Mailslot

For the postal delivery receptacle, see Letter box.

Mailslot is a one-way interprocess communication mechanism, available on the Microsoft Windows operating system that allows communication between processes locally. Mailslots also supported communication over a network. The use of Mailslots is generally simpler than named pipes or sockets when a relatively small number of relatively short messages is expected to be transmitted, such as for example infrequent state-change messages, or as part of a peer-discovery protocol. Up until Windows 11 24H2, the Mailslot mechanism also allowed for short message broadcasts ("datagrams") to all listening computers across a given network domain.[1][2] The mechanism originated in LAN Manager, a component of OS/2, an operating system co-developed by IBM and Microsoft.[3] Mailslots were discontinued in Windows 11 24H2.[4][5]

Features

Mailslots function as a server-client interface. A server can create a Mailslot, and a client could write to it by name. Only the server can read the mailslot, as such mailslots represented a one-way communication mechanism. A server-client interface could consist of two processes communicating locally or across a network. Mailslots operated over the RPC protocol and worked across all computers in the same network domain. Mailslots offered no confirmation that a message had been received. Mailslots were generally a good choice when one client process need to broadcast a message to multiple server processes.[1][2]

Uses

The most widely known use of the Mailslot IPC mechanism was the Windows Messenger service that was part of the Windows NT-line of products, including Windows XP. The Messenger Service, not to be confused with the MSN Messenger internet chat service, was essentially a Mailslot server that waited for a message to arrive. When a message arrived, it was displayed in a popup onscreen. The NET SEND command was therefore a type of Mailslot client, because it wrote to specified mailslots on a network.

A number of programs also used Mailslots to communicate. Generally these were amateur chat clients and other such programs[citation needed]. Commercial programs usually preferred pipes or sockets.

Mailslots were implemented as files in a mailslot file system (MSFS). Examples of Mailslots included:

  • MAILSLOT\Messngr - Microsoft NET SEND Protocol
  • MAILSLOT\Browse - Microsoft Browser Protocol
  • MAILSLOT\Alerter
  • MAILSLOT\53cb31a0\UnimodemNotifyTSP
  • MAILSLOT\HydraLsServer - Microsoft Terminal Services Licensing
  • MAILSLOT\CheyenneDS - CA BrightStor Discovery Service

References

  1. ^ a b Brain, Marshall; Ron Reeves (2000). Win32 System Services: The Heart of Windows 98 and Windows 2000. Prentice Hall PTR. pp. 230–243. ISBN 0130225576 – via Google Books.
  2. ^ a b Mitchell, Stan (1997). Inside the Windows 95 File System. O'Reilly. pp. 292–295. ISBN 156592200X – via Google Books.
  3. ^ Plateau, Brigitte; Denis Trystram; Jacek Blazewicz; Klaus Ecker, eds. (2013). Handbook on Parallel and Distributed Processing. Springer Berlin Heidelberg. p. 249. ISBN 3662043033 – via Google Books.
  4. ^ Burt, Jeff (March 13, 2023). "Windows 11 puts 'disgusting' Remote Mailslots protocol out of its misery". The Register. Archived from the original on June 5, 2026.
  5. ^ Jawad, Usama (March 10, 2023). "Microsoft is getting rid of the Remote Mailslot legacy protocol in Windows 11 soon". Neowin. Archived from the original on June 15, 2025.
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