented by using locks on the file. The linux provides two lock APIs, exclusive lock and shared lock.

  • Exclusive lock: A process that writes to the file must gain an exclusive lock before writing. An exclusive lock can be held only by one process at most, this prevents a race condition because no other process can access the file until the lock is released.
  • Shared lock: A process that reads the file should gain at least a shared lock before reading. Multiple processes can hold a shared lock for reading the file. Any process intend to write to the file must wait until all the shared locks are released.

This locking mechanism applies to shared variables too. The only downside to this IPC mechanism is that file access and locking are slow operations. Hence not suitable for high performance communication.

Shared Memory

Shared memory is designed to be used in large memory sharing between processes. It is faster than file based sharing because of memory based data access instead of file. Linux provides two separate APIs for shared memory: System V (read as System Five) and POSIX. These two APIs are very different and hence should not mixed in an application.

Semaphores are used to sync the shared memory access between processes. A semaphore is a signalling mechanism and mutex is a locking mechanism. Both are used to sync the shared memory access. A mutex is always first taken and then released. Semaphores are used to either signal or wait and not both.

Channel based IPC

Channels connect processes for communication. A channel has a write end and a read end. It follows FIFO (first in, first out) order when writing or reading bytes. One process writes to the channel at the write end and a different process reads from this same channel at the read end. Pipes and message queues are the channel based IPC mechanisms in linux.

Pipes

Pipes can be named or unnamed. They can be used either interactively from the command line or within programs. Pipes have strict FIFO behavior, the first byte written is the first byte read and then the second byte, third byte and so on.

Message queues

Message queues are also FIFO based channel, but are flexible. Messages in the queue can be accessed out of FIFO order by using the message type of the message. A message queue is a sequence of messages and each message has two parts: payload to store the message and type for message retrieval. By using the message type the messages can be retrieved in any order from the message queue.

The pipes and message queues are fundamentally unidirectional, one process writes and another reads. For bidirectional communication it is better to use stream based IPC.

Stream based IPC

Most applications often deal with streaming data with very large size. Shared files or memory is not suited for this purpose. Pipes can be used for streaming large data but this is generally unidirectional. For bidirectional streaming sockets are used in linux.

Sockets

Sockets are available in linux as:

  • IPC sockets: used for communication between processes on the same host. Uses a local file as a socket address.
  • Network sockets: used for communication between processes on different hosts. Network sockets need support from an underlying network protocol such as TCP or UDP.

The IPC socket and the Network socket are same at the API with differences in the internal implementation.

Sockets configured as streams are bidirectional and control follows a client-server pattern. The server listens on a specific host and port. The clients try to connect to the server. On a successful connection the requests from client and the corresponding responses from server then can flow through the channel until it is closed on either end. A new connection is required to be established when previous connection is closed to communicate further.

References

For more information and code examples refer to below links.

  1. https://en.wikipedia.org/wiki/Inter-process_communication
  2. https://opensource.com/article/19/4/interprocess-communication-linux-networking
  3. https://barrgroup.com/embedded-systems/how-to/rtos-mutex-semaphore

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