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authorLinus Torvalds <torvalds@ppc970.osdl.org>2005-04-16 15:20:36 -0700
committerLinus Torvalds <torvalds@ppc970.osdl.org>2005-04-16 15:20:36 -0700
commit1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 (patch)
tree0bba044c4ce775e45a88a51686b5d9f90697ea9d /Documentation/DocBook/procfs-guide.tmpl
downloadlinux-stericsson-2.6.12-rc2.tar.gz
Linux-2.6.12-rc2v2.6.12-rc2
Initial git repository build. I'm not bothering with the full history, even though we have it. We can create a separate "historical" git archive of that later if we want to, and in the meantime it's about 3.2GB when imported into git - space that would just make the early git days unnecessarily complicated, when we don't have a lot of good infrastructure for it. Let it rip!
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+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
+ "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" [
+<!ENTITY procfsexample SYSTEM "procfs_example.xml">
+]>
+
+<book id="LKProcfsGuide">
+ <bookinfo>
+ <title>Linux Kernel Procfs Guide</title>
+
+ <authorgroup>
+ <author>
+ <firstname>Erik</firstname>
+ <othername>(J.A.K.)</othername>
+ <surname>Mouw</surname>
+ <affiliation>
+ <orgname>Delft University of Technology</orgname>
+ <orgdiv>Faculty of Information Technology and Systems</orgdiv>
+ <address>
+ <email>J.A.K.Mouw@its.tudelft.nl</email>
+ <pob>PO BOX 5031</pob>
+ <postcode>2600 GA</postcode>
+ <city>Delft</city>
+ <country>The Netherlands</country>
+ </address>
+ </affiliation>
+ </author>
+ </authorgroup>
+
+ <revhistory>
+ <revision>
+ <revnumber>1.0&nbsp;</revnumber>
+ <date>May 30, 2001</date>
+ <revremark>Initial revision posted to linux-kernel</revremark>
+ </revision>
+ <revision>
+ <revnumber>1.1&nbsp;</revnumber>
+ <date>June 3, 2001</date>
+ <revremark>Revised after comments from linux-kernel</revremark>
+ </revision>
+ </revhistory>
+
+ <copyright>
+ <year>2001</year>
+ <holder>Erik Mouw</holder>
+ </copyright>
+
+
+ <legalnotice>
+ <para>
+ This documentation is free software; you can redistribute it
+ and/or modify it under the terms of the GNU General Public
+ License as published by the Free Software Foundation; either
+ version 2 of the License, or (at your option) any later
+ version.
+ </para>
+
+ <para>
+ This documentation is distributed in the hope that it will be
+ useful, but WITHOUT ANY WARRANTY; without even the implied
+ warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
+ PURPOSE. See the GNU General Public License for more details.
+ </para>
+
+ <para>
+ You should have received a copy of the GNU General Public
+ License along with this program; if not, write to the Free
+ Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
+ MA 02111-1307 USA
+ </para>
+
+ <para>
+ For more details see the file COPYING in the source
+ distribution of Linux.
+ </para>
+ </legalnotice>
+ </bookinfo>
+
+
+
+
+ <toc>
+ </toc>
+
+
+
+
+ <preface>
+ <title>Preface</title>
+
+ <para>
+ This guide describes the use of the procfs file system from
+ within the Linux kernel. The idea to write this guide came up on
+ the #kernelnewbies IRC channel (see <ulink
+ url="http://www.kernelnewbies.org/">http://www.kernelnewbies.org/</ulink>),
+ when Jeff Garzik explained the use of procfs and forwarded me a
+ message Alexander Viro wrote to the linux-kernel mailing list. I
+ agreed to write it up nicely, so here it is.
+ </para>
+
+ <para>
+ I'd like to thank Jeff Garzik
+ <email>jgarzik@pobox.com</email> and Alexander Viro
+ <email>viro@parcelfarce.linux.theplanet.co.uk</email> for their input,
+ Tim Waugh <email>twaugh@redhat.com</email> for his <ulink
+ url="http://people.redhat.com/twaugh/docbook/selfdocbook/">Selfdocbook</ulink>,
+ and Marc Joosen <email>marcj@historia.et.tudelft.nl</email> for
+ proofreading.
