Steps
--------
- A module is made to register with kernel for the file /dev/myown withe major number 254.
- Then open/write function were implemeneted for file_operations structure.
- Two character device files were created with the same major number and different minor no under /dev using the following command,
mknod /dev/myown c 254 0
mknod /dev/myown1 c 25 0
- Two application programs were written to open the file /dev/mywon & /dev/myown1
- Run the exes of the two application programs, you will get the printk messages from the kernel for both of these programs. (i.e from sample_write function of driver )
Observation:
------------ ---
- The driver is said not to be associated with the device file /dev/myown but to the device files who are all having major no 254.
- A driver never actually knows the name of the device being opened, just the device number�and users can play on this indiffer ence to names by aliasing new names to a single device for their own convenience. If you create two special files with the same major/minor pair, the devices are one and the same, and there is no way to differentiate between them. The same effect can be obtained using a symbolic or hard link, and the preferr ed way to implement aliasing is creating a symbolic link
- From LDD - Linux Device Driver book.
Driver code
driver.c
----------
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/fs.h>
static int sample_write(struct file *file,const char* data,int len,loff_t *off);
static int sample_open(struct inode* inode,struct file* filep);
static struct file_operations rwstruct={
open:sample_open,
write:sample_write,
owner:THIS_MODULE
};
static int major;
int init_module(void)
{
printk(" <0>inside init_module");
major = register_chrdev(254,"myown",&rwstruct);
printk(KERN_EMERG "The major umber assigned is %d",major);
return 0;
}
void cleanup_module(void)
{
int cleanup = unregister_chrdev(major,"myown");
printk(KERN_EMERG "The major number cleaned up is %d",cleanup);
}
static int sample_open(struct inode* inode,struct file* file)
{
printk(KERN_EMERG "Inside open %d \n",inode->i_rdev);
return 0;
}
static int sample_write(struct file *file,const char* data,int len,loff_t *off)
{
printk(KERN_EMERG "Inside sample_write lengths is %d \n",len);
return 0;
}
Application code
appl1.c
----------
#include<stdio.h>
#include<unistd.h>
int main()
{
int fd;
fd = open("/dev/myown",O_WRONLY);
while(1)
{
printf("Writig a myown character");
write(fd,"c",1);
sleep(3);
}
close(fd);
}
appl2.c
---------
#include<stdio.h>
#include<unistd.h>
int main()
{
int fd;
printf("Pgrogram starts \n");
fd= open("/dev/myown1",O_RDONLY);
while(1)
{
printf("Writing myown1 character");
write(fd,"MANI",1);
sleep(3);
}
close(fd);
}