diff --git a/libc/stdio/fileext.h b/libc/stdio/fileext.h
index 25b7bda..75230cd 100644
--- a/libc/stdio/fileext.h
+++ b/libc/stdio/fileext.h
@@ -33,6 +33,7 @@
 #define _FILEEXT_H_
 
 #include <pthread.h>
+#include <stdbool.h>
 
 __BEGIN_DECLS
 
@@ -40,9 +41,10 @@
  * file extension
  */
 struct __sfileext {
-	struct	__sbuf _ub; /* ungetc buffer */
+	struct	__sbuf _ub;		/* ungetc buffer */
 	struct wchar_io_data _wcio;	/* wide char io status */
-	pthread_mutex_t _lock; /* file lock */
+	pthread_mutex_t _lock;		/* file lock */
+	bool _stdio_handles_locking;	/* __fsetlocking support */
 };
 
 #define _EXT(fp) ((struct __sfileext *)((fp)->_ext._base))
@@ -54,7 +56,8 @@
 	_UB(fp)._base = NULL; \
 	_UB(fp)._size = 0; \
 	WCIO_INIT(fp); \
-        _FLOCK(fp).value = __PTHREAD_RECURSIVE_MUTEX_INIT_VALUE; \
+	_FLOCK(fp).value = __PTHREAD_RECURSIVE_MUTEX_INIT_VALUE; \
+	_EXT(fp)->_stdio_handles_locking = true; \
 } while (0)
 
 #define _FILEEXT_SETUP(f, fext) \
diff --git a/libc/stdio/local.h b/libc/stdio/local.h
index 46b11f1..7033eda 100644
--- a/libc/stdio/local.h
+++ b/libc/stdio/local.h
@@ -111,8 +111,8 @@
 	(fp)->_lb._base = NULL; \
 }
 
-#define FLOCKFILE(fp)   flockfile(fp)
-#define FUNLOCKFILE(fp) funlockfile(fp)
+#define FLOCKFILE(fp)   if (_EXT(fp)->_stdio_handles_locking) flockfile(fp)
+#define FUNLOCKFILE(fp) if (_EXT(fp)->_stdio_handles_locking) funlockfile(fp)
 
 #define FLOATING_POINT
 #define PRINTF_WIDE_CHAR
diff --git a/libc/stdio/stdio_ext.cpp b/libc/stdio/stdio_ext.cpp
index bfdecb8..fea44f6 100644
--- a/libc/stdio/stdio_ext.cpp
+++ b/libc/stdio/stdio_ext.cpp
@@ -27,13 +27,10 @@
  */
 
 #include <stdio_ext.h>
+#include <stdlib.h>
 
-#include <stdio.h>
 #include "local.h"
-
-#define FSETLOCKING_QUERY 0
-#define FSETLOCKING_INTERNAL 1
-#define FSETLOCKING_BYCALLER 2
+#include "private/libc_logging.h"
 
 size_t __fbufsize(FILE* fp) {
   return fp->_bf._size;
@@ -76,11 +73,19 @@
   fflush(NULL);
 }
 
-int __fsetlocking(FILE*, int) {
-  // We don't currently have an implementation that would obey this,
-  // so make setting the state a no-op and always return "we handle locking for you".
-  // http://b/17154740 suggests ways we could fix this.
-  return FSETLOCKING_INTERNAL;
+int __fsetlocking(FILE* fp, int type) {
+  int old_state = _EXT(fp)->_stdio_handles_locking ? FSETLOCKING_INTERNAL : FSETLOCKING_BYCALLER;
+  if (type == FSETLOCKING_QUERY) {
+    return old_state;
+  }
+
+  if (type != FSETLOCKING_INTERNAL && type != FSETLOCKING_BYCALLER) {
+    // The API doesn't let us report an error, so blow up.
+    __libc_fatal("Bad type (%d) passed to __fsetlocking", type);
+  }
+
+  _EXT(fp)->_stdio_handles_locking = (type == FSETLOCKING_INTERNAL);
+  return old_state;
 }
 
 void clearerr_unlocked(FILE* fp) {