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double messages in mailbox with Win32 API

Posted on 1997-08-13
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Last Modified: 2013-12-04
I don't understand why one of my task is receiving a message twice in its mailslot when the message is sent once.

I use VC++ 4.2 and NT 4.0.
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Question by:didede
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davmarc earned 100 total points
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It is a known problem - Microsoft considers it "by design".
This comes from KB article Q127905 "Messages Sent to Mailslot Are Duplicated":


SYMPTOMS
One application creates a mailslot using CreateMailSlot() and reads from it using ReadFile(). A second application opens the mailslot using CreateFile() and writes to it using WriteFile(). The second application writes one message to the mailslot, but the first application receives three duplicates of the message.

CAUSE
This is expected behavior if you have three network transports loaded. There is no way to know which transport should be used to deliver to a given mailslot on a remote machine, so all transports are used.

RESOLUTION
Send a unique ID at the beginning of each message. The listening end can detect duplicates and delete them. If you have multiple clients sending messages, their messages may be interleaved in the mailslot. You may need to track which client sent which message last, in order to successfully detect duplicates.

STATUS
This behavior is by design.


Davide Marcato.
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