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a115433



Joined: 05 Mar 2010
Posts: 144
a115433
does APC has its purpose, or it just exist to make kernel look bigger and more complex?

Whats the purpose of switching threads to address space of other process?
Whats the reason of implementing apc in kernel?
I would understand it if there wasnt CreateRemoteThread option, it works simmilar, you just switch your thread to another context.
It seems so useless to me...
Post 13 Apr 2010, 19:10
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LocoDelAssembly
Your code has a bug


Joined: 06 May 2005
Posts: 4633
Location: Argentina
LocoDelAssembly
http://msdn.microsoft.com/en-us/library/ms681951%28VS.85%29.aspx

asmcoder wrote:

I would understand it if there wasnt CreateRemoteThread option, it works simmilar, you just switch your thread to another context.

Care to explain the similarity with the explanation given in the link above? Especially the queuing and the synchronization parts...
Post 13 Apr 2010, 20:55
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a115433



Joined: 05 Mar 2010
Posts: 144
a115433
so what can i use apc for? nothing! its exactly same as creating new thread, but i use existing one.
Post 14 Apr 2010, 06:51
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baldr



Joined: 19 Mar 2008
Posts: 1651
baldr
a115433,

Overlapped I/O and timers. QueueUserAPC() is simply implementation detail being exposed to API user at almost no cost.

Exactly the same? Think again. CreateRemoteThread() won't help much if you need to access thread context (e.g. thread-local variables).

Do you think threads are cheap?
Post 14 Apr 2010, 08:07
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a115433



Joined: 05 Mar 2010
Posts: 144
a115433
Quote:
QueueUserAPC() is simply implementation detail being exposed to API user at almost no cost.

ok thats the reason i guess.
about overlapped io, its processed in other thread as apc, right?
With method_buffered, because it use buffers loaded to global pages (mapping isnt flushed on context switch), all processes can access this memory.

i have 1 last question about io, when actually does it end? after call to IoCompleteRequest(), or returning from IRP_MJ_XXX handler?

io functions such ReadFile return what exactly? Status of returning from irp handler?
when i will queue overlapped io, and get error_io_pending, that means it was sent to other thread as apc, right? And when it complete, IoCompleteRequest() is called, event set to signaled state, and i can continue. when i terminate process, what happens? irp gets canceled? apc gets canceled? or termination wait untill all irp is completed by IoCompleteRequest()?
Post 14 Apr 2010, 08:22
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baldr



Joined: 19 Mar 2008
Posts: 1651
baldr
a115433 wrote:
about overlapped io, its processed in other thread as apc, right?
Why do you think so? Overlapped I/O is processed by NTOSKRNL almost in the same way as synchronous I/O, only calling thread isn't put in wait state until I/O is complete. I emphasize that: most of I/O request (be it sync or async) is done in the context of calling thread. This request can be processed by driver in context of any thread. The result can be indicated to calling thread by APC (which is done in the context of calling thread).

Most questions are answered in "Inside Microsoft Windows 2000" by David A. Solomon & Mark E. Russinovich.
Post 14 Apr 2010, 15:36
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a115433



Joined: 05 Mar 2010
Posts: 144
a115433
yes so when i call async operation, i ask kernel to do it in other thread (apc)?
Post 14 Apr 2010, 16:13
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baldr



Joined: 19 Mar 2008
Posts: 1651
baldr
a115433,

No, NTOSKRNL!ReadFile() executes in context of calling thread and doesn't use APC. I/O manager can post APC to calling thread's APC queue on I/O completion.
Post 14 Apr 2010, 16:32
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a115433



Joined: 05 Mar 2010
Posts: 144
a115433
duno ive read some time ago in official msdn that ReadFile with overlapped only queue read task to other thread, so it can return and let other thread process operation.

when i call io function in overlapped mode, i have control over my thread immidietly after executing function, and event is signaled when its finished. so ReadFile execute in context of calling thread, of course, but IO processing (copying buffer from/to dma, reading/writing to port) is executed by other thread, from worker pool.
Post 14 Apr 2010, 18:58
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