The Linux enthusiast should find in this book enough food for her mind to start playing with the code base and should be able to join the group of developers that is continuously working on new capabilities and performance enhancements. Stacking in the parallel port subsystem is shown in Figure ; the arrows show the communications between the modules and with the kernel programming interface. Proper management of concurrency is required to write correct kernel code; for that reason, every sample driver in this book has been written with concurrency in mind. Macros that designate a module’s initialization and cleanup functions. Every chapter includes source code and points to sample drivers that you can run on any Linux computer.
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This edition of this book only concerns itself with one major version of the kernel, so you do not often see version tests in our example code. Either of the above macros makes the given symbol available outside the module.
Linux Device Drivers, Third Edition
Entries contain the module name, the amount of memory each module occupies, and the usage count. All the programs are available at ftp: The techniques used are explained as we come to them; Chapter 5 has also been dedicated to this issue and the kernel primitives available for concurrency management.
Several free software tools are needed to build the kernel, and you often need specific versions of these tools. Even in previous devicw which were not preemptivethis assumption was not valid on multiprocessor systems.
Linux Device Drivers, 3rd Edition
Writing code that handles concurrency and avoids race conditions situations in which an unfortunate order of execution causes undesirable behavior requires thought and can be tricky. Additionally, any software product derived from a product covered by the GPL must, if it is redistributed at all, be released under the GPL. We’ll point things out as we get to them over the course of the book, but there are a few fundamental issues which, while not warranting a section of their own, are worth a mention.
Then, there are licensing issues to be considered, as we discussed in Section 1.
A character char device is one that can be accessed as a stream of bytes like a file ; a char driver is in charge of implementing this behavior. Unlike the linker, however, the kernel doesn’t modify the module’s disk file, but rather an in-memory copy. Stacking in the parallel port subsystem is shown in Figure ; the arrows show the communications between the modules and with the kernel programming interface.
Once you have everything set up, creating a makefile for your module is straightforward. The only relevant difference between a char device and a regular file is that you can always move back and forth in the regular file, whereas most char devices are just data channels, which you can only access sequentially.
As you can imagine, device number registration is just the first of many tasks that driver code must carry out.
The exact behavior of the kernel when a NULL pointer is specified is different ldc3 each function, as the list later in this section shows. Examples include the tunelp program, which adjusts how the parallel port printer driver operates, and the graphical cardctl utility that is part of the PCMCIA driver package. But the user-space approach to device eevice has a number of drawbacks. Note that “a Linux box” is a wider concept than “a PC running Linux,” as many platforms are supported by our operating system, and kernel programming is by no means bound to a specific platform.
Linux Device Drivers, Third Edition 
Your functions must share that stack with the entire kernel-space call chain. The Linux device model, which is new in 2.
All major kernel developers, from Linus Torvalds on down, subscribe to this list. The technical reviewers for the second edition were Allan B. We sometimes made an exception to this rule and called Linus by name; we hope he doesn’t mind.
Additionally, all the routing and address resolution issues are implemented within the kernel.
Before dealing with hardware management, we dissect a few more of the kernel’s software interfaces: As a general rule, distributing things in source form is an easier way to make your way in the world. The changes from 2. Do not register any facility until all of your internal initialization needed to support that facility lniux been completed.
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If your module needs to export symbols for other modules to use, the following macros should be used. We’ll examine these files as we come to them, but there are a few that are specific to modules, and must appear in every loadable module. The “hello world” example at the beginning of this chapter included a brief demonstration of building a module and loading it into the system. Naturally, Linux systems run multiple processes, more than one of which can be trying to use your driver at the same time.
Before digging into programming, we should comment on the version numbering scheme used in Linux and which versions are covered by this book.