Chapter 3, "Processes," discussed the creation of processes and showed how one process can obtain the exit status of a child process. That's the simplest form of communication between two processes, but it's by no means the most powerful. The mechanisms of Chapter 3 don't provide any way for the parent to communicate with the child except via command-line arguments and environment variables, nor any way for the child to communicate with the parent except via the child's exit status. None of these mechanisms provides any means for communicating with the child process while it is actually running, nor do these mechanisms allow communication with a process outside the parent-child relationship.
This chapter describes means for interprocess communication that circumvent these limitations. We will present various ways for communicating between parents and children, between "unrelated" processes, and even between processes on different machines.
Interprocess communication (IPC) is the transfer of data among processes. For example, a Web browser may request a Web page from a Web server, which then sends HTML data. This transfer of data usually uses sockets in a telephone-like connection. In another example, you may want to print the filenames in a directory using a command such as ls | lpr. The shell creates an ls process and a separate lpr process, connecting the two with a pipe, represented by the "|" symbol. A pipe permits one-way communication between two related processes. The ls process writes data into the pipe, and the lpr process reads data from the pipe.
In this chapter, we discuss five types of interprocess communication:
· Shared memory permits processes to communicate by simply reading and writing to a specified memory location.
· Mapped memory is similar to shared memory, except that it is associated with a file in the filesystem.
· Pipes permit sequential communication from one process to a related process.
· FIFOs are similar to pipes, except that unrelated processes can communicate because the pipe is given a name in the filesystem.
· Sockets support communication between unrelated processes even on different computers.
These types of IPC differ by the following criteria:
· Whether they restrict communication to related processes (processes with a common ancestor), to unrelated processes sharing the same filesystem, or to any computer connected to a network
· Whether a communicating process is limited to only write data or only read data
· The number of processes permitted to communicate
· Whether the communicating processes are synchronized by the IPC—for example, a reading process halts until data is available to read
In this chapter, we omit discussion of IPC permitting communication only a limited number of times, such as communicating via a child's exit value.