         let umbrellaHeader = path.appending(component: module.c99name + ".h")
         let invalidUmbrellaHeader = path.appending(component: module.name + ".h")
         if module.c99name != module.name && fileSystem.isFile(invalidUmbrellaHeader) {
-            print("warning: \(invalidUmbrellaHeader) should be renamed to \(umbrellaHeader) to be used as an umbrella header")
+            warningStream <<< "warning: \(invalidUmbrellaHeader) should be renamed to \(umbrellaHeader) to be used as an umbrella header"
+            warningStream.flush()
         }
     }
 

    /// The file system to be used.
    private var fileSystem: FileSystem

    public init(for module: ClangModule, fileSystem: FileSystem = localFileSystem) {
        self.module = module
        self.fileSystem = fileSystem
    }

    /// A link-declaration specifies a library or framework

        let includeDir = module.includeDir
        // Warn and return if no include directory.
        guard fileSystem.isDirectory(includeDir) else {
            warningStream <<< "warning: No include directory found for module '\(module.name)'. A library can not be imported without any public headers."
            warningStream.flush()
            return
        }
        
