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[C++] How do I know FILE_FLAG_OVERLAPPED has been set in the file handle ?

Posted on 2013-12-29
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Last Modified: 2013-12-30
Hello Experts,

I want to know FILE_FLAG_OVERLAPPED flag has been set in the file handle.

I got a file handle from DuplicateHandle() and that is HANDLE_TYPE_FILE and
my target file name.
I want to read that file by using duplicated handle, but I don't know
FILE_FLAG_OVERLAPPED flag has been set or not into that file handle.

If I know that flag information, I can call ReadFile() with/without OVERLAPPED
structure pointer.

Any hints welcome.

Windows 7 32bit
C++Builder

Nobuo Miwa
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Question by:NobMiwa
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jkr earned 500 total points
ID: 39747239
>>I want to read that file by using duplicated handle, but I don't know
>>FILE_FLAG_OVERLAPPED flag has been set or not into that file handle.

>>If I know that flag information, I can call ReadFile() with/without OVERLAPPED
>>structure pointer.

It's actually quite easy: Even though there is no documented means to find out if that flag has been set, you can deal with this in both ways

- just don't pass an OVERLAPPED pointer, use NULL instead. The operation will be carried out synchronously (setting that flag does not mean you have to use the HANDLE in any async calls)

- pass an OVERLAPPED pointer anyway. If the flag was not set, the operation will be carried out synchronously. If it was set, use the means of async I/O.

A little more here: http://blogs.msdn.com/b/oldnewthing/archive/2012/04/05/10290954.aspx ("You can use an OVERLAPPED structure with synchronous I/O, too")
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Author Closing Comment

by:NobMiwa
ID: 39747343
Great thanks !
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