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$M_MBX | Device is a mailbox. |
DVI$_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.
