VMS Help DCX Routines, DCX$COMPRESS_DATA, Description  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    The DCX$COMPRESS_DATA routine compresses a data record. Call this
    routine for each data record to be compressed. As you compress
    each record, write the compressed record to the file containing
    the compression/expansion map. For each record, write the length
    of the record and substring string containing the record to the
    same file.