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