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diff --git a/Documentation/DocBook/procfs-guide.tmpl b/Documentation/DocBook/procfs-guide.tmpl new file mode 100644 index 00000000000..45cad23efef --- /dev/null +++ b/Documentation/DocBook/procfs-guide.tmpl @@ -0,0 +1,591 @@ +<?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 </revnumber> + <date>May 30, 2001</date> + <revremark>Initial revision posted to linux-kernel</revremark> + </revision> + <revision> + <revnumber>1.1 </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 <linux/proc_fs.h> + </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> |