--- liolib.c
+++ liolib.c
@@ -4,6 +4,12 @@
 ** See Copyright Notice in lua.h
 */
 
+/***
+ * Input and output facilities.
+ *
+ * @module io
+ */
+
 #define liolib_c
 #define LUA_LIB
 
@@ -16,6 +22,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <unistd.h>
 
 #include "lua.h"
 
@@ -732,6 +739,23 @@
   return luaL_fileresult(L, res == 0, NULL);
 }
 
+/***
+ * Returns whether the stream points to a valid TTY device.
+ *
+ * @function file:istty
+ * @usage
+local f = io.open("/etc/fstab", 'r');
+-- a regular file will never be a TTY device
+assert(not f:istty())
+-- stdin, stdout and stderr can often be TTYs
+print(io.stdout:istty())
+ */
+static int f_istty (lua_State *L) {
+  int fd = fileno(tofile(L));
+  lua_pushboolean(L, isatty(fd));
+  return 1;
+}
+
 
 
 static int io_flush (lua_State *L) {
@@ -778,6 +802,7 @@
   {"seek", f_seek},
   {"close", f_close},
   {"setvbuf", f_setvbuf},
+  {"istty", f_istty},
   {NULL, NULL}
 };