diff -r 242907849850 -r 76175defb87b stage4/generate_c/generate_c.cc
--- a/stage4/generate_c/generate_c.cc	Sat May 07 21:17:49 2016 +0100
+++ b/stage4/generate_c/generate_c.cc	Sun May 08 20:01:15 2016 +0100
@@ -893,19 +893,28 @@
       }
       s4o.print(";\n\n");
       
-      s4o.print(s4o.indent_spaces + "// Control execution\n");
-      s4o.print(s4o.indent_spaces + "if (!EN) {\n");
-      s4o.indent_right();
-      s4o.print(s4o.indent_spaces + "if (__ENO != NULL) {\n");
-      s4o.indent_right();
-      s4o.print(s4o.indent_spaces + "*__ENO = __BOOL_LITERAL(FALSE);\n");
-      s4o.indent_left();
-      s4o.print(s4o.indent_spaces + "}\n");
-      s4o.print(s4o.indent_spaces + "return ");
-      symbol->derived_function_name->accept(print_base);
-      s4o.print(";\n");
-      s4o.indent_left();
-      s4o.print(s4o.indent_spaces + "}\n");
+      
+      // Only generate the code that controls the execution of the function's body if the
+      // function contains a declaration of both the EN and ENO variables
+      search_var_instance_decl_c search_var(symbol);
+      identifier_c  en_var("EN");
+      identifier_c eno_var("ENO");
+      if (   (search_var.get_vartype(& en_var) == search_var_instance_decl_c::input_vt)
+          && (search_var.get_vartype(&eno_var) == search_var_instance_decl_c::output_vt)) {
+        s4o.print(s4o.indent_spaces + "// Control execution\n");
+        s4o.print(s4o.indent_spaces + "if (!EN) {\n");
+        s4o.indent_right();
+        s4o.print(s4o.indent_spaces + "if (__ENO != NULL) {\n");
+        s4o.indent_right();
+        s4o.print(s4o.indent_spaces + "*__ENO = __BOOL_LITERAL(FALSE);\n");
+        s4o.indent_left();
+        s4o.print(s4o.indent_spaces + "}\n");
+        s4o.print(s4o.indent_spaces + "return ");
+        symbol->derived_function_name->accept(print_base);
+        s4o.print(";\n");
+        s4o.indent_left();
+        s4o.print(s4o.indent_spaces + "}\n");
+      }
     
       /* (C) Function body */
       generate_c_SFC_IL_ST_c generate_c_code(&s4o, symbol->derived_function_name, symbol);
@@ -1071,31 +1080,40 @@
       } else {
         s4o.print(" {\n");
         s4o.indent_right();
-      
-        s4o.print(s4o.indent_spaces + "// Control execution\n");
-        s4o.print(s4o.indent_spaces + "if (!");
-        s4o.print(GET_VAR);
-        s4o.print("(");
-        s4o.print(FB_FUNCTION_PARAM);
-        s4o.print("->EN)) {\n");
-        s4o.indent_right();
-        s4o.print(s4o.indent_spaces);
-        s4o.print(SET_VAR);
-        s4o.print("(");
-        s4o.print(FB_FUNCTION_PARAM);
-        s4o.print("->,ENO,,__BOOL_LITERAL(FALSE));\n");
-        s4o.print(s4o.indent_spaces + "return;\n");
-        s4o.indent_left();
-        s4o.print(s4o.indent_spaces + "}\n");
-        s4o.print(s4o.indent_spaces + "else {\n");
-        s4o.indent_right();
-        s4o.print(s4o.indent_spaces);
-        s4o.print(SET_VAR);
-        s4o.print("(");
-        s4o.print(FB_FUNCTION_PARAM);
-        s4o.print("->,ENO,,__BOOL_LITERAL(TRUE));\n");
-        s4o.indent_left();
-        s4o.print(s4o.indent_spaces + "}\n");
+
+        // Only generate the code that controls the execution of the function's body if the
+        // function contains a declaration of both the EN and ENO variables
+        search_var_instance_decl_c search_var(symbol);
+        identifier_c  en_var("EN");
+        identifier_c eno_var("ENO");
+        if (   (search_var.get_vartype(& en_var) == search_var_instance_decl_c::input_vt)
+            && (search_var.get_vartype(&eno_var) == search_var_instance_decl_c::output_vt)) {
+
+          s4o.print(s4o.indent_spaces + "// Control execution\n");
+          s4o.print(s4o.indent_spaces + "if (!");
+          s4o.print(GET_VAR);
+          s4o.print("(");
+          s4o.print(FB_FUNCTION_PARAM);
+          s4o.print("->EN)) {\n");
+          s4o.indent_right();
+          s4o.print(s4o.indent_spaces);
+          s4o.print(SET_VAR);
+          s4o.print("(");
+          s4o.print(FB_FUNCTION_PARAM);
+          s4o.print("->,ENO,,__BOOL_LITERAL(FALSE));\n");
+          s4o.print(s4o.indent_spaces + "return;\n");
+          s4o.indent_left();
+          s4o.print(s4o.indent_spaces + "}\n");
+          s4o.print(s4o.indent_spaces + "else {\n");
+          s4o.indent_right();
+          s4o.print(s4o.indent_spaces);
+          s4o.print(SET_VAR);
+          s4o.print("(");
+          s4o.print(FB_FUNCTION_PARAM);
+          s4o.print("->,ENO,,__BOOL_LITERAL(TRUE));\n");
+          s4o.indent_left();
+          s4o.print(s4o.indent_spaces + "}\n");
+        }
       