+ </para>
+
+ <para>
+ This documentation was written while working on the LART
+ computing board (<ulink
+ url="http://www.lart.tudelft.nl/">http://www.lart.tudelft.nl/</ulink>),
+ which is sponsored by the Mobile Multi-media Communications
+ (<ulink
+ url="http://www.mmc.tudelft.nl/">http://www.mmc.tudelft.nl/</ulink>)
+ and Ubiquitous Communications (<ulink
+ url="http://www.ubicom.tudelft.nl/">http://www.ubicom.tudelft.nl/</ulink>)
+ projects.
+ </para>
+
+ <para>
+ Erik
+ </para>
+ </preface>
+
+
+
+
+ <chapter id="intro">
+ <title>Introduction</title>
+
+ <para>
+ The <filename class="directory">/proc</filename> file system
+ (procfs) is a special file system in the linux kernel. It's a
+ virtual file system: it is not associated with a block device
+ but exists only in memory. The files in the procfs are there to
+ allow userland programs access to certain information from the
+ kernel (like process information in <filename
+ class="directory">/proc/[0-9]+/</filename>), but also for debug
+ purposes (like <filename>/proc/ksyms</filename>).
+ </para>
+
+ <para>
+ This guide describes the use of the procfs file system from
+ within the Linux kernel. It starts by introducing all relevant
+ functions to manage the files within the file system. After that
+ it shows how to communicate with userland, and some tips and
+ tricks will be pointed out. Finally a complete example will be
+ shown.
+ </para>
+
+ <para>
+ Note that the files in <filename
+ class="directory">/proc/sys</filename> are sysctl files: they
+ don't belong to procfs and are governed by a completely
+ different API described in the Kernel API book.
+ </para>
+ </chapter>
+
+
+
+
+ <chapter id="managing">
+ <title>Managing procfs entries</title>
+
+ <para>
+ This chapter describes the functions that various kernel
+ components use to populate the procfs with files, symlinks,
+ device nodes, and directories.
+ </para>
+
+ <para>
+ A minor note before we start: if you want to use any of the
+ procfs functions, be sure to include the correct header file!
+ This should be one of the first lines in your code:
+ </para>
+
+ <programlisting>
+#include &lt;linux/proc_fs.h&gt;
+ </programlisting>
+
+
+
+
+ <sect1 id="regularfile">
+ <title>Creating a regular file</title>
+
+ <funcsynopsis>
+ <funcprototype>
+ <funcdef>struct proc_dir_entry* <function>create_proc_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>
+ </funcprototype>
+ </funcsynopsis>
+
+ <para>
+ This function creates a regular file with the name
+ <parameter>name</parameter>, file mode
+ <parameter>mode</parameter> in the directory
+ <parameter>parent</parameter>. To create a file in the root of
+ the procfs, use <constant>NULL</constant> as
+ <parameter>parent</parameter> parameter. When successful, the
+ function will return a pointer to the freshly created
+ <structname>struct proc_dir_entry</structname>; otherwise it
+ will return <constant>NULL</constant>. <xref
+ linkend="userland"/> describes how to do something useful with
+ regular files.
+ </para>
+
+ <para>
+ Note that it is specifically supported that you can pass a
+ path that spans multiple directories. For example
+ <function>create_proc_entry</function>(<parameter>"drivers/via0/info"</parameter>)
+ will create the <filename class="directory">via0</filename>
+ directory if necessary, with standard
+ <constant>0755</constant> permissions.
+ </para>
+
+ <para>
+ If you only want to be able to read the file, the function
+ <function>create_proc_read_entry</function> described in <xref
+ linkend="convenience"/> may be used to create and initialise
+ the procfs entry in one single call.
+ </para>
+ </sect1>
+
+
+
+
+ <sect1>
+ <title>Creating a symlink</title>
+
+ <funcsynopsis>
+ <funcprototype>
+ <funcdef>struct proc_dir_entry*
+ <function>proc_symlink</function></funcdef> <paramdef>const
+ char* <parameter>name</parameter></paramdef>
+ <paramdef>struct proc_dir_entry*
+ <parameter>parent</parameter></paramdef> <paramdef>const
+ char* <parameter>dest</parameter></paramdef>
+ </funcprototype>
+ </funcsynopsis>
+
+ <para>
+ This creates a symlink in the procfs directory
+ <parameter>parent</parameter> that points from
+ <parameter>name</parameter> to
+ <parameter>dest</parameter>. This translates in userland to
+ <literal>ln -s</literal> <parameter>dest</parameter>
+ <parameter>name</parameter>.
