Allows you to read and change the default directory string for
    the process. Do not include any device or file name strings in
    the default directory string; include only the [DIR] syntax.
    Modify the device portion of the current default directory
    location through the specification of the SYS$DISK logical name.
    Redefine this logical name using either the $CRELNM edtsystem
    service or the LIB$SET_LOGICAL RTL routine.

    Format

      SYS$SETDDIR  [new-dir-addr] ,[length-addr] ,[cur-dir-addr]

    C Prototype

      int sys$setddir  (void *newdiraddr, unsigned short int

                       *lengthaddr, void *curdiraddr);