linux kernel character drivers

Question

I have registered a character driver using the api register_chrdrv().

The driver creates an entry in /dev/myfile. User space applications can access my driver by opening the /dev/myfile. Now I want to make my character driver such that only one user space application can access my driver at a time; only when that user space application closes the file can another user space program access my driver.

Can this be done in the kernel level?

I don't have control of the number of applications running in user space so i have to make changes only in kernel level.

Answer 1

In the open method of your driver: use a global flag to say whether the device is in use. Unset when closed (in the release method). Pseudo-code:

am_open=False

open() method:
if (!am_open)
  am_open=True
  ...proceed...
else
  return -EAGAIN; // try again later
 -or-
  return -EBUSY;


release() method:
  am_open=False

Having said that, drivers (usually) should be a 100% reentrant :)

Answer 2

With a kernel char device you usually have the following methods.

.open
.read
.write
.release

(struct file_operations fops which will be used in the method register_chrdev(...,&fops).)

I would create your clients in a way that they open the device when they get started, and release it upon exiting. Like this you could easily handle / hinder a new (second) program of doing reads and writes on this device.

There may be other possibilities as well.

Answer 3

As per your question you want to synchronize your application so that at a time only one process can access it. You can go for semaphore in kernel or locking methods in drivers that may help you. You may refer this http://www.cs.fsu.edu/~baker/devices/lxr/http/source/linux/kernel/semaphore.c