+++ /dev/null
- 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.
-
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