mirror of
https://github.com/reactos/reactos.git
synced 2024-05-15 17:51:58 +00:00
37 lines
1.8 KiB
Plaintext
37 lines
1.8 KiB
Plaintext
APC
|
|
|
|
Asynchronous procedure call
|
|
|
|
An APC is a Kernel-defined control object representing a procedure
|
|
that is called asynchronously. APCs are thread-context dependent; that
|
|
is, they are queued to a particular thread for execution.
|
|
|
|
There are three different kinds of APCs in NT:
|
|
|
|
User APCs are used by certain asynchronous NT system services to allow
|
|
user-mode applications or protected subsystems to synchronize the
|
|
execution of a thread with the completion of an operation or the
|
|
occurrence of an event such as a timers expiration. User APCs are, by
|
|
default, disabled. That is, they are queued to the user-mode thread,
|
|
but they are not executed except at well-defined points in the
|
|
program. Specifically, they can only be executed when an application
|
|
or protected subsystem has called a wait service and has enabled
|
|
alerts to occur, or if it has called the test-alert service.
|
|
|
|
Kernel APCs are normal kernel-mode APCs. They are much like a normal
|
|
user APC except that they are executable by default. That is, they are
|
|
enabled except when the thread is already executing a Kernel APC.
|
|
(Note that a special Kernel APC always preempts these.)
|
|
|
|
Special Kernel APCs cannot be blocked except by running at a raised
|
|
IRQL. They are executed at APC_LEVEL IRQL (see IDT), in kernel mode.
|
|
These types of APCs are used by the system to force a thread to
|
|
execute a procedure in the threads context. An example of this is I/O
|
|
completion: the I/O Manager needs to get back into the context of the
|
|
original requestor of the I/O operation so that it can copy buffers,
|
|
and so forth. In order to do this, the I/O Manager must be able to
|
|
access the virtual address space of the thread/process, and the most
|
|
efficient way to complete the operation is to be in the calling
|
|
threads context.
|
|
|