diff --git a/libc/bionic/c16rtomb.cpp b/libc/bionic/c16rtomb.cpp
new file mode 100644
index 0000000..77512be
--- /dev/null
+++ b/libc/bionic/c16rtomb.cpp
@@ -0,0 +1,67 @@
+/*
+ * Copyright (C) 2014 The Android Open Source Project
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *  * Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ *  * Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in
+ *    the documentation and/or other materials provided with the
+ *    distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+ * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+ * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
+ * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
+ * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+ * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#include <errno.h>
+#include <uchar.h>
+#include <wchar.h>
+
+#include "private/bionic_mbstate.h"
+
+static inline constexpr bool is_high_surrogate(char16_t c16) {
+  return c16 >= 0xd800 && c16 < 0xdc00;
+}
+
+static inline constexpr bool is_low_surrogate(char16_t c16) {
+  return c16 >= 0xdc00 && c16 < 0xe000;
+}
+
+size_t c16rtomb(char* s, char16_t c16, mbstate_t* ps) {
+  static mbstate_t __private_state;
+  mbstate_t* state = (ps == NULL) ? &__private_state : ps;
+  if (mbsinit(state)) {
+    if (is_high_surrogate(c16)) {
+      char32_t c32 = (c16 & ~0xd800) << 10;
+      mbstate_set_byte(state, 3, (c32 & 0xff0000) >> 16);
+      mbstate_set_byte(state, 2, (c32 & 0x00ff00) >> 8);
+      return 0;
+    } else if (is_low_surrogate(c16)) {
+      return reset_and_return_illegal(EINVAL, state);
+    } else {
+      return c32rtomb(s, static_cast<char32_t>(c16), state);
+    }
+  } else {
+    if (!is_low_surrogate(c16)) {
+      return reset_and_return_illegal(EINVAL, state);
+    }
+
+    char32_t c32 = ((mbstate_get_byte(state, 3) << 16) |
+                    (mbstate_get_byte(state, 2) << 8) |
+                    (c16 & ~0xdc00)) + 0x10000;
+    return reset_and_return(c32rtomb(s, c32, NULL), state);
+  }
+}