Be extra explicit with include guards.

I believe that this is only a problem for Arduino because Arduino brings
in header files that are not actually #included. This solution is a
little bit of a hack because it should not be necessary to add this
extra #ifndef, but it solves the problem and it should be mostly
harmless for users that do not target Arduino.

core/src/main/java/org/lflang/generator/c/CGenerator.java

@@ -948,7 +948,7 @@ private void generateReactorClass(TypeParameterizedReactor tpr) throws IOExcepti
     CodeBuilder header = new CodeBuilder();
     CodeBuilder src = new CodeBuilder();
     final String headerName = CUtil.getName(tpr) + ".h";
-    var guardMacro = headerName.toUpperCase().replace(".", "_");
+    var guardMacro = CUtil.internalIncludeGuard(tpr);
     header.pr("#ifndef " + guardMacro);
     header.pr("#define " + guardMacro);
     generateReactorClassHeaders(tpr, headerName, header, src);

core/src/main/java/org/lflang/generator/c/CReactorHeaderFileGenerator.java

@@ -55,7 +55,7 @@ private static String generateHeaderFile(
       GenerateAuxiliaryStructs generator,
       String topLevelPreamble) {
     CodeBuilder builder = new CodeBuilder();
-    appendIncludeGuard(builder, tpr);
+    appendIncludeGuards(builder, tpr);
     builder.pr(topLevelPreamble);
     appendPoundIncludes(builder);
     tpr.doDefines(builder);
@@ -64,14 +64,20 @@ private static String generateHeaderFile(
     for (Reaction reaction : tpr.reactor().getReactions()) {
       appendSignature(builder, types, reaction, tpr);
     }
-    builder.pr("#endif");
+    closeIncludeGuards(builder);
     return builder.getCode();
   }
 
-  private static void appendIncludeGuard(CodeBuilder builder, TypeParameterizedReactor r) {
+  private static void appendIncludeGuards(CodeBuilder builder, TypeParameterizedReactor r) {
     String macro = CUtil.getName(r) + "_H";
     builder.pr("#ifndef " + macro);
     builder.pr("#define " + macro);
+    builder.pr("#ifndef " + CUtil.internalIncludeGuard(r));
+  }
+
+  private static void closeIncludeGuards(CodeBuilder builder) {
+    builder.pr("#endif");
+    builder.pr("#endif");
   }
 
   private static void appendPoundIncludes(CodeBuilder builder) {

core/src/main/java/org/lflang/generator/c/CUtil.java

@@ -148,6 +148,14 @@ public static String getName(TypeParameterizedReactor reactor) {
     return name;
   }
 
+  /**
+   * Return the name used in the internal (non-user-facing) include guard for the given reactor.
+   */
+  public static String internalIncludeGuard(TypeParameterizedReactor tpr) {
+    final String headerName = CUtil.getName(tpr) + ".h";
+    return headerName.toUpperCase().replace(".", "_");
+  }
+
   /**
    * Return a reference to the specified port.
    *

test/C/src/arduino/DualReactorBlink.lf

@@ -0,0 +1,35 @@
+target C {
+  platform: {
+      name: "arduino",
+      board: "arduino:avr:mega"
+  }
+}
+
+reactor Blinker {
+  input in_port:bool
+
+  reaction(startup) {=
+      pinMode(LED_BUILTIN, OUTPUT);
+  =}
+
+  reaction(in_port) {=
+      digitalWrite(LED_BUILTIN, in_port->value ? HIGH : LOW);
+  =}
+}
+
+reactor Timer {
+  timer t1(0, 500 msec)
+  state on_off:bool = false
+  output out_port:bool
+
+  reaction (t1) -> out_port {=
+      self->on_off = !self->on_off;
+      lf_set(out_port, self->on_off);
+  =}
+}
+
+main reactor {
+  the_blinker = new Blinker()
+  the_timer = new Timer()
+  the_timer.out_port -> the_blinker.in_port;
+}