7.1. Talking to Device Files (writes and IOCTLs)

/*
 *  chardev.c - Create an input/output character device
 */

#include <linux/kernel.h>	/* We're doing kernel work */
#include <linux/module.h>	/* Specifically, a module */
#include <linux/fs.h>
#include <asm/uaccess.h>	/* for get_user and put_user */

#include "chardev.h"
#define SUCCESS 0
#define DEVICE_NAME "char_dev"
#define BUF_LEN 80

/* 
 * Is the device open right now? Used to prevent
 * concurent access into the same device 
 */
static int Device_Open = 0;

/* 
 * The message the device will give when asked 
 */
static char Message[BUF_LEN];

/* 
 * How far did the process reading the message get?
 * Useful if the message is larger than the size of the
 * buffer we get to fill in device_read. 
 */
static char *Message_Ptr;

/* 
 * This is called whenever a process attempts to open the device file 
 */
static int device_open(struct inode *inode, struct file *file)
{
#ifdef DEBUG
	printk(KERN_INFO "device_open(%p)\n", file);
#endif

	/* 
	 * We don't want to talk to two processes at the same time 
	 */
	if (Device_Open)
		return -EBUSY;

	Device_Open++;
	/*
	 * Initialize the message 
	 */
	Message_Ptr = Message;
	try_module_get(THIS_MODULE);
	return SUCCESS;
}

static int device_release(struct inode *inode, struct file *file)
{
#ifdef DEBUG
	printk(KERN_INFO "device_release(%p,%p)\n", inode, file);
#endif

	/* 
	 * We're now ready for our next caller 
	 */
	Device_Open--;

	module_put(THIS_MODULE);
	return SUCCESS;
}

/* 
 * This function is called whenever a process which has already opened the
 * device file attempts to read from it.
 */
static ssize_t device_read(struct file *file,	/* see include/linux/fs.h   */
			   char __user * buffer,	/* buffer to be
							 * filled with data */
			   size_t length,	/* length of the buffer     */
			   loff_t * offset)
{
	/* 
	 * Number of bytes actually written to the buffer 
	 */
	int bytes_read = 0;

#ifdef DEBUG
	printk(KERN_INFO "device_read(%p,%p,%d)\n", file, buffer, length);
#endif

	/* 
	 * If we're at the end of the message, return 0
	 * (which signifies end of file) 
	 */
	if (*Message_Ptr == 0)
		return 0;

	/* 
	 * Actually put the data into the buffer 
	 */
	while (length && *Message_Ptr) {

		/* 
		 * Because the buffer is in the user data segment,
		 * not the kernel data segment, assignment wouldn't
		 * work. Instead, we have to use put_user which
		 * copies data from the kernel data segment to the
		 * user data segment. 
		 */
		put_user(*(Message_Ptr++), buffer++);
		length--;
		bytes_read++;
	}

#ifdef DEBUG
	printk(KERN_INFO "Read %d bytes, %d left\n", bytes_read, length);
#endif

	/* 
	 * Read functions are supposed to return the number
	 * of bytes actually inserted into the buffer 
	 */
	return bytes_read;
}

/* 
 * This function is called when somebody tries to
 * write into our device file. 
 */
static ssize_t
device_write(struct file *file,
	     const char __user * buffer, size_t length, loff_t * offset)
{
	int i;

#ifdef DEBUG
	printk(KERN_INFO "device_write(%p,%s,%d)", file, buffer, length);
#endif

	for (i = 0; i < length && i < BUF_LEN; i++)
		get_user(Message[i], buffer + i);

	Message_Ptr = Message;

