mutexes

/*
**	This "device" reverses strings that are written to it
*/
#include <linux/module.h>
#include <linux/init.h>
#include <linux/kernel.h>
#include <linux/cdev.h>
#include <linux/mutex.h> //for mutexes
#include <linux/types.h>   // for dev_t typedef
#include <linux/kdev_t.h>  // for format_dev_t
#include <linux/fs.h>      // for alloc_chrdev_region()

#include <asm/uaccess.h>   // for copy_to_user, copy_from_user

#define BUFSZ 6

static dev_t mydev;             // (major,minor) value
static struct cdev my_cdev;
static struct mutex write;
static struct mutex read;
static struct semaphore bufempty;
static struct semaphore buffull;
static int access_count = 0;
static char internal_buffer[BUFSZ];

/*********************************************************************
**	The set of file operations that are implemented in this driver
**********************************************************************/
static ssize_t device_read(struct file *filp, char __user *buf,
			size_t len, loff_t *f_pos);
static ssize_t device_write(struct file *filp, const char __user *buf,
			size_t len, loff_t *f_pos);

static int device_open( struct inode *, struct file *);
static int device_close( struct inode *, struct file *);

static struct file_operations my_fops = {
    .owner = THIS_MODULE,
    .open = device_open,
    .read = device_read,
    .write = device_write,
    .release = device_close
};


/*********************************************************************
**	Called when the device is opened (system call open)
**********************************************************************/
static int device_open(struct inode *inodep, struct file *filep)
{
	//mutex_lock_interruptible(&reject); //rejecting mutex read
	access_count++;
	printk(KERN_ALERT "Reverse device opened %d time(s)\n", access_count);

	if ( filep->f_mode & FMODE_READ )
		printk(KERN_ALERT "Reverse device opened for read\n" );
	if ( filep->f_mode & FMODE_WRITE )
		printk(KERN_ALERT "Reverse device opened for write\n" );

	return 0;
}


/**********************************************************************
**	Called when the device is read from
**********************************************************************/
static ssize_t device_read( struct file *filp, char __user *buf,
				size_t len, loff_t *f_pos )
{
	// print some information
	mutex_lock_interruptible(&read);
	printk( KERN_INFO "In reverse read routine.\n" );
	printk( KERN_INFO "Length field is %lu.\n", (long unsigned int) len );
	printk( KERN_INFO "Offset is %lu.\n", (long unsigned int) *f_pos );

	if ( internal_buffer[*f_pos] == '\0' )
	{
		printk( KERN_INFO "End of string, returning zero.\n" );
		return 0;
	}

	if ( copy_to_user( buf, &internal_buffer[*f_pos], 1 ) != 0 )
		return -EFAULT;

	*f_pos += 1;
	mutex_unlock(&read);
	mutex_unlock(&write);
	return 1;      // return a single character

}


/**********************************************************************
**	Called when the device is written to
**********************************************************************/
static ssize_t device_write( struct file *filep, const char *buf,
				size_t len, loff_t *f_pos )
{
	mutex_lock_interruptible(&write); //rejecting mutex write
	int i;
	int count = 0;
	i = 0;  // start at the end to reverse

	// print some information
	printk( KERN_INFO "In reverse write routine.\n" );
	printk( KERN_INFO "Length field is %lu.\n", (long unsigned int) len );
	printk( KERN_INFO "Offset is %lu.\n", (long unsigned int) *f_pos );

	// clear the buffer
	memset( internal_buffer, 0, BUFSZ );
	*f_pos = 0;

	if(len>BUFSZ){ //checking if the input is bigger than the size of buffer
		int k=0;
		for(k=0; k<BUFSZ-1; k++){ //only BUFSZ-1
			if ( copy_from_user( &internal_buffer[count++], buf+i, 1 ) != 0 )
				return -EFAULT;
			i++;
			len--;
		}
		printk( KERN_INFO "FULL DEVICE.\n" );
		mutex_lock_interruptible(&write);
		//mutex_unlock(&read);
	}
	else{
	// copy the data from the user into the internal buffer
	while ( len > 0 )
	{
		if ( copy_from_user( &internal_buffer[count++], buf+i, 1 ) != 0 )
			return -EFAULT;
		i++;
		len--;
	}
	printk( KERN_INFO "DEVICE is not full.\n" );
	}
	mutex_unlock(&write);
	mutex_unlock(&read);

	return count;
}

/**********************************************************************
**	Called when the device is closed
**********************************************************************/
static int device_close(struct inode *inodep, struct file *filep)
{
	//mutex_unlock(&reject);


   printk(KERN_ALERT "reverse device closed !\n");
	 //rejecting mutex unlock
   return 0;
}


/**********************************************************************
**	Called when this driver module is loaded
**********************************************************************/
static int __init chardrv_in(void)
{
	int result;
	printk(KERN_INFO "module reverse driver being loaded.\n");
	mutex_init(&write);
	mutex_init(&read);
	sema_init(&bufempty, BUFSZ);
	sema_init(&buffull, BUFSZ);
	result = alloc_chrdev_region(&mydev, 0, 1, "rday");
	if ( result < 0 )
	{
    		printk(KERN_WARNING "Failed to allocate major/minor numbers");
    		return result;
	}

	cdev_init(&my_cdev, &my_fops);
	my_cdev.owner = THIS_MODULE;
	result = cdev_add(&my_cdev, mydev, 1);
	if ( result < 0 )
	{
    		printk(KERN_WARNING "Failed to register the cdev structure");
    		return result;
	}

	return 0;
}


/*********************************************************************
**	Called when this driver module is unloaded
*********************************************************************/
static void __exit chardrv_out(void)
{
	printk(KERN_INFO "module chardrv being unloaded.\n");

	cdev_del(&my_cdev);
	unregister_chrdev_region(mydev, 1);

}

module_init(chardrv_in);
module_exit(chardrv_out);

MODULE_AUTHOR("Robert P. J. Day, http://crashcourse.ca");
MODULE_LICENSE("GPL");