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Linux Character Device Example
/*
* chardev.c: Creates a read-only char device that says how many times you've
* read from the dev file.
*
* You can have some fun with this by removing the module_get/put calls,
* allowing the module to be removed while the file is still open.
*
* Compile with `make`. Load with `sudo insmod chardev.ko`. Check `dmesg | tail`
* output to see the assigned device number and command to create a device file.
*
* From TLDP.org's LKMPG book.
*/
#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
*/
int init_module(void);
void cleanup_module(void);
static int device_open(struct inode *, struct file *);
static int device_release(struct inode *, struct file *);
static ssize_t device_read(struct file *, char *, size_t, loff_t *);
static ssize_t device_write(struct file *, const char *, size_t, loff_t *);
#define SUCCESS 0
#define DEVICE_NAME "chardev"
#define BUF_LEN 80
/*
* Global variables are declared as static, so are global within the file.
*/
static int Major;
static int Device_Open = 0;
static char msg[BUF_LEN];
static char *msg_Ptr;
static struct file_operations fops = {
.read = device_read,
.write = device_write,
.open = device_open,
.release = device_release
};
/*
* This function is called when the module is loaded
*/
int init_module(void)
{
Major = register_chrdev(0, DEVICE_NAME, &fops);
if (Major < 0) {
printk(KERN_ALERT "Registering char device failed with %d\n", Major);
return Major;
}
printk(KERN_INFO "I was assigned major number %d. To talk to\n", Major);
printk(KERN_INFO "the driver, create a dev file with\n");
printk(KERN_INFO "'mknod /dev/%s c %d 0'.\n", DEVICE_NAME, Major);
printk(KERN_INFO "Try various minor numbers. Try to cat and echo to\n");
printk(KERN_INFO "the device file.\n");
printk(KERN_INFO "Remove the device file and module when done.\n");
return SUCCESS;
}
/*
* This function is called when the module is unloaded
*/
void cleanup_module(void)
{
/*
* Unregister the device
*/
unregister_chrdev(Major, DEVICE_NAME);
}
/*
* Methods
*/
/*
* Called when a process tries to open the device file, like
* "cat /dev/mycharfile"
*/
static int device_open(struct inode *inode, struct file *filp)
{
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;
/*
* TODO: comment out the line below to have some fun!
*/
try_module_get(THIS_MODULE);
return SUCCESS;
}
/*
* Called when a process closes the device file.
*/
static int device_release(struct inode *inode, struct file *filp)
{
Device_Open--;
/*
* Decrement the usage count, or else once you opened the file, you'll never
* get rid of the module.
*
* TODO: comment out the line below to have some fun!
*/
module_put(THIS_MODULE);
return SUCCESS;
}
/*
* Called when a process, which already opened the dev file, attempts to read
* from it.
*/
static ssize_t device_read(struct file *filp, /* see include/linux/fs.h */
char *buffer, /* buffer to fill with data */
size_t length, /* length of the buffer */
loff_t *offset)
{
/*
* 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 *buf, size_t len, loff_t *off)
{
printk(KERN_ALERT "Sorry, this operation isn't supported.\n");
return -EINVAL;
}
obj-m += chardev.o
all:
make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules
clean:
make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean
@cirosantilli

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