Dear Friends,

As per my knowledge, one can use systemcall to call the driver functions for example using open, close... Now is there any way to call driver functions directly from application without using system call?

Have a Good Day,
Thank you,
Ankur.

Using system call is the conventional way to call driver functionalities such as the driver open, close, and ioctls. If your application or program is running in kernel space, then you can directly call your driver functions by exporting them in the driver. But if your program is running in user space, then you have to go through system calls.

Dear Friend Archieval,

Thank you for the response. I had one question pertaining to it, Is there any reason why we can access the kernel space device driver function from user space through system call only ?

In my opinion, as devices are treated as file, thats why its must to use systemcall to access device driver present in kernel space?

Have a Good Day,
Thank you,
Ankur

Hi Ankur,

Because you really cannot access them. Kernel space driver functions are only visible on kernel space. This is important to ensure the stability of the system. Kernel space functions do not share their process address space with the user space programs. So an address of variable being used in a kernel space, cannot be accessed, used, or supplied by a user space program, because if it is possible to do that, it would be easy to destroy the kernel by passing invalid addresses. This separation of user and kernel space is to prevent invalid executions in the kernel space. Kernel space modules or drivers are designed (ideally) to always work, never fail. Because any failure of kernel space functions such as segmentation fault are not recoverable and will cause kernel panic or kernel oops. It is not safe to fail in the kernel space, whereas in the user space it is ok since it does not control the whole system.

So by providing a way or means of accessing the kernel space modules or drivers through system calls, the possible errors that can be committed are reduced or restricted. System calls return proper error codes and the driver also do proper error handling in cases of invalid arguments.

Sorry for the messy explanation, but to summarize it, you cannot afford to fail in the kernel space, so you should go through the proper way of accessing the kernel space driver functionality.

By the way, device files are not ordinary files like text files which can be accessed by fopen, fprintf, etc.

Regards,
archieval

Dear Friend Archieval,

I appreciate your kind response. Thank you for wonderful and detailed explanation. My doubts are clear.

Have a Blessed Day,
Thank you,
Ankur