Earlier in this chapter, you saw that by creating a thread using
CreateRemoteThread, you can invoke functionality in a remote process. However, thread
creation requires overhead, so it would be more efficient to invoke a function on an existing thread. This capability exists in Windows as the asynchronous
procedure call (APC).
APCs can direct a thread to execute some other code prior to executing its regular execution
path. Every thread has a queue of APCs attached to it, and these are processed when the thread is in
an alertable state, such as when they call functions like
SleepEx. These functions essentially give the thread a chance to
process the waiting APCs.
If an application ...