diff --git a/libc/config/linux/x86_64/entrypoints.txt b/libc/config/linux/x86_64/entrypoints.txt
index 1451bd286d8aa..e8f59c9d5cd73 100644
--- a/libc/config/linux/x86_64/entrypoints.txt
+++ b/libc/config/linux/x86_64/entrypoints.txt
@@ -1268,6 +1268,9 @@ if(LLVM_LIBC_FULL_BUILD)
     libc.src.wchar.mbtowc
     libc.src.wchar.wcrtomb
     libc.src.wchar.wctomb
+    libc.src.wchar.wcstombs
+    libc.src.wchar.wcsrtombs
+    libc.src.wchar.wcsnrtombs
   )
 endif()
 
diff --git a/libc/include/wchar.yaml b/libc/include/wchar.yaml
index daf05cdcd00c2..0285f1924a2fa 100644
--- a/libc/include/wchar.yaml
+++ b/libc/include/wchar.yaml
@@ -204,6 +204,25 @@ functions:
       - type: wchar_t *__restrict
       - type: const wchar_t *__restrict
       - type: size_t
+  - name: wcsnrtombs
+    standards:
+      - stdc
+    return_type: size_t
+    arguments:
+      - type: char *__restrict
+      - type: const wchar_t **__restrict
+      - type: size_t
+      - type: size_t
+      - type: mbstate_t
+  - name: wcsrtombs
+    standards:
+      - stdc
+    return_type: size_t
+    arguments:
+      - type: char *__restrict
+      - type: const wchar_t **__restrict
+      - type: size_t
+      - type: mbstate_t
   - name: wcrtomb
     standards:
       - stdc
@@ -279,6 +298,14 @@ functions:
       - type: const wchar_t *__restrict
       - type: wchar_t **__restrict
       - type: int
+  - name: wcstombs
+    standards:
+      - stdc
+    return_type: size_t
+    arguments:
+      - type: char *__restrict
+      - type: const wchar_t *__restrict
+      - type: size_t
   - name: wcstoul
     standards:
       - stdc
diff --git a/libc/src/__support/wchar/CMakeLists.txt b/libc/src/__support/wchar/CMakeLists.txt
index cf3e641d2d2db..e363ad397079c 100644
--- a/libc/src/__support/wchar/CMakeLists.txt
+++ b/libc/src/__support/wchar/CMakeLists.txt
@@ -69,3 +69,20 @@ add_object_library(
   .character_converter
   .mbstate
 )
+
+add_header_library(
+  wcsnrtombs
+  HDRS
+    wcsnrtombs.h
+  DEPENDS
+    libc.hdr.errno_macros
+    libc.hdr.types.char8_t
+    libc.hdr.types.char32_t
+    libc.hdr.types.size_t
+    libc.hdr.types.wchar_t
+    libc.src.__support.error_or
+    libc.src.__support.common
+    .string_converter
+    .character_converter
+    .mbstate
+)
diff --git a/libc/src/__support/wchar/wcsnrtombs.h b/libc/src/__support/wchar/wcsnrtombs.h
new file mode 100644
index 0000000000000..433097c937a42
--- /dev/null
+++ b/libc/src/__support/wchar/wcsnrtombs.h
@@ -0,0 +1,69 @@
+//===-- Implementation header for wcsnrtombs ------------------------------===//
+//
+// 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__SUPPORT_WCHAR_WCSNRTOMBS_H
+#define LLVM_LIBC_SRC__SUPPORT_WCHAR_WCSNRTOMBS_H
+
+#include "hdr/types/char32_t.h"
+#include "hdr/types/size_t.h"
+#include "hdr/types/wchar_t.h"
+#include "src/__support/common.h"
+#include "src/__support/libc_errno.h"
+#include "src/__support/macros/config.h"
+#include "src/__support/macros/null_check.h"
+#include "src/__support/wchar/mbstate.h"
+#include "src/__support/wchar/string_converter.h"
+
+namespace LIBC_NAMESPACE_DECL {
+namespace internal {
+
+LIBC_INLINE static ErrorOr<size_t>
+wcsnrtombs(char *__restrict dest, const wchar_t **__restrict ptr_to_src,
+           size_t num_src_widechars, size_t dest_len, mbstate *ps) {
+  LIBC_CRASH_ON_NULLPTR(ptr_to_src);
+  LIBC_CRASH_ON_NULLPTR(ps);
+
+  CharacterConverter cr(ps);
+  if (!cr.isValidState())
+    return Error(EINVAL);
+
+  if (dest == nullptr)
+    dest_len = SIZE_MAX;
+
+  StringConverter<char32_t> str_conv(
+      reinterpret_cast<const char32_t *>(*ptr_to_src), ps, dest_len,
+      num_src_widechars);
+  size_t dst_idx = 0;
+  ErrorOr<char8_t> converted = str_conv.popUTF8();
+  while (converted.has_value()) {
+    if (dest != nullptr)
+      dest[dst_idx] = converted.value();
+
+    if (converted.value() == '\0') {
+      if (dest != nullptr)
+        *ptr_to_src = nullptr;
+      return dst_idx;
+    }
+
+    dst_idx++;
+    converted = str_conv.popUTF8();
+  }
+
+  if (dest != nullptr)
+    *ptr_to_src += str_conv.getSourceIndex();
+
+  if (converted.error() == -1) // if we hit conversion limit
+    return dst_idx;
+
+  return Error(converted.error());
+}
+
+} // namespace internal
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SRC__SUPPORT_WCHAR_WCSNRTOMBS_H
diff --git a/libc/src/wchar/CMakeLists.txt b/libc/src/wchar/CMakeLists.txt
index 25319837bdc70..43f44a941d451 100644
--- a/libc/src/wchar/CMakeLists.txt
+++ b/libc/src/wchar/CMakeLists.txt
@@ -169,6 +169,47 @@ add_entrypoint_object(
     libc.src.__support.wchar.mbstate
 )
 
