4.2 Mailbox Driver Device Information
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You can obtain information on mailbox characteristics by using the Get Device/Volume Information ($GETDVI) system service. (See the HP OpenVMS System Services Reference Manual.)
$GETDVI returns mailbox characteristics when you specify the item code DVI$_DEVCHAR. Table 4-1 lists these characteristics, which are defined by the $DEVDEF macro.
Table 4-1 Mailbox Characteristics
| Characteristic[1] | Meaning |
|---|---|
| Dynamic Bits (Conditionally Set) | |
DEV$M_SHR | Device is shareable. |
DEV$M_AVL | Device is available. |
| Static Bits (Always Set) | |
DEV$M_REC | Device is record-oriented. |
DEV$M_IDV | Device is capable of input. |
DEV$M_ODV | Device is capable of output. |
DEV$M_MBX | Device is a mailbox. |
[1] Defined by the $DEVDEF macro. | |
DVI$_DEVCLASS and DVI$_DEVTYPE return the device class and device type names, which are defined by the $DCDEF macro. The device class for mailboxes is DC$_MAILBOX. The device type is DT$_MBX (or DT$_SHRMBX if the mailbox is a shared memory mailbox). DVI$_DEVBUFSIZ returns the buffer size, which is the maximum message size in bytes.
DVI$_DEVDEPEND returns a longword field in which the two low-order bytes contain the number of messages in the mailbox. (The two high-order bytes are not used and should be ignored.) This information can also be obtained through the Get Mailbox Information function (see “Get Mailbox Information”).
DVI$_UNIT returns the mailbox unit number. Using mailbox to hold a termination message for a subprocess or a detached process requires that the parent process obtain this number to pass to the mbxunt argument of the $CREPRC system service.