Index: lib/profile/WindowsMMap.c
===================================================================
--- lib/profile/WindowsMMap.c
+++ lib/profile/WindowsMMap.c
@@ -120,9 +120,33 @@
 }
 
 COMPILER_RT_VISIBILITY
-int flock(int fd, int operation)
-{
-  return -1; /* Not supported. */
+int flock(int fd, int operation) {
+  HANDLE handle = (HANDLE)_get_osfhandle(fd);
+  if (handle == INVALID_HANDLE_VALUE)
+    return -1;
+
+  switch (operation) {
+  case LOCK_EX: {
+    OVERLAPPED overlapped = { 0 };
+    if (!LockFileEx(handle, LOCKFILE_EXCLUSIVE_LOCK, 0, MAXDWORD, MAXDWORD,
+                    &overlapped))
+      return -1;
+
+    break;
+  }
+
+  case LOCK_UN: {
+    if (!UnlockFile(handle, 0, 0, MAXDWORD, MAXDWORD))
+      return -1;
+
+    break;
+  }
+
+  default:
+    return -1; /* Not supported. */
+  }
+
+  return 0;
 }
 
 #undef DWORD_HI