Product: Zoo 4.0
Summary: All about mailslots.

A mailslot is a mechanism for one-way interprocess communications (IPC). Applications can store messages in a mailslot. The owner of the mailslot can retrieve messages that are stored there. These messages are typically sent over a network to either a specified computer or to all computers in a specified domain. A domain is a group of workstations and servers that share a group name.

Mailslots are a simple way for a process to either send or broadcast messages to one or more processes. One important consideration is that mailslots broadcast messages using datagrams. 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 that can prevent a datagram from reaching a particular destination.

A mailslot is a pseudofile that resides in memory, and you use standard file functions to access it. The data in a mailslot message can be in any form, but cannot be larger than 424 bytes when sent between computers. Unlike disk files, mailslots are temporary. When all handles to a mailslot are closed, the mailslot and all the data it contains are deleted.

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. 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 any existing 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.