Slackware Linux Basics - Huihoo

Chapter 4. General concepts
This chapter gives an introduction to some general UNIX and GNU/Linux concepts. It is important to read this chapter
thoroughly if you do not have any UNIX or GNU/Linux experience. Many concepts covered in this chapter are used
in this book and in GNU/Linux.
4.1. Multitasking
Introduction
One of UNIX's traditional strengths is multitasking. Multitasking means that multiple programs can be run at the same
time. You may wonder why this is important, because most people use only one application at a time. Multitasking is a
bare necessity for UNIX-like systems. Even if you have not started any applications, there are programs that run in the
background. For instance, some programs provide network services, while others show a login prompt and wait until
a user logs in on a (virtual) terminal. Programs that are running in the background are often called daemon processes 1 .
Processes and threads
After a program is loaded from a storage medium, an instance of the program is started. This instance is called a
process. A process has its own protected memory, named the process address space. The process address space has
two important areas: the text area and the data area. The text area is the actual program code; it is used to tell the system
what to do. The data area is used to store constant and runtime data. The operating system gives every process time to
execute. On single processor systems processes are not really running simultaneously. In reality a smart scheduler in
the kernel divides CPU time among processes, giving the illusion that processes run simultaneously. This process is
called time-sharing. On systems with more than one CPU or CPU cores, more than one process can run simultaneously,
but time-sharing is still used to divide the available CPU time among processes.
New processes are created by duplicating a running process with the fork system call. Figure 4.1, “Forking of a
process” shows a fork() call in action schematically. The parent process issues a fork() call. The kernel will respond
to this call by duplicating the process, and naming one process the parent, and the other process the child.
Figure 4.1. Forking of a process
1 The word daemon should not to be confused with the word demon, the word daemon refers to supernatural beings in Greek mythology.
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