+ </para>
+ </sect1>
+
+ <sect1>
+ <title>Creating a directory</title>
+
+ <funcsynopsis>
+ <funcprototype>
+ <funcdef>struct proc_dir_entry* <function>proc_mkdir</function></funcdef>
+ <paramdef>const char* <parameter>name</parameter></paramdef>
+ <paramdef>struct proc_dir_entry* <parameter>parent</parameter></paramdef>
+ </funcprototype>
+ </funcsynopsis>
+
+ <para>
+ Create a directory <parameter>name</parameter> in the procfs
+ directory <parameter>parent</parameter>.
+ </para>
+ </sect1>
+
+
+
+
+ <sect1>
+ <title>Removing an entry</title>
+
+ <funcsynopsis>
+ <funcprototype>
+ <funcdef>void <function>remove_proc_entry</function></funcdef>
+ <paramdef>const char* <parameter>name</parameter></paramdef>
+ <paramdef>struct proc_dir_entry* <parameter>parent</parameter></paramdef>
+ </funcprototype>
+ </funcsynopsis>
+
+ <para>
+ Removes the entry <parameter>name</parameter> in the directory
+ <parameter>parent</parameter> from the procfs. Entries are
+ removed by their <emphasis>name</emphasis>, not by the
+ <structname>struct proc_dir_entry</structname> returned by the
+ various create functions. Note that this function doesn't
+ recursively remove entries.
+ </para>
+
+ <para>
+ Be sure to free the <structfield>data</structfield> entry from
+ the <structname>struct proc_dir_entry</structname> before
+ <function>remove_proc_entry</function> is called (that is: if
+ there was some <structfield>data</structfield> allocated, of
+ course). See <xref linkend="usingdata"/> for more information
+ on using the <structfield>data</structfield> entry.
+ </para>
+ </sect1>
+ </chapter>
+
+
+
+
+ <chapter id="userland">
+ <title>Communicating with userland</title>
+
+ <para>
+ Instead of reading (or writing) information directly from
+ kernel memory, procfs works with <emphasis>call back
+ functions</emphasis> for files: functions that are called when
+ a specific file is being read or written. Such functions have
+ to be initialised after the procfs file is created by setting
+ the <structfield>read_proc</structfield> and/or
+ <structfield>write_proc</structfield> fields in the
+ <structname>struct proc_dir_entry*</structname> that the
+ function <function>create_proc_entry</function> returned:
+ </para>
+
+ <programlisting>
+struct proc_dir_entry* entry;
+
+entry->read_proc = read_proc_foo;
+entry->write_proc = write_proc_foo;
+ </programlisting>
+
+ <para>
+ If you only want to use a the
+ <structfield>read_proc</structfield>, the function
+ <function>create_proc_read_entry</function> described in <xref
+ linkend="convenience"/> may be used to create and initialise the
+ procfs entry in one single call.
+ </para>
+
+
+
+ <sect1>
+ <title>Reading data</title>
+
+ <para>
+ The read function is a call back function that allows userland
+ processes to read data from the kernel. The read function
+ should have the following format:
+ </para>
+
+ <funcsynopsis>
+ <funcprototype>
+ <funcdef>int <function>read_func</function></funcdef>
+ <paramdef>char* <parameter>page</parameter></paramdef>
+ <paramdef>char** <parameter>start</parameter></paramdef>
+ <paramdef>off_t <parameter>off</parameter></paramdef>
+ <paramdef>int <parameter>count</parameter></paramdef>
+ <paramdef>int* <parameter>eof</parameter></paramdef>
+ <paramdef>void* <parameter>data</parameter></paramdef>
+ </funcprototype>
+ </funcsynopsis>
+
+ <para>
+ The read function should write its information into the
+ <parameter>page</parameter>. For proper use, the function
+ should start writing at an offset of
+ <parameter>off</parameter> in <parameter>page</parameter> and
+ write at most <parameter>count</parameter> bytes, but because
+ most read functions are quite simple and only return a small
+ amount of information, these two parameters are usually
+ ignored (it breaks pagers like <literal>more</literal> and
+ <literal>less</literal>, but <literal>cat</literal> still
+ works).
