diff --git a/include/unistd.h b/include/unistd.h
index 667ed0cb5..fb401be89 100644
--- a/include/unistd.h
+++ b/include/unistd.h
@@ -88,7 +88,11 @@ int __fastcall__ rmdir (const char* name);
 /* Others */
 unsigned __fastcall__ sleep (unsigned seconds);
 int __fastcall__ getopt (int argc, char* const* argv, const char* optstring);
+
+/* Non standard: */
+#if __CC65_STD__ == __CC65_STD_CC65__
 int __fastcall__ exec (const char* progname, const char* cmdline);
+#endif