Bad Data Values

Constants with names of the form VAL__BAD$<$T$>$ represent the Starlink-defined bad (or ``magic'') values to be used for flagging bad or undefined data. The use of these constants is discussed in SGP/38, which also defines the values to be used on VAX machines. These are:

Constant	Type	Value	Hexadecimal Pattern
VAL__BADUB	_UBYTE	255	FF
VAL__BADB	_BYTE	$-$128	80
VAL__BADUW	_UWORD	65535	FFFF
VAL__BADW	_WORD	$-$32768	8000
VAL__BADI	_INTEGER	$-$2147483648	80000000
VAL__BADR	_REAL	$-$1.7014117E$+$38	FFFFFFFF
VAL__BADD	_DOUBLE	$-$1.701411834604923D$+$38	FFFFFFFFFFFFFFFF

Note that the operations permitted on bad values are restricted to:

• Assigning the value of a ``bad constant'' to a variable;

• Testing a variable for equality with a ``bad constant'';

• Assigning a non-bad value to a variable to replace one which was previously bad.

Software which aims to be portable may assume that the bad values described by these constants will always lie at the extreme end of the number range which can be represented by each data type. However, no assumptions should be made about which end of the range (upper or lower) this will be -- an explicit test should be made if it is necessary to determine this.