Using Win32 functions in Visual FoxPro Image Gallery
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Peer-to-peer LAN messenger built with Mailslot API functions

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Before you begin:
Run this example simultaneously on several computers on a network. All users will be able to exchange plain text messages.

A light-weight solution with no central server, no file sharing, and no access violation, but with automatic user detection.

Note that the communication may require ports 137 to 139 opened on each side. No further configuration is required except the setting a name for the mailslot. Same name have to be used by all participating applications.

A timer control is set to check for inbound messages.

See also:
  • Using mailslots to send messages on the network
  • Using the NetMessageBufferSend to send messages on the network
  • How to create non-blocking Winsock server
  • A client for testing non-blocking Winsock server
  • Using named pipes to pass data between processes
  • Using WM_COPYDATA for interprocess communication
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    8758 bytes  
    Created: 2003-06-06 14:46:00  
    Modified: 2011-12-10 09:20:22  
    Visits in 7 days: 35  
    Listed functions:
    CloseHandle
    CreateFile
    CreateMailslot
    GetLastError
    GetMailslotInfo
    GetTickCount
    ReadFile
    WriteFile
    Printer friendly API declarations
    My comment:
    Mailslot name -- in this sample code "foxapp" -- practically keeps all users together. Any other valid file name is appropriate, except some reserved names like "Messengr", "BROWSER" etc.

    A mailslot with name \\.\mailslot\foxapp is created on each computer. That turns such a computer into server receiving messages sent to this mailslot from other computers.

    A computer name is obtained through SYS(0) function.

    * * *
    On start the Tmsg instance creates a mailslot with a name "foxapp", same name for all users in this group should be used: \\.\maislot\foxapp.

    Then Tmsg broadcasts ONLINE and DETECT messages to all available network computers, using wildcard character * as a destination mailslot name: \\*\mailslot\foxapp.

    On receiving DETECT message any existing Tmsg instance automatically sends ONLINE message back to the sender. That is how the new object gets know who else is online.

    On termination the Tmsg instance broadcasts OFFLINE message to all users -- again the wildcard character is used -- to let everyone know that this user is disconnected.

    Maislots provide no confirmation of receipt. For that reason the Tmsg object on receiving a message automatically sends a confirmation back to the sender.

    For sending a message to another computer -- say with DESK3011 name -- the Tmsg object creates file handle using mailslot name \\DESK3011\Maislot\foxapp, and writes message data to this handle using WriteFile function.

    A message has a simple format:
  • sender - 24 bytes -- local computer name
  • confirmation flag - 1 byte
  • system message flag - 1 byte
  • message body - the rest of the message

    The GetMessages method is invoked on each Timer event checking for incoming messages.
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    User Contributed Notes:
    How to open port 137 to 139 | 2010-06-20 22:41:42
    Please tell me how to open communication port 137 to 139
    Thank You

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