627 lines
20 KiB
XML
627 lines
20 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
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"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" [
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<!ENTITY procfsexample SYSTEM "procfs_example.xml">
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]>
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<book id="LKProcfsGuide">
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<bookinfo>
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<title>Linux Kernel Procfs Guide</title>
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<authorgroup>
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<author>
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<firstname>Erik</firstname>
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<othername>(J.A.K.)</othername>
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<surname>Mouw</surname>
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<affiliation>
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<address>
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<email>mouw@nl.linux.org</email>
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</address>
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</affiliation>
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</author>
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<othercredit>
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<contrib>
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This software and documentation were written while working on the
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LART computing board
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(<ulink url="http://www.lartmaker.nl/">http://www.lartmaker.nl/</ulink>),
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which was sponsored by the Delt University of Technology projects
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Mobile Multi-media Communications and Ubiquitous Communications.
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</contrib>
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</othercredit>
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</authorgroup>
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<revhistory>
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<revision>
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<revnumber>1.0</revnumber>
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<date>May 30, 2001</date>
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<revremark>Initial revision posted to linux-kernel</revremark>
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</revision>
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<revision>
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<revnumber>1.1</revnumber>
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<date>June 3, 2001</date>
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<revremark>Revised after comments from linux-kernel</revremark>
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</revision>
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</revhistory>
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<copyright>
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<year>2001</year>
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<holder>Erik Mouw</holder>
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</copyright>
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<legalnotice>
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<para>
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This documentation is free software; you can redistribute it
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and/or modify it under the terms of the GNU General Public
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License as published by the Free Software Foundation; either
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version 2 of the License, or (at your option) any later
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version.
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</para>
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<para>
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This documentation is distributed in the hope that it will be
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useful, but WITHOUT ANY WARRANTY; without even the implied
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warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
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PURPOSE. See the GNU General Public License for more details.
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</para>
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<para>
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You should have received a copy of the GNU General Public
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License along with this program; if not, write to the Free
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Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
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MA 02111-1307 USA
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</para>
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<para>
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For more details see the file COPYING in the source
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distribution of Linux.
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</para>
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</legalnotice>
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</bookinfo>
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<toc>
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</toc>
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<preface id="Preface">
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<title>Preface</title>
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<para>
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This guide describes the use of the procfs file system from
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within the Linux kernel. The idea to write this guide came up on
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the #kernelnewbies IRC channel (see <ulink
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url="http://www.kernelnewbies.org/">http://www.kernelnewbies.org/</ulink>),
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when Jeff Garzik explained the use of procfs and forwarded me a
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message Alexander Viro wrote to the linux-kernel mailing list. I
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agreed to write it up nicely, so here it is.
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</para>
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<para>
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I'd like to thank Jeff Garzik
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<email>jgarzik@pobox.com</email> and Alexander Viro
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<email>viro@parcelfarce.linux.theplanet.co.uk</email> for their input,
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Tim Waugh <email>twaugh@redhat.com</email> for his <ulink
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url="http://people.redhat.com/twaugh/docbook/selfdocbook/">Selfdocbook</ulink>,
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and Marc Joosen <email>marcj@historia.et.tudelft.nl</email> for
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proofreading.
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</para>
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<para>
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Erik
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</para>
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</preface>
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<chapter id="intro">
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<title>Introduction</title>
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<para>
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The <filename class="directory">/proc</filename> file system
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(procfs) is a special file system in the linux kernel. It's a
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virtual file system: it is not associated with a block device
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but exists only in memory. The files in the procfs are there to
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allow userland programs access to certain information from the
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kernel (like process information in <filename
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class="directory">/proc/[0-9]+/</filename>), but also for debug
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purposes (like <filename>/proc/ksyms</filename>).
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</para>
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<para>
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This guide describes the use of the procfs file system from
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within the Linux kernel. It starts by introducing all relevant
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functions to manage the files within the file system. After that
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it shows how to communicate with userland, and some tips and
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tricks will be pointed out. Finally a complete example will be
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shown.
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</para>
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<para>
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Note that the files in <filename
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class="directory">/proc/sys</filename> are sysctl files: they
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don't belong to procfs and are governed by a completely
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different API described in the Kernel API book.
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</para>
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</chapter>
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<chapter id="managing">
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<title>Managing procfs entries</title>
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<para>
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This chapter describes the functions that various kernel
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components use to populate the procfs with files, symlinks,
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device nodes, and directories.
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</para>
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<para>
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A minor note before we start: if you want to use any of the
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procfs functions, be sure to include the correct header file!
