Linux System Programming

Chapter 5. Process Management

As mentioned in Chapter 1, processes are the most fundamental abstraction in a Unix system, after files. As object code in execution—active, alive, running programs—processes are more than just assembly language; they consist of data, resources, state, and a virtualized computer.

In this chapter, we will look at the fundamentals of the process, from creation to termination. The basics have remained relatively unchanged since the earliest days of Unix. It is here, in the subject of process management, that the longevity and forward thinking of Unix’s original design shines brightest. Unix took an interesting path, one seldom traveled, and separated the act of creating a new process from the act of loading a new binary image. Although the two tasks are performed in tandem most of the time, the division has allowed a great deal of freedom for experimentation and evolution for each of the tasks. This road less traveled has survived to this day, and while most operating systems offer a single system call to start up a new program, Unix requires two: a fork and an exec. But before we cover those system calls, let’s look more closely at the process itself.

The Process ID

Each process is represented by a unique identifier, the process ID (frequently shortened to pid). The pid is guaranteed to be unique at any single point in time. That is, while at time t0 there can be only one process with the pid 770 (if any process at all exists with such a value), there is ...