Linux Kernel Procfs Guide

By Thomas Price,2014-09-30 13:43
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Linux Kernel Procfs Guide

Linux Kernel Procfs Guide

    Erik (J.A.K.) Mouw

    Delft University of Technology

    Faculty of Information Technology and Systems


     PO BOX 5031

     2600 GA


     The Netherlands

Copyright ? 2001 Erik Mouw

    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.

    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.

    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

    For more details see the file COPYING in the source distribution of Linux.

Revision History

    Revision 1.0 May 30, 2001

    Initial revision posted to linux-kernel

    Revision 1.1 June 3, 2001

    Revised after comments from linux-kernel


Table of Contents


    1. Introduction

    2. Managing procfs entries

    Creating a regular file

Creating a symlink

    Creating a directory

    Removing an entry

    3. Communicating with userland

    Reading data

    Writing data

    A single call back for many files

    4. Tips and tricks

    Convenience functions


    Mode and ownership

    5. Example


    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 http://www.kernelnewbies.org/), 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.

    I'd like to thank Jeff Garzik <jgarzik@pobox.com> and Alexander Viro <viro@parcelfarce.linux.theplanet.co.uk> for their input, Tim Waugh <twaugh@redhat.com> for his Selfdocbook, and Marc Joosen

    <marcj@historia.et.tudelft.nl> for proofreading.

    This documentation was written while working on the LART computing board (http://www.lart.tudelft.nl/), which is sponsored by the Mobile Multi-media Communications (http://www.mmc.tudelft.nl/) and Ubiquitous Communications (http://www.ubicom.tudelft.nl/) projects.


Chapter 1. Introduction

    The /proc 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 /proc/[0-9]+/), but also for debug purposes (like /proc/ksyms).

    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.

    Note that the files in /proc/sys are sysctl files: they don't belong to procfs and

    are governed by a completely different API described in the Kernel API book.

Chapter 2.Managing procfs entries

    Table of Contents

Creating a regular file

    Creating a symlink

    Creating a directory

    Removing an entry

    This chapter describes the functions that various kernel components use to populate the procfs with files, symlinks, device nodes, and directories.

    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:

#include <linux/proc_fs.h>

Creating a regular file

    struct proc_dir_entry* create_proc_entry( const char* name,

     mode_t mode,

     struct proc_dir_entry* parent);

    This function creates a regular file with the name name, file mode mode in the directory parent. To create a file in the root of the procfs, use NULL as parent parameter. When successful, the function will return a pointer to the freshly created struct proc_dir_entry; otherwise it will return NULL. Chapter 3, Communicating with userland describes how to do something useful with regular files.

    Note that it is specifically supported that you can pass a path that spans multiple directories. For example create_proc_entry("drivers/via0/info") will create the via0 directory if necessary, with standard 0755 permissions.

    If you only want to be able to read the file, the function create_proc_read_entry described in the section called “Convenience functions” may be used to create and initialise the procfs entry in one single call.

Creating a symlink

    struct proc_dir_entry* proc_symlink( const char* name,

     struct proc_dir_entry* parent,

     const char* dest);

    This creates a symlink in the procfs directory parent that points from name to dest. This translates in userland to ln -s dest name.

Creating a directory

    struct proc_dir_entry* proc_mkdir(const char* name, struct proc_dir_entry* parent);

Create a directory name in the procfs directory parent.

Removing an entry

    void remove_proc_entry(const char* name, struct proc_dir_entry* parent);

    Removes the entry name in the directory parent from the procfs. Entries are removed by their name, not by the struct proc_dir_entry returned by the various create functions. Note that this function doesn't recursively remove entries.

    Be sure to free the data entry from the struct proc_dir_entry before remove_proc_entry is called (that is: if there was some data allocated, of course). See the section called “A single call back for many files” for more information on using the data entry.

Chapter 3. Communicating with userland

    Table of Contents

Reading data

    Writing data

    A single call back for many files

    Instead of reading (or writing) information directly from kernel memory, procfs works with call back functions 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 read_proc and/or write_proc fields in the struct proc_dir_entry* that the function create_proc_entry returned:

struct proc_dir_entry* entry;

entry->read_proc = read_proc_foo;

    entry->write_proc = write_proc_foo;

    If you only want to use a the read_proc, the function create_proc_read_entry described in the section called “Convenience functions” may be used to create and initialise the procfs entry in one single call.

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