error in sample Module

[jesusitoLinux]

Hello Friends! I have this problem. I'm following a tutorial for developing drivers in Linux using Modules. First of all I type "mknod /dev/tutorial c 203 1". Then I compile my module, "tutorial.c":

#include <linux/device.h>
#include <linux/init.h>
#include <linux/module.h>
#include <linux/cdev.h>
#include <linux/fs.h>

MODULE_LICENSE("GPL");

static int major=203;
static int minor=1;
dev_t dev;
static int tutorial_open(struct inode *inode, struct file *file);

struct file_operations tutorial_fops={

  .owner = THIS_MODULE,
  .open = tutorial_open,
};





static int tutorial_open(struct inode *inode, struct file *file)
{
  file->private_data=dev;
  printk(KERN_INFO "abriendo dispositivo\n");
  
  
 return 0; 
}

static int tutorial_init(void) {
  struct cdev *mycdev;
  int result;
  printk(KERN_INFO "Init module\n");
  
  dev=MKDEV(major, minor);
  result = register_chrdev_region(dev,1, "tutorial");

  
  if(result < 0) {
    printk(KERN_INFO "Cant get major nr %d\n", major);
  }
  else
  {
    printk(KERN_INFO "get major nr %d\n", major);
    mycdev=cdev_alloc();
    cdev_init(mycdev,&tutorial_fops);
    mycdev->owner = THIS_MODULE;
    mycdev->ops = &tutorial_fops;

    int rc = cdev_add(mycdev,dev, 1);
    if(rc<0)
      printk(KERN_INFO "Error on cdev_add\n");

  }
  
  return 0;
}


static void tutorial_exit(void) {
printk(KERN_INFO "Exit module\n");
}
module_init(tutorial_init);
module_exit(tutorial_exit);

And after getting "tutorial.ko", I type "insmod ./tutorial.ko". When I take a look at "/var/log/messages" I see "get major nr 203". But when I try to open my driver with "cat /dev/tutorial" to execute the "open" function, I get "invalid argument ". Then I remove the driver "rmmod tutorial", and reload with "insmod ./tutorial.ko", and when y try to open again with "cat /dev/tutorial", i get "killed", and my computer gets entirely blocked, I cant use neither the keyboard or the mouse!! Why ??

Thank you very much!!!

[shivanth]

If you run cat ,you need to implement the read function as well in the file operations structure . Cat tries to read the contents of the file after opening it . So implement that and your code'll run safe

[jesusitoLinux]

Ey Shivant!! You were right!! now it works!! Thank you very much!! I'm new at developing with kernel, but i find it amazing!!

[jesusitoLinux]

I have other problem. This is the new code

#include <linux/device.h>
#include <linux/init.h>
#include <linux/module.h>
#include <linux/cdev.h>
#include <linux/fs.h>

MODULE_LICENSE("GPL");

static int major=203;
static int minor=1;
dev_t dev;
static int tutorial_open(struct inode *inode, struct file *file);
static ssize_t tutorial_read(struct file *filp, char __user *buf, size_t count,loff_t *f_pos);
struct file_operations tutorial_fops={

  .owner = THIS_MODULE,
  .open = tutorial_open,
  .read = tutorial_read,
};


ssize_t tutorial_read(struct file *filp, char __user *buf, size_t count,loff_t *f_pos)
{
  printk(KERN_INFO "Read from device\n");
  return 1;
}



static int tutorial_open(struct inode *inode, struct file *file)
{
  file->private_data=dev;
  printk(KERN_INFO "open device\n");
  
  
 return 0; 
}

static int tutorial_init(void) {
  struct cdev *mycdev;
  int result;
  printk(KERN_INFO "Init module\n");
  
  dev=MKDEV(major, minor);
  result = register_chrdev_region(dev,1, "tutorial");

  
  if(result < 0) {
    printk(KERN_INFO "Cant get major nr %d\n", major);
  }
  else
  {
    printk(KERN_INFO "get major nr %d\n", major);
    mycdev=cdev_alloc();
    cdev_init(mycdev,&tutorial_fops);
    mycdev->owner = THIS_MODULE;
    mycdev->ops = &tutorial_fops;

    int rc = cdev_add(mycdev,dev, 1);
    if(rc<0)
      printk(KERN_INFO "Error on cdev_add\n");

  }
  
  return 0;
}


static void tutorial_exit(void) {
  printk(KERN_INFO "Exit module\n");
  unregister_chrdev_region(dev,1);

}
module_init(tutorial_init);
module_exit(tutorial_exit);

I load the module, and when i type "cat /dev/tutorial" it enter in an infinte loop in "tutorial_read", cause i can see the infinite message from /var/log/messages. So I type Cntrl^C, remove module with "rmmod tutorial". And when i reload the module "insmod ./tutorial.ko", and type "cat /dev/tutorial", i got "killed", and my computer gets entirely blocked again!!!

[shivanth]

Your read function should return 0 to indicate EOF . That'll do

[jesusitoLinux]

Yes shivanth! you were right! returning 0 didn't enter in an infinite loop. But i still getting the other error. When i remove "rmmod tutorial" and reload with "insmod ./tutorial.ko" and type "cat /dev/tutorial" I got "killed", and the system gets block.

Thank you very much!

[shivanth]

You need to implement a close function as well to for proper unloadingl of the driver . Unregister the device etc is done in this function so that the driver is unloaded properly . If its not unloaded properly , the next time you try to load it , the driver is seen as registered causing the kill signal delivered to insmod

[jesusitoLinux]

Ok shivanth. I have modified the exit function "tutorial_exit". This is the new code:

static void tutorial_exit(void) {
  printk(KERN_INFO "Exit module\n");
  unregister_chrdev_region(dev,1);
  void cdev_del(mycdev);


}

I add the "cdev_del" function to properly remove the driver. But I still getting the error. When I reload the driver the second, i don't get the killed signal with "insmod ./tutorial.ko", i get the killed signal when I read for the second time "cat /dev/tutorial".

Thank you very much

[shivanth]

You still haven't implemented the release method . In release, int this case , u just need to return a zero for successful closing of the device .

And a piece of advice , google for linux device drivers 3d edition . All i just posted was straight from that . It'll surely help you . And do not stop posting if anything comes in your way