Sometimes it is necessary to pass a parameter to a procedure even though the data type is not precisely known to the called procedure, (for example, different types of strings). In these instances you can use operational descriptors to provide descriptive information to the called procedure regarding the form of the parameter. The additional information allows the procedure to properly interpret the string. You should only use operational descriptors when they are expected by the called procedure.
You can request operational descriptors for both prototyped and non-prototyped parameters. For prototyped calls, you specify the keyword OPDESC on the prototype definition. For non-prototyped parameters, you specify (D) as the operation code extender of the CALLB operation. In either case, operational descriptors are then built by the calling procedure and passed as hidden parameters to the called procedure.
When you have specified the OPDESC keyword for your own procedure, you can call APIs to find out information about the length and type of some of the parameters. These APIs require you to pass a parameter number to identify which parameter you are interested in. Usually, the number of a parameter can be obtained by simply counting the parameters in the prototype or procedure interface. However, when the RTNPARM keyword is specified, the number of each parameter is one higher than its apparent number. Use the %PARMNUM built-in function to get the number of a particular parameter instead of using a numeric literal. For more information, see OPDESC, RTNPARM and %PARMNUM (Return Parameter Number).