+add_entrypoint_object(
+  wcstombs
+  SRCS
+    wcstombs.cpp
+  HDRS
+    wcstombs.h
+  DEPENDS
+    libc.hdr.types.wchar_t
+    libc.src.__support.wchar.mbstate
+    libc.src.__support.wchar.wcsnrtombs
+    libc.src.__support.libc_errno
+)
+
+add_entrypoint_object(
+  wcsrtombs
+  SRCS
+    wcsrtombs.cpp
+  HDRS
+    wcsrtombs.h
+  DEPENDS
+    libc.hdr.types.wchar_t
+    libc.hdr.types.mbstate_t
+    libc.src.__support.wchar.mbstate
+    libc.src.__support.wchar.wcsnrtombs
+    libc.src.__support.libc_errno
+)
+
+add_entrypoint_object(
+  wcsnrtombs
+  SRCS
+    wcsnrtombs.cpp
+  HDRS
+    wcsnrtombs.h
+  DEPENDS
+    libc.hdr.types.wchar_t
+    libc.hdr.types.mbstate_t
+    libc.src.__support.wchar.mbstate
+    libc.src.__support.wchar.wcsnrtombs
+    libc.src.__support.libc_errno
+)
+
 add_entrypoint_object(
   mblen
   SRCS
diff --git a/libc/src/wchar/wcsnrtombs.cpp b/libc/src/wchar/wcsnrtombs.cpp
new file mode 100644
index 0000000000000..7f25b248a0863
--- /dev/null
+++ b/libc/src/wchar/wcsnrtombs.cpp
@@ -0,0 +1,40 @@
+//===-- Implementation of wcsnrtombs --------------------------------------===//
+//
+// 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/wchar/wcsnrtombs.h"
+
+#include "hdr/types/char32_t.h"
+#include "hdr/types/mbstate_t.h"
+#include "hdr/types/size_t.h"
+#include "hdr/types/wchar_t.h"
+#include "src/__support/common.h"
+#include "src/__support/libc_errno.h"
+#include "src/__support/macros/config.h"
+#include "src/__support/wchar/mbstate.h"
+#include "src/__support/wchar/wcsnrtombs.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+LLVM_LIBC_FUNCTION(size_t, wcsnrtombs,
+                   (char *__restrict s, const wchar_t **__restrict pwcs,
+                    size_t nwc, size_t len, mbstate_t *ps)) {
+  LIBC_CRASH_ON_NULLPTR(pwcs);
+  static internal::mbstate internal_mbstate;
+  auto result = internal::wcsnrtombs(
+      s, pwcs, nwc, len,
+      ps == nullptr ? &internal_mbstate
+                    : reinterpret_cast<internal::mbstate *>(ps));
+  if (!result.has_value()) {
+    libc_errno = result.error();
+    return -1;
+  }
+
+  return result.value();
+}
+
+} // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/src/wchar/wcsnrtombs.h b/libc/src/wchar/wcsnrtombs.h
new file mode 100644
index 0000000000000..bf8add75b2951
--- /dev/null
+++ b/libc/src/wchar/wcsnrtombs.h
@@ -0,0 +1,24 @@
+//===-- Implementation header for wcsnrtombs ------------------------------===//
+//
+// 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_WCHAR_WCSNRTOMBS_H
+#define LLVM_LIBC_SRC_WCHAR_WCSNRTOMBS_H
+
+#include "hdr/types/mbstate_t.h"
+#include "hdr/types/size_t.h"
+#include "hdr/types/wchar_t.h"
+#include "src/__support/macros/config.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+size_t wcsnrtombs(char *__restrict s, const wchar_t **__restrict pwcs,
+                  size_t nwc, size_t len, mbstate_t *ps);
+
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SRC_WCHAR_WCSNRTOMBS_H
diff --git a/libc/src/wchar/wcsrtombs.cpp b/libc/src/wchar/wcsrtombs.cpp
new file mode 100644
index 0000000000000..9d2508cb81a8c
--- /dev/null
+++ b/libc/src/wchar/wcsrtombs.cpp
@@ -0,0 +1,40 @@
+//===-- Implementation of wcsrtombs ---------------------------------------===//
+//
+// 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/wchar/wcsrtombs.h"
+
+#include "hdr/types/char32_t.h"
+#include "hdr/types/mbstate_t.h"
+#include "hdr/types/size_t.h"
+#include "hdr/types/wchar_t.h"
+#include "src/__support/common.h"
+#include "src/__support/libc_errno.h"
+#include "src/__support/macros/config.h"
+#include "src/__support/wchar/mbstate.h"
+#include "src/__support/wchar/wcsnrtombs.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+LLVM_LIBC_FUNCTION(size_t, wcsrtombs,
+                   (char *__restrict s, const wchar_t **__restrict pwcs,
+                    size_t n, mbstate_t *ps)) {
+  LIBC_CRASH_ON_NULLPTR(pwcs);
+  static internal::mbstate internal_mbstate;
+  auto result = internal::wcsnrtombs(
+      s, pwcs, SIZE_MAX, n,
+      ps == nullptr ? &internal_mbstate
+                    : reinterpret_cast<internal::mbstate *>(ps));
+  if (!result.has_value()) {
+    libc_errno = result.error();
+    return -1;
+  }
+
+  return result.value();
+}
+
+} // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/src/wchar/wcsrtombs.h b/libc/src/wchar/wcsrtombs.h
new file mode 100644
index 0000000000000..d23573f5b9418
--- /dev/null
+++ b/libc/src/wchar/wcsrtombs.h
@@ -0,0 +1,24 @@
+//===-- Implementation header for wcsrtombs -------------------------------===//
+//
+// 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_WCHAR_WCSRTOMBS_H