	/* 
	 * Again, return the number of input characters used 
	 */
	return i;
}

/* 
 * This function is called whenever a process tries to do an ioctl on our
 * device file. We get two extra parameters (additional to the inode and file
 * structures, which all device functions get): the number of the ioctl called
 * and the parameter given to the ioctl function.
 *
 * If the ioctl is write or read/write (meaning output is returned to the
 * calling process), the ioctl call returns the output of this function.
 *
 */
int device_ioctl(struct inode *inode,	/* see include/linux/fs.h */
		 struct file *file,	/* ditto */
		 unsigned int ioctl_num,	/* number and param for ioctl */
		 unsigned long ioctl_param)
{
	int i;
	char *temp;
	char ch;

	/* 
	 * Switch according to the ioctl called 
	 */
	switch (ioctl_num) {
	case IOCTL_SET_MSG:
		/* 
		 * Receive a pointer to a message (in user space) and set that
		 * to be the device's message.  Get the parameter given to 
		 * ioctl by the process. 
		 */
		temp = (char *)ioctl_param;

		/* 
		 * Find the length of the message 
		 */
		get_user(ch, temp);
		for (i = 0; ch && i < BUF_LEN; i++, temp++)
			get_user(ch, temp);

		device_write(file, (char *)ioctl_param, i, 0);
		break;

	case IOCTL_GET_MSG:
		/* 
		 * Give the current message to the calling process - 
		 * the parameter we got is a pointer, fill it. 
		 */
		i = device_read(file, (char *)ioctl_param, 99, 0);

		/* 
		 * Put a zero at the end of the buffer, so it will be 
		 * properly terminated 
		 */
		put_user('\0', (char *)ioctl_param + i);
		break;

	case IOCTL_GET_NTH_BYTE:
		/* 
		 * This ioctl is both input (ioctl_param) and 
		 * output (the return value of this function) 
		 */
		return Message[ioctl_param];
		break;
	}

	return SUCCESS;
}

/* Module Declarations */

/* 
 * This structure will hold the functions to be called
 * when a process does something to the device we
 * created. Since a pointer to this structure is kept in
 * the devices table, it can't be local to
 * init_module. NULL is for unimplemented functions. 
 */
struct file_operations Fops = {
	.read = device_read,
	.write = device_write,
	.ioctl = device_ioctl,
	.open = device_open,
	.release = device_release,	/* a.k.a. close */
};

/* 
 * Initialize the module - Register the character device 
 */
int init_module()
{
	int ret_val;
	/* 
	 * Register the character device (atleast try) 
	 */
	ret_val = register_chrdev(MAJOR_NUM, DEVICE_NAME, &Fops);

	/* 
	 * Negative values signify an error 
	 */
	if (ret_val < 0) {
		printk(KERN_ALERT "%s failed with %d\n",
		       "Sorry, registering the character device ", ret_val);
		return ret_val;
	}

	printk(KERN_INFO "%s The major device number is %d.\n",
	       "Registeration is a success", MAJOR_NUM);
	printk(KERN_INFO "If you want to talk to the device driver,\n");
	printk(KERN_INFO "you'll have to create a device file. \n");
	printk(KERN_INFO "We suggest you use:\n");
	printk(KERN_INFO "mknod %s c %d 0\n", DEVICE_FILE_NAME, MAJOR_NUM);
	printk(KERN_INFO "The device file name is important, because\n");
	printk(KERN_INFO "the ioctl program assumes that's the\n");
	printk(KERN_INFO "file you'll use.\n");

	return 0;
}

/* 
 * Cleanup - unregister the appropriate file from /proc 
 */
void cleanup_module()
{
	int ret;

	/* 
	 * Unregister the device 
	 */
	ret = unregister_chrdev(MAJOR_NUM, DEVICE_NAME);

	/* 
	 * If there's an error, report it 
	 */
	if (ret < 0)
		printk(KERN_ALERT "Error: unregister_chrdev: %d\n", ret);
}

/*
 *  chardev.h - the header file with the ioctl definitions.
 *
 *  The declarations here have to be in a header file, because
 *  they need to be known both to the kernel module
 *  (in chardev.c) and the process calling ioctl (ioctl.c)
 */

