Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Radius
PRIM_CYLINDER_RADIUSParameter: Height
PRIM_CYLINDER_HEIGHTParameter: Height Segments
PRIM_CYLINDER_HSUBParameter: Rotation Segments
PRIM_CYLINDER_SEGParameter: Switch to Updated Cylinder
PRIM_CYLINDER_SWITCH_TO_UPDATEDParameter: Caps
PRIM_CYLINDER_CAPSParameter: Cap Segments
PRIM_CYLINDER_CSUBParameter: Fillet
PRIM_CYLINDER_FILLETParameter: Fillet Segments
PRIM_CYLINDER_FSUBParameter: Fillet Radius
PRIM_CYLINDER_FILLETRADIUS