reactos/media/doc/apc

   1   APC
   2
   3    Asynchronous procedure call
   4
   5    An APC is a Kernel-defined control object representing a procedure
   6    that is called asynchronously. APCs are thread-context dependent; that
   7    is, they are queued to a particular thread for execution.
   8
   9    There are three different kinds of APCs in NT:
  10
  11    User APCs are used by certain asynchronous NT system services to allow
  12    user-mode applications or protected subsystems to synchronize the
  13    execution of a thread with the completion of an operation or the
  14    occurrence of an event such as a timers expiration. User APCs are, by
  15    default, disabled. That is, they are queued to the user-mode thread,
  16    but they are not executed except at well-defined points in the
  17    program. Specifically, they can only be executed when an application
  18    or protected subsystem has called a wait service and has enabled
  19    alerts to occur, or if it has called the test-alert service.
  20
  21    Kernel APCs are normal kernel-mode APCs. They are much like a normal
  22    user APC except that they are executable by default. That is, they are
  23    enabled except when the thread is already executing a Kernel APC.
  24    (Note that a special Kernel APC always preempts these.)
  25
  26    Special Kernel APCs cannot be blocked except by running at a raised
  27    IRQL. They are executed at APC_LEVEL IRQL (see IDT), in kernel mode.
  28    These types of APCs are used by the system to force a thread to
  29    execute a procedure in the threads context. An example of this is I/O
  30    completion: the I/O Manager needs to get back into the context of the
  31    original requestor of the I/O operation so that it can copy buffers,
  32    and so forth. In order to do this, the I/O Manager must be able to
  33    access the virtual address space of the thread/process, and the most
  34    efficient way to complete the operation is to be in the calling
  35    threads context.
  36