Query the value of a double-valued model attribute.
A non-zero return value indicates that a problem occurred while querying the attribute. Refer to the Error Code table for a list of possible return values. Details on the error can be obtained by calling GRBgeterrormsg.
attrname: The name of a double-valued model attribute. Available attributes are listed and described in the Attributes section of this document.
valueP: The location in which the current value of the requested attribute should be placed.
Note that this method should be used for scalar attributes only (i.e., model attributes). To query a single element of an array attribute, use GRBgetdblattrelement instead.
error = GRBgetdblattr(model, "ObjCon", &objcon);