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Writing a GEOM Class

<email>ivoras@FreeBSD.org</email>

<personname> <firstname>Ivan</firstname> <surname>Voras</surname> </personname> <affiliation> <_:address-1/> </affiliation>

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$FreeBSD: head/en_US.ISO8859-1/articles/geom-class/article.xml 54291 2020-06-23 13:48:26Z emaste $

This text documents some starting points in developing GEOM classes, and kernel modules in general. It is assumed that the reader is familiar with C userland programming.

Introduction

Documentation

Documentation on kernel programming is scarce — it is one of few areas where there is nearly nothing in the way of friendly tutorials, and the phrase <quote>use the source!</quote> really holds true. However, there are some bits and pieces (some of them seriously outdated) floating around that should be studied before beginning to code:

The <link xlink:href="@@URL_RELPREFIX@@/doc/en_US.ISO8859-1/books/developers-handbook/index.html">FreeBSD Developer's Handbook</link> — part of the documentation project, it does not contain anything specific to kernel programming, but rather some general useful information.

The <link xlink:href="@@URL_RELPREFIX@@/doc/en_US.ISO8859-1/books/arch-handbook/index.html">FreeBSD Architecture Handbook</link> — also from the documentation project, contains descriptions of several low-level facilities and procedures. The most important chapter is 13, <link xlink:href="@@URL_RELPREFIX@@/doc/en_US.ISO8859-1/books/arch-handbook/driverbasics.html">Writing FreeBSD device drivers</link>.

The Blueprints section of <link xlink:href="http://www.freebsddiary.org">FreeBSD Diary</link> web site — contains several interesting articles on kernel facilities.

The man pages in section 9 — for important documentation on kernel functions.

The <citerefentry><refentrytitle>geom</refentrytitle><manvolnum>4</manvolnum></citerefentry> man page and <link xlink:href="http://phk.freebsd.dk/pubs/">PHK's GEOM slides</link> — for general introduction of the GEOM subsystem.

Man pages <citerefentry><refentrytitle>g_bio</refentrytitle><manvolnum>9</manvolnum></citerefentry>, <citerefentry><refentrytitle>g_event</refentrytitle><manvolnum>9</manvolnum></citerefentry>, <citerefentry><refentrytitle>g_data</refentrytitle><manvolnum>9</manvolnum></citerefentry>, <citerefentry><refentrytitle>g_geom</refentrytitle><manvolnum>9</manvolnum></citerefentry>, <citerefentry><refentrytitle>g_provider</refentrytitle><manvolnum>9</manvolnum></citerefentry> <citerefentry><refentrytitle>g_consumer</refentrytitle><manvolnum>9</manvolnum></citerefentry>, <citerefentry><refentrytitle>g_access</refentrytitle><manvolnum>9</manvolnum></citerefentry> &amp; others linked from those, for documentation on specific functionalities.

The <citerefentry><refentrytitle>style</refentrytitle><manvolnum>9</manvolnum></citerefentry> man page — for documentation on the coding-style conventions which must be followed for any code which is to be committed to the FreeBSD tree.

Preliminaries