Documentation/DocBook/procfs-guide.tmpl - fp2-dev/kernel/msm - Gitiles

 <?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>buffer</parameter></paramdef>
 	  <paramdef>char** <parameter>start</parameter></paramdef>
 	  <paramdef>off_t <parameter>off</parameter></paramdef>
 	  <paramdef>int <parameter>count</parameter></paramdef>
 	  <paramdef>int* <parameter>peof</parameter></paramdef>
 	  <paramdef>void* <parameter>data</parameter></paramdef>
 	</funcprototype>
       </funcsynopsis>

       <para>
         The read function should write its information into the
         <parameter>buffer</parameter>, which will be exactly
         <literal>PAGE_SIZE</literal> bytes long.
       </para>

       <para>
         The parameter
         <parameter>peof</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>peof</parameter> points to.
       </para>

       <para>
         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 rest of the parameters and the return value are described
 	by a comment in <filename>fs/proc/generic.c</filename> as follows:
       </para>

       <blockquote>
         <para>
 	You have three ways to return data:
        	</para>
         <orderedlist>
           <listitem>
             <para>
 	      Leave <literal>*start = NULL</literal>.  (This is the default.)
 	      Put the data of the requested offset at that
 	      offset within the buffer.  Return the number (<literal>n</literal>)
 	      of bytes there are from the beginning of the
 	      buffer up to the last byte of data.  If the
 	      number of supplied bytes (<literal>= n - offset</literal>) is
 	      greater than zero and you didn't signal eof
 	      and the reader is prepared to take more data
 	      you will be called again with the requested
 	      offset advanced by the number of bytes
 	      absorbed.  This interface is useful for files
 	      no larger than the buffer.
 	    </para>
 	  </listitem>
 	  <listitem>
             <para>
 	      Set <literal>*start</literal> to an unsigned long value less than
 	      the buffer address but greater than zero.
 	      Put the data of the requested offset at the
 	      beginning of the buffer.  Return the number of
 	      bytes of data placed there.  If this number is
 	      greater than zero and you didn't signal eof
 	      and the reader is prepared to take more data
 	      you will be called again with the requested
 	      offset advanced by <literal>*start</literal>.  This interface is
 	      useful when you have a large file consisting
 	      of a series of blocks which you want to count
 	      and return as wholes.
 	      (Hack by Paul.Russell@rustcorp.com.au)
 	    </para>
 	  </listitem>
 	  <listitem>
             <para>
 	      Set <literal>*start</literal> to an address within the buffer.
 	      Put the data of the requested offset at <literal>*start</literal>.
 	      Return the number of bytes of data placed there.
 	      If this number is greater than zero and you
 	      didn't signal eof and the reader is prepared to
 	      take more data you will be called again with the
 	      requested offset advanced by the number of bytes
 	      absorbed.
 	    </para>
 	  </listitem>
 	</orderedlist>
       </blockquote>

       <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>
	<?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>buffer</parameter></paramdef>
	<paramdef>char** <parameter>start</parameter></paramdef>
	<paramdef>off_t <parameter>off</parameter></paramdef>
	<paramdef>int <parameter>count</parameter></paramdef>
	<paramdef>int* <parameter>peof</parameter></paramdef>
	<paramdef>void* <parameter>data</parameter></paramdef>
	</funcprototype>
	</funcsynopsis>

	<para>
	The read function should write its information into the
	<parameter>buffer</parameter>, which will be exactly
	<literal>PAGE_SIZE</literal> bytes long.
	</para>

	<para>
	The parameter
	<parameter>peof</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>peof</parameter> points to.
	</para>

	<para>
	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 rest of the parameters and the return value are described
	by a comment in <filename>fs/proc/generic.c</filename> as follows:
	</para>

	<blockquote>
	<para>
	You have three ways to return data:
	</para>
	<orderedlist>
	<listitem>
	<para>
	Leave <literal>*start = NULL</literal>. (This is the default.)
	Put the data of the requested offset at that
	offset within the buffer. Return the number (<literal>n</literal>)
	of bytes there are from the beginning of the
	buffer up to the last byte of data. If the
	number of supplied bytes (<literal>= n - offset</literal>) is
	greater than zero and you didn't signal eof
	and the reader is prepared to take more data
	you will be called again with the requested
	offset advanced by the number of bytes
	absorbed. This interface is useful for files
	no larger than the buffer.
	</para>
	</listitem>
	<listitem>
	<para>
	Set <literal>*start</literal> to an unsigned long value less than
	the buffer address but greater than zero.
	Put the data of the requested offset at the
	beginning of the buffer. Return the number of
	bytes of data placed there. If this number is
	greater than zero and you didn't signal eof
	and the reader is prepared to take more data
	you will be called again with the requested
	offset advanced by <literal>*start</literal>. This interface is
	useful when you have a large file consisting
	of a series of blocks which you want to count
	and return as wholes.
	(Hack by Paul.Russell@rustcorp.com.au)
	</para>
	</listitem>
	<listitem>
	<para>
	Set <literal>*start</literal> to an address within the buffer.
	Put the data of the requested offset at <literal>*start</literal>.
	Return the number of bytes of data placed there.
	If this number is greater than zero and you
	didn't signal eof and the reader is prepared to
	take more data you will be called again with the
	requested offset advanced by the number of bytes
	absorbed.
	</para>
	</listitem>
	</orderedlist>
	</blockquote>

	<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>