mirror of
https://github.com/edk2-porting/linux-next.git
synced 2024-12-21 19:53:59 +08:00
6b86e854f7
The procfs-guide claims that 'the parameter start doesn't seem to be used anywhere in the kernel'. This is out of date. In linux/fs/proc/generic.c we find a very nice description of the parameters to read_func. The appended patch replaces the bogus description with this (as far as I know) accurate one. Cc: "Randy.Dunlap" <rdunlap@xenotime.net> Cc: "Eric W. Biederman" <ebiederm@xmission.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
636 lines
20 KiB
XML
636 lines
20 KiB
XML
<?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>
|