+#define LLVM_LIBC_SRC_WCHAR_WCSRTOMBS_H
+
+#include "hdr/types/mbstate_t.h"
+#include "hdr/types/size_t.h"
+#include "hdr/types/wchar_t.h"
+#include "src/__support/macros/config.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+size_t wcsrtombs(char *__restrict s, const wchar_t **__restrict pwcs, size_t n,
+                 mbstate_t *ps);
+
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SRC_WCHAR_WCSRTOMBS_H
diff --git a/libc/src/wchar/wcstombs.cpp b/libc/src/wchar/wcstombs.cpp
new file mode 100644
index 0000000000000..c3793cbe912cd
--- /dev/null
+++ b/libc/src/wchar/wcstombs.cpp
@@ -0,0 +1,38 @@
+//===-- Implementation of wcstombs ----------------------------------------===//
+//
+// 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/wchar/wcstombs.h"
+
+#include "hdr/types/char32_t.h"
+#include "hdr/types/size_t.h"
+#include "hdr/types/wchar_t.h"
+#include "src/__support/common.h"
+#include "src/__support/libc_errno.h"
+#include "src/__support/macros/config.h"
+#include "src/__support/wchar/mbstate.h"
+#include "src/__support/wchar/wcsnrtombs.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+LLVM_LIBC_FUNCTION(size_t, wcstombs,
+                   (char *__restrict s, const wchar_t *__restrict wcs,
+                    size_t n)) {
+  LIBC_CRASH_ON_NULLPTR(wcs);
+  static internal::mbstate internal_mbstate;
+  const wchar_t *wcs_ptr_copy = wcs;
+  auto result =
+      internal::wcsnrtombs(s, &wcs_ptr_copy, SIZE_MAX, n, &internal_mbstate);
+  if (!result.has_value()) {
+    libc_errno = result.error();
+    return -1;
+  }
+
+  return result.value();
+}
+
+} // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/src/wchar/wcstombs.h b/libc/src/wchar/wcstombs.h
new file mode 100644
index 0000000000000..cd0008a168d90
--- /dev/null
+++ b/libc/src/wchar/wcstombs.h
@@ -0,0 +1,22 @@
+//===-- Implementation header for wcstombs --------------------------------===//
+//
+// 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_WCHAR_WCSTOMBS_H
+#define LLVM_LIBC_SRC_WCHAR_WCSTOMBS_H
+
+#include "hdr/types/size_t.h"
+#include "hdr/types/wchar_t.h"
+#include "src/__support/macros/config.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+size_t wcstombs(char *__restrict s, const wchar_t *__restrict pwcs, size_t n);
+
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SRC_WCHAR_WCSTOMBS_H
diff --git a/libc/test/src/__support/wchar/CMakeLists.txt b/libc/test/src/__support/wchar/CMakeLists.txt
index f0727451736f9..c112c83dbe9af 100644
--- a/libc/test/src/__support/wchar/CMakeLists.txt
+++ b/libc/test/src/__support/wchar/CMakeLists.txt
@@ -34,3 +34,20 @@ add_libc_test(
     libc.hdr.errno_macros
     libc.hdr.types.char32_t
 )
+
+add_libc_test(
+  wcsnrtombs_test
+  SUITE
+    libc-support-tests
+  SRCS
+    wcsnrtombs_test.cpp
+  DEPENDS
+    libc.src.__support.wchar.string_converter
+    libc.src.__support.wchar.character_converter
+    libc.src.__support.wchar.mbstate
+    libc.src.__support.error_or
+    libc.src.__support.wchar.wcsnrtombs
+    libc.hdr.errno_macros
+    libc.hdr.types.char32_t
+    libc.hdr.types.char8_t
+)
diff --git a/libc/test/src/__support/wchar/wcsnrtombs_test.cpp b/libc/test/src/__support/wchar/wcsnrtombs_test.cpp
new file mode 100644
index 0000000000000..2d431eddf4a6f
--- /dev/null
+++ b/libc/test/src/__support/wchar/wcsnrtombs_test.cpp
@@ -0,0 +1,213 @@
+//===-- Unittests for wcsnrtombs ------------------------------------------===//
+//
+// 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 "hdr/errno_macros.h"
+#include "hdr/types/size_t.h"
+#include "hdr/types/wchar_t.h"
+#include "src/__support/error_or.h"
+#include "src/__support/macros/null_check.h"
+#include "src/__support/macros/properties/os.h"
+#include "src/__support/wchar/mbstate.h"
+#include "src/__support/wchar/wcsnrtombs.h"
+#include "test/UnitTest/Test.h"
+
+// TODO: add support for 16-bit widechars to remove this macro
+#ifdef LIBC_TARGET_OS_IS_WINDOWS
+TEST(LlvmLibcStringConverterTest, Windows) {
+  // pass on windows for now
+}
+
+#else
+
+TEST(LlvmLibcWcsnrtombs, AllMultibyteLengths) {
+  LIBC_NAMESPACE::internal::mbstate state;
+
+  /// clown emoji, sigma symbol, y with diaeresis, letter A
+  const wchar_t src[] = {static_cast<wchar_t>(0x1f921),
+                         static_cast<wchar_t>(0x2211),
+                         static_cast<wchar_t>(0xff), static_cast<wchar_t>(0x41),
+                         static_cast<wchar_t>(0x0)};
+  const wchar_t *cur = src;
+  char mbs[11];
+
+  auto res = LIBC_NAMESPACE::internal::wcsnrtombs(mbs, &cur, 5, 11, &state);
+  ASSERT_TRUE(res.has_value());
