Index: libc/config/CMakeLists.txt
===================================================================
--- libc/config/CMakeLists.txt
+++ libc/config/CMakeLists.txt
@@ -1 +1,2 @@
 add_subdirectory(linux)
+add_subdirectory(darwin)
Index: libc/config/darwin/CMakeLists.txt
===================================================================
--- /dev/null
+++ libc/config/darwin/CMakeLists.txt
@@ -0,0 +1 @@
+
Index: libc/config/darwin/platfrom_defs.h.inc
===================================================================
--- /dev/null
+++ libc/config/darwin/platfrom_defs.h.inc
@@ -0,0 +1,15 @@
+//===----- Definition of platform specific macros for Mach-O platforms -------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+%%begin()
+
+#define ENTRYPOINT_SECTION_ATTRIBUTE(name) \
+    __attribute__((section(".libc.entrypoint,"#name)))
+#define LLVM_LIBC_ENTRYPOINT(name) ENTRYPOINT_SECTION_ATTRIBUTE(name) name
+
+// Note the comma in the section name!!!
\ No newline at end of file
Index: libc/config/darwin/x86_64/entrypoints.txt
===================================================================
--- /dev/null
+++ libc/config/darwin/x86_64/entrypoints.txt
@@ -0,0 +1,41 @@
+set(TARGET_LIBC_ENTRYPOINTS
+    # errno.h entrypoints
+    libc.src.errno.__errno_location
+
+    # string.h entrypoints
+    libc.src.string.bzero
+    libc.src.string.memcpy
+    libc.src.string.memset
+    libc.src.string.strcpy
+    libc.src.string.strcat
+    libc.src.string.strlen
+)
+
+set(TARGET_LIBM_ENTRYPOINTS
+    # math.h entrypoints
+    libc.src.math.copysign
+    libc.src.math.copysignf
+    libc.src.math.ceil
+    libc.src.math.ceilf
+    libc.src.math.cosf
+    libc.src.math.expf
+    libc.src.math.exp2f
+    libc.src.math.fabs
+    libc.src.math.fabsf
+    libc.src.math.fabsl
+    libc.src.math.floor
+    libc.src.math.floorf
+    libc.src.math.frexp
+    libc.src.math.frexpf
+    libc.src.math.logb
+    libc.src.math.logbf
+    libc.src.math.modf
+    libc.src.math.modff
+    libc.src.math.round
+    libc.src.math.roundf
+    libc.src.math.sincosf
+    libc.src.math.sinf
+    libc.src.math.trunc
+    libc.src.math.truncf
+    libc.src.math.truncl
+)
Index: libc/config/darwin/x86_64/headers.txt
===================================================================
--- /dev/null
+++ libc/config/darwin/x86_64/headers.txt
@@ -0,0 +1,6 @@
+set(PUBLIC_HEADERS
+    libc.include.errno
+    libc.include.math
+    libc.include.sys_syscall
+)
+
Index: libc/config/linux/platfrom_defs.h.inc
===================================================================
--- libc/config/linux/platfrom_defs.h.inc
+++ libc/config/linux/platfrom_defs.h.inc
@@ -1,4 +1,4 @@
-//===----- Definition of platform specific macros for ELF paltforms -------===//
+//===----- Definition of platform specific macros for ELF platforms -------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
Index: libc/src/string/bzero.h
===================================================================
--- libc/src/string/bzero.h
+++ libc/src/string/bzero.h
@@ -10,6 +10,7 @@
 #define LLVM_LIBC_SRC_STRING_BZERO_H
 
 #include "include/string.h"
+#include <stddef.h> // size_t
 
 namespace __llvm_libc {
 
Index: libc/src/string/memset.h
===================================================================
--- libc/src/string/memset.h
+++ libc/src/string/memset.h
@@ -10,6 +10,7 @@
 #define LLVM_LIBC_SRC_STRING_MEMSET_H
 
 #include "include/string.h"
+#include <stddef.h> // size_t
 
 namespace __llvm_libc {
 
Index: libc/src/string/strlen.h
===================================================================
--- libc/src/string/strlen.h
+++ libc/src/string/strlen.h
@@ -10,6 +10,7 @@
 #define LLVM_LIBC_SRC_STRING_STRLEN_H
 
 #include "include/string.h"
+#include <stddef.h> // size_t
 
 namespace __llvm_libc {
 
Index: libc/src/string/strlen.cpp
===================================================================
--- libc/src/string/strlen.cpp
+++ libc/src/string/strlen.cpp
@@ -9,6 +9,7 @@
 #include "src/string/strlen.h"
 
 #include "src/__support/common.h"
+#include <stddef.h> // size_t
 
 namespace __llvm_libc {
 
Index: libc/test/config/darwin/CmakeLists.txt
===================================================================
--- /dev/null
+++ libc/test/config/darwin/CmakeLists.txt
@@ -0,0 +1 @@
+
Index: libc/test/config/darwin/x86_64/CMakeLists.txt
===================================================================
--- /dev/null
+++ libc/test/config/darwin/x86_64/CMakeLists.txt
@@ -0,0 +1,9 @@
+add_libc_unittest(
+  libc_darwin_x86_64_strlen_test
+  SUITE
+    libc_darwin_unittests
+  SRCS
+    strlen_test.cpp
+  DEPENDS
+    libc.src.string.strlen
+)
Index: libc/test/config/darwin/x86_64/darwin.cpp
===================================================================
--- /dev/null
+++ libc/test/config/darwin/x86_64/darwin.cpp
@@ -0,0 +1,24 @@
+//===-- Unittests for strlen          -------------------------------------===//
+//
+// 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/string/strlen.h"
+#include "utils/UnitTest/Test.h"
+
+TEST(StrLenTest, EmptyString) {
+  const char *empty = "";
+
+  size_t result = __llvm_libc::strlen(empty);
+  ASSERT_EQ((size_t)0, result);
+}
+
+TEST(StrLenTest, AnyString) {
+  const char *any = "Hello World!";
+
+  size_t result = __llvm_libc::strlen(any);
+  ASSERT_EQ((size_t)12, result);
+}