Setting a NULL Indicator (Embedded SQL Guide)
From InterBase
If a parameter or select-list item can contain a NULL value, the sqlind field is used to indicate its NULL status. Appropriate storage space must be allocated for sqlind before values can be stored there.
On insertion, set sqlind to –1 to indicate that NULL values are legal. Otherwise set sqlind to 0.
On selection, a sqlind of –1 indicates a field contains a NULL value. Other values indicate a field contains non-NULL data.