+  ASSERT_EQ(res.value(), static_cast<size_t>(10));
+  ASSERT_EQ(cur, nullptr);
+  ASSERT_EQ(mbs[0], '\xF0'); // clown begin
+  ASSERT_EQ(mbs[1], '\x9F');
+  ASSERT_EQ(mbs[2], '\xA4');
+  ASSERT_EQ(mbs[3], '\xA1');
+  ASSERT_EQ(mbs[4], '\xE2'); // sigma begin
+  ASSERT_EQ(mbs[5], '\x88');
+  ASSERT_EQ(mbs[6], '\x91');
+  ASSERT_EQ(mbs[7], '\xC3'); // y diaeresis begin
+  ASSERT_EQ(mbs[8], '\xBF');
+  ASSERT_EQ(mbs[9], '\x41'); // A begin
+  ASSERT_EQ(mbs[10], '\0');  // null terminator
+}
+
+TEST(LlvmLibcWcsnrtombs, DestLimit) {
+  LIBC_NAMESPACE::internal::mbstate state1;
+
+  /// clown emoji, sigma symbol, y with diaeresis, letter A
+  const wchar_t src[] = {static_cast<wchar_t>(0x1f921),
+                         static_cast<wchar_t>(0x2211),
+                         static_cast<wchar_t>(0xff), static_cast<wchar_t>(0x41),
+                         static_cast<wchar_t>(0x0)};
+  const wchar_t *cur = src;
+
+  char mbs[11];
+  for (int i = 0; i < 11; ++i)
+    mbs[i] = '\x01'; // dummy initial values
+
+  auto res = LIBC_NAMESPACE::internal::wcsnrtombs(mbs, &cur, 5, 4, &state1);
+  ASSERT_TRUE(res.has_value());
+  ASSERT_EQ(res.value(), static_cast<size_t>(4));
+  ASSERT_EQ(cur, src + 1);
+  ASSERT_EQ(mbs[0], '\xF0');
+  ASSERT_EQ(mbs[1], '\x9F');
+  ASSERT_EQ(mbs[2], '\xA4');
+  ASSERT_EQ(mbs[3], '\xA1');
+  ASSERT_EQ(mbs[4], '\x01'); // didn't write more than 4 bytes
+
+  for (int i = 0; i < 11; ++i)
+    mbs[i] = '\x01'; // dummy initial values
+  LIBC_NAMESPACE::internal::mbstate state2;
+
+  // not enough bytes to convert the second character, so only converts one
+  cur = src;
+  res = LIBC_NAMESPACE::internal::wcsnrtombs(mbs, &cur, 5, 6, &state2);
+  ASSERT_TRUE(res.has_value());
+  ASSERT_EQ(res.value(), static_cast<size_t>(4));
+  ASSERT_EQ(cur, src + 1);
+  ASSERT_EQ(mbs[0], '\xF0');
+  ASSERT_EQ(mbs[1], '\x9F');
+  ASSERT_EQ(mbs[2], '\xA4');
+  ASSERT_EQ(mbs[3], '\xA1');
+  ASSERT_EQ(mbs[4], '\x01');
+}
+
+TEST(LlvmLibcWcsnrtombs, SrcLimit) {
+  LIBC_NAMESPACE::internal::mbstate state;
+
+  /// clown emoji, sigma symbol, y with diaeresis, letter A
+  const wchar_t src[] = {static_cast<wchar_t>(0x1f921),
+                         static_cast<wchar_t>(0x2211),
+                         static_cast<wchar_t>(0xff), static_cast<wchar_t>(0x41),
+                         static_cast<wchar_t>(0x0)};
+  const wchar_t *cur = src;
+
+  char mbs[11];
+  for (int i = 0; i < 11; ++i)
+    mbs[i] = '\x01'; // dummy initial values
+
+  auto res = LIBC_NAMESPACE::internal::wcsnrtombs(mbs, &cur, 2, 11, &state);
+  ASSERT_TRUE(res.has_value());
+  ASSERT_EQ(res.value(), static_cast<size_t>(7));
+  ASSERT_EQ(cur, src + 2);
+  ASSERT_EQ(mbs[0], '\xF0'); // clown begin
+  ASSERT_EQ(mbs[1], '\x9F');
+  ASSERT_EQ(mbs[2], '\xA4');
+  ASSERT_EQ(mbs[3], '\xA1');
+  ASSERT_EQ(mbs[4], '\xE2'); // sigma begin
+  ASSERT_EQ(mbs[5], '\x88');
+  ASSERT_EQ(mbs[6], '\x91');
+  ASSERT_EQ(mbs[7], '\x01');
+
+  res = LIBC_NAMESPACE::internal::wcsnrtombs(mbs + res.value(), &cur, 100, 11,
+                                             &state);
+  ASSERT_TRUE(res.has_value());
+  ASSERT_EQ(res.value(), static_cast<size_t>(3));
+  ASSERT_EQ(cur, nullptr);
+  ASSERT_EQ(mbs[0], '\xF0'); // clown begin
+  ASSERT_EQ(mbs[1], '\x9F');
+  ASSERT_EQ(mbs[2], '\xA4');
+  ASSERT_EQ(mbs[3], '\xA1');
+  ASSERT_EQ(mbs[4], '\xE2'); // sigma begin
+  ASSERT_EQ(mbs[5], '\x88');
+  ASSERT_EQ(mbs[6], '\x91');
+  ASSERT_EQ(mbs[7], '\xC3'); // y diaeresis begin
+  ASSERT_EQ(mbs[8], '\xBF');
+  ASSERT_EQ(mbs[9], '\x41'); // A begin
+  ASSERT_EQ(mbs[10], '\0');  // null terminator
+}
+
+TEST(LlvmLibcWcsnrtombs, NullDest) {
+  LIBC_NAMESPACE::internal::mbstate state1;
+
+  const wchar_t src[] = {static_cast<wchar_t>(0x1f921),
+                         static_cast<wchar_t>(0x2211),
+                         static_cast<wchar_t>(0xff), static_cast<wchar_t>(0x41),
+                         static_cast<wchar_t>(0x0)};
+  const wchar_t *cur = src;
+
+  // n parameter ignored when dest is null
+  auto res = LIBC_NAMESPACE::internal::wcsnrtombs(nullptr, &cur, 5, 1, &state1);
+  ASSERT_TRUE(res.has_value());
+  ASSERT_EQ(res.value(), static_cast<size_t>(10));
+  ASSERT_EQ(cur, src); // pointer not updated when dest = null
+
+  LIBC_NAMESPACE::internal::mbstate state2;
+  res = LIBC_NAMESPACE::internal::wcsnrtombs(nullptr, &cur, 5, 100, &state2);
+  ASSERT_TRUE(res.has_value());
+  ASSERT_EQ(res.value(), static_cast<size_t>(10));
+  ASSERT_EQ(cur, src);
+}
+
+TEST(LlvmLibcWcsnrtombs, InvalidState) {
+  // this is more thoroughly tested by CharacterConverter
+  LIBC_NAMESPACE::internal::mbstate state;
