diff --git a/libc/stdio/fileext.h b/libc/stdio/fileext.h
index 2d07043..b36a448 100644
--- a/libc/stdio/fileext.h
+++ b/libc/stdio/fileext.h
@@ -29,24 +29,41 @@
  * $Citrus$
  */
 
+#include <pthread.h>
+#include "wcio.h"
+
 /*
  * file extension
  */
 struct __sfileext {
 	struct	__sbuf _ub; /* ungetc buffer */
 	struct wchar_io_data _wcio;	/* wide char io status */
+	pthread_mutex_t _lock; /* file lock */
 };
 
+#define _FILEEXT_INITIALIZER  {{NULL,0},{0},PTHREAD_RECURSIVE_MUTEX_INITIALIZER}
+
 #define _EXT(fp) ((struct __sfileext *)((fp)->_ext._base))
 #define _UB(fp) _EXT(fp)->_ub
+#define _FLOCK(fp)  _EXT(fp)->_lock
 
 #define _FILEEXT_INIT(fp) \
 do { \
 	_UB(fp)._base = NULL; \
 	_UB(fp)._size = 0; \
 	WCIO_INIT(fp); \
+	_FLOCK_INIT(fp); \
 } while (0)
 
+/* Helper macros to avoid a function call when you know that fp is not NULL.
+ * Notice that we keep _FLOCK_INIT() fast by slightly breaking our pthread
+ * encapsulation.
+ */
+#define _FLOCK_INIT(fp)    _FLOCK(fp).value = __PTHREAD_RECURSIVE_MUTEX_INIT_VALUE
+#define _FLOCK_LOCK(fp)    pthread_mutex_lock(&_FLOCK(fp))
+#define _FLOCK_TRYLOCK(fp) pthread_mutex_trylock(&_FLOCK(fp))
+#define _FLOCK_UNLOCK(fp)  pthread_mutex_unlock(&_FLOCK(fp))
+
 #define _FILEEXT_SETUP(f, fext) \
 do { \
 	(f)->_ext._base = (unsigned char *)(fext); \