It's possible with the following trick: Arguments SortLightningArgs[]={ {Title, "NLDN Sort Routine (US only)",NULL,INTEGER4, {RealX8: 0}}, {UserDefined,"Minimum propagation delay",".001",REAL8, {RealX8: 0}}, {UserDefined,"Maximum propagation delay","1.0",REAL8, {RealX8: 0}}, {UserDefined,"Minimum latitude","20.0", REAL8, {RealX8: 0}}, {UserDefined,"Maximum latitude","50.0", REAL8, {RealX8: 0}}, {UserDefined,"Minimum longitude","-50.0", REAL8, {RealX8: 0}}, {UserDefined,"Maximum longitude","-150.0", REAL8, {RealX8: 0}}, {UserDefined,"Input file containing NLDN event times","lightning.times.251_253", STRING,{RealX8: 0}}, {UserDefined,"Output file containing list of time differences and intensities", "HistoTimeDifs.txt", STRING,{RealX8: 0}}, {0,NULL,NULL,0,{Str: NULL}} };