List the different inter process communication (IPC) mechanisms (with examples).

Question:

List the different inter process communication (IPC) mechanisms (with examples).

Answer:

1. Shared Memory: Shared memory allows one or more processes to communicate via memory that appears in all of their virtual address spaces. Each process that wishes to share the memory must attach to that virtual memory via a system call. The process can choose where in its virtual address space the shared memory goes or it can let Linux choose a free area large enough. When processes no longer wish to share the virtual memory, they detach from it. So long as other processes are still using the memory the detach only affects the current process. Its data structure is removed from the shared memory data structure and de-allocated. The current process's page tables are updated to invalidate the area of virtual memory that it used to share. When the last process sharing the memory detaches from it, the pages of the shared memory current in physical memory are freed.

2. Pipe: A pipe allows for data flow in one direction; when bidirectional communication is needed, two pipes need to be created.

Only related processes (those in the same branch of the process tree) can communicate through a pipe.

The pipe is created by system call pipe():

Example: int pipe(int filedes[2]);

- creates a pair of file descriptors, pointing to a pipe inode, and places them in the array pointed to by filedes. filedes[0] is for reading, filedes[1] is for writing.

3. Message Queues: Differs from pipes in that the caller need not (but can) read the messages in FIFO manner; it can select the message it wants to acquire instead.

4. Signals: 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.

Example: signal(), kill(), alarm()

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