6.10 File Checksum Subroutines

The following routines either compute or validate the checksums for the CHDU. The DATASUM keyword is used to store the numerical value of the 32-bit, 1's complement checksum for the data unit alone. If there is no data unit then the value is set to zero. The numerical value is stored as an ASCII string of digits, enclosed in quotes, because the value may be too large to represent as a 32-bit signed integer. The CHECKSUM keyword is used to store the ASCII encoded COMPLEMENT of the checksum for the entire HDU. Storing the complement, rather than the actual checksum, forces the checksum for the whole HDU to equal zero. If the file has been modified since the checksums were computed, then the HDU checksum will usually not equal zero. These checksum keyword conventions are based on a paper by Rob Seaman published in the proceedings of the ADASS IV conference in Baltimore in November 1994 and a later revision in June 1995.

1

Compute and write the DATASUM and CHECKSUM keyword values for the CHDU into the current header. The DATASUM value is the 32-bit checksum for the data unit, expressed as a decimal integer enclosed in single quotes. The CHECKSUM keyword value is a 16-character string which is the ASCII-encoded value for the complement of the checksum for the whole HDU. If these keywords already exist, their values will be updated only if necessary (i.e., if the file has been modified since the original keyword values were computed).

        FTPCKS(unit, > status)

2

Update the CHECKSUM keyword value in the CHDU, assuming that the DATASUM keyword exists and already has the correct value. This routine calculates the new checksum for the current header unit, adds it to the data unit checksum, encodes the value into an ASCII string, and writes the string to the CHECKSUM keyword.

        FTUCKS(unit, > status)

3

Verify the CHDU by computing the checksums and comparing them with the keywords. The data unit is verified correctly if the computed checksum equals the value of the DATASUM keyword. The checksum for the entire HDU (header plus data unit) is correct if it equals zero. The output DATAOK and HDUOK parameters in this subroutine are integers which will have a value = 1 if the data or HDU is verified correctly, a value = 0 if the DATASUM or CHECKSUM keyword is not present, or value = -1 if the computed checksum is not correct.

        FTVCKS(unit, > dataok,hduok,status)

4

Compute and return the checksum values for the CHDU (as double precision variables) without creating or modifying the CHECKSUM and DATASUM keywords. This routine is used internally by FTVCKS, but may be useful in other situations as well.

        FTGCKS(unit, > datasum,hdusum,status)

5

Encode a checksum value (stored in a double precision variable) into a 16-character string. If COMPLEMENT = .true. then the 32-bit sum value will be complemented before encoding.

        FTESUM(sum,complement, > checksum)

6

Decode a 16 character checksum string into a double precision value. If COMPLEMENT = .true. then the 32-bit sum value will be complemented after decoding.

        FTDSUM(checksum,complement, > sum)