Inter process communication

INTER PROCESS COMMUNICATION: Process communicates with each other is called IPC. Process frequently needs to communicate, for instance in a shell pipeline, the output of the first process need to pass to the second one and so on to the other process. It is even possible for the two processes to be running on different machines.

REQUESTING SYSTEM SERVICE: There are two ways by which a program executing in the user mode can request the kernel services:

1.      System call                               2. Message passing

OS are designed with one of the two but not both. First assume that the user process wishes to invoke a particular target system function. For the system call approach, the user process uses the TRAP instruction. The idea is that the system call should appear to be an ordinary procedure call to the application program; the OS provide the library of user functions with names corresponding to each actual system call. Each of these function contains the TRAP instruction, which switches the CPU to kernel mode and then branches to the entry point of the function which is to be invoked. When the function completes, it switches the processor to user mode and then returns control to the user process.