Allocates a device for exclusive use by a process and its
    subprocesses. No other process can allocate the device or assign
    channels to it until the image that called $ALLOC exits or
    explicitly deallocates the device with the Deallocate Device
    ($DALLOC) service.

    Format

      SYS$ALLOC  devnam ,[phylen] ,[phybuf] ,[acmode] ,[flags]

    C Prototype

      int sys$alloc  (void *devnam, unsigned short int *phylen, void

                     *phybuf, unsigned int acmode, unsigned int

                     flags);

1  –  Arguments

 devnam

    OpenVMS usage:device_name
    type:         character-coded text string
    access:       read only
    mechanism:    by descriptor-fixed-length string descriptor
    Device name of the device to be allocated. The devnam argument
    is the address of a character string descriptor pointing to the
    device name string.

    The string can be either a physical device name or a logical
    name. If it is a logical name, it must translate to a physical
    device name.

 phylen

    OpenVMS usage:word_unsigned
    type:         word (unsigned)
    access:       write only
    mechanism:    by reference
    Word into which $ALLOC writes the length of the device name
    string for the device it has allocated. The phylen argument is
    the address of this word.

 phybuf

    OpenVMS usage:device_name
    type:         character-coded text string
    access:       write only
    mechanism:    by descriptor-fixed-length string descriptor
    Buffer into which $ALLOC writes the device name string for the
    device it has allocated. The phybuf argument is the address of a
    character string descriptor pointing to this buffer.

 acmode

    OpenVMS usage:access_mode
    type:         longword (unsigned)
    access:       read only
    mechanism:    by value
    Access mode to be associated with the allocated device. The
    acmode argument is a longword containing the access mode.

    The most privileged access mode used is the access mode of the
    caller. Only equal or more privileged access modes can deallocate
    the device.

 flags

    OpenVMS usage:mask_longword
    type:         longword (unsigned)
    access:       read only
    mechanism:    by value
    Longword of status flags indicating whether to interpret the
    devnam argument as the type of device to be allocated. Only one
    flag exists, bit 0. When it is set, the $ALLOC service allocates
    the first available device that has the type specified in the
    devnam argument.

    This feature is available for the following mass storage devices:

    RA60    RA80    RA81    RC25
    RCF25   RK06    RK07    RL01
    RL02    RM03    RM05    RM80
    RP04    RP05    RP06    RP07
    RX01    RX02    TA78    TA81
    TS11    TU16    TU58    TU77
    TU78    TU80    TU81