Re: ReadFile problem (overlapped I/O)

"Scott McPhillips [MVP]" <org-dot-mvps-at-scottmcp>
Fri, 03 Aug 2007 12:34:09 -0400
Alan Williams-Key wrote:

Yes it is truly asynchronous.

    ReadEvent[bufno] = CreateEvent(NULL,TRUE,FALSE,NULL);
    OVERLAPPED over;
    over.Offset = 0;
    over.OffsetHigh = 0;
    over.hEvent = ReadEvent[bufno];

then I test for completion

ready = WaitForSingleObject(ReadEvent[nextbuf],0);

and if ready does not have value WAIT_OBJECT_0 the program continues with
other processing. Is there anything else I need to include?


The error you reported sounds like normal behavior. Look at the sample
code in MSDN ReadFile for overlapped reads. ReadFile can return error,
with GetLastError returning ERROR_IO_PENDING. This simply means the
read is still in progress and ReadFile returned quickly because you
requested overlapped operation. So do NOT reissue the read in this

That's the meaning of asynchronous. It gives your code the opportunity
to do some work during the read, and then check for the read completion

Scott McPhillips [MVP VC++]

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