Index: libc/src/CMakeLists.txt
===================================================================
--- libc/src/CMakeLists.txt
+++ libc/src/CMakeLists.txt
@@ -4,6 +4,7 @@
 add_subdirectory(signal)
 add_subdirectory(stdlib)
 add_subdirectory(string)
+add_subdirectory(uchar)
 # TODO: Add this target conditional to the target OS.
 add_subdirectory(sys)
 add_subdirectory(threads)
Index: libc/src/uchar/CMakeLists.txt
===================================================================
--- /dev/null
+++ libc/src/uchar/CMakeLists.txt
@@ -0,0 +1,35 @@
+add_entrypoint_object(
+  mbrtoc16
+  SRCS
+    mbrtoc16.cpp
+  HDRS
+    mbrtoc16.h
+  #DEPENDS
+)
+
+add_entrypoint_object(
+  c16rtomb
+  SRCS
+    c16rtomb.cpp
+  HDRS
+    c16rtomb.h
+  #DEPENDS
+)
+
+add_entrypoint_object(
+  mbrtoc32
+  SRCS
+    mbrtoc32.cpp
+  HDRS
+    mbrtoc32.h
+  #DEPENDS
+)
+
+add_entrypoint_object(
+  c32rtomb
+  SRCS
+    c32rtomb.cpp
+  HDRS
+    c32rtomb.h
+  #DEPENDS
+)
Index: libc/src/uchar/c16rtomb.h
===================================================================
--- /dev/null
+++ libc/src/uchar/c16rtomb.h
@@ -0,0 +1,20 @@
+//===--------------- Implementation header for c16rtomb -----------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===--------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_UCHAR_C16RTOMB_H
+#define LLVM_LIBC_SRC_UCHAR_C16RTOMB_H
+
+#include "include/uchar.h"
+
+namespace __llvm_libc {
+
+size_t c16rtomb(char *restrict s, char16_t c16, mbstate_t *restrict ps);
+
+} // namespace __llvm_libc
+
+#endif // LLVM_LIBC_SRC_UCHAR_C16RTOMB_H
Index: libc/src/uchar/c16rtomb.cpp
===================================================================
--- /dev/null
+++ libc/src/uchar/c16rtomb.cpp
@@ -0,0 +1,39 @@
+//===-------------------- Implementation of c16rtomb
+//-----------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/uchar/c16rtomb.h"
+
+#include "src/__support/common.h"
+
+namespace __llvm_libc {
+
+size_t LLVM_LIBC_ENTRYPOINT(c16rtomb)(char *restrict s, char16_t c16,
+                                      mbstate_t *restrict ps) {
+  size_t StringSize = 0;
+  if (c16 >= 0xD800 && c16 <= 0xDFFF) {
+    c16 = 0xFFFD; // Invalid Replacement Character
+  }
+
+  if (c16 <= 0x7F) {
+    StringSize = 1;
+    s[0] = c16 & 0x7F;
+  } else if (c16 <= 0x7FF) {
+    StringSize = 2;
+    s[0] = 0xC0 | (c16 & ((0x1F << 6) >> 6));
+    s[1] = 0x80 | (c16 & 0x3F);
+  } else if (c16 <= 0xFFFF) {
+    StringSize = 3;
+    s[0] = 0xE0 | (c16 & ((0x0F << 12) >> 12));
+    s[1] = 0x80 | (c16 & ((0x3F << 6) >> 6));
+    s[2] = 0x80 | (c16 & 0x3F);
+  }
+  return StringSize;
+}
+
+} // namespace __llvm_libc
Index: libc/src/uchar/c32rtomb.h
===================================================================
--- /dev/null
+++ libc/src/uchar/c32rtomb.h
@@ -0,0 +1,20 @@
+//===--------------- Implementation header for c32rtomb -----------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===--------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_UCHAR_C32RTOMB_H
+#define LLVM_LIBC_SRC_UCHAR_C32RTOMB_H
+
+#include "include/uchar.h"
+
+namespace __llvm_libc {
+
+size_t c16rtomb(char *restrict s, char16_t c16, mbstate_t *restrict ps);
+
+} // namespace __llvm_libc
+
+#endif // LLVM_LIBC_SRC_UCHAR_C16RTOMB_H
Index: libc/src/uchar/c32rtomb.cpp
===================================================================
--- /dev/null
+++ libc/src/uchar/c32rtomb.cpp
@@ -0,0 +1,41 @@
+//===-------------------- Implementation of c32rtomb
+//-----------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/uchar/c32rtomb.h"
+
+#include "src/__support/common.h"
+
+namespace __llvm_libc {
+
+size_t LLVM_LIBC_ENTRYPOINT(c32rtomb)(char *restrict s, char32_t c32,
+                                      mbstate_t *restrict ps) {
+  size_t StringSize = 0;
+  if (c32 <= 0x7F) {
+    StringSize = 1;
+    s[0] = c32 & 0x7F;
+  } else if (c32 <= 0x7FF) {
+    StringSize = 2;
+    s[0] = 0xC0 | (c32 & ((0x1F << 6) >> 6));
+    s[1] = 0x80 | (c32 & 0x3F);
+  } else if (c32 <= 0xFFFF) {
+    StringSize = 3;
+    s[0] = 0xE0 | (c32 & ((0x0F << 12) >> 12));
+    s[1] = 0x80 | (c32 & ((0x3F << 6) >> 6));
+    s[2] = 0x80 | (c32 & 0x3F);
+  } else if (c32 <= 0x10FFFF) {
+    StringSize = 4;
+    s[0] = 0xF0 | (c32 & 0x1C0000) >> 18;
+    s[1] = 0x80 | (c32 & 0x3F000) >> 12;