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This should be one of the first lines in your code:
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</para>
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<programlisting>
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#include <linux/proc_fs.h>
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</programlisting>
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<sect1 id="regularfile">
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<title>Creating a regular file</title>
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<funcsynopsis>
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<funcprototype>
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<funcdef>struct proc_dir_entry* <function>create_proc_entry</function></funcdef>
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<paramdef>const char* <parameter>name</parameter></paramdef>
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<paramdef>mode_t <parameter>mode</parameter></paramdef>
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<paramdef>struct proc_dir_entry* <parameter>parent</parameter></paramdef>
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</funcprototype>
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</funcsynopsis>
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<para>
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This function creates a regular file with the name
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<parameter>name</parameter>, file mode
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<parameter>mode</parameter> in the directory
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<parameter>parent</parameter>. To create a file in the root of
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the procfs, use <constant>NULL</constant> as
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<parameter>parent</parameter> parameter. When successful, the
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function will return a pointer to the freshly created
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<structname>struct proc_dir_entry</structname>; otherwise it
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will return <constant>NULL</constant>. <xref
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linkend="userland"/> describes how to do something useful with
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regular files.
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</para>
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<para>
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Note that it is specifically supported that you can pass a
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path that spans multiple directories. For example
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<function>create_proc_entry</function>(<parameter>"drivers/via0/info"</parameter>)
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will create the <filename class="directory">via0</filename>
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directory if necessary, with standard
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<constant>0755</constant> permissions.
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</para>
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<para>
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If you only want to be able to read the file, the function
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<function>create_proc_read_entry</function> described in <xref
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linkend="convenience"/> may be used to create and initialise
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the procfs entry in one single call.
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</para>
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</sect1>
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<sect1 id="Creating_a_symlink">
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<title>Creating a symlink</title>
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<funcsynopsis>
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<funcprototype>
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<funcdef>struct proc_dir_entry*
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<function>proc_symlink</function></funcdef> <paramdef>const
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char* <parameter>name</parameter></paramdef>
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<paramdef>struct proc_dir_entry*
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<parameter>parent</parameter></paramdef> <paramdef>const
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char* <parameter>dest</parameter></paramdef>
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</funcprototype>
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</funcsynopsis>
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<para>
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This creates a symlink in the procfs directory
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<parameter>parent</parameter> that points from
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<parameter>name</parameter> to
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<parameter>dest</parameter>. This translates in userland to
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<literal>ln -s</literal> <parameter>dest</parameter>
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<parameter>name</parameter>.
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</para>
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</sect1>
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<sect1 id="Creating_a_directory">
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<title>Creating a directory</title>
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<funcsynopsis>
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<funcprototype>
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<funcdef>struct proc_dir_entry* <function>proc_mkdir</function></funcdef>
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<paramdef>const char* <parameter>name</parameter></paramdef>
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<paramdef>struct proc_dir_entry* <parameter>parent</parameter></paramdef>
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</funcprototype>
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</funcsynopsis>
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<para>
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Create a directory <parameter>name</parameter> in the procfs
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directory <parameter>parent</parameter>.
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</para>
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</sect1>
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<sect1 id="Removing_an_entry">
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<title>Removing an entry</title>
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<funcsynopsis>
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<funcprototype>
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<funcdef>void <function>remove_proc_entry</function></funcdef>
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<paramdef>const char* <parameter>name</parameter></paramdef>
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<paramdef>struct proc_dir_entry* <parameter>parent</parameter></paramdef>
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</funcprototype>
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</funcsynopsis>
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<para>
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Removes the entry <parameter>name</parameter> in the directory
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<parameter>parent</parameter> from the procfs. Entries are
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removed by their <emphasis>name</emphasis>, not by the
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<structname>struct proc_dir_entry</structname> returned by the
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various create functions. Note that this function doesn't
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recursively remove entries.
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</para>
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<para>
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Be sure to free the <structfield>data</structfield> entry from
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the <structname>struct proc_dir_entry</structname> before
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<function>remove_proc_entry</function> is called (that is: if
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there was some <structfield>data</structfield> allocated, of
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course). See <xref linkend="usingdata"/> for more information
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on using the <structfield>data</structfield> entry.
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</para>
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</sect1>
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</chapter>
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<chapter id="userland">
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<title>Communicating with userland</title>
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<para>
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Instead of reading (or writing) information directly from
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kernel memory, procfs works with <emphasis>call back
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functions</emphasis> for files: functions that are called when
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a specific file is being read or written. Such functions have
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to be initialised after the procfs file is created by setting
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the <structfield>read_proc</structfield> and/or
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<structfield>write_proc</structfield> fields in the
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<structname>struct proc_dir_entry*</structname> that the
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function <function>create_proc_entry</function> returned:
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</para>
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<programlisting>
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struct proc_dir_entry* entry;
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entry->read_proc = read_proc_foo;
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entry->write_proc = write_proc_foo;
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</programlisting>
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<para>
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If you only want to use a the
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<structfield>read_proc</structfield>, the function
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<function>create_proc_read_entry</function> described in <xref
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linkend="convenience"/> may be used to create and initialise the
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procfs entry in one single call.
