diff --git a/libc/cmake/modules/LLVMLibCCompileOptionRules.cmake b/libc/cmake/modules/LLVMLibCCompileOptionRules.cmake
index 2eb0f066e19e0..042350546b1c1 100644
--- a/libc/cmake/modules/LLVMLibCCompileOptionRules.cmake
+++ b/libc/cmake/modules/LLVMLibCCompileOptionRules.cmake
@@ -170,6 +170,19 @@ function(_get_common_compile_options output_var flags)
           list(APPEND compile_options "-idirafter${LIBC_KERNEL_HEADERS}")
         endif()
       endif()
+      if(LIBC_TARGET_OS_IS_DARWIN)
+        execute_process(
+          COMMAND xcrun --sdk macosx --show-sdk-path
+          OUTPUT_VARIABLE MACOSX_SDK_PATH
+          RESULT_VARIABLE MACOSX_SDK_PATH_RESULT
+          OUTPUT_STRIP_TRAILING_WHITESPACE
+        )
+        if(MACOSX_SDK_PATH_RESULT EQUAL 0)
+          list(APPEND compile_options "-I" "${MACOSX_SDK_PATH}/usr/include")
+        else()
+          message(WARNING "Could not find macOS SDK path. `xcrun --sdk macosx --show-sdk-path` failed.")
+        endif()
+      endif()
     endif()
 
     if(LIBC_COMPILER_HAS_FIXED_POINT)
diff --git a/libc/config/darwin/aarch64/entrypoints.txt b/libc/config/darwin/aarch64/entrypoints.txt
index e3c6c2b30c415..3909417f9730d 100644
--- a/libc/config/darwin/aarch64/entrypoints.txt
+++ b/libc/config/darwin/aarch64/entrypoints.txt
@@ -111,6 +111,7 @@ if(LLVM_LIBC_FULL_BUILD)
     libc.src.setjmp.setjmp
     libc.src.setjmp.siglongjmp
     libc.src.setjmp.sigsetjmp
+    libc.src.stdlib._Exit
   )
 endif()
 
diff --git a/libc/include/llvm-libc-macros/darwin/CMakeLists.txt b/libc/include/llvm-libc-macros/darwin/CMakeLists.txt
new file mode 100644
index 0000000000000..ea08c63c00301
--- /dev/null
+++ b/libc/include/llvm-libc-macros/darwin/CMakeLists.txt
@@ -0,0 +1,5 @@
+add_header(
+  time_macros
+  HDR
+    time-macros.h
+)
diff --git a/libc/include/llvm-libc-macros/darwin/time-macros.h b/libc/include/llvm-libc-macros/darwin/time-macros.h
new file mode 100644
index 0000000000000..477dfa8eda85f
--- /dev/null
+++ b/libc/include/llvm-libc-macros/darwin/time-macros.h
@@ -0,0 +1,14 @@
+//===-- Definition of macros from time.h ---------------------------------===//
+//
+// 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_MACROS_LINUX_TIME_MACROS_H
+#define LLVM_LIBC_MACROS_LINUX_TIME_MACROS_H
+
+#include <_time.h>
+
+#endif // LLVM_LIBC_MACROS_LINUX_TIME_MACROS_H
diff --git a/libc/include/llvm-libc-macros/time-macros.h b/libc/include/llvm-libc-macros/time-macros.h
index 30e0a310a5485..c026df29b1e7f 100644
--- a/libc/include/llvm-libc-macros/time-macros.h
+++ b/libc/include/llvm-libc-macros/time-macros.h
@@ -7,6 +7,8 @@
 #include "linux/time-macros.h"
 #elif defined(__ELF__)
 #include "baremetal/time-macros.h"
+#elif defined(__APPLE__)
+#include "darwin/time-macros.h"
 #else
 #define CLOCKS_PER_SEC 1000000
 #endif
diff --git a/libc/include/llvm-libc-types/clockid_t.h b/libc/include/llvm-libc-types/clockid_t.h
index 4b059599502c4..926948717c664 100644
--- a/libc/include/llvm-libc-types/clockid_t.h
+++ b/libc/include/llvm-libc-types/clockid_t.h
@@ -9,6 +9,12 @@
 #ifndef LLVM_LIBC_TYPES_CLOCKID_T_H
 #define LLVM_LIBC_TYPES_CLOCKID_T_H
 
