diff --git a/src/de/inetsoftware/jwebassembly/JWebAssembly.java b/src/de/inetsoftware/jwebassembly/JWebAssembly.java
index 6a19050..8d432b1 100644
--- a/src/de/inetsoftware/jwebassembly/JWebAssembly.java
+++ b/src/de/inetsoftware/jwebassembly/JWebAssembly.java
@@ -258,5 +258,6 @@ public class JWebAssembly {
             ClassFile classFile = new ClassFile( new BufferedInputStream( url.openStream() ) );
             generator.write( classFile );
         }
+        generator.finish();
     }
 }
diff --git a/src/de/inetsoftware/jwebassembly/module/FunctionManager.java b/src/de/inetsoftware/jwebassembly/module/FunctionManager.java
new file mode 100644
index 0000000..d45c0ee
--- /dev/null
+++ b/src/de/inetsoftware/jwebassembly/module/FunctionManager.java
@@ -0,0 +1,53 @@
+/*
+ * Copyright 2018 Volker Berlin (i-net software)
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package de.inetsoftware.jwebassembly.module;
+
+import java.util.HashSet;
+
+/**
+ * Manage the required function/methods
+ * 
+ * @author Volker Berlin
+ */
+public class FunctionManager {
+
+    private HashSet<String>       writtenFunctions = new HashSet<>();
+
+    private HashSet<FunctionName> toWriteLater     = new HashSet<>();
+
+    /**
+     * Mark the a function as written to the wasm file.
+     * 
+     * @param name
+     *            the function name
+     */
+    void writeFunction( FunctionName name ) {
+        toWriteLater.remove( name );
+        writtenFunctions.add( name.signatureName );
+    }
+
+    /**
+     * Mark a function as used/called.
+     * 
+     * @param name
+     *            the function name
+     */
+    void functionCall( FunctionName name ) {
+        if( !writtenFunctions.contains( name.signatureName ) ) {
+            toWriteLater.add( name );
+        }
+    }
+}
diff --git a/src/de/inetsoftware/jwebassembly/module/ModuleGenerator.java b/src/de/inetsoftware/jwebassembly/module/ModuleGenerator.java
index a0d32d8..7f959f0 100644
--- a/src/de/inetsoftware/jwebassembly/module/ModuleGenerator.java
+++ b/src/de/inetsoftware/jwebassembly/module/ModuleGenerator.java
@@ -53,6 +53,8 @@ public class ModuleGenerator {
 
     private String                          className;
 
+    private FunctionManager                 functions = new FunctionManager();
+
     /**
      * Create a new generator.
      * 
@@ -98,6 +100,23 @@ public class ModuleGenerator {
         iterateMethods( classFile, m -> writeMethod( m ) );
     }
 
+    /**
+     * Finish the code generation.
+     */
+    public void finish() {
+        
+    }
+
+    /**
+     * Iterate over all methods of the classFile and run the handler.
+     * 
+     * @param classFile
+     *            the classFile
+     * @param handler
+     *            the handler
+     * @throws WasmException
+     *             if some Java code can't converted
+     */
     private void iterateMethods( ClassFile classFile, Consumer<MethodInfo> handler ) throws WasmException {
         sourceFile = null; // clear previous value for the case an IO exception occur
         className = null;
@@ -131,6 +150,7 @@ public class ModuleGenerator {
             FunctionName name = new FunctionName( method );
             Map<String,Object> annotationValues = method.getAnnotation( JWebAssembly.IMPORT_ANNOTATION );
             if( annotationValues != null ) {
+                functions.writeFunction( name );
                 String impoarModule = (String)annotationValues.get( "module" );
                 String importName = (String)annotationValues.get( "name" );
                 writer.prepareImport( name, impoarModule, importName );
@@ -179,10 +199,13 @@ public class ModuleGenerator {
             FunctionName name = new FunctionName( method );
             writeExport( name, method );
             writer.writeMethodStart( name );
-
+            functions.writeFunction( name );
             writeMethodSignature( signature, code.getLocalVariableTable(), codeBuilder );
 
             for( WasmInstruction instruction : codeBuilder.getInstructions() ) {
+                if( instruction instanceof WasmCallInstruction ) {
+                    functions.functionCall( ((WasmCallInstruction)instruction).getFunctionName() );
+                }
                 instruction.writeTo( writer );
             }
             writer.writeMethodFinish();