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</para>
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<sect1 id="Reading_data">
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<title>Reading data</title>
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<para>
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The read function is a call back function that allows userland
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processes to read data from the kernel. The read function
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should have the following format:
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</para>
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<funcsynopsis>
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<funcprototype>
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<funcdef>int <function>read_func</function></funcdef>
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<paramdef>char* <parameter>buffer</parameter></paramdef>
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<paramdef>char** <parameter>start</parameter></paramdef>
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<paramdef>off_t <parameter>off</parameter></paramdef>
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<paramdef>int <parameter>count</parameter></paramdef>
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<paramdef>int* <parameter>peof</parameter></paramdef>
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<paramdef>void* <parameter>data</parameter></paramdef>
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</funcprototype>
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</funcsynopsis>
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<para>
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The read function should write its information into the
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<parameter>buffer</parameter>, which will be exactly
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<literal>PAGE_SIZE</literal> bytes long.
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</para>
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<para>
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The parameter
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<parameter>peof</parameter> should be used to signal that the
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end of the file has been reached by writing
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<literal>1</literal> to the memory location
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<parameter>peof</parameter> points to.
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</para>
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<para>
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The <parameter>data</parameter>
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parameter can be used to create a single call back function for
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several files, see <xref linkend="usingdata"/>.
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</para>
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<para>
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The rest of the parameters and the return value are described
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by a comment in <filename>fs/proc/generic.c</filename> as follows:
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</para>
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<blockquote>
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<para>
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You have three ways to return data:
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</para>
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<orderedlist>
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<listitem>
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<para>
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Leave <literal>*start = NULL</literal>. (This is the default.)
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Put the data of the requested offset at that
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offset within the buffer. Return the number (<literal>n</literal>)
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of bytes there are from the beginning of the
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buffer up to the last byte of data. If the
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number of supplied bytes (<literal>= n - offset</literal>) is
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greater than zero and you didn't signal eof
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and the reader is prepared to take more data
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you will be called again with the requested
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offset advanced by the number of bytes
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absorbed. This interface is useful for files
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no larger than the buffer.
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</para>
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</listitem>
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<listitem>
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<para>
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Set <literal>*start</literal> to an unsigned long value less than
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the buffer address but greater than zero.
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Put the data of the requested offset at the
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beginning of the buffer. Return the number of
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bytes of data placed there. If this number is
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greater than zero and you didn't signal eof
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and the reader is prepared to take more data
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you will be called again with the requested
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offset advanced by <literal>*start</literal>. This interface is
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useful when you have a large file consisting
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of a series of blocks which you want to count
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and return as wholes.
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(Hack by Paul.Russell@rustcorp.com.au)
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</para>
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</listitem>
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<listitem>
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<para>
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Set <literal>*start</literal> to an address within the buffer.
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Put the data of the requested offset at <literal>*start</literal>.
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Return the number of bytes of data placed there.
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If this number is greater than zero and you
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didn't signal eof and the reader is prepared to
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take more data you will be called again with the
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requested offset advanced by the number of bytes
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absorbed.
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</para>
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</listitem>
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</orderedlist>
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</blockquote>
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<para>
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<xref linkend="example"/> shows how to use a read call back
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function.
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</para>
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</sect1>
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<sect1 id="Writing_data">
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<title>Writing data</title>
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<para>
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The write call back function allows a userland process to write
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data to the kernel, so it has some kind of control over the
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kernel. The write function should have the following format:
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</para>
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<funcsynopsis>
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<funcprototype>
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<funcdef>int <function>write_func</function></funcdef>
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<paramdef>struct file* <parameter>file</parameter></paramdef>
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<paramdef>const char* <parameter>buffer</parameter></paramdef>
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<paramdef>unsigned long <parameter>count</parameter></paramdef>
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<paramdef>void* <parameter>data</parameter></paramdef>
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</funcprototype>
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</funcsynopsis>
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<para>
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The write function should read <parameter>count</parameter>
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bytes at maximum from the <parameter>buffer</parameter>. Note
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that the <parameter>buffer</parameter> doesn't live in the
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kernel's memory space, so it should first be copied to kernel
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space with <function>copy_from_user</function>. The
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<parameter>file</parameter> parameter is usually
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ignored. <xref linkend="usingdata"/> shows how to use the
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<parameter>data</parameter> parameter.
