Inter-Process Communication, which in short is known as IPC, deals mainly with the techniques and mechanisms that facilitate communication between processes. Now, why do we need special separate mechanisms or techniques for communicating between processes? Why isn't it possible to have information shared between two processes without using such special mechanisms?
Let us start from something primitive. Imagine you have two glasses completely filled with water. One glass contains hot water and the other contains cold water. What can you do to make the temperature of water in both the glasses equal? The simplest answer will be to mix the water from both the glasses in a glass with much bigger capacity. Once water is mixed, the temperature becomes equal after some time. If one can remember, this will be framed as a problem with some numerical data in a High-School Physics examination. If we go by principles, then the phenomenon here is conduction. If we go by our topic of IPC, then we can say that since the two glasses were full, we had to use another glass with a larger capacity to mix the contents in order to balance their heat energy.
Have you ever wondered about the communication medium used in telephones? What about the blood transporting system in the human body which communicates blood to different parts of the body? What about my fingers which are typing this document? My brain is doing so many things at a time. How is it directing one of my fingers to hit one key and some other finger to hit another key? How is it synchronizing the typing work that is done by both my hands? How is it also directing me to type the letters of a word that are actually coming to my mind?
Don't worry. I am not going to give a class in Biology. But it would be good if one can imagine a few more situations where we are using inter-process communication, though not necessarily in the human body or in a computer program.
So, where are we now? We know that some medium or other is required for communication between different processes. Similarly, when it comes to computer programs, we need some mechanism or medium for communication. Primarily, processes can use the available memory to communicate with each other. But then, the memory is completely managed by the operating system. A process will be allotted some part of the available memory for execution. Then each process will have its own unique user space. In no way will the memory allotted for one process overlap with the memory allotted for another process. Imagine what would happen otherwise!
So, now the question - how do different processes with unique address space communicate with each other? The operating system's kernel, which has access to all the memory available, will act as the communication channel. Similar to our earlier example, where the glass with hot water is one process address space, the glass with cold water is another, and the glass with the larger capacity is the kernel address space, so that we pour both hot water and cold water into the glass with larger capacity.
The IPC mechanisms can be classified into the following categories as given below:
- Pipes
- FIFOs
- Shared memory
- Mapped memory
- Message Queues
- Sockets
