Mac OS X Internals: A Systems Approach

In a typical operating system, a process represents a program in execution along with associated system resources, which may be physical (such as processor cycles and memory) or abstract^[1] (such as the number of files the process can open). The kernel provides an illusion of concurrent execution by scheduling resources between ready-to-run processes. On a multiprocessor or multicore system, more than one process may execute truly concurrently.

^[1] Abstract resources are often directly or indirectly limited by physical resources.

In their landmark 1965 paper,^[2] Jack B. Dennis and Earl C. Van Horn defined a process as a "locus of control within an instruction sequence . . . an abstract entity which moves through the instructions of a procedure as the procedure is executed by a processor."

^[2] "Programming Semantics for Multiprogrammed Computations," by Jack B. Dennis and Earl C. Van Horn (The ACM Conference on Programming Languages and Pragmatics, San Dimas, California, August 1965).

In earlier chapters, we saw that the Mac OS X kernel divides the traditional process abstraction into multiple related abstractions. In this chapter, we will look at both kernel-level and user-level details of the Mac OS X process subsystem.