/* * sleepy.c -- the writers awake the readers * * Copyright (C) 2001 Alessandro Rubini and Jonathan Corbet * Copyright (C) 2001 O'Reilly & Associates * * The source code in this file can be freely used, adapted, * and redistributed in source or binary form, so long as an * acknowledgment appears in derived source files.  The citation * should list that the code comes from the book "Linux Device * Drivers" by Alessandro Rubini and Jonathan Corbet, published * by O'Reilly & Associates.   No warranty is attached; * we cannot take responsibility for errors or fitness for use. * * $Id: sleepy.c,v 1.7 2004/09/26 07:02:43 gregkh Exp $ */#include <linux/module.h>#include <linux/init.h>#include <linux/sched.h>  /* current and everything */#include <linux/kernel.h> /* printk() */#include <linux/fs.h>     /* everything... */#include <linux/types.h>  /* size_t */#include <linux/wait.h>MODULE_LICENSE("Dual BSD/GPL");static int sleepy_major = 0;static DECLARE_WAIT_QUEUE_HEAD(wq);static int flag = 0;ssize_t sleepy_read (struct file *filp, char __user *buf, size_t count, loff_t *pos){	printk(KERN_DEBUG "process %i (%s) going to sleep\n",			current->pid, current->comm);	wait_event_interruptible(wq, flag != 0);	flag = 0;	printk(KERN_DEBUG "awoken %i (%s)\n", current->pid, current->comm);	return 0; /* EOF */}ssize_t sleepy_write (struct file *filp, const char __user *buf, size_t count,		loff_t *pos){	printk(KERN_DEBUG "process %i (%s) awakening the readers...\n",			current->pid, current->comm);	flag = 1;	wake_up_interruptible(&wq);	return count; /* succeed, to avoid retrial */}struct file_operations sleepy_fops = {	.owner = THIS_MODULE,	.read =  sleepy_read,	.write = sleepy_write,};int sleepy_init(void){	int result;	/*	 * Register your major, and accept a dynamic number	 */	result = register_chrdev(sleepy_major, "sleepy", &sleepy_fops);	if (result < 0)		return result;	if (sleepy_major == 0)		sleepy_major = result; /* dynamic */	return 0;}void sleepy_cleanup(void){	unregister_chrdev(sleepy_major, "sleepy");}module_init(sleepy_init);module_exit(sleepy_cleanup);