The file header characteristic XAB (XABFHC) contains file header
    information that is output by the Open service and the Display
    service. The Create service can output information in this XAB
    when it opens an existing file through use of the Create-if
    option.

    The only input field is the longest record length (XAB$W_LRL)
    field. The Create service uses this field when it creates a
    sequential file that does not use a fixed-length record format.

    Note that, for unshared sequential files or sequential files
    shared using the FAB$V_UPI option, the values in the end-of-
    file block (XAB$L_EBK), first free byte in the end-of-file
    block (XAB$W_FFB), and longest record length (XAB$W_LRL) fields
    correspond to the values at the time of the last Close or Flush
    service.

    The following table lists the symbolic offset, the size in bytes,
    and a brief description of each field.

    Offset        Size    Description

    XAB$B_ATR     1       Record attributes; equivalent to FAB$B_RAT
    XAB$B_BKZ     1       Bucket size; equivalent to FAB$B_BKS
    XAB$B_BLN     1       Block length
    XAB$B_COD     1       Type code
    XAB$W_DXQ     2       Default file extension quantity; equivalent
                          to FAB$W_DEQ
    XAB$L_EBK     4       End-of-file block
    XAB$W_FFB     2       First free byte in the end-of-file block
    XAB$W_GBC     2       Default global buffer count
    XAB$L_HBK     4       Highest virtual block in the file;
                          equivalent to FAB$L_ALQ
    XAB$B_HSZ     1       Fixed-length control header size;
                          equivalent to FAB$B_FSZ
    XAB$W_LRL     2       Longest record length
    XAB$W_MRZ     2       Maximum record size; equivalent to FAB$W_
                          MRS
    XAB$L_NXT     4       Next XAB address
    XAB$B_RFO     1       File organization and record format;
                          combines FAB$B_RFM and FAB$B_ORG
    XAB$L_SBN     4       Starting logical block number for the file
                          if it is contiguous; otherwise this field
                          is 0
    XAB$W_        2       Version limit for the file
    VERLIMIT