+ </para>
+
+ <para>
+ If the <parameter>off</parameter> and
+ <parameter>count</parameter> parameters are properly used,
+ <parameter>eof</parameter> should be used to signal that the
+ end of the file has been reached by writing
+ <literal>1</literal> to the memory location
+ <parameter>eof</parameter> points to.
+ </para>
+
+ <para>
+ The parameter <parameter>start</parameter> doesn't seem to be
+ used anywhere in the kernel. The <parameter>data</parameter>
+ parameter can be used to create a single call back function for
+ several files, see <xref linkend="usingdata"/>.
+ </para>
+
+ <para>
+ The <function>read_func</function> function must return the
+ number of bytes written into the <parameter>page</parameter>.
+ </para>
+
+ <para>
+ <xref linkend="example"/> shows how to use a read call back
+ function.
+ </para>
+ </sect1>
+
+
+
+
+ <sect1>
+ <title>Writing data</title>
+
+ <para>
+ The write call back function allows a userland process to write
+ data to the kernel, so it has some kind of control over the
+ kernel. The write function should have the following format:
+ </para>
+
+ <funcsynopsis>
+ <funcprototype>
+ <funcdef>int <function>write_func</function></funcdef>
+ <paramdef>struct file* <parameter>file</parameter></paramdef>
+ <paramdef>const char* <parameter>buffer</parameter></paramdef>
+ <paramdef>unsigned long <parameter>count</parameter></paramdef>
+ <paramdef>void* <parameter>data</parameter></paramdef>
+ </funcprototype>
+ </funcsynopsis>
+
+ <para>
+ The write function should read <parameter>count</parameter>
+ bytes at maximum from the <parameter>buffer</parameter>. Note
+ that the <parameter>buffer</parameter> doesn't live in the
+ kernel's memory space, so it should first be copied to kernel
+ space with <function>copy_from_user</function>. The
+ <parameter>file</parameter> parameter is usually
+ ignored. <xref linkend="usingdata"/> shows how to use the
+ <parameter>data</parameter> parameter.
+ </para>
+
+ <para>
+ Again, <xref linkend="example"/> shows how to use this call back
+ function.
+ </para>
+ </sect1>
+
+
+
+
+ <sect1 id="usingdata">
+ <title>A single call back for many files</title>
+
+ <para>
+ When a large number of almost identical files is used, it's
+ quite inconvenient to use a separate call back function for
+ each file. A better approach is to have a single call back
+ function that distinguishes between the files by using the
+ <structfield>data</structfield> field in <structname>struct
+ proc_dir_entry</structname>. First of all, the
+ <structfield>data</structfield> field has to be initialised:
+ </para>
+
+ <programlisting>
+struct proc_dir_entry* entry;
+struct my_file_data *file_data;
+
+file_data = kmalloc(sizeof(struct my_file_data), GFP_KERNEL);
+entry->data = file_data;
+ </programlisting>
+
+ <para>
+ The <structfield>data</structfield> field is a <type>void
+ *</type>, so it can be initialised with anything.
+ </para>
+
+ <para>
+ Now that the <structfield>data</structfield> field is set, the
+ <function>read_proc</function> and
+ <function>write_proc</function> can use it to distinguish
+ between files because they get it passed into their
+ <parameter>data</parameter> parameter:
+ </para>
+
+ <programlisting>
+int foo_read_func(char *page, char **start, off_t off,
+ int count, int *eof, void *data)
+{
+ int len;
+
+ if(data == file_data) {
+ /* special case for this file */
+ } else {
+ /* normal processing */
+ }
+
+ return len;
+}
+ </programlisting>
+
+ <para>
+ Be sure to free the <structfield>data</structfield> data field
+ when removing the procfs entry.
+ </para>
+ </sect1>
+ </chapter>
+
+
+
+
+ <chapter id="tips">
+ <title>Tips and tricks</title>
+
+
+
+
+ <sect1 id="convenience">
+ <title>Convenience functions</title>
+
+ <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>
+ <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>
+ <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>