This chapter explains how User Mode processes can synchronize themselves and exchange data. We have already covered a lot of synchronization topics in Chapter 11, 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 of synchronization to User Mode processes. These processes must rely on the kernel to synchronize themselves and to exchange data.
As we saw in Section 12.6.1 in Chapter 12, a crude form of synchronization among User Mode processes can be achieved by creating a (possibly empty) file and by making use of suitable VFS system calls to lock and unlock it. Similarly, data sharing among processes can be obtained by storing data in temporary files protected by locks. This approach is costly since it requires accesses to the disk filesystem. For that reason, all Unix kernels include a set of system calls that supports process communication without interacting with the filesystem; furthermore, several wrapper functions have been 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, and Linux in particular, offer to allow interprocess communication:
Pipes and FIFOs (named pipes)
Best suited to implement ...