First Linux Char driver

1. Driver Source Code

#include   
#include
#include
#include
#include

MODULE_LICENSE("GPL");                    

#define MAJOR_NUM 254                     

static ssize_t globalvar_read(struct file *, char *, size_t, loff_t*);         
static ssize_t globalvar_write(struct file *, const char *, size_t, loff_t*);

struct file_operations globalvar_fops =
{
  .read=globalvar_read,
  .write=globalvar_write,
};

 

static int global_var = 0;                 

static int __init globalvar_init(void)
{
 int ret;
 ret= register_chrdev(MAJOR_NUM,"globalvar",&globalvar_fops);
 if(ret)
 {
  printk("globalvar register failure"); 
 } 
 else
 {
  printk("globalvar register sucess");  
 }
 return ret;
}

 

static void __exit globalvar_exit(void)
{
  int ret;
  ret=unregister_chrdev(MAJOR_NUM,"globalvar");
  if(ret)
  {
   printk("globalvar unregister failure"); 
  }
  else
  {
   printk("globalvar unregister sucess");
  } 
 
}

static ssize_t globalvar_read(struct file *filp, char *buf, size_t len, loff_t *off)
{
 if(copy_to_user(buf,&global_var,sizeof(int)))
 {
  return -EFAULT;
 }
   return (sizeof(int)); 
}

static ssize_t globalvar_write(struct file *filp, const char *buf, size_t len, loff_t *off)
{
 if(copy_from_user(&global_var,buf,sizeof(int)))
 {
  return -EFAULT;
 }
   return sizeof(int); 
}

module_init(globalvar_init);        
module_exit(globalvar_exit);

================================================Driver Source Code Ends

2. App source code

 

#include
#include
#include
#include

main()
{
 int fd,num1,num2;
 fd = open("/dev/chrdev",O_RDWR,S_IRUSR | S_IWUSR);
 if(fd!=-1)
 {
  read(fd,&num,sizeof(int));
  printf("the globalvar is %d",num);
  printf("please input the num written to globalvar");
  scanf("%d",&num1);
  write(fd,&num1,sizeof(int));
  read(fd,&num2,sizeof(int));
  printf("the globalvar is %d",num2);
  close(fd); 
 }
 else
 {
  printf("device open failure");
 } 
}

App can be compiled by command: gcc -o chrdevapp chrdevApp.c

=========================================================App source code end

 

3. This Module can be compile in Linux 2.4 kernel(Red Hat 9.0). With tool GCC

the command is:

gcc -DMODULE -D__KERNLE__ -I /usr/src/linux-2.4.20-8/include -c chrdev.c

Then, there will be a module file chrdev.o

==> Use command: insmod chrdev, to add this module to kernel, this command will call register_chrdev() method to register this module to kernel. if sucessful, check /proc/modules, chrdev module will show there.

 

==> the resource that driver declare can be released when this module is removed by command: rmmod chrdev, this command will call unregister_chrdev() method.

 

==>We must use the command: insmod chrdev.o to add this module to kernel, then run the App. If not, when run app, it will show "Device open error"