Understanding the Linux Kernel, Second Edition

Chapter 9. System Calls

Operating systems offer processes running in User Mode a set of interfaces to interact with hardware devices such as the CPU, disks, and printers. Putting an extra layer between the application and the hardware has several advantages. First, it makes programming easier by freeing users from studying low-level programming characteristics of hardware devices. Second, it greatly increases system security, since the kernel can check the accuracy of the request at the interface level before attempting to satisfy it. Last but not least, these interfaces make programs more portable since they can be compiled and executed correctly on any kernel that offers the same set of interfaces.

Unix systems implement most interfaces between User Mode processes and hardware devices by means of system calls issued to the kernel. This chapter examines in detail how Linux implements system calls that User Mode programs issue to the kernel.

POSIX APIs and System Calls

Let’s start by stressing the difference between an application programmer interface (API) and a system call. The former is a function definition that specifies how to obtain a given service, while the latter is an explicit request to the kernel made via a software interrupt.

Unix systems include several libraries of functions that provide APIs to programmers. Some of the APIs defined by the libc standard C library refer to wrapper routines (routines whose only purpose is to issue a system call). Usually, each ...