System Calls: Function, Importance & Categories

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  • 0:04 System Calls
  • 1:15 Importance
  • 1:33 Categories
  • 3:39 Lesson Summary
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Lesson Transcript
Instructor: Martin Gibbs

Martin has 16 years experience in Human Resources Information Systems and has a PhD in Information Technology Management. He is an adjunct professor of computer science and computer programming.

In this lesson, learn how an operating system handles system calls, or interfaces from applications to the underlying processes. Five types of system calls are defined, with examples of each.

System Calls

Think of a remote starting system for your vehicle. When you press a button, a message is sent to the vehicle and the engine is started. The doors remain locked, the trunk doesn't open, nor does the car put itself in drive. It just starts.

A system call is like a remote start application. It's an interface to an operating system, but it doesn't have direct access to the operating system (the engine). A program accesses the service through an Application Programming Interface (API).

There are common APIs available for most operating systems or virtual machines:

API Operating System
Win32 API Windows
POSIX API Unix, Linux, Mac OS X
Java API Java Virtual Machine (JVM)

A processor in an operating system will have two modes for running: user mode and kernel mode. User mode has fewer permissions to the underlying operating system, but a process in kernel mode can work with operating system files or data. In the car-starting program, the actual fob does not have access to the engine, but the software calls the starting routine and that is run in kernel mode.

Importance

The most important benefit of a system call is simplicity. You should not have to write a complex program in order to open or save a file to the disk, or print a document. Further, you don't want to have anything become compromised in the operating system, such as device drivers or other system components.

Categories

There are five types of system calls in an operating system. Let's go over them now.

1. Process Control

Sometimes programs and processes get stuck, or need to be closed. Process control system calls allow you to create, exit, or wait for a process. If a process is terminated abnormally, there is usually a dump of data you can view to see what went wrong.

2. File Management

This is one of the most heavily used system calls, since almost every application will open, read, and save a file. The application developer shouldn't have to write elaborate code for opening files. Instead, a simple system call allows you to work with files.

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