The user-supplied interactive input routine, which has the name INSUB in the above example call to HLP_HELP but which can have any name, is an integer function subprogram with arguments STRING, PROMPT, L. The argument STRING receives the line input, PROMPT is the string to output prior to reading the line, and L the number of characters input. If the call is successful, a function value of +1 is returned.
The HLP package contains a simple example of an input routine. It is called HLP_INSUB and is the one used in the TSTHLP demonstration program.