+#if defined(__APPLE__)
+// Darwin provides its own defintion for clockid_t . Use that to prevent
+// redeclaration errors and correctness.
+#include <_time.h>
+#else
 typedef int clockid_t;
+#endif // __APPLE__
 
 #endif // LLVM_LIBC_TYPES_CLOCKID_T_H
diff --git a/libc/include/llvm-libc-types/struct_timespec.h b/libc/include/llvm-libc-types/struct_timespec.h
index 28b5a571f6790..8993ecc7db8f0 100644
--- a/libc/include/llvm-libc-types/struct_timespec.h
+++ b/libc/include/llvm-libc-types/struct_timespec.h
@@ -9,6 +9,11 @@
 #ifndef LLVM_LIBC_TYPES_STRUCT_TIMESPEC_H
 #define LLVM_LIBC_TYPES_STRUCT_TIMESPEC_H
 
+#if defined(__APPLE__)
+// Darwin provides its own definition for struct timespec. Include it directly
+// to ensure type compatibility and avoid redefinition errors.
+#include <sys/_types/_timespec.h>
+#else
 #include "time_t.h"
 
 struct timespec {
@@ -16,5 +21,6 @@ struct timespec {
   /* TODO: BIG_ENDIAN may require padding. */
   long tv_nsec; /* Nanoseconds.  */
 };
+#endif // __APPLE__
 
 #endif // LLVM_LIBC_TYPES_STRUCT_TIMESPEC_H
diff --git a/libc/include/llvm-libc-types/struct_timeval.h b/libc/include/llvm-libc-types/struct_timeval.h
index 9595d85a46c8f..41f0b4e92932e 100644
--- a/libc/include/llvm-libc-types/struct_timeval.h
+++ b/libc/include/llvm-libc-types/struct_timeval.h
@@ -12,9 +12,15 @@
 #include "suseconds_t.h"
 #include "time_t.h"
 
+#if defined(__APPLE__)
+// Darwin provides its own definition for struct timeval. Include it directly
+// to ensure type compatibility and avoid redefinition errors.
+#include <sys/_types/_timeval.h>
+#else
 struct timeval {
   time_t tv_sec;       // Seconds
   suseconds_t tv_usec; // Micro seconds
 };
+#endif // __APPLE__
 
 #endif // LLVM_LIBC_TYPES_STRUCT_TIMEVAL_H
diff --git a/libc/include/llvm-libc-types/suseconds_t.h b/libc/include/llvm-libc-types/suseconds_t.h
index 8e926e8401f5c..acc1822cb59e1 100644
--- a/libc/include/llvm-libc-types/suseconds_t.h
+++ b/libc/include/llvm-libc-types/suseconds_t.h
@@ -14,6 +14,12 @@
 // types...] and suseconds_t are no greater than the width of type long.
 
 // The kernel expects 64 bit suseconds_t at least on x86_64.
+#if defined(__APPLE__)
+// Darwin provides its own definition for suseconds_t. Include it directly
+// to ensure type compatibility and avoid redefinition errors.
+#include <sys/_types/_suseconds_t.h>
+#else
 typedef long suseconds_t;
+#endif // __APPLE__
 
 #endif // LLVM_LIBC_TYPES_SUSECONDS_T_H
diff --git a/libc/include/llvm-libc-types/time_t_32.h b/libc/include/llvm-libc-types/time_t_32.h
index 2c415f6fa9dca..8d7a81e5ce7f7 100644
--- a/libc/include/llvm-libc-types/time_t_32.h
+++ b/libc/include/llvm-libc-types/time_t_32.h
@@ -9,6 +9,12 @@
 #ifndef LLVM_LIBC_TYPES_TIME_T_32_H
 #define LLVM_LIBC_TYPES_TIME_T_32_H
 
