SAS variables that contain missing values are converted according to how the function that is being called has mapped missing values when using the PROTO procedure. All variables that are returned from the function are checked for the mapped missing values and converted to SAS missing values.

For example, if an argument to a function is missing, and the argument is to be converted to an integer, and an integer was mapped to –99, then –99 is passed to the function. If the same function returns an integer with the value –99, then the variable that this value is returned to would have a value of missing.