When run from C, log to stderr.

src/main/java/uvm/refimpl/nat/CInitiater.java

 package uvm.refimpl.nat;
 
+import java.io.ByteArrayOutputStream;
+
+import org.slf4j.LoggerFactory;
+
+import ch.qos.logback.classic.Logger;
+import ch.qos.logback.classic.LoggerContext;
+import ch.qos.logback.classic.encoder.PatternLayoutEncoder;
+import ch.qos.logback.classic.spi.ILoggingEvent;
+import ch.qos.logback.core.ConsoleAppender;
 import uvm.refimpl.MicroVM;
 import uvm.refimpl.MicroVM$;
 
 /** Static functions for the convenient of C programs that start Mu via JNI. */
 public class CInitiater {
+    
+    private static boolean isLogConfigured = false;
+    
+    private static void configureLog() {
+        if (isLogConfigured) {
+            return;
+        }
+        isLogConfigured = true;
+        
+        LoggerContext lc = (LoggerContext) LoggerFactory.getILoggerFactory();
+        
+        lc.reset();
+
+        ConsoleAppender<ILoggingEvent> ca = new ConsoleAppender<ILoggingEvent>();
+        ca.setContext(lc);
+        ca.setName("console");
+        ca.setTarget("System.err");
+        PatternLayoutEncoder pl = new PatternLayoutEncoder();
+        pl.setContext(lc);
+        pl.setPattern("%d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n");
+        pl.start();
+
+        ca.setEncoder(pl);
+        ca.start();
+        Logger rootLogger = lc.getLogger(Logger.ROOT_LOGGER_NAME);
+        rootLogger.addAppender(ca);
+    }
+    
     /** Called by the native program, this function creates a Mu instance. */
     static long mu_refimpl2_new() {
+        configureLog();
         MicroVM mvm = new MicroVM(MicroVM$.MODULE$.DEFAULT_HEAP_SIZE(), MicroVM$.MODULE$.DEFAULT_GLOBAL_SIZE(),
                 MicroVM$.MODULE$.DEFAULT_STACK_SIZE());
         long fak = NativeClientSupport$.MODULE$.exposeMicroVM(mvm);
@@ -18,6 +56,7 @@ public class CInitiater {
      * extra arguments.
      */
     static long mu_refimpl2_new_ex(long heap_size, long global_size, long stack_size) {
+        configureLog();
         MicroVM mvm = new MicroVM(heap_size, global_size, stack_size);
         long fak = NativeClientSupport$.MODULE$.exposeMicroVM(mvm);
         return fak;