Question

I’m trying to write a simple device driver, and use the Dev_Read() and Dev_Write() functions, which I have already defined.
My driver registers a character device tied to a
given major number this way

# mknod /dev/mydev c 250 0

And then, from my user program I open the device this way

int fd;
if ((fd = open("/dev/mydev", O_RDWR)) < 0)
{
  perror("open /dev/mydev");
  exit(EXIT_FAILURE);
}

after I invoke the Read() function

int read_bytes = read (fd, buffer, 1);

and successfully get kernel space information, that my Dev_Read function works.
My problem is that I don’t understand how to implement my Dev_Write function.
How can I write someting to my file, to see that Dev_Write function works ?
THANK YOU for help.

Answer 1

It would help to know what you’ve tried which didn’t work.

One thing to be aware of, and not necessarily intuitive, is that your driver’s write function must copy the write buffer from the caller’s space into kernel space. An example of that can be seen in the tutorial at http://www.freesoftwaremagazine.com/articles/drivers_linux –

ssize_t memory_write( struct file *filp, char *buf,
                      size_t count, loff_t *f_pos) {

  char *tmp;

  tmp=buf+count-1;
  copy_from_user(memory_buffer,tmp,1);
  return 1;
}

where memory_buffer is space you’ve allocated within your driver.