<programlisting>
/*
 * procfs_example.c: an example proc interface
 *
 * Copyright (C) 2001, Erik Mouw (J.A.K.Mouw@its.tudelft.nl)
 *
 * This file accompanies the procfs-guide in the Linux kernel
 * source. Its main use is to demonstrate the concepts and
 * functions described in the guide.
 *
 * This software has been developed 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.
 *
 * The author can be reached at:
 *
 *  Erik Mouw
 *  Information and Communication Theory Group
 *  Faculty of Information Technology and Systems
 *  Delft University of Technology
 *  P.O. Box 5031
 *  2600 GA Delft
 *  The Netherlands
 *
 *
 * This program 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 program 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
 *
 */

#include &lt;linux/module.h&gt;
#include &lt;linux/kernel.h&gt;
#include &lt;linux/init.h&gt;
#include &lt;linux/proc_fs.h&gt;
#include &lt;linux/jiffies.h&gt;
#include &lt;asm/uaccess.h&gt;


#define MODULE_VERS "1.0"
#define MODULE_NAME "procfs_example"

#define FOOBAR_LEN 8

struct fb_data_t {
        char name[FOOBAR_LEN + 1];
        char value[FOOBAR_LEN + 1];
};


static struct proc_dir_entry *example_dir, *foo_file,
        *bar_file, *jiffies_file, *symlink;


struct fb_data_t foo_data, bar_data;


static int proc_read_jiffies(char *page, char **start,
                             off_t off, int count,
                             int *eof, void *data)
{
        int len;

        len = sprintf(page, "jiffies = %ld\n",
                      jiffies);

        return len;
}


static int proc_read_foobar(char *page, char **start,
                            off_t off, int count, 
                            int *eof, void *data)
{
        int len;
        struct fb_data_t *fb_data = (struct fb_data_t *)data;

        /* DON'T DO THAT - buffer overruns are bad */
        len = sprintf(page, "%s = '%s'\n", 
                      fb_data-&gt;name, fb_data-&gt;value);

        return len;
}


static int proc_write_foobar(struct file *file,
                             const char *buffer,
                             unsigned long count, 
                             void *data)
{
        int len;
        struct fb_data_t *fb_data = (struct fb_data_t *)data;

        if(count &gt; FOOBAR_LEN)
                len = FOOBAR_LEN;
        else
                len = count;

        if(copy_from_user(fb_data-&gt;value, buffer, len))
                return -EFAULT;

        fb_data-&gt;value[len] = '\0';

        return len;
}


static int __init init_procfs_example(void)
{
        int rv = 0;

        /* create directory */
        example_dir = proc_mkdir(MODULE_NAME, NULL);
        if(example_dir == NULL) {
                rv = -ENOMEM;
                goto out;
        }
        
        example_dir-&gt;owner = THIS_MODULE;
        
        /* create jiffies using convenience function */
        jiffies_file = create_proc_read_entry("jiffies", 
                                              0444, example_dir, 
                                              proc_read_jiffies,
                                              NULL);
        if(jiffies_file == NULL) {
                rv  = -ENOMEM;
                goto no_jiffies;
        }

        jiffies_file-&gt;owner = THIS_MODULE;

        /* create foo and bar files using same callback
         * functions 
         */
        foo_file = create_proc_entry("foo", 0644, example_dir);
        if(foo_file == NULL) {
                rv = -ENOMEM;
                goto no_foo;
        }

        strcpy(foo_data.name, "foo");
        strcpy(foo_data.value, "foo");
        foo_file-&gt;data = &amp;foo_data;
        foo_file-&gt;read_proc = proc_read_foobar;
        foo_file-&gt;write_proc = proc_write_foobar;
        foo_file-&gt;owner = THIS_MODULE;
                
        bar_file = create_proc_entry("bar", 0644, example_dir);
        if(bar_file == NULL) {
                rv = -ENOMEM;
                goto no_bar;
        }

        strcpy(bar_data.name, "bar");
        strcpy(bar_data.value, "bar");
        bar_file-&gt;data = &amp;bar_data;
        bar_file-&gt;read_proc = proc_read_foobar;
        bar_file-&gt;write_proc = proc_write_foobar;
        bar_file-&gt;owner = THIS_MODULE;
                
        /* create symlink */
        symlink = proc_symlink("jiffies_too", example_dir, 
                               "jiffies");
        if(symlink == NULL) {
                rv = -ENOMEM;
                goto no_symlink;
        }

        symlink-&gt;owner = THIS_MODULE;

        /* everything OK */
        printk(KERN_INFO "%s %s initialised\n",
               MODULE_NAME, MODULE_VERS);
        return 0;

no_symlink:
        remove_proc_entry("tty", example_dir);
no_tty:
        remove_proc_entry("bar", example_dir);
no_bar:
        remove_proc_entry("foo", example_dir);
no_foo:
        remove_proc_entry("jiffies", example_dir);
no_jiffies:                           
        remove_proc_entry(MODULE_NAME, NULL);
out:
        return rv;
}


static void __exit cleanup_procfs_example(void)
{
        remove_proc_entry("jiffies_too", example_dir);
        remove_proc_entry("tty", example_dir);
        remove_proc_entry("bar", example_dir);
        remove_proc_entry("foo", example_dir);
        remove_proc_entry("jiffies", example_dir);
        remove_proc_entry(MODULE_NAME, NULL);

        printk(KERN_INFO "%s %s removed\n",
               MODULE_NAME, MODULE_VERS);
}


module_init(init_procfs_example);
module_exit(cleanup_procfs_example);

MODULE_AUTHOR("Erik Mouw");
MODULE_DESCRIPTION("procfs examples");
</programlisting>
