diff --git a/luaconf.h b/luaconf.h
index afc1b8b5..bd394650 100644
--- a/luaconf.h
+++ b/luaconf.h
@@ -87,6 +87,11 @@
 #endif
 
 
+#if defined(LUA_USE_C89) && defined(LUA_USE_POSIX)
+#error "Posix is not compatible with C89"
+#endif
+
+
 /*
 @@ LUAI_IS32INT is true iff 'int' has (at least) 32 bits.
 */
diff --git a/makefile b/makefile
index b37fdb28..58de5ddb 100644
--- a/makefile
+++ b/makefile
@@ -69,7 +69,9 @@ CWARNS= $(CWARNSCPP) $(CWARNSC) $(CWARNGCC)
 LOCAL = $(TESTS) $(CWARNS)
 
 
-# enable Linux goodies
+# To enable Linux goodies, -DLUA_USE_LINUX
+# For C89, "-std=c89 -DLUA_USE_C89"
+# Note that Linux/Posix options are not compatible with C89
 MYCFLAGS= $(LOCAL) -std=c99 -DLUA_USE_LINUX
 MYLDFLAGS= $(LOCAL) -Wl,-E
 MYLIBS= -ldl