+    s[2] = 0x80 | (c32 & 0xFC0) >> 6;
+    s[3] = 0x80 | (c32 & 0x3F);
+  }
+  return StringSize;
+}
+
+} // namespace __llvm_libc
Index: libc/src/uchar/mbrtoc16.h
===================================================================
--- /dev/null
+++ libc/src/uchar/mbrtoc16.h
@@ -0,0 +1,20 @@
+//===---------------- Implementation header for mbrtoc16 -----------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===---------------------------------------------------------------------===//
+#ifndef LLVM_LIBC_SRC_UCHAR_MBRTOC16_H
+#define LLVM_LIBC_SRC_UCHAR_MBRTOC16_H
+
+#include "include/uchar.h"
+
+namespace __llvm_libc {
+
+size_t mbrtoc16(char16_t *restrict pc16, const char *restrict s, size_t n,
+                mbstate_t *restrict ps);
+
+} // namespace __llvm_libc
+
+#endif // LLVM_LIBC_SRC_UCHAR_MBRTOC16_H
Index: libc/src/uchar/mbrtoc16.cpp
===================================================================
--- /dev/null
+++ libc/src/uchar/mbrtoc16.cpp
@@ -0,0 +1,54 @@
+//===-------------------- Implementation of mbrtoc16
+//-----------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/uchar/mbrtoc16.h"
+
+#include "src/__support/common.h"
+
+namespace __llvm_libc {
+
+size_t LLVM_LIBC_ENTRYPOINT(mbrtoc16)(char16_t *restrict pc16,
+                                      const char *restrict s, size_t n,
+                                      mbstate_t *restrict ps) {
+  size_t StringSize = 0;
+  char32_t Decoded = 0;
+
+  switch (n) {
+  case 1:
+    Decoded = s[0] & 0x7F;
+    break;
+  case 2:
+    Decoded |= (s[0] & 0x1F) << 6;
+    Decoded |= (s[1] & 0x3F) << 0;
+    break;
+  case 3:
+    Decoded |= (s[0] & 0x0F) << 12;
+    Decoded |= (s[1] & 0x1F) << 6;
+    Decoded |= (s[2] & 0x1F) << 0;
+    break;
+  case 4:
+    Decoded |= (s[0] & 0x07) << 18;
+    Decoded |= (s[1] & 0x3F) << 12;
+    Decoded |= (s[2] & 0x3F) << 6;
+    Decoded |= (s[3] & 0x3F) << 0;
+    break;
+  }
+
+  if (Decoded <= 0xFFFF) {
+    StringSize = 1;
+    pc16[0] = Decoded & 0xFFFF;
+  } else if (Decoded <= 0x10FFFF) {
+    StringSize = 2;
+    pc16[0] = 0xD800 + ((Decoded & 0xFFC00) >> 10);
+    pc16[1] = 0xDC00 + (Decoded & 0x3FF);
+  }
+  return StringSize;
+}
+
+} // namespace __llvm_libc
Index: libc/src/uchar/mbrtoc32.h
===================================================================
--- /dev/null
+++ libc/src/uchar/mbrtoc32.h
@@ -0,0 +1,21 @@
+//===---------------- Implementation header for mbrtoc32 -----------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===---------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_UCHAR_MBRTOC32_H
+#define LLVM_LIBC_SRC_UCHAR_MBRTOC32_H
+
+#include "include/uchar.h"
+
+namespace __llvm_libc {
+
+size_t mbrtoc32(char32_t *restrict pc32, const char *restrict s, size_t n,
+                mbstate_t *restrict ps);
+
+} // namespace __llvm_libc
+
+#endif // LLVM_LIBC_SRC_UCHAR_MBRTOC32_H
Index: libc/src/uchar/mbrtoc32.cpp
===================================================================
--- /dev/null
+++ libc/src/uchar/mbrtoc32.cpp
@@ -0,0 +1,44 @@
+//===-------------------- Implementation of mbrtoc32
+//-----------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/uchar/mbrtoc32.h"
+
+#include "src/__support/common.h"
+
+namespace __llvm_libc {
+
+size_t LLVM_LIBC_ENTRYPOINT(mbrtoc32)(char32_t *restrict pc32,
+                                      const char *restrict s, size_t n,
+                                      mbstate_t *restrict ps) {
+  size_t StringSize = 1;
+
+  switch (n) {
+  case 1:
+    pc32[0] = s[0] & 0x7F;
+    break;
+  case 2:
+    pc32[0] |= (s[0] & 0x1F) << 6;
+    pc32[0] |= (s[1] & 0x3F) << 0;
+    break;
+  case 3:
+    pc32[0] |= (s[0] & 0x0F) << 12;
+    pc32[0] |= (s[1] & 0x1F) << 6;
+    pc32[0] |= (s[2] & 0x1F) << 0;
+    break;
+  case 4:
+    pc32[0] |= (s[0] & 0x07) << 18;
+    pc32[0] |= (s[1] & 0x3F) << 12;
+    pc32[0] |= (s[2] & 0x3F) << 6;
+    pc32[0] |= (s[3] & 0x3F) << 0;
+    break;
+  }
+  return StringSize;
+}
+
+} // namespace __llvm_libc
Index: libc/test/src/CMakeLists.txt
===================================================================
--- libc/test/src/CMakeLists.txt
+++ libc/test/src/CMakeLists.txt
@@ -5,3 +5,5 @@
 add_subdirectory(string)
 add_subdirectory(sys)
 add_subdirectory(threads)
+add_subdirectory(uchar)
+add_subdirectory(threads)