Book Image

Linux Kernel Programming

By : Kaiwan N. Billimoria
Book Image

Linux Kernel Programming

By: Kaiwan N. Billimoria

Overview of this book

Linux Kernel Programming is a comprehensive introduction for those new to Linux kernel and module development. This easy-to-follow guide will have you up and running with writing kernel code in next-to-no time. This book uses the latest 5.4 Long-Term Support (LTS) Linux kernel, which will be maintained from November 2019 through to December 2025. By working with the 5.4 LTS kernel throughout the book, you can be confident that your knowledge will continue to be valid for years to come. You’ll start the journey by learning how to build the kernel from the source. Next, you’ll write your first kernel module using the powerful Loadable Kernel Module (LKM) framework. The following chapters will cover key kernel internals topics including Linux kernel architecture, memory management, and CPU scheduling. During the course of this book, you’ll delve into the fairly complex topic of concurrency within the kernel, understand the issues it can cause, and learn how they can be addressed with various locking technologies (mutexes, spinlocks, atomic, and refcount operators). You’ll also benefit from more advanced material on cache effects, a primer on lock-free techniques within the kernel, deadlock avoidance (with lockdep), and kernel lock debugging techniques. By the end of this kernel book, you’ll have a detailed understanding of the fundamentals of writing Linux kernel module code for real-world projects and products.
Table of Contents (19 chapters)
Section 1: The Basics
Writing Your First Kernel Module - LKMs Part 2
Section 2: Understanding and Working with the Kernel
Kernel Memory Allocation for Module Authors - Part 1
Kernel Memory Allocation for Module Authors - Part 2
Section 3: Delving Deeper
About Packt

The ERR_PTR and PTR_ERR macros

On the discussion of return values, you now understand that the kernel module's init routine must return an integer. What if you wish to return a pointer instead? The ERR_PTR() inline function comes to our rescue, allowing us to return a pointer disguised as an integer simply by typecasting it as void *. It actually gets better: you can check for an error using the IS_ERR() inline function (which really just figures out whether the value is in the range [-1 to -4095]), encodes a negative error value into a pointer via the ERR_PTR() inline function, and retrieves this value from the pointer using the converse routine PTR_ERR().

As a simple example, see the callee code given here. This time, we have the (sample) function myfunc() return a pointer (to a structure named mystruct) and not an integer:

struct mystruct * myfunc(void)
struct mystruct *mys = NULL;
mys = kzalloc(sizeof(struct mystruct), GFP_KERNEL);
if (!mys)
return ERR_PTR...