+  state.total_bytes = 100;
+
+  const wchar_t src[] = {static_cast<wchar_t>(0x1f921),
+                         static_cast<wchar_t>(0x2211),
+                         static_cast<wchar_t>(0xff), static_cast<wchar_t>(0x41),
+                         static_cast<wchar_t>(0x0)};
+  const wchar_t *cur = src;
+
+  // n parameter ignored when dest is null
+  auto res = LIBC_NAMESPACE::internal::wcsnrtombs(nullptr, &cur, 5, 1, &state);
+  ASSERT_FALSE(res.has_value());
+  ASSERT_EQ(res.error(), EINVAL);
+}
+
+TEST(LlvmLibcWcsnrtombs, InvalidCharacter) {
+  LIBC_NAMESPACE::internal::mbstate state1;
+
+  const wchar_t src[] = {static_cast<wchar_t>(0x1f921),
+                         static_cast<wchar_t>(0x2211),
+                         static_cast<wchar_t>(0x12ffff), // invalid widechar
+                         static_cast<wchar_t>(0x0)};
+  const wchar_t *cur = src;
+  char mbs[11];
+
+  // n parameter ignored when dest is null
+  auto res = LIBC_NAMESPACE::internal::wcsnrtombs(mbs, &cur, 5, 7, &state1);
+  ASSERT_TRUE(res.has_value());
+  ASSERT_EQ(res.value(), static_cast<size_t>(7));
+
+  LIBC_NAMESPACE::internal::mbstate state2;
+  cur = src;
+  res = LIBC_NAMESPACE::internal::wcsnrtombs(mbs, &cur, 5, 11, &state2);
+  ASSERT_FALSE(res.has_value());
+  ASSERT_EQ(res.error(), EILSEQ);
+}
+
+#if defined(LIBC_ADD_NULL_CHECKS) && !defined(LIBC_HAS_SANITIZER)
+TEST(LlvmLibcWcsnrtombs, NullSrc) {
+  EXPECT_DEATH(
+      [] {
+        LIBC_NAMESPACE::internal::mbstate state;
+        char mbs[10];
+        LIBC_NAMESPACE::internal::wcsnrtombs(mbs, nullptr, 1, 1, &state);
+      },
+      WITH_SIGNAL(-1));
+}
+#endif // LIBC_HAS_ADDRESS_SANITIZER
+#endif
diff --git a/libc/test/src/wchar/CMakeLists.txt b/libc/test/src/wchar/CMakeLists.txt
index 9b0c63ad8e07b..f420ecc465a53 100644
--- a/libc/test/src/wchar/CMakeLists.txt
+++ b/libc/test/src/wchar/CMakeLists.txt
@@ -139,6 +139,46 @@ add_libc_test(
     libc.hdr.types.wchar_t
 )
 
+add_libc_test(
+  wcstombs_test
+  SUITE
+    libc_wchar_unittests
+  SRCS
+    wcstombs_test.cpp
+  DEPENDS
+    libc.src.wchar.wcstombs
+    libc.test.UnitTest.ErrnoCheckingTest
+    libc.hdr.types.wchar_t
+)
+
+add_libc_test(
+  wcsrtombs_test
+  SUITE
+    libc_wchar_unittests
+  SRCS
+    wcsrtombs_test.cpp
+  DEPENDS
+    libc.src.wchar.wcsrtombs
+    libc.test.UnitTest.ErrnoCheckingTest
+    libc.hdr.types.wchar_t
+    libc.src.string.memset
+    libc.hdr.types.mbstate_t
+)
+
+add_libc_test(
+  wcsnrtombs_test
+  SUITE
+    libc_wchar_unittests
+  SRCS
+    wcsnrtombs_test.cpp
+  DEPENDS
+    libc.src.wchar.wcsnrtombs
+    libc.test.UnitTest.ErrnoCheckingTest
+    libc.hdr.types.wchar_t
+    libc.src.string.memset
+    libc.hdr.types.mbstate_t
+)
+
 add_libc_test(
   wmemset_test 
   SUITE
diff --git a/libc/test/src/wchar/wcsnrtombs_test.cpp b/libc/test/src/wchar/wcsnrtombs_test.cpp
new file mode 100644
index 0000000000000..04cf426d31cc7
--- /dev/null
+++ b/libc/test/src/wchar/wcsnrtombs_test.cpp
@@ -0,0 +1,192 @@
+//===-- Unittests for wcsnrtombs ------------------------------------------===//
+//
+// 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 "hdr/types/mbstate_t.h"
+#include "src/__support/macros/null_check.h"
+#include "src/string/memset.h"
+#include "src/wchar/wcsnrtombs.h"
+#include "test/UnitTest/ErrnoCheckingTest.h"
+#include "test/UnitTest/Test.h"
+
+using LlvmLibcWcsnrtombs = LIBC_NAMESPACE::testing::ErrnoCheckingTest;
+
+// these tests are fairly simple as this function just calls into the internal
+// wcsnrtombs which is more thoroughly tested
+
+TEST_F(LlvmLibcWcsnrtombs, AllMultibyteLengths) {
+  mbstate_t state;
+  LIBC_NAMESPACE::memset(&state, 0, sizeof(mbstate_t));
+
+  /// clown emoji, sigma symbol, y with diaeresis, letter A
+  const wchar_t src[] = {static_cast<wchar_t>(0x1f921),
+                         static_cast<wchar_t>(0x2211),
+                         static_cast<wchar_t>(0xff), static_cast<wchar_t>(0x41),
+                         static_cast<wchar_t>(0x0)};
+  const wchar_t *cur = src;
+  char mbs[11];
+
+  ASSERT_EQ(LIBC_NAMESPACE::wcsnrtombs(mbs, &cur, 5, 11, &state),
+            static_cast<size_t>(10));
+  ASSERT_ERRNO_SUCCESS();
+  ASSERT_EQ(cur, nullptr);
+  ASSERT_EQ(mbs[0], '\xF0'); // clown begin
+  ASSERT_EQ(mbs[1], '\x9F');
+  ASSERT_EQ(mbs[2], '\xA4');
+  ASSERT_EQ(mbs[3], '\xA1');
+  ASSERT_EQ(mbs[4], '\xE2'); // sigma begin
+  ASSERT_EQ(mbs[5], '\x88');
+  ASSERT_EQ(mbs[6], '\x91');
+  ASSERT_EQ(mbs[7], '\xC3'); // y diaeresis begin
+  ASSERT_EQ(mbs[8], '\xBF');
+  ASSERT_EQ(mbs[9], '\x41'); // A begin
+  ASSERT_EQ(mbs[10], '\0');  // null terminator