+#if defined(__APPLE__)
+// Darwin provides its own definition for time_t. Include it directly
+// to ensure type compatibility and avoid redefinition errors.
+#include <sys/_types/_time_t.h>
+#else
 typedef __INT32_TYPE__ time_t;
+#endif // __APPLE__
 
 #endif // LLVM_LIBC_TYPES_TIME_T_32_H
diff --git a/libc/include/llvm-libc-types/time_t_64.h b/libc/include/llvm-libc-types/time_t_64.h
index 8f7fd3233646e..c8267abe31289 100644
--- a/libc/include/llvm-libc-types/time_t_64.h
+++ b/libc/include/llvm-libc-types/time_t_64.h
@@ -9,6 +9,12 @@
 #ifndef LLVM_LIBC_TYPES_TIME_T_64_H
 #define LLVM_LIBC_TYPES_TIME_T_64_H
 
+#if defined(__APPLE__)
+// Darwin provides its own definition for time_t. Include it directly
+// to ensure type compatibility and avoid redefinition errors.
+#include <sys/_types/_time_t.h>
+#else
 typedef __INT64_TYPE__ time_t;
+#endif // __APPLE__
 
 #endif // LLVM_LIBC_TYPES_TIME_T_64_H
diff --git a/libc/include/sys/syscall.h.def b/libc/include/sys/syscall.h.def
index 60e5024e500e3..112fdd2d80369 100644
--- a/libc/include/sys/syscall.h.def
+++ b/libc/include/sys/syscall.h.def
@@ -9,7 +9,11 @@
 #ifndef LLVM_LIBC_SYS_SYSCALL_H
 #define LLVM_LIBC_SYS_SYSCALL_H
 
-//TODO: Handle non-linux syscalls
+#if defined(__APPLE__)
+
+#include <sys/syscall.h>
+
+#elif defined(__linux__)
 
 #include <asm/unistd.h>
 
@@ -2361,5 +2365,6 @@
 #define SYS_writev __NR_writev
 #endif
 
+#endif // __linux__
 
 #endif // LLVM_LIBC_SYS_SYSCALL_H
diff --git a/libc/src/__support/OSUtil/darwin/CMakeLists.txt b/libc/src/__support/OSUtil/darwin/CMakeLists.txt
index 4241bb37684f7..9e69bf7d0cbab 100644
--- a/libc/src/__support/OSUtil/darwin/CMakeLists.txt
+++ b/libc/src/__support/OSUtil/darwin/CMakeLists.txt
@@ -4,13 +4,16 @@ endif()
 
 add_subdirectory(${LIBC_TARGET_ARCHITECTURE})
 
