Is it valid to write below ESP?

Question

For a 32-bit windows application is it valid to use stack memory below ESP for temporary swap space without explicitly decrementing ESP?

Consider a function that ret

Answer 1

Several answers here mention APCs (Asynchronous Procedure Calls), saying that they can only be delivered when the process is in an "alertable state", and are not truly asynchronous with respect to normal execution the way Unix signal handlers are

Windows 10 version 1809 introduces Special User APCs, which can fire at any moment just like Unix signals. See this article for low level details.

The Special User APC is a mechanism that was added in RS5 (and exposed through NtQueueApcThreadEx), but lately (in an insider build) was exposed through a new syscall - NtQueueApcThreadEx2. If this type of APC is used, the thread is signaled in the middle of the execution to execute the special APC.