This chapter explains how User Mode processes can synchronize their actions and exchange data. We already covered several synchronization topics in Chapter 5, but the actors there were kernel control paths, not User Mode programs. We are now ready, after having discussed I/O management and filesystems at length, to extend the discussion to User Mode processes. These processes must rely on the kernel to facilitate interprocess synchronization and communication.
As we saw in Section 12.7.1, a form of synchronization among User Mode processes can be achieved by creating a (possibly empty) file and using suitable VFS system calls to lock and unlock it. While processes can similarly share data via temporary files protected by locks, this approach is costly because it requires accesses to the disk filesystem. For this reason, all Unix kernels include a set of system calls that supports process communication without interacting with the filesystem; furthermore, several wrapper functions were developed and inserted in suitable libraries to expedite how processes issue their synchronization requests to the kernel.
As usual, application programmers have a variety of needs that call for different communication mechanisms. Here are the basic mechanisms that Unix systems offer to allow interprocess communication:
Pipes and FIFOs (named pipes)
Best suited to implement producer/consumer interactions among processes. Some processes fill the pipe with data, while others extract data from the pipe.
Represent, as the name implies, the User Mode version of the kernel semaphores discussed in Section 5.3.6.
Allow processes to exchange messages (short blocks of data) by reading and writing them in predefined message queues.
Allow processes to exchange information via a shared block of memory. In applications that must share large amounts of data, this can be the most efficient form of process communication.
Allow processes on different computers to exchange data through a network, as described in Chapter 18. Sockets can also be used as a communication tool for processes located on the same host computer; the X Window System graphic interface, for instance, uses a socket to allow client programs to exchange data with the X server.
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