Combine sugar, salt, cornstarch and flour. Stir in milk. Cook over moderate heat, stirring constantly, until mixture thickens and boils. Boil 1 minute and remove from heat. Slowly stir in egg yolks beaten slightly. Boil 1 minute more, stirring constantly.
Remove from heat and blend in butter, vanilla and coconut. Pour into pie shell. Beat egg whites with cream of tartar until stiff. Spread over pie.