+}
+
+TEST_F(LlvmLibcWcsnrtombs, DestLimit) {
+  mbstate_t state;
+  LIBC_NAMESPACE::memset(&state, 0, sizeof(mbstate_t));
+
+  /// clown emoji, sigma symbol, y with diaeresis, letter A
+  const wchar_t src[] = {static_cast<wchar_t>(0x1f921),
+                         static_cast<wchar_t>(0x2211),
+                         static_cast<wchar_t>(0xff), static_cast<wchar_t>(0x41),
+                         static_cast<wchar_t>(0x0)};
+  const wchar_t *cur = src;
+
+  char mbs[11];
+  LIBC_NAMESPACE::memset(mbs, '\x01', 11); // dummy initial values
+
+  ASSERT_EQ(LIBC_NAMESPACE::wcsnrtombs(mbs, &cur, 5, 4, &state),
+            static_cast<size_t>(4));
+  ASSERT_ERRNO_SUCCESS();
+  ASSERT_EQ(cur, src + 1);
+  ASSERT_EQ(mbs[0], '\xF0');
+  ASSERT_EQ(mbs[1], '\x9F');
+  ASSERT_EQ(mbs[2], '\xA4');
+  ASSERT_EQ(mbs[3], '\xA1');
+  ASSERT_EQ(mbs[4], '\x01'); // didn't write more than 4 bytes
+
+  LIBC_NAMESPACE::memset(mbs, '\x01', 11); // dummy initial values
+  LIBC_NAMESPACE::memset(&state, 0, sizeof(mbstate_t));
+  cur = src;
+
+  // not enough bytes to convert the second character, so only converts one
+  ASSERT_EQ(LIBC_NAMESPACE::wcsnrtombs(mbs, &cur, 5, 6, &state),
+            static_cast<size_t>(4));
+  ASSERT_ERRNO_SUCCESS();
+  ASSERT_EQ(cur, src + 1);
+  ASSERT_EQ(mbs[0], '\xF0');
+  ASSERT_EQ(mbs[1], '\x9F');
+  ASSERT_EQ(mbs[2], '\xA4');
+  ASSERT_EQ(mbs[3], '\xA1');
+  ASSERT_EQ(mbs[4], '\x01');
+}
+
+TEST(LlvmLibcWcsnrtombs, SrcLimit) {
+  mbstate_t state;
+  LIBC_NAMESPACE::memset(&state, 0, sizeof(mbstate_t));
+
+  /// clown emoji, sigma symbol, y with diaeresis, letter A
+  const wchar_t src[] = {static_cast<wchar_t>(0x1f921),
+                         static_cast<wchar_t>(0x2211),
+                         static_cast<wchar_t>(0xff), static_cast<wchar_t>(0x41),
+                         static_cast<wchar_t>(0x0)};
+  const wchar_t *cur = src;
+
+  char mbs[11];
+  LIBC_NAMESPACE::memset(mbs, '\x01', 11); // dummy initial values
+
+  auto res = LIBC_NAMESPACE::wcsnrtombs(mbs, &cur, 2, 11, &state);
+  ASSERT_ERRNO_SUCCESS();
+  ASSERT_EQ(res, static_cast<size_t>(7));
+  ASSERT_EQ(cur, src + 2);
+  ASSERT_EQ(mbs[0], '\xF0'); // clown begin
+  ASSERT_EQ(mbs[1], '\x9F');
+  ASSERT_EQ(mbs[2], '\xA4');
+  ASSERT_EQ(mbs[3], '\xA1');
+  ASSERT_EQ(mbs[4], '\xE2'); // sigma begin
+  ASSERT_EQ(mbs[5], '\x88');
+  ASSERT_EQ(mbs[6], '\x91');
+  ASSERT_EQ(mbs[7], '\x01');
+
+  res = LIBC_NAMESPACE::wcsnrtombs(mbs + res, &cur, 100, 11, &state);
+  ASSERT_ERRNO_SUCCESS();
+  ASSERT_EQ(res, static_cast<size_t>(3));
+  ASSERT_EQ(cur, nullptr);
+  ASSERT_EQ(mbs[0], '\xF0'); // clown begin
+  ASSERT_EQ(mbs[1], '\x9F');
+  ASSERT_EQ(mbs[2], '\xA4');
+  ASSERT_EQ(mbs[3], '\xA1');
+  ASSERT_EQ(mbs[4], '\xE2'); // sigma begin
+  ASSERT_EQ(mbs[5], '\x88');
+  ASSERT_EQ(mbs[6], '\x91');
+  ASSERT_EQ(mbs[7], '\xC3'); // y diaeresis begin
+  ASSERT_EQ(mbs[8], '\xBF');
+  ASSERT_EQ(mbs[9], '\x41'); // A begin
+  ASSERT_EQ(mbs[10], '\0');  // null terminator
+}
+
+TEST_F(LlvmLibcWcsnrtombs, ErrnoTest) {
+  mbstate_t state;
+  LIBC_NAMESPACE::memset(&state, 0, sizeof(mbstate_t));
+
+  const wchar_t src[] = {static_cast<wchar_t>(0x1f921),
+                         static_cast<wchar_t>(0x2211),
+                         static_cast<wchar_t>(0x12ffff), // invalid widechar
+                         static_cast<wchar_t>(0x0)};
+  const wchar_t *cur = src;
+  char mbs[11];
+
+  // n parameter ignored when dest is null
+  ASSERT_EQ(LIBC_NAMESPACE::wcsnrtombs(mbs, &cur, 5, 7, &state),
+            static_cast<size_t>(7));
+  ASSERT_ERRNO_SUCCESS();
+
+  LIBC_NAMESPACE::memset(&state, 0, sizeof(mbstate_t));
+  ASSERT_EQ(LIBC_NAMESPACE::wcsnrtombs(mbs, &cur, 5, 100, &state),
+            static_cast<size_t>(-1));
+  ASSERT_ERRNO_EQ(EILSEQ);
+}
+
+TEST_F(LlvmLibcWcsnrtombs, NullState) {
+  // this test is the same as DestLimit except it uses a nullptr mbstate*
+
+  /// clown emoji, sigma symbol, y with diaeresis, letter A
+  const wchar_t src[] = {static_cast<wchar_t>(0x1f921),
+                         static_cast<wchar_t>(0x2211),
+                         static_cast<wchar_t>(0xff), static_cast<wchar_t>(0x41),
+                         static_cast<wchar_t>(0x0)};
+  const wchar_t *cur = src;
+
+  char mbs[11];
+  LIBC_NAMESPACE::memset(mbs, '\x01', 11); // dummy initial values
+
+  ASSERT_EQ(LIBC_NAMESPACE::wcsnrtombs(mbs, &cur, 5, 4, nullptr),
+            static_cast<size_t>(4));
+  ASSERT_ERRNO_SUCCESS();
+  ASSERT_EQ(cur, src + 1);
+  ASSERT_EQ(mbs[0], '\xF0');
+  ASSERT_EQ(mbs[1], '\x9F');
+  ASSERT_EQ(mbs[2], '\xA4');
+  ASSERT_EQ(mbs[3], '\xA1');