-add_header_library(
+add_object_library(
   darwin_util
+  SRCS
+    exit.cpp
   HDRS
     io.h
     syscall.h
   DEPENDS
-    .${LIBC_TARGET_ARCHITECTURE}.darwin_util
+  .${LIBC_TARGET_ARCHITECTURE}.darwin_${LIBC_TARGET_ARCHITECTURE}_util
     libc.src.__support.common
     libc.src.__support.CPP.string_view
+    libc.include.sys_syscall
 )
diff --git a/libc/src/__support/OSUtil/darwin/aarch64/CMakeLists.txt b/libc/src/__support/OSUtil/darwin/aarch64/CMakeLists.txt
index 5ab95b01758c8..b36fe22017f34 100644
--- a/libc/src/__support/OSUtil/darwin/aarch64/CMakeLists.txt
+++ b/libc/src/__support/OSUtil/darwin/aarch64/CMakeLists.txt
@@ -1,5 +1,5 @@
 add_header_library(
-  darwin_util
+  darwin_aarch64_util
   HDRS
     syscall.h
   DEPENDS
diff --git a/libc/src/__support/OSUtil/darwin/exit.cpp b/libc/src/__support/OSUtil/darwin/exit.cpp
new file mode 100644
index 0000000000000..7439db2ef38b0
--- /dev/null
+++ b/libc/src/__support/OSUtil/darwin/exit.cpp
@@ -0,0 +1,24 @@
+//===------------ MacOS implementation of an exit function ------*- C++ -*-===//
+//
+// 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/__support/OSUtil/darwin/syscall.h" // syscall_impl
+#include "src/__support/common.h"
+#include "src/__support/macros/config.h"
+#include "sys/syscall.h" // For syscall numbers.
+
+namespace LIBC_NAMESPACE_DECL {
+namespace internal {
+
+[[noreturn]] void exit(int status) {
+  for (;;) {
+    LIBC_NAMESPACE::syscall_impl<long>(SYS_exit, status);
+  }
+}
+
+} // namespace internal
+} // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/src/__support/threads/darwin/CMakeLists.txt b/libc/src/__support/threads/darwin/CMakeLists.txt
new file mode 100644
index 0000000000000..2a7ce0676f68f
--- /dev/null
+++ b/libc/src/__support/threads/darwin/CMakeLists.txt
@@ -0,0 +1,11 @@
+if(NOT TARGET libc.src.__support.OSUtil.osutil)
+  return()
+endif()
+
+add_header_library(
+  mutex
+  HDRS
+    mutex.h
+  DEPENDS
+    libc.src.__support.threads.mutex_common
+)
diff --git a/libc/src/__support/threads/darwin/mutex.h b/libc/src/__support/threads/darwin/mutex.h
new file mode 100644
index 0000000000000..f48803ebe1e7d
--- /dev/null
+++ b/libc/src/__support/threads/darwin/mutex.h
@@ -0,0 +1,132 @@
+//===--- Implementation of a Darwin mutex class ------------------*- C++
+//-*-===//
+//
+// 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_THREADS_DARWIN_MUTEX_H
+#define LLVM_LIBC_SRC___SUPPORT_THREADS_DARWIN_MUTEX_H
+
+#include "src/__support/libc_assert.h"
+#include "src/__support/macros/config.h"
+#include "src/__support/threads/mutex_common.h"
+#include "src/__support/threads/sleep.h" // For sleep_briefly
+#include "src/__support/time/linux/abs_timeout.h"
+
+#include <mach/mach_init.h> // For mach_thread_self
+#include <mach/mach_port.h> // For mach_port_t and MACH_PORT_NULL
+#include <os/lock.h>        // For os_unfair_lock
+#include <time.h>           // For clock_gettime
+
+namespace LIBC_NAMESPACE_DECL {
+
+// This file is an implementation of `LIBC_NAMESPACE::mutex` for Darwin-based
+// platforms. It is a wrapper around `os_unfair_lock`, which is a low-level,
+// high-performance locking primitive provided by the kernel.
+//
+// `os_unfair_lock` is a non-recursive, thread-owned lock that blocks waiters
+// efficiently in the kernel. As the name implies, it is "unfair," meaning
+// it does not guarantee the order in which waiting threads acquire the lock.
+// This trade-off allows for higher performance in contended scenarios.
+//
