FCHECKSUM (May95)             ftools.futils            FCHECKSUM (May95)



NAME
    fchecksum -- Verify or update the checksum keywords in a FITS file.
    
    
USAGE
    fchecksum filename
    
    
DESCRIPTION
    This  task  verifies or updates the CHECKSUM and DATASUM keywords in
    each HDU of the input FITS file.  These keywords are used  to  embed
    a  32-bit  1's  complement  checksum  into each HDU of the FITS file
    which can be  used  to  verify  the  integrity  of  the  file.   The
    CHECKSUM  keyword  gives  an  ASCII  encoded  representation  of the
    complement of the checksum for the entire  HDU.   A  valid  CHECKSUM
    keyword  value causes the checksum for the whole HDU to have a value
    of zero.  The DATASUM keyword  gives  the  32-bit  unsigned  integer
    value  for  the  checksum of the data unit alone, stored as an ASCII
    string since the value may be too large to  represent  as  a  32-bit
    signed  integer.   Further  details  of this checksum algorithm have
    been published by Rob Seaman in  the  Astronomical  Society  of  the
    Pacific  Conference  Series,  Volume 77, "Astronomical Data Analysis
    Software and Systems IV" and in a revised draft dated June 1995.
    
    This task may be used to either verify the checksums in a file or to
    update  the  checksum keyword values if they do not exist or are out
    of date.  If the 'update' parameter has the default  value  of  'no'
    then  the  input  file  is not modified and the checksums are merely
    verified; a warning message is issued if  the  checksum  values  are
    not  correct,  thus  indicating  that  the file has been modified or
    corrupted since the checksum keywords were originally written.
    
    If an existing FITS file does not contain the checksum  keywords  or
    if the keyword values are out of date, then this task may be used to
    compute the new checksum keyword  values  by  specifying  'update  =
    yes'.   If  the  FITS  file  already has the correct DATASUM keyword
    value in each HDU and only the header(s)  have  been  modified  then
    the  CHECKSUM keywords can be quickly updated by specifying 'datasum
    = no'.  Specifying 'yes' for this parameter will force  the  DATASUM
    keyword  to  be  recomputed (which may take a while for large files)
    and updated if necessary before  recomputing  the  CHECKSUM  keyword
    value in each HDU.
    
    Note  that  the  fverify task also checks the values of the checksum
    keywords if they are present.
    
    
PARAMETERS
    
    infile [file name]
        Name of the FITS file to be checksummed.  Can use  @filename  to
        specify a text file containing a list of files to process.
    
    (update = no) [boolean]
        Should  the  checksum  keywords be updated if they are no longer
        correct? By default the checksum keywords  are  simply  verified
        and a warning message is issued if they are not correct.
    
    (datasum = yes) [boolean]
        Should  the  DATASUM  keyword  be  verified  and  updated before
        recomputing the  CHECKSUM  keyword  value?   If  not,  then  the
        DATASUM  keyword  will  be assumed to be correct and will simply
        be added to the checksum for the header to  generate  the  total
        checksum for the HDU.
    
    (ok ) [boolean]
        Returns  the  status of checksum verification. This parameter is
        set  to  NO  if  there  were  problems  found  while   verifying 
        checksums. For a succesful checksum test, it returns YES.
    
    
EXAMPLES
    1.  Verify the checksum values, without updating the input file:
    
            ft> fchecksum test.fits
    
    2.   Update  both  the  CHECKSUM  and  DATASUM  keywords in the file
    test.fits:
    
            ft> fchecksum test.fits update+ datasum+
    
    
SEE ALSO
    ftchecksum, ftverify