+  ASSERT_EQ(mbs[4], '\x01'); // didn't write more than 4 bytes
+
+  LIBC_NAMESPACE::memset(mbs, '\x01', 11); // dummy initial values
+
+  // not enough bytes to convert the second character, so only converts one
+  cur = src;
+  ASSERT_EQ(LIBC_NAMESPACE::wcsnrtombs(mbs, &cur, 5, 6, nullptr),
+            static_cast<size_t>(4));
+  ASSERT_ERRNO_SUCCESS();
+  ASSERT_EQ(cur, src + 1);
+  ASSERT_EQ(mbs[0], '\xF0');
+  ASSERT_EQ(mbs[1], '\x9F');
+  ASSERT_EQ(mbs[2], '\xA4');
+  ASSERT_EQ(mbs[3], '\xA1');
+  ASSERT_EQ(mbs[4], '\x01');
+}
diff --git a/libc/test/src/wchar/wcsrtombs_test.cpp b/libc/test/src/wchar/wcsrtombs_test.cpp
new file mode 100644
index 0000000000000..65c69e63aee0c
--- /dev/null
+++ b/libc/test/src/wchar/wcsrtombs_test.cpp
@@ -0,0 +1,150 @@
+//===-- Unittests for wcsrtombs -------------------------------------------===//
+//
+// 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 "hdr/types/mbstate_t.h"
+#include "src/__support/macros/null_check.h"
+#include "src/string/memset.h"
+#include "src/wchar/wcsrtombs.h"
+#include "test/UnitTest/ErrnoCheckingTest.h"
+#include "test/UnitTest/Test.h"
+
+using LlvmLibcWcsrtombs = LIBC_NAMESPACE::testing::ErrnoCheckingTest;
+
+// these tests are fairly simple as this function just calls into the internal
+// wcsnrtombs which is more thoroughly tested
+
+TEST_F(LlvmLibcWcsrtombs, AllMultibyteLengths) {
+  mbstate_t state;
+  LIBC_NAMESPACE::memset(&state, 0, sizeof(mbstate_t));
+
+  /// clown emoji, sigma symbol, y with diaeresis, letter A
+  const wchar_t src[] = {static_cast<wchar_t>(0x1f921),
+                         static_cast<wchar_t>(0x2211),
+                         static_cast<wchar_t>(0xff), static_cast<wchar_t>(0x41),
+                         static_cast<wchar_t>(0x0)};
+  const wchar_t *cur = src;
+  char mbs[11];
+
+  ASSERT_EQ(LIBC_NAMESPACE::wcsrtombs(mbs, &cur, 11, &state),
+            static_cast<size_t>(10));
+  ASSERT_ERRNO_SUCCESS();
+  ASSERT_EQ(cur, nullptr);
+  ASSERT_EQ(mbs[0], '\xF0'); // clown begin
+  ASSERT_EQ(mbs[1], '\x9F');
+  ASSERT_EQ(mbs[2], '\xA4');
+  ASSERT_EQ(mbs[3], '\xA1');
+  ASSERT_EQ(mbs[4], '\xE2'); // sigma begin
+  ASSERT_EQ(mbs[5], '\x88');
+  ASSERT_EQ(mbs[6], '\x91');
+  ASSERT_EQ(mbs[7], '\xC3'); // y diaeresis begin
+  ASSERT_EQ(mbs[8], '\xBF');
+  ASSERT_EQ(mbs[9], '\x41'); // A begin
+  ASSERT_EQ(mbs[10], '\0');  // null terminator
+}
+
+TEST_F(LlvmLibcWcsrtombs, DestLimit) {
+  mbstate_t state;
+  LIBC_NAMESPACE::memset(&state, 0, sizeof(mbstate_t));
+
+  /// clown emoji, sigma symbol, y with diaeresis, letter A
+  const wchar_t src[] = {static_cast<wchar_t>(0x1f921),
+                         static_cast<wchar_t>(0x2211),
+                         static_cast<wchar_t>(0xff), static_cast<wchar_t>(0x41),
+                         static_cast<wchar_t>(0x0)};
+  const wchar_t *cur = src;
+
+  char mbs[11];
+  LIBC_NAMESPACE::memset(mbs, '\x01', 11); // dummy initial values
+
+  ASSERT_EQ(LIBC_NAMESPACE::wcsrtombs(mbs, &cur, 4, &state),
+            static_cast<size_t>(4));
+  ASSERT_ERRNO_SUCCESS();
+  ASSERT_EQ(cur, src + 1);
+  ASSERT_EQ(mbs[0], '\xF0');
+  ASSERT_EQ(mbs[1], '\x9F');
+  ASSERT_EQ(mbs[2], '\xA4');
+  ASSERT_EQ(mbs[3], '\xA1');
+  ASSERT_EQ(mbs[4], '\x01'); // didn't write more than 4 bytes
+
+  LIBC_NAMESPACE::memset(mbs, '\x01', 11); // dummy initial values
+  LIBC_NAMESPACE::memset(&state, 0, sizeof(mbstate_t));
+
+  // not enough bytes to convert the second character, so only converts one
+  cur = src;
+  ASSERT_EQ(LIBC_NAMESPACE::wcsrtombs(mbs, &cur, 6, &state),
+            static_cast<size_t>(4));
+  ASSERT_ERRNO_SUCCESS();
+  ASSERT_EQ(cur, src + 1);
+  ASSERT_EQ(mbs[0], '\xF0');
+  ASSERT_EQ(mbs[1], '\x9F');
+  ASSERT_EQ(mbs[2], '\xA4');
+  ASSERT_EQ(mbs[3], '\xA1');
+  ASSERT_EQ(mbs[4], '\x01');
+}
+
+TEST_F(LlvmLibcWcsrtombs, ErrnoTest) {
+  mbstate_t state;
+  LIBC_NAMESPACE::memset(&state, 0, sizeof(mbstate_t));
+
+  const wchar_t src[] = {static_cast<wchar_t>(0x1f921),
+                         static_cast<wchar_t>(0x2211),
+                         static_cast<wchar_t>(0x12ffff), // invalid widechar
+                         static_cast<wchar_t>(0x0)};
+  const wchar_t *cur = src;
+  char mbs[11];
+
+  // n parameter ignored when dest is null
+  ASSERT_EQ(LIBC_NAMESPACE::wcsrtombs(mbs, &cur, 7, &state),
+            static_cast<size_t>(7));
+  ASSERT_ERRNO_SUCCESS();
+
+  LIBC_NAMESPACE::memset(&state, 0, sizeof(mbstate_t));
+  cur = src;
+
+  ASSERT_EQ(LIBC_NAMESPACE::wcsrtombs(mbs, &cur, 100, &state),
+            static_cast<size_t>(-1));
+  ASSERT_ERRNO_EQ(EILSEQ);
+}
+
+TEST_F(LlvmLibcWcsrtombs, NullState) {
