Linux Kernel

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The kernel is written mostly in C, with some architecture-dependent parts written in assembly. A good understanding of C is required for kernel development. Assembly (any architecture) is not required unless you plan to do low-level development for that architecture. Though they are not a good substitute for a solid C education and/or years of
experience, the following books are good for, if anything, reference:
- "The C Programming Language" by Kernighan and Ritchie [Prentice Hall]
- "Practical C Programming" by Steve Oualline [O'Reilly]
- "C: A Reference Manual" by Harbison and Steele [Prentice Hall]

The kernel is written using GNU C and the GNU toolchain. While it adheres to the ISO C89 standard, it uses a number of extensions that arenot featured in the standard. The kernel is a freestanding C environment, with no reliance on the standard C library, so some portions of the C standard are not supported. Arbitrary long long divisions and floating point are not allowed. It can sometimes be difficult to understand the assumptions the kernel has on the toolchain and the extensions that it uses, and unfortunately there is no definitive reference for them. Please check the gcc info pages (`info gcc`) for some information on them.

Please remember that you are trying to learn how to work with the existing development community. It is a diverse group of people, with high standards for coding, style and procedure. These standards have been created over time based on what they have found to work best for such a large and geographically dispersed team. Try to learn as much as possible about these standards ahead of time, as they are well documented; do not expect people to adapt to you or your company's way of doing things.

Legal Issues
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The Linux kernel source code is released under the GPL. Please see the file, COPYING, in the main directory of the source tree, for details on the license. If you have further questions about the license, please contact a lawyer, and do not ask on the Linux kernel mailing list. The people on the mailing lists are not lawyers, and you should not rely on their statements on legal matters.

For common questions and answers about the GPL, please see:
http://www.gnu.org/licenses/gpl-faq.html

Documentation
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The Linux kernel source tree has a large range of documents that are invaluable for learning how to interact with the kernel community. When new features are added to the kernel, it is recommended that new documentation files are also added which explain how to use the feature. When a kernel change causes the interface that the kernel exposes to userspace to change, it is recommended that you send the information or a patch to the manual pages explaining the change to the manual pages maintainer at mtk.manpages@gmail.com, and CC the list linux-api@vger.kernel.org.

Here is a list of files that are in the kernel source tree that are required reading:
README
This file gives a short background on the Linux kernel and describes what is necessary to do to configure and build the kernel. People who are new to the kernel should start here.

Documentation/Changes
This file gives a list of the minimum levels of various software packages that are necessary to build and run the kernel successfully.

Documentation/CodingStyle
This describes the Linux kernel coding style, and some of the rationale behind it. All new code is expected to follow the guidelines in this document. Most maintainers will only accept patches if these rules are followed, and many people will only review code if it is in the proper style.

Documentation/SubmittingPatches
Documentation/SubmittingDrivers
These files describe in explicit detail how to successfully create and send a patch, including (but not limited to):
- Email contents
- Email format
- Who to send it to
Following these rules will not guarantee success (as all patches are subject to scrutiny for content and style), but not following them will almost always prevent it.

Other excellent descriptions of how to create patches properly are:
"The Perfect Patch"
http://www.ozlabs.org/~akpm/stuff/tpp.txt
"Linux kernel patch submission format"
http://linux.yyz.us/patch-format.html

Documentation/stable_api_nonsense.txt
This file describes the rationale behind the conscious decision to not have a stable API within the kernel, including things like:
- Subsystem shim-layers (for compatibility?)
- Driver portability between Operating Systems.
- Mitigating rapid change within the kernel source tree (or
preventing rapid change)
This document is crucial for understanding the Linux development philosophy and is very important for people moving to Linux from development on other Operating Systems.

Documentation/SecurityBugs
If you feel you have found a security problem in the Linux kernel, please follow the steps in this document to help notify the kernel developers, and help solve the issue.

Documentation/ManagementStyle
This document describes how Linux kernel maintainers operate and the shared ethos behind their methodologies. This is important reading for anyone new to kernel development (or anyone simply curious about it), as it resolves a lot of common misconceptions and confusion about the unique behavior of kernel maintainers.

Documentation/stable_kernel_rules.txt
This file describes the rules on how the stable kernel releases happen, and what to do if you want to get a change into one of these releases.

Documentation/kernel-docs.txt
A list of external documentation that pertains to kernel development. Please consult this list if you do not find what you are looking for within the in-kernel documentation.

Documentation/applying-patches.txt
A good introduction describing exactly what a patch is and how to apply it to the different development branches of the kernel.

The kernel also has a large number of documents that can be automatically generated from the source code itself. This includes a full description of the in-kernel API, and rules on how to handle locking properly. The documents will be created in the Documentation/DocBook/ directory and can be generated as PDF, Postscript, HTML, and man pages by running:
make pdfdocs
make psdocs
make htmldocs
make mandocs
respectively from the main kernel source directory.

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