4.1.1 Creating Mailboxes

To create a mailbox and assign a channel and logical name to it, a process uses the Create Mailbox and Assign Channel ($CREMBX) system service. A logical name can optionally be associated with the mailbox. If a logical name is specified for the mailbox, the system enters the logical name in a logical name table and gives it an equivalence name of MBAn, where n is a unique unit number.

$CREMBX also establishes the characteristics of the mailbox. These characteristics include a protection mask, a permanence indicator, maximum message size, buffer quota, and direction in which I/O can be performed (read, write, or read/write). A mailbox is created as either temporary or permanent; both types require privilege to create. Applications and restrictions on how to use temporary and permanent mailboxes are described in the following sections. (See the HP OpenVMS System Services Reference Manual for additional information on creating mailboxes.)

Other processes can assign additional channels to a mailbox using either the $CREMBX or the Assign I/O Channel ($ASSIGN) system service. The mailbox is identified by its logical name both when it is created and when it is assigned channels by cooperating processes. Channels assigned to the mailbox can specify the direction that I/O can be performed on the channel.

Figure 4-1 shows the use of $CREMBX and $ASSIGN.

Figure 4-1 Multiple Mailbox Channels

If sufficient dynamic memory for the mailbox data structure is not available when a mailbox is created, a resource wait occurs if resource wait mode is enabled.

When a mailbox is created, a certain amount of space is specified for buffering messages that have been written to the mailbox but have not yet been read. The bufquo argument to the $CREMBX system service specifies this amount or quota. If that argument is omitted, its value defaults to the system parameter DEFMBXBUFQUO.

A message written to a mailbox, in the absence of an outstanding read request, is queued to the mailbox, and the size of the message (the QIO P2 argument) is subtracted from the available buffering space. After the message is read, it is added back to the available buffering space.

If a process attempts to write to a mailbox that is full or has insufficient buffering space and if the process has resource wait enabled (which is the default case), the process is placed in miscellaneous resource wait mode until sufficient space is available in the mailbox. If resource wait is not enabled, the I/O completes with the status return SS$_MBFULL in the I/O status block (IOSB).

Channels can be assigned to mailboxes as bidirectional (read/write), read only, or write only. This allows for greater synchronization between users of the mailbox. To specify a unidirectional channel to the mailbox, specify the flags argument for the $CREMBX or $ASSIGN system services.

The flags argument is a longword bit mask that enables you to specify that the channel assigned to the mailbox is a read-only or write-only channel. If the flags argument is not specified, the default channel behavior is read/write. A channel assigned to the mailbox as read only is considered a reader. A channel assigned to the mailbox as write only is considered a writer. A channel assigned to the mailbox as read/write is considered both a reader and a writer.

For the $ASSIGN system service, the $AGNDEF macro defines a symbolic name for each flag bit. These flags are as follows:

For the $CREMBX system service, the $CMBDEF macro defines a symbolic name for each flag bit. These flags are as follows:

See the HP OpenVMS System Services Reference Manual for a syntax description of the $CREMBX and $ASSIGN system services.

The programming examples at the end of this section (“Mailbox Driver Programming Examples”) show mailbox creation, interprocess communication, and synchronization.

4.1.2 Deleting Mailboxes

As each process finishes using a mailbox, it deassigns the channel using the Deassign I/O Channel ($DASSGN) system service. Temporary mailboxes or permanent mailboxes that have been marked for deletion are actually deleted when no more channels are assigned to them.

If a mailbox channel is deassigned, any incomplete I/O requests on the mailbox channel for the process deassigning the channel are removed.

Permanent mailboxes that have not been marked for deletion must be explicitly deleted using the Delete Mailbox ($DELMBX) system service. An explicit deletion can occur at any time. As is true for temporary mailboxes, the mailbox is deleted when no processes have channels assigned to it.

When a temporary mailbox is deleted, its message buffer quota is returned to the process that created it. (No quota charge is made for permanent mailboxes.)

4.1.3 Mailbox Protection

Mailboxes (both temporary and permanent) are protected by a code, or mask, that is similar to the code used in protecting volumes. As with volumes, four types of users (defined by UIC) can gain access to a mailbox: SYSTEM, OWNER, GROUP, and WORLD; however, only three types of access—logical I/O, read, and write—are meaningful to users of a mailbox. Therefore, when creating a mailbox, you can specify logical I/O, read, and write access to the mailbox separately for each type of user. Logical I/O access is required for any mailbox operation. The set protection function modifier provides additional control of mailbox access (see “Set Protection”).

For additional information on temporary mailboxes and mailbox protection, see the description of the $CREMBX system service in the HP OpenVMS System Services Reference Manual.

4.1.4 Mailbox Message Format

There is no standardized format for mailbox messages and none is imposed on users.