diff --git a/include/mbedtls/threading.h b/include/mbedtls/threading.h
index d400abe..309727f 100644
--- a/include/mbedtls/threading.h
+++ b/include/mbedtls/threading.h
@@ -55,7 +55,11 @@
 
 /**
  * \brief           Set your alternate threading implementation function
- *                  pointers
+ *                  pointers and initialize global mutexes. If used, this
+ *                  function must be called once in the main thread before any
+ *                  other mbed TLS function is called, and
+ *                  mbedtls_threading_free_alt() must be called once in the main
+ *                  thread after all other mbed TLS functions.
  *
  * \note            mutex_init() and mutex_free() don't return a status code.
  *                  If mutex_init() fails, it should leave its argument (the
@@ -66,13 +70,16 @@
  * \param mutex_free    the free function implementation
  * \param mutex_lock    the lock function implementation
  * \param mutex_unlock  the unlock function implementation
- *
- * \return              0 if successful
  */
-int mbedtls_threading_set_alt( void (*mutex_init)( mbedtls_threading_mutex_t * ),
+void mbedtls_threading_set_alt( void (*mutex_init)( mbedtls_threading_mutex_t * ),
                        void (*mutex_free)( mbedtls_threading_mutex_t * ),
                        int (*mutex_lock)( mbedtls_threading_mutex_t * ),
                        int (*mutex_unlock)( mbedtls_threading_mutex_t * ) );
+
+/**
+ * \brief               Free global mutexes.
+ */
+void mbedtls_threading_free_alt( void );
 #endif /* MBEDTLS_THREADING_ALT */
 
 /*
@@ -85,6 +92,11 @@
 extern int (*mbedtls_mutex_lock)( mbedtls_threading_mutex_t *mutex );
 extern int (*mbedtls_mutex_unlock)( mbedtls_threading_mutex_t *mutex );
 
+/*
+ * Global mutexes
+ */
+extern mbedtls_threading_mutex_t mbedtls_threading_readdir_mutex;
+
 #ifdef __cplusplus
 }
 #endif