block_type

    OpenVMS usage longword_unsigned
    type          longword (unsigned)
    access        read only
    mechanism     by value
    Block type in range 0 - 255 (usually extracted from xxx$b_type
    field).

 block_subtype

    OpenVMS usage longword_unsigned
    type          longword (unsigned)
    access        read only
    mechanism     by value
    Block subtype in range 0 - 255 (ignored if the given block type
    has no subtypes).

 buffer_ptr

    OpenVMS usage char_string
    type          character string
    access        write only
    mechanism     by reference
    Address of buffer to save block name, which is returned as a
    zero-terminated string.

 buffer_len

    OpenVMS usage longword_unsigned
    type          longword (unsigned)
    access        read only
    mechanism     by value
    Length of buffer to receive block name.