+// The lock must be unlocked from the same thread that locked it. Attempting
+// to unlock from a different thread will result in a runtime error.
+//
+// This implementation is suitable for simple critical sections where fairness
+// and reentrancy are not concerns.
+
+class Mutex final {
+  os_unfair_lock_s lock_val = OS_UNFAIR_LOCK_INIT;
+  mach_port_t owner = MACH_PORT_NULL;
+
+  // API compatibility fields.
+  unsigned char timed;
+  unsigned char recursive;
+  unsigned char robust;
+  unsigned char pshared;
+
+public:
+  LIBC_INLINE constexpr Mutex(bool is_timed, bool is_recursive, bool is_robust,
+                              bool is_pshared)
+      : owner(MACH_PORT_NULL), timed(is_timed), recursive(is_recursive),
+        robust(is_robust), pshared(is_pshared) {}
+
+  LIBC_INLINE constexpr Mutex()
+      : owner(MACH_PORT_NULL), timed(0), recursive(0), robust(0), pshared(0) {}
+
+  LIBC_INLINE static MutexError init(Mutex *mutex, bool is_timed, bool is_recur,
+                                     bool is_robust, bool is_pshared) {
+    mutex->lock_val = OS_UNFAIR_LOCK_INIT;
+    mutex->owner = MACH_PORT_NULL;
+    mutex->timed = is_timed;
+    mutex->recursive = is_recur;
+    mutex->robust = is_robust;
+    mutex->pshared = is_pshared;
+    return MutexError::NONE;
+  }
+
+  LIBC_INLINE static MutexError destroy(Mutex *lock) {
+    LIBC_ASSERT(lock->owner == MACH_PORT_NULL &&
+                "Mutex destroyed while locked.");
+    return MutexError::NONE;
+  }
+
+  LIBC_INLINE MutexError lock() {
+    os_unfair_lock_lock(&lock_val);
+    owner = mach_thread_self();
+    return MutexError::NONE;
+  }
+
+  LIBC_INLINE MutexError timed_lock(internal::AbsTimeout abs_time) {
+    while (true) {
+      if (try_lock() == MutexError::NONE) {
+        return MutexError::NONE;
+      }
+
+      // Manually check if the timeout has expired.
+      struct timespec now;
+      // The clock used here must match the clock used to create the
+      // absolute timeout.
+      clock_gettime(abs_time.is_realtime() ? CLOCK_REALTIME : CLOCK_MONOTONIC,
+                    &now);
+      const timespec &target_ts = abs_time.get_timespec();
+
+      if (now.tv_sec > target_ts.tv_sec || (now.tv_sec == target_ts.tv_sec &&
+                                            now.tv_nsec >= target_ts.tv_nsec)) {
+        // We might have acquired the lock between the last try_lock() and now.
+        // To avoid returning TIMEOUT incorrectly, we do one last try_lock().
+        if (try_lock() == MutexError::NONE)
+          return MutexError::NONE;
+        return MutexError::TIMEOUT;
+      }
+
+      sleep_briefly();
+    }
+  }
+
+  LIBC_INLINE MutexError unlock() {
+    // This check is crucial. It prevents both double-unlocks and unlocks
+    // by threads that do not own the mutex.
+    if (owner != mach_thread_self()) {
+      return MutexError::UNLOCK_WITHOUT_LOCK;
+    }
+    owner = MACH_PORT_NULL;
+    os_unfair_lock_unlock(&lock_val);
+    return MutexError::NONE;
+  }
+
+  LIBC_INLINE MutexError try_lock() {
+    if (os_unfair_lock_trylock(&lock_val)) {
+      owner = mach_thread_self();
+      return MutexError::NONE;
+    }
+    return MutexError::BUSY;
+  }
+};
+
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SRC___SUPPORT_THREADS_DARWIN_MUTEX_H
diff --git a/libc/src/__support/threads/mutex.h b/libc/src/__support/threads/mutex.h
index f64f7e7b40082..e2401507d86ac 100644
--- a/libc/src/__support/threads/mutex.h
+++ b/libc/src/__support/threads/mutex.h
@@ -42,7 +42,9 @@
 
