About Mailslots

A mailslot is a pseudofile; it resides in memory, and standard Windows file functions write to it. Unlike disk files, however, mailslots are temporary. When every handle of a mailslot is closed, the mailslot and all the data it contains are deleted. The data in a mailslot message can be in any form.

A mailslot server is a process that creates and owns a mailslot. When the server creates a mailslot, it receives a mailslot handle. This handle must be used when a process reads messages from the mailslot. Only the process that creates a mailslot (or has obtained the handle by some other mechanism, such as inheritance) can read from the mailslot. A mailslot exists until all server handles to it have been closed or all server processes have exited. All mailslots are local to the process that creates them; a process cannot create a remote mailslot.

A mailslot client is a process that writes a message to a mailslot. Any process that has the name of a mailslot can put a message there. New messages follow preexisting messages in the mailslot.

Mailslots can broadcast messages within a domain. If several processes in a domain each create a mailslot using the same name, every message that is addressed to that mailslot and sent to the domain is received by the participating processes. Because one process can control both a server mailslot handle and the client handle retrieved when the mailslot is opened for a write operation, applications can easily implement a simple message-passing facility within a domain.

Some developers choose to use named pipes instead of mailslots for interprocess communications. Named pipes are a simple way for two processes to exchange messages. Mailslots, on the other hand, are a simple way for a process to broadcast messages to many other processes. For more information about named pipes, see Pipes.

Another issue in the choice between mailslots and named pipes is that mailslots use datagrams, and named pipes do not. A datagram is a small packet of information that the network sends along the wire. Like a radio or television broadcast, a datagram offers no confirmation of receipt; there is no way to guarantee that a datagram has been received. Just as mountains, large buildings, or interfering signals might cause a radio or television signal to get lost, there are things the sender has no knowledge of that can prevent a datagram from reaching a particular destination. Named pipes are more like simple one-to-one telephone calls: you can talk only to one party, but you know that the conversation is occurring. So, if an application requires guaranteed reception, it should use a named pipe; but named pipes cannot broadcast. Mailslots do not guarantee reception, but they can broadcast.

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