         /* (C.4) Initialize TEMP variables */
         /* function body */
@@ -1380,6 +1398,15 @@
   s4o.print("/*     FILE GENERATED BY iec2c             */\n");
   s4o.print("/* Editing this file is not recommended... */\n");
   s4o.print("/*******************************************/\n\n");
+  
+  if (runtime_options.disable_implicit_en_eno) {
+    // If we are not generating the EN and ENO parameters for functions and FB,
+    //   then make sure we use the standard library version compiled without these parameters too!
+    s4o.print("#ifndef DISABLE_EN_ENO_PARAMETERS\n");
+    s4o.print("#define DISABLE_EN_ENO_PARAMETERS\n");
+    s4o.print("#endif\n");
+  }
+  
   s4o.print("#include \"iec_std_lib.h\"\n\n");
   s4o.print("#include \"accessor.h\"\n\n"); 
   s4o.print("#include \"POUS.h\"\n\n");
@@ -1685,6 +1712,15 @@
       s4o.print("/*     FILE GENERATED BY iec2c             */\n");
       s4o.print("/* Editing this file is not recommended... */\n");
       s4o.print("/*******************************************/\n\n");
+  
+      if (runtime_options.disable_implicit_en_eno) {
+        // If we are not generating the EN and ENO parameters for functions and FB,
+        //   then make sure we use the standard library version compiled without these parameters too!
+        s4o.print("#ifndef DISABLE_EN_ENO_PARAMETERS\n");
+        s4o.print("#define DISABLE_EN_ENO_PARAMETERS\n");
+        s4o.print("#endif\n");
+      }
+      
       s4o.print("#include \"iec_std_lib.h\"\n\n");
       
       /* (A) resource declaration... */
@@ -2118,7 +2154,17 @@
 /* B 0 - Programming Model */
 /***************************/
     void *visit(library_c *symbol) {
-      pous_incl_s4o.print("#ifndef __POUS_H\n#define __POUS_H\n\n#include \"accessor.h\"\n#include \"iec_std_lib.h\"\n\n");
+      pous_incl_s4o.print("#ifndef __POUS_H\n#define __POUS_H\n\n");
+      
+      if (runtime_options.disable_implicit_en_eno) {
+        // If we are not generating the EN and ENO parameters for functions and FB,
+        //   then make sure we use the standard library version compiled without these parameters too!
+        pous_incl_s4o.print("#ifndef DISABLE_EN_ENO_PARAMETERS\n");
+        pous_incl_s4o.print("#define DISABLE_EN_ENO_PARAMETERS\n");
+        pous_incl_s4o.print("#endif\n");
+      }
+      
+      pous_incl_s4o.print("#include \"accessor.h\"\n#include \"iec_std_lib.h\"\n\n");
 
       for(int i = 0; i < symbol->n; i++) {
         symbol->elements[i]->accept(*this);