Allownull indicates that an actual parameter may also be 0. No matter what type the formal parameter is.
dll "some.dll" "DoAllowNull" function DoAllowNull(nParam1 : out number allownull, nParam2 : in number); dcltrans transaction TMain begin // although a variable should be set as first parameter, // we can also // specify 0, as the formal parameter // is declared allownull DoAllowNull(0, 12); end TMain;