You have two or more processes running on the same machine that need to communicate with each other.
Modern operating systems support a variety of interprocess communications primitives that vary from system to system. If you intend to make your program portable among different platforms and even different implementations of Unix, your best bet is to use sockets. All modern operating systems support the Berkeley socket interface for TCP/IP at a minimum, while most—if not all—Unix implementations also support Unix domain sockets.
Many operating systems support various methods of allowing two or more processes to communicate with each other. Most systems (including both Unix and Windows) support anonymous and named pipes. Many Unix systems (including BSD) also support message queues, which have their origins in AT&T’s System V Unix. Windows systems have a similar construct known as mailslots. Unix systems also have Unix domain sockets, which share the Berkeley socket interface with TCP/IP sockets. Here’s an overview of common mechanisms:
Anonymous pipes are useful for communication between a parent and a child process. The parent can create the two endpoints of the pipe before spawning the child process, and the child process will inherit the file descriptors from the parent. There are ways on both Unix and Windows for two otherwise unrelated processes to exchange file descriptors, but ...