Re: Overlapped file IO

.NET async file IO is a subset of Win32 overlapped IO.
 
Looks like I am going to have to implement it myself.

________________________________

From: Discussion of development on the .NET platform using any managed language on behalf of Dan Farino
Sent: Mon 11/19/2007 18:18
To: [email protected]
Subject: Re: [DOTNET-CLR] Overlapped file IO



Hmm...sounds like our definitions of "overlapped" are a bit different
here.

I interpreted it as just meaning passing in the flag
FILE_FLAG_OVERLAPPED (which is the same numeric value as
FileOptions.Asynchronous) to CreateFile and using the OVERLAPPED
structure for performing async I/O with ReadFile and WriteFile.

So while you can't touch the fields of the OVERLAPPED struct yourself,
FileStream is using it under the covers to accomplish the async I/O.

-----Original Message-----
From: Discussion of development on the .NET platform using any managed
language [mailto:[email protected]] On Behalf Of Alex
Ivanoff
Sent: Monday, November 19, 2007 3:56 PM
To: [email protected]
Subject: Re: [DOTNET-CLR] Overlapped file IO

FileStream.BeginXXX/EndXXX methods are asynchronous, but not overlapped.
You cannot specify where to start reading/writing (Win32
OVERLAPPED.Offset/OffsetHigh members) nor can you issue multiple
read/write operations against the same file handle at the same time.
From MSDN:

"EndRead must be called exactly for every call to BeginRead. Failing to
end a read process before beginning another read can cause undesirable
behavior such as deadlock."

This is not overlapped at all.


-----Original Message-----
From: Discussion of development on the .NET platform using any managed
language [mailto:[email protected]] On Behalf Of Dan Farino
Sent: Monday, November 19, 2007 17:42
To: [email protected]
Subject: Re: [DOTNET-CLR] Overlapped file IO

Yes it does (except on Windows 9x). For FileStream, use the constructor:

public FileStream(string path, FileMode mode, FileSystemRights rights,
FileShare share, int bufferSize, FileOptions options)

Make sure you pass in FileOptions.Asynchronous. Then, use
BeginRead/Write and EndRead/Write. This uses overlapped I/O. (The only
part that is still synchronous is the constructor since there is no
async version of the underlying CreateFile Win32 call.)

Other classes such as Socket also use overlapped I/O when you use
BeginXXX/EndXXX. In the case of Socket and others, you don't need to
pass any special flags in like you have to with FileStream's
FileOptions.

-----Original Message-----
From: Discussion of development on the .NET platform using any managed
language [mailto:[email protected]] On Behalf Of Alex
Ivanoff
Sent: Monday, November 19, 2007 2:44 PM
To: [email protected]
Subject: [DOTNET-CLR] Overlapped file IO

Does .NET 2.0 have support for overlapped IO or do I need to implement
one myself?

Alex

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