#ifndef CHARDEV_H
#define CHARDEV_H

#include <linux/ioctl.h>

/* 
 * The major device number. We can't rely on dynamic 
 * registration any more, because ioctls need to know 
 * it. 
 */
#define MAJOR_NUM 100

/* 
 * Set the message of the device driver 
 */
#define IOCTL_SET_MSG _IOR(MAJOR_NUM, 0, char *)
/*
 * _IOR means that we're creating an ioctl command 
 * number for passing information from a user process
 * to the kernel module. 
 *
 * The first arguments, MAJOR_NUM, is the major device 
 * number we're using.
 *
 * The second argument is the number of the command 
 * (there could be several with different meanings).
 *
 * The third argument is the type we want to get from 
 * the process to the kernel.
 */

/* 
 * Get the message of the device driver 
 */
#define IOCTL_GET_MSG _IOR(MAJOR_NUM, 1, char *)
/* 
 * This IOCTL is used for output, to get the message 
 * of the device driver. However, we still need the 
 * buffer to place the message in to be input, 
 * as it is allocated by the process.
 */

/* 
 * Get the n'th byte of the message 
 */
#define IOCTL_GET_NTH_BYTE _IOWR(MAJOR_NUM, 2, int)
/* 
 * The IOCTL is used for both input and output. It 
 * receives from the user a number, n, and returns 
 * Message[n]. 
 */

/* 
 * The name of the device file 
 */
#define DEVICE_FILE_NAME "char_dev"

#endif

/*
 *  ioctl.c - the process to use ioctl's to control the kernel module
 *
 *  Until now we could have used cat for input and output.  But now
 *  we need to do ioctl's, which require writing our own process.
 */

/* 
 * device specifics, such as ioctl numbers and the
 * major device file. 
 */
#include "chardev.h"

#include <fcntl.h>		/* open */
#include <unistd.h>		/* exit */
#include <sys/ioctl.h>		/* ioctl */

/* 
 * Functions for the ioctl calls 
 */

ioctl_set_msg(int file_desc, char *message)
{
	int ret_val;

	ret_val = ioctl(file_desc, IOCTL_SET_MSG, message);

	if (ret_val < 0) {
		printf("ioctl_set_msg failed:%d\n", ret_val);
		exit(-1);
	}
}

ioctl_get_msg(int file_desc)
{
	int ret_val;
	char message[100];

	/* 
	 * Warning - this is dangerous because we don't tell
	 * the kernel how far it's allowed to write, so it
	 * might overflow the buffer. In a real production
	 * program, we would have used two ioctls - one to tell
	 * the kernel the buffer length and another to give
	 * it the buffer to fill
	 */
	ret_val = ioctl(file_desc, IOCTL_GET_MSG, message);

	if (ret_val < 0) {
		printf("ioctl_get_msg failed:%d\n", ret_val);
		exit(-1);
	}

	printf("get_msg message:%s\n", message);
}

ioctl_get_nth_byte(int file_desc)
{
	int i;
	char c;

	printf("get_nth_byte message:");

	i = 0;
	while (c != 0) {
		c = ioctl(file_desc, IOCTL_GET_NTH_BYTE, i++);

		if (c < 0) {
			printf
			    ("ioctl_get_nth_byte failed at the %d'th byte:\n",
			     i);
			exit(-1);
		}

		putchar(c);
	}
	putchar('\n');
}

/* 
 * Main - Call the ioctl functions 
 */
main()
{
	int file_desc, ret_val;
	char *msg = "Message passed by ioctl\n";

	file_desc = open(DEVICE_FILE_NAME, 0);
	if (file_desc < 0) {
		printf("Can't open device file: %s\n", DEVICE_FILE_NAME);
		exit(-1);
	}

	ioctl_get_nth_byte(file_desc);
	ioctl_get_msg(file_desc);
	ioctl_set_msg(file_desc, msg);

	close(file_desc);
}