 #if defined(__linux__)
 #include "src/__support/threads/linux/mutex.h"
-#endif // __linux__
+#elif defined(__APPLE__)
+#include "src/__support/threads/darwin/mutex.h"
+#endif
 
 #elif LIBC_THREAD_MODE == LIBC_THREAD_MODE_SINGLE
 
diff --git a/libc/src/__support/time/darwin/CMakeLists.txt b/libc/src/__support/time/darwin/CMakeLists.txt
new file mode 100644
index 0000000000000..a06a41289a41c
--- /dev/null
+++ b/libc/src/__support/time/darwin/CMakeLists.txt
@@ -0,0 +1,12 @@
+add_object_library(
+  clock_gettime
+  SRCS
+    clock_gettime.cpp
+  HDRS
+    ../clock_gettime.h
+  DEPENDS
+    libc.src.__support.common
+    libc.src.__support.error_or
+    libc.hdr.types.struct_timeval
+    libc.hdr.types.struct_timespec
+)
diff --git a/libc/src/__support/time/darwin/clock_gettime.cpp b/libc/src/__support/time/darwin/clock_gettime.cpp
new file mode 100644
index 0000000000000..158737a6251a0
--- /dev/null
+++ b/libc/src/__support/time/darwin/clock_gettime.cpp
@@ -0,0 +1,44 @@
+//===-- Darwin implementation of internal clock_gettime -------------------===//
+//
+// 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/__support/time/clock_gettime.h"
+#include "hdr/errno_macros.h" // For EINVAL
+#include "hdr/time_macros.h"
+#include "hdr/types/struct_timespec.h"
+#include "hdr/types/struct_timeval.h"
+#include "src/__support/OSUtil/syscall.h" // For syscall_impl
+#include "src/__support/common.h"
+#include "src/__support/error_or.h"
+#include <sys/syscall.h> // For SYS_gettimeofday
+#include <sys/time.h>    // For struct timezone
+
+namespace LIBC_NAMESPACE_DECL {
+namespace internal {
+
+ErrorOr<int> clock_gettime(clockid_t clockid, struct timespec *ts) {
+  if (clockid != CLOCK_REALTIME) {
+    return Error(EINVAL);
+  }
+  struct timeval tv;
+  // The second argument to gettimeofday is a timezone pointer
+  // The third argument is mach_absolute_time
+  // Both of these, we don't need here, so they are 0
+  long ret = LIBC_NAMESPACE::syscall_impl<long>(
+      SYS_gettimeofday, reinterpret_cast<long>(&tv), 0, 0);
+  if (ret != 0) {
+    // The syscall returns -1 on error and sets errno.
+    return Error(EINVAL);
+  }
+
+  ts->tv_sec = tv.tv_sec;
+  ts->tv_nsec = tv.tv_usec * 1000;
+  return 0;
+}
+
+} // namespace internal
+} // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/src/time/darwin/CMakeLists.txt b/libc/src/time/darwin/CMakeLists.txt
new file mode 100644
index 0000000000000..6d68086c72584
--- /dev/null
+++ b/libc/src/time/darwin/CMakeLists.txt
@@ -0,0 +1,10 @@
+add_entrypoint_object(
+  clock_gettime
+  SRCS
+    clock_gettime.cpp
+  HDRS
+    # The public header is part of the parent directory's library.
+  DEPENDS
+    libc.src.__support.time.clock_gettime
+    libc.src.errno.errno
+)
diff --git a/libc/src/time/darwin/clock_gettime.cpp b/libc/src/time/darwin/clock_gettime.cpp
new file mode 100644
index 0000000000000..f717d05451aca
--- /dev/null
+++ b/libc/src/time/darwin/clock_gettime.cpp
@@ -0,0 +1,29 @@
+//===---------- Darwin implementation of the POSIX clock_gettime function
+//--===//
+//
+// 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/time/clock_gettime.h"
+
+#include "src/__support/common.h"
+#include "src/__support/libc_errno.h"
+#include "src/__support/macros/config.h"
+#include "src/__support/time/clock_gettime.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+LLVM_LIBC_FUNCTION(int, clock_gettime,
+                   (clockid_t clockid, struct timespec *ts)) {
+  auto result = internal::clock_gettime(clockid, ts);
+  if (!result.has_value()) {
+    libc_errno = result.error();
+    return -1;
+  }
+  return 0;
+}
+
+} // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/test/UnitTest/CMakeLists.txt b/libc/test/UnitTest/CMakeLists.txt
index 31d1e9dce8204..028e8e3a2ea67 100644
--- a/libc/test/UnitTest/CMakeLists.txt
+++ b/libc/test/UnitTest/CMakeLists.txt
@@ -83,7 +83,7 @@ add_unittest_framework_library(
 )
 
 set(libc_death_test_srcs LibcDeathTestExecutors.cpp)
-if(${LIBC_TARGET_OS} STREQUAL "linux")
+if(${LIBC_TARGET_OS} STREQUAL "linux" OR ${LIBC_TARGET_OS} STREQUAL "darwin")
   list(APPEND libc_death_test_srcs ExecuteFunctionUnix.cpp)
 endif()
 
diff --git a/libc/test/UnitTest/ExecuteFunctionUnix.cpp b/libc/test/UnitTest/ExecuteFunctionUnix.cpp
index c0e85c2144005..7c2eb7c6e887c 100644
--- a/libc/test/UnitTest/ExecuteFunctionUnix.cpp
+++ b/libc/test/UnitTest/ExecuteFunctionUnix.cpp
@@ -57,9 +57,7 @@ ProcessStatus invoke_in_subprocess(FunctionCaller *func, int timeout_ms) {
   }
   ::close(pipe_fds[1]);
 
