Re: How to understand the macro __init?

On Thu, Aug 16, 2012 at 8:43 PM, 王哲 <wangzhe5004@xxxxxxxxx> wrote:

2012/8/16 Amarnath Revanna <amarnath.revanna@xxxxxxxxx>

Hi Vijay,

Is this a loadable kernel module or a built-in module?
If it is a loadable module, everything still remains in the memory without being freed.

Functions and data defined with __init gets freed after their execution only if these are part of built-in kernel modules.

In your case, if the module is a loadable one, you can still access the print_k() from hello_exit() as its still residing in memory.

Regards,
-Amar

Hi Amar

Thank you very much for reply. I often write loadable modules,but know little about

built-in modules,can you tell something about built-in modules? or,how i can take a built-in
module into the kernel?

When I say built-in module, I meant to say the driver module is going to be part of kernel itself, i.e. the module is compiled as part of kernel and not explicitly loaded later.

The __init macros are applicable only for these modules that are initialized as part of the kernel boot process.

At the end of the boot process, the kernel identifies all these memory areas that are part of the init section that were required only during initialization phase, and will continue to free these memory region to re-use.

If you have a linux machine, you can do a dmesg to see this memory freeing message that looks something something like:

" [1.011596] Freeing unused kernel memory: 664k freed "

This is exactly the freeing of all __init data sections from different drivers/modules that were part of the kernel image. The freeing was done at the end of the boot process, after they had been used.

On the other hand, any other kernel module that you load using insmod or modprobe comes after this stage, wherein the kernel was already booted, and hence, no memory area of __init will ever be freed.

Regards,

-Amar

On Thu, Aug 16, 2012 at 5:23 PM, Vijay Chauhan <kernel.vijay@xxxxxxxxx> wrote:

Hi,

On Tue, Aug 14, 2012 at 9:34 AM, Mulyadi Santosa
<mulyadi.santosa@xxxxxxxxx> wrote:
> Hi.. :)
>
> On Tue, Aug 14, 2012 at 9:14 AM, 王哲 <wangzhe5004@xxxxxxxxx> wrote:
>> i use the __init for function print_k.
>> in my opinion after the fisrt invoking the print_k in the hello_init.
>> the memory of print_k will be freed,and the second invoking will
>> not be executed.but the result of second invoking is executing .
>>
>> why?
>
> because you're still in module_init.... :)
>
> right after modul init stage is done, _init marked function is thrown away...

Even if we call print_k() function inside hello_exit, it still works.
At that point __init hello_init execution is over. How its still
working?

Regards,
Vijay

>
> --
> regards,
>
> Mulyadi Santosa
> Freelance Linux trainer and consultant
>
> blog: the-hydra.blogspot.com
> training: mulyaditraining.blogspot.com
>
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