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A mailbox is a pseudo device used for interprocess communication and synchronization. The device code for mailboxes is MBAn.

Mailbox characteristics

Mailboxes can store a certain amount of information to be shared between processes, signal to a process when there is information to be read by that process. There is also a queueing mechanism for when multiple messages are being written to a mailbox. Multiple channels can be assigned to a mailbox. Mailboxes can be permanent and temporary. Temporary mailboxes are deleted when all channels to the mailbox have been deassigned; permanent mailboxes must be explicitly deleted. Mailboxes can have many writers and many readers. Mailboxes have a "size" determined by the buffer quota.


Mailboxes have four kinds of access: read (R), write (W), logical I/O (L), and or physical I/O (P).


  • TMPMBX is required to create temporary mailboxes
  • PRMMBX is required to create permanent mailboxes
  • SYSNAM or GRPNAM are required to place the logical name for the mailbox in the appropriate table

See also