+  // this test is the same as DestLimit except it uses a nullptr mbstate*
+
+  /// clown emoji, sigma symbol, y with diaeresis, letter A
+  const wchar_t src[] = {static_cast<wchar_t>(0x1f921),
+                         static_cast<wchar_t>(0x2211),
+                         static_cast<wchar_t>(0xff), static_cast<wchar_t>(0x41),
+                         static_cast<wchar_t>(0x0)};
+  const wchar_t *cur = src;
+
+  char mbs[11];
+  LIBC_NAMESPACE::memset(mbs, '\x01', 11); // dummy initial values
+
+  ASSERT_EQ(LIBC_NAMESPACE::wcsrtombs(mbs, &cur, 4, nullptr),
+            static_cast<size_t>(4));
+  ASSERT_ERRNO_SUCCESS();
+  ASSERT_EQ(cur, src + 1);
+  ASSERT_EQ(mbs[0], '\xF0');
+  ASSERT_EQ(mbs[1], '\x9F');
+  ASSERT_EQ(mbs[2], '\xA4');
+  ASSERT_EQ(mbs[3], '\xA1');
+  ASSERT_EQ(mbs[4], '\x01'); // didn't write more than 4 bytes
+
+  LIBC_NAMESPACE::memset(mbs, '\x01', 11); // dummy initial values
+
+  // not enough bytes to convert the second character, so only converts one
+  cur = src;
+  ASSERT_EQ(LIBC_NAMESPACE::wcsrtombs(mbs, &cur, 6, nullptr),
+            static_cast<size_t>(4));
+  ASSERT_ERRNO_SUCCESS();
+  ASSERT_EQ(cur, src + 1);
+  ASSERT_EQ(mbs[0], '\xF0');
+  ASSERT_EQ(mbs[1], '\x9F');
+  ASSERT_EQ(mbs[2], '\xA4');
+  ASSERT_EQ(mbs[3], '\xA1');
+  ASSERT_EQ(mbs[4], '\x01');
+}
diff --git a/libc/test/src/wchar/wcstombs_test.cpp b/libc/test/src/wchar/wcstombs_test.cpp
new file mode 100644
index 0000000000000..61e0873dc9711
--- /dev/null
+++ b/libc/test/src/wchar/wcstombs_test.cpp
@@ -0,0 +1,84 @@
+//===-- Unittests for wcstombs --------------------------------------------===//
+//
+// 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/wchar/wcstombs.h"
+#include "test/UnitTest/ErrnoCheckingTest.h"
+#include "test/UnitTest/Test.h"
+
+using LlvmLibcWcstombs = LIBC_NAMESPACE::testing::ErrnoCheckingTest;
+
+// these tests are fairly simple as this function just calls into the internal
+// wcsnrtombs which is more thoroughly tested
+
+TEST_F(LlvmLibcWcstombs, AllMultibyteLengths) {
+  /// clown emoji, sigma symbol, y with diaeresis, letter A
+  const wchar_t src[] = {static_cast<wchar_t>(0x1f921),
+                         static_cast<wchar_t>(0x2211),
+                         static_cast<wchar_t>(0xff), static_cast<wchar_t>(0x41),
+                         static_cast<wchar_t>(0x0)};
+  char mbs[11];
+
+  ASSERT_EQ(LIBC_NAMESPACE::wcstombs(mbs, src, 11), static_cast<size_t>(10));
+  ASSERT_ERRNO_SUCCESS();
+  ASSERT_EQ(mbs[0], '\xF0'); // clown begin
+  ASSERT_EQ(mbs[1], '\x9F');
+  ASSERT_EQ(mbs[2], '\xA4');
+  ASSERT_EQ(mbs[3], '\xA1');
+  ASSERT_EQ(mbs[4], '\xE2'); // sigma begin
+  ASSERT_EQ(mbs[5], '\x88');
+  ASSERT_EQ(mbs[6], '\x91');
+  ASSERT_EQ(mbs[7], '\xC3'); // y diaeresis begin
+  ASSERT_EQ(mbs[8], '\xBF');
+  ASSERT_EQ(mbs[9], '\x41'); // A begin
+  ASSERT_EQ(mbs[10], '\0');  // null terminator
+}
+
+TEST_F(LlvmLibcWcstombs, DestLimit) {
+  /// clown emoji, sigma symbol, y with diaeresis, letter A
+  const wchar_t src[] = {static_cast<wchar_t>(0x1f921),
+                         static_cast<wchar_t>(0x2211),
+                         static_cast<wchar_t>(0xff), static_cast<wchar_t>(0x41),
+                         static_cast<wchar_t>(0x0)};
+  char mbs[11];
+  for (int i = 0; i < 11; ++i)
+    mbs[i] = '\x01'; // dummy initial values
+
+  ASSERT_EQ(LIBC_NAMESPACE::wcstombs(mbs, src, 4), static_cast<size_t>(4));
+  ASSERT_ERRNO_SUCCESS();
+  ASSERT_EQ(mbs[0], '\xF0');
+  ASSERT_EQ(mbs[1], '\x9F');
+  ASSERT_EQ(mbs[2], '\xA4');
+  ASSERT_EQ(mbs[3], '\xA1');
+  ASSERT_EQ(mbs[4], '\x01'); // didn't write more than 4 bytes
+
+  for (int i = 0; i < 11; ++i)
+    mbs[i] = '\x01'; // dummy initial values
+
+  // not enough bytes to convert the second character, so only converts one
+  ASSERT_EQ(LIBC_NAMESPACE::wcstombs(mbs, src, 6), static_cast<size_t>(4));
+  ASSERT_ERRNO_SUCCESS();
+  ASSERT_EQ(mbs[0], '\xF0');
+  ASSERT_EQ(mbs[1], '\x9F');
+  ASSERT_EQ(mbs[2], '\xA4');
+  ASSERT_EQ(mbs[3], '\xA1');
+  ASSERT_EQ(mbs[4], '\x01');
+}
+
+TEST_F(LlvmLibcWcstombs, ErrnoTest) {
+  const wchar_t src[] = {static_cast<wchar_t>(0x1f921),
+                         static_cast<wchar_t>(0x2211),
+                         static_cast<wchar_t>(0x12ffff), // invalid widechar
+                         static_cast<wchar_t>(0x0)};
+  char mbs[11];
+
+  // n parameter ignored when dest is null
+  ASSERT_EQ(LIBC_NAMESPACE::wcstombs(mbs, src, 7), static_cast<size_t>(7));
+  ASSERT_ERRNO_SUCCESS();
+  ASSERT_EQ(LIBC_NAMESPACE::wcstombs(mbs, src, 100), static_cast<size_t>(-1));
+  ASSERT_ERRNO_EQ(EILSEQ);
+}