Question

In this article, section ‘More Waiting…’, when author talked about synchronize i/o with thread, the 1st example he gives is:

hFile = CreateFile(....,FILE_IO_OVERLAPPED,...);
ReadFile(hFile,...)
< Do some computation.>
WaitForSingleObject(hFile,INFINITE);

I had the impression that both ‘CreateFile’ and ‘ReadFile’ are synchronous APIs, no? Why would a ‘WaitForSingleObject’ needed here? Wouldn’t ‘CreateFile’ and ‘ReadFile’ block until the operation is over and then return?

Then some more confusion coming when author talks about ‘alertable waiting’. What is it, a thread state after WaitForSingleObjectEx()? then why author give:

while (WaitForSingleObject(hObject,TRUE)==WAIT_IO_COMPLETION);

as example instead of

while (WaitForSingleObjectEx(hObject,TRUE)==WAIT_IO_COMPLETION);

?

and should ‘ex’ functions be together like:

hFile = CreateFile(....,FILE_IO_OVERLAPPED,...);
ReadFileEx(hFile,...)
< Do some computation.>
while (WaitForSingleObjectEx(hObject,TRUE)==WAIT_IO_COMPLETION);

Any one can explain a little more?

Thanks,

Answer 1

The document also states following:

Asynchronous I/O is a way the operating system provides for letting
I/O instructions execute in the background, the idea being roughly
that a call to ReadFile or WriteFile will return immediately to the
caller and overlap the I/O operation itself with the computation of
the thread that called it in the first place. This technique only
works for I/O devices whose drivers support asynchronous I/O and
manifests itself to the programmer through the flag FILE_IO_OVERLAPPED ^(*)
that can be passed to the CreateFile call. I/O performed on such a
file object will behave quite differently from synchronous I/O.

The methods are synchronous/asynchronous depending on parameters passed to CreateFile

^{* _{should read FILE_FLAG_OVERLAPPED}}