How to understand the macro __init?

[Amarnath Revanna]

Hi Wanny,

On Thu, Aug 16, 2012 at 8:43 PM, 王哲 <wangzhe5004 at gmail.com> wrote:

>
>
> 2012/8/16 Amarnath Revanna <amarnath.revanna at gmail.com>
>
>> 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 at gmail.com>wrote:
>>
>>> Hi,
>>>
>>> On Tue, Aug 14, 2012 at 9:34 AM, Mulyadi Santosa
>>> <mulyadi.santosa at gmail.com> wrote:
>>> > Hi.. :)
>>> >
>>> > On Tue, Aug 14, 2012 at 9:14 AM, 王哲 <wangzhe5004 at gmail.com> 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
>>> >
>>> > _______________________________________________
>>> > Kernelnewbies mailing list
>>> > Kernelnewbies at kernelnewbies.org
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>>>
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>>
>>
>
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