Inter-Process Communication (IPC) Methods

1. File - (All operating systems)

A computer file is a block of arbitrary information, or resource for storing information, which is available to a computer program and is usually based on some kind of durable storage. A file is durable in the sense that it remains available for programs to use after the current program has finished. Computer files can be considered as the modern counterpart of paper documents which traditionally were kept in offices' and libraries' files, which are the source of the term.

2. Signal - (Most operating systems; some systems, such as Windows, only implement signals in the C run-time library and do not actually provide support for their use as an IPC technique)

A signal is a limited form of inter-process communication used in Unix, Unix-like, and other POSIX-compliant operating systems. Essentially it is an asynchronous notification sent to a process in order to notify it of an event that occurred. When a signal is sent to a process, the operating system interrupts the process' normal flow of execution. Execution can be interrupted during any non-atomic instruction. If the process has previously registered a signal handler, that routine is executed. Otherwise the default signal handler is executed.

3. Socket - (Most operating systems)

Sockets are just like "worm holes" in science fiction. When things go into one end, they (should) come out of the other. Different kinds of sockets have different properties. Sockets are either connection-oriented or connectionless. Connection-oriented sockets allow for data to flow back and forth as needed, while connectionless sockets (also known as datagram sockets) allow only one message at a time to be transmitted, without an open connection. There are also different socket families. The two most common are AF_INET for internet connections, and AF_UNIX for unix IPC (interprocess communication).

Of the various forms of IPC (Inter Process Communication), sockets are by far the most popular. On any given platform, there are likely to be other forms of IPC that are faster, but for cross-platform communication, sockets are about the only game in town.

4. Pipe - (All POSIX systems)

In Unix-like computer operating systems, a pipeline is the original software pipeline: a set of processes chained by their standard streams, so that the output of each process (stdout) feeds directly as input (stdin) of the next one. Each connection is implemented by an anonymous pipe. Filter programs are often used in this configuration.

5. Named pipe - (All POSIX systems)

In computing, a named pipe (also FIFO for its behavior) is an extension to the traditional pipe concept on Unix and Unix-like systems, and is one of the methods of inter-process communication. The concept is also found in Microsoft Windows, although the semantics differ substantially. A traditional pipe is "unnamed" because it exists anonymously and persists only for as long as the process is running. A named pipe is system-persistent and exists beyond the life of the process and must be "unlinked" or deleted once it is no longer being used. Processes generally attach to the named pipe (usually appearing as a file) to perform IPC (inter-process communication).

6. Semaphore - (All POSIX systems)

A semaphore is nothing but a term used in UNIX for a variable which acts as a counter. So the next question that comes in mind is what for we need this variable. It’s so simple.

For instance there may be times when two processes try to access the same file simultaneously. In this event we must control the access of the file when the other process is accessing. This is done by assigning value to semaphore.

A semaphore, in computer science, is a protected variable (an entity storing a value) or abstract data type (an entity grouping several variables that may or may not be numerical) which constitutes the classic method for restricting access to shared resources, such as shared memory, in a multiprogramming environment (a system where several programs may be executing, or taking turns to execute, at once).

Semaphores exist in many variants, though usually the term refers to a counting semaphore, since a binary semaphore is better known as a mutex. A counting semaphore is a counter for a set of available resources, rather than a locked/unlocked flag of a single resource.

Semaphores are the classic solution to preventing race conditions in the dining philosophers problem, although they do not prevent resource deadlocks.


7. Shared memory - (All POSIX systems)

What is Shared Memory?

Shared Memory is an efficient means of passing data between programs. One program will create a memory portion which other processes (if permitted) can access. Shared memory is another method of interprocess communication (IPC) whereby 2 or more processes share a single chunk of memory to communicate. The shared memory system can also be used to set permissions on memory, allowing for things like malloc debuggers to be written.



Types of Shared memory available

Basically there are two different types of shared memory available for most flavors of UNIX. As you may have guessed, each of the two original ancestors of modern UNIX have their own implementation, although almost all modern UNIX flavors implement both. The names of the respective implementations are System V IPC, and BSD mmap.

8. Message queue - (Most operating systems)

In computer science, a message queue is a software-engineering component used for interprocess communication or inter-thread communication within the same process. It uses a queue for messaging – the passing of control or of content. Group communication systems provide similar kinds of functionality.









Message queues provide an asynchronous communications protocol, meaning that the sender and receiver of the message do not need to interact with the message queue at the same time. Messages placed onto the queue are stored until the recipient retrieves them.

Examples of commercial implementations of this kind of message queueing software (also known as Message Oriented Middleware) include IBM's WebSphere MQ (formerly MQ Series), Oracle Advanced Queuing (AQ) within an Oracle database, and Microsoft's MSMQ. There is a Java standard called Java Message Service, which has, associated with it, a number of implementations, both proprietary and free software.

Synchronous vs. asynchronous

Many of the more widely-known communications protocols in use operate synchronously. The HTTP protocol – used in the World Wide Web and in web services – offers an obvious example.

In a synchronous model, one system makes a connection to another, sends a request and waits for a reply.