    /*
     * 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 <linux/module.h>
    #include <linux/kernel.h>
    #include <linux/init.h>
    #include <linux/proc_fs.h>
    #include <linux/jiffies.h>
    #include <asm/uaccess.h>

    #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->name, fb_data->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 > FOOBAR_LEN)
                    len = FOOBAR_LEN;
            else
                    len = count;

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

            fb_data->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->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->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->data = &foo_data;
            foo_file->read_proc = proc_read_foobar;
            foo_file->write_proc = proc_write_foobar;
            foo_file->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->data = &bar_data;
            bar_file->read_proc = proc_read_foobar;
            bar_file->write_proc = proc_write_foobar;
            bar_file->owner = THIS_MODULE;

            /* create symlink */
            symlink = proc_symlink("jiffies_too", example_dir,
                                   "jiffies");
            if(symlink == NULL) {
                    rv = -ENOMEM;
                    goto no_symlink;
            }

            symlink->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");