diff --git a/tcc.h b/tcc.h
index 98f28db..5ed3e21 100644
--- a/tcc.h
+++ b/tcc.h
@@ -389,9 +389,10 @@ typedef struct AttributeDef {
       func_export   : 1,
       func_import   : 1,
       func_args     : 5,
+      func_proto    : 1,
       mode          : 4,
       weak          : 1,
-      fill          : 11;
+      fill          : 10;
     struct Section *section;
     int alias_target;    /* token */
 } AttributeDef;
@@ -401,6 +402,7 @@ typedef struct AttributeDef {
 #define FUNC_EXPORT(r) (((AttributeDef*)&(r))->func_export)
 #define FUNC_IMPORT(r) (((AttributeDef*)&(r))->func_import)
 #define FUNC_ARGS(r) (((AttributeDef*)&(r))->func_args)
+#define FUNC_PROTO(r) (((AttributeDef*)&(r))->func_proto)
 #define FUNC_ALIGN(r) (((AttributeDef*)&(r))->aligned)
 #define FUNC_PACKED(r) (((AttributeDef*)&(r))->packed)
 #define ATTR_MODE(r)  (((AttributeDef*)&(r))->mode)
diff --git a/tccgen.c b/tccgen.c
index 4849b6c..a937358 100644
--- a/tccgen.c
+++ b/tccgen.c
@@ -5818,6 +5818,10 @@ static int decl0(int l, int is_for_loop_init)
                         goto func_error1;
 
                     r = sym->type.ref->r;
+
+                    if (!FUNC_PROTO(r))
+                        tcc_error("redefinition of '%s'", get_tok_str(v, NULL));
+
                     /* use func_call from prototype if not defined */
                     if (FUNC_CALL(r) != FUNC_CDECL
                      && FUNC_CALL(type.ref->r) == FUNC_CDECL)
@@ -5836,6 +5840,7 @@ static int decl0(int l, int is_for_loop_init)
                         tcc_error("incompatible types for redefinition of '%s'", 
                               get_tok_str(v, NULL));
                     }
+                    FUNC_PROTO(type.ref->r) = 0;
                     /* if symbol is already defined, then put complete type */
                     sym->type = type;
                 } else {
@@ -5901,6 +5906,7 @@ static int decl0(int l, int is_for_loop_init)
                     if ((type.t & VT_BTYPE) == VT_FUNC) {
                         /* external function definition */
                         /* specific case for func_call attribute */
+                        ad.func_proto = 1;
                         type.ref->r = INT_ATTR(&ad);
                     } else if (!(type.t & VT_ARRAY)) {
                         /* not lvalue if array */