Boris wrote:
Robert Ramey wrote:
Windows has the concept of "Overlapped I/O" which permits dispatch of an i/o request that invokes a call back when done.
Posix has aio which functions (I believe) in a similar way.
So one has the concept of asyncronous i/o without any explicit reference to threads at least at the application level.
I am not sure if I understand correctly. Just as Don I think we can't get around threads when we have callbacks. I don't know about Windows but the callback in aio (Posix) is done by creating a new thread.
From the Microsoft documentation of ReadFileEx (a Windows API function):
"If the function succeeds, and the file reading operation completes, but the calling thread is not in an alertable wait state, the system queues the completion routine call, holding the call until the calling thread enters an alertable wait state." Very simply the calling thread starts off the read operation, and then either polls the i/o state or waits. In that wait operation lies the call to the i/o completion routine (the callback), which is called on this thread.
From the caller's point of view there's only one thread.
However, the abstraction involved in waiting on completion is the same as waiting for a thread, so the i/o operation is perhaps best modelled as having at least part of the interface of a thread (but this just gut feeling).