:
 *,:
struct file_operations fops = {
read: device_read,
write: device_write,
open: device_open,
release: device_release
};:
struct file_operations fops = {
.read = device_read,
.write = device_write,
.open = device_open,
.release = device_release
};/modules.h which let you increase, decrease and display this counter:
try_module_get(THIS_MODULE): Increment the use count.
try.
Example 4-1. chardev.c
/*
* chardev.c: Creates a read-only char device that says how many times
* you've read from the dev file
*/
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/fs.h>
#include <asm/uaccess.h> /* for put_user */
/*
* Prototypes - this would normally go in a .h file
*/ as it appears in /proc/devices */
#define BUF_LEN 80 /* Max length of the message from the device */
/*
* Global variables are declared as static, so are global within the file.
*/
static int Major; /* Major number assigned to our device driver */
static int Device_Open = 0; /* Is device open?
* Used to prevent multiple access to "Error in unregister_chrdev: %d\n", ret);
}
/*
* Methods
*/
/*
* Called when a process tries to open the device file, like
* "cat /dev/mycharfile"
*/
static int device_open(struct inode *inode, struct file *file)
{
static int counter = 0;
if (Device_Open)
return -EBUSY;
Device_Open++;
sprintf(msg, "I already told you %d times Hello world!\n", counter++);
msg_Ptr = msg;
try_module_get(THIS_MODULE);
return SUCCESS;
}
/*
* Called when a process closes the device file.
*/
static int device_release(struct inode *inode, struct file *file)
{
Device_Open--; /* We're now ready for our next caller */
/*
* Decrement the usage count, or else once you opened the file, you'll
* never get get rid of)
{
/*
* Number of bytes actually written to the buffer
*/
int bytes_read = 0;
/*
* If we're at the end of the message,
* return 0 signifying end of file
*/
if (*msg_Ptr == 0)
return 0;
/*
* Actually put the data into the buffer
*/
while (length && *msg_Ptr) {
/*
* The buffer is in the user data segment, not the kernel
* segment so "*" assignment won't work. We have to use
* put_user which copies data from the kernel data segment to
* the user data segment.
*/
put_user(*(msg_Ptr++), buffer++);
length--;
bytes_read++;
}
/*
* Most read functions return the number of bytes put into the buffer
*/
return bytes_read;
}
/*
* Called when a process writes to dev file: echo "hi" > /dev/hello
*/
static ssize_t
device_write(struct file *filp, const char *buff, size_t len, loff_t * off)
{
printk(KERN_ALERT "Sorry, this operation isn't supported.\n");
return -EINVAL;
}.
This is by convention. When writing a driver, it's OK to put the device
file in your current directory. Just make sure you place it in /dev for a
production driver