VMS Help System Services, $CANEXH, Argument  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 desblk

    OpenVMS usage:exit_handler_block
    type:         longword (unsigned)
    access:       read only
    mechanism:    by reference
    Control block describing the exit handler to be canceled. If you
    do not specify the desblk argument or specify it as 0, all exit
    control blocks are canceled for the current access mode. The
    desblk argument is the address of this control block.