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</para>
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<para>
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Again, <xref linkend="example"/> shows how to use this call back
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function.
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</para>
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</sect1>
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<sect1 id="usingdata">
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<title>A single call back for many files</title>
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<para>
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When a large number of almost identical files is used, it's
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quite inconvenient to use a separate call back function for
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each file. A better approach is to have a single call back
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function that distinguishes between the files by using the
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<structfield>data</structfield> field in <structname>struct
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proc_dir_entry</structname>. First of all, the
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<structfield>data</structfield> field has to be initialised:
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</para>
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<programlisting>
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struct proc_dir_entry* entry;
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struct my_file_data *file_data;
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file_data = kmalloc(sizeof(struct my_file_data), GFP_KERNEL);
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entry->data = file_data;
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</programlisting>
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<para>
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The <structfield>data</structfield> field is a <type>void
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*</type>, so it can be initialised with anything.
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</para>
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<para>
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Now that the <structfield>data</structfield> field is set, the
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<function>read_proc</function> and
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<function>write_proc</function> can use it to distinguish
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between files because they get it passed into their
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<parameter>data</parameter> parameter:
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</para>
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<programlisting>
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int foo_read_func(char *page, char **start, off_t off,
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int count, int *eof, void *data)
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{
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int len;
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if(data == file_data) {
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/* special case for this file */
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} else {
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/* normal processing */
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}
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return len;
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}
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</programlisting>
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|
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<para>
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Be sure to free the <structfield>data</structfield> data field
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when removing the procfs entry.
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</para>
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</sect1>
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</chapter>
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<chapter id="tips">
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<title>Tips and tricks</title>
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<sect1 id="convenience">
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<title>Convenience functions</title>
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|
|
|
<funcsynopsis>
|
|
<funcprototype>
|
|
<funcdef>struct proc_dir_entry* <function>create_proc_read_entry</function></funcdef>
|
|
<paramdef>const char* <parameter>name</parameter></paramdef>
|
|
<paramdef>mode_t <parameter>mode</parameter></paramdef>
|
|
<paramdef>struct proc_dir_entry* <parameter>parent</parameter></paramdef>
|
|
<paramdef>read_proc_t* <parameter>read_proc</parameter></paramdef>
|
|
<paramdef>void* <parameter>data</parameter></paramdef>
|
|
</funcprototype>
|
|
</funcsynopsis>
|
|
|
|
<para>
|
|
This function creates a regular file in exactly the same way
|
|
as <function>create_proc_entry</function> from <xref
|
|
linkend="regularfile"/> does, but also allows to set the read
|
|
function <parameter>read_proc</parameter> in one call. This
|
|
function can set the <parameter>data</parameter> as well, like
|
|
explained in <xref linkend="usingdata"/>.
|
|
</para>
|
|
</sect1>
|
|
|
|
|
|
|
|
<sect1 id="Modules">
|
|
<title>Modules</title>
|
|
|
|
<para>
|
|
If procfs is being used from within a module, be sure to set
|
|
the <structfield>owner</structfield> field in the
|
|
<structname>struct proc_dir_entry</structname> to
|
|
<constant>THIS_MODULE</constant>.
|
|
</para>
|
|
|
|
<programlisting>
|
|
struct proc_dir_entry* entry;
|
|
|
|
entry->owner = THIS_MODULE;
|
|
</programlisting>
|
|
</sect1>
|
|
|
|
|
|
|
|
|
|
<sect1 id="Mode_and_ownership">
|
|
<title>Mode and ownership</title>
|
|
|
|
<para>
|
|
Sometimes it is useful to change the mode and/or ownership of
|
|
a procfs entry. Here is an example that shows how to achieve
|
|
that:
|
|
</para>
|
|
|
|
<programlisting>
|
|
struct proc_dir_entry* entry;
|
|
|
|
entry->mode = S_IWUSR |S_IRUSR | S_IRGRP | S_IROTH;
|
|
entry->uid = 0;
|
|
entry->gid = 100;
|
|
</programlisting>
|
|
|
|
</sect1>
|
|
</chapter>
|
|
|
|
|
|
|
|
|
|
<chapter id="example">
|
|
<title>Example</title>
|
|
|
|
<!-- be careful with the example code: it shouldn't be wider than
|
|
approx. 60 columns, or otherwise it won't fit properly on a page
|
|
-->
|
|
|
|
&procfsexample;
|
|
|
|
</chapter>
|
|
</book>
|