-  struct pollfd poll_fd {
-    pipe_fds[0], 0, 0
-  };
+  struct pollfd poll_fd{pipe_fds[0], POLLIN, 0};
   // No events requested so this call will only return after the timeout or if
   // the pipes peer was closed, signaling the process exited.
   if (::poll(&poll_fd, 1, timeout_ms) == -1) {
diff --git a/libc/test/src/__support/threads/darwin/CMakeLists.txt b/libc/test/src/__support/threads/darwin/CMakeLists.txt
new file mode 100644
index 0000000000000..50b8259a9fde9
--- /dev/null
+++ b/libc/test/src/__support/threads/darwin/CMakeLists.txt
@@ -0,0 +1,9 @@
+add_libc_test(
+  mutex_test
+  SUITE
+    libc-support-threads-tests
+  SRCS
+    mutex_test.cpp
+  DEPENDS
+    libc.src.__support.threads.darwin.mutex
+)
diff --git a/libc/test/src/__support/threads/darwin/mutex_test.cpp b/libc/test/src/__support/threads/darwin/mutex_test.cpp
new file mode 100644
index 0000000000000..e8b46fdb7ebe3
--- /dev/null
+++ b/libc/test/src/__support/threads/darwin/mutex_test.cpp
@@ -0,0 +1,78 @@
+//===-- Unittests for Darwin's Mutex ------------------------------------===//
+//
+// 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/__support/threads/darwin/mutex.h"
+#include "src/__support/threads/mutex_common.h"
+#include "test/UnitTest/Test.h"
+
+TEST(LlvmLibcSupportThreadsMutexTest, SmokeTest) {
+  LIBC_NAMESPACE::Mutex mutex;
+  ASSERT_EQ(mutex.lock(), LIBC_NAMESPACE::MutexError::NONE);
+  ASSERT_EQ(mutex.unlock(), LIBC_NAMESPACE::MutexError::NONE);
+  ASSERT_EQ(mutex.try_lock(), LIBC_NAMESPACE::MutexError::NONE);
+  ASSERT_EQ(mutex.try_lock(), LIBC_NAMESPACE::MutexError::BUSY);
+  ASSERT_EQ(mutex.unlock(), LIBC_NAMESPACE::MutexError::NONE);
+  ASSERT_EQ(mutex.unlock(), LIBC_NAMESPACE::MutexError::UNLOCK_WITHOUT_LOCK);
+}
+
+// TEST(LlvmLibcSupportThreadsRawMutexTest, Timeout) {
+//   LIBC_NAMESPACE::RawMutex mutex;
+//   ASSERT_TRUE(mutex.lock());
+//   timespec ts;
+//   LIBC_NAMESPACE::internal::clock_gettime(CLOCK_MONOTONIC, &ts);
+//   ts.tv_sec += 1;
+//   // Timeout will be respected when deadlock happens.
+//   auto timeout = LIBC_NAMESPACE::internal::AbsTimeout::from_timespec(ts,
+//   false); ASSERT_TRUE(timeout.has_value());
+//   // The following will timeout
+//   ASSERT_FALSE(mutex.lock(*timeout));
+//   ASSERT_TRUE(mutex.unlock());
+//   // Test that the mutex works after the timeout.
+//   ASSERT_TRUE(mutex.lock());
+//   ASSERT_TRUE(mutex.unlock());
+//   // If a lock can be acquired directly, expired timeout will not count.
+//   // Notice that the timeout is already reached during preivous deadlock.
+//   ASSERT_TRUE(mutex.lock(*timeout));
+//   ASSERT_TRUE(mutex.unlock());
+// }
+//
+// TEST(LlvmLibcSupportThreadsRawMutexTest, PSharedLock) {
+//   struct SharedData {
+//     LIBC_NAMESPACE::RawMutex mutex;
+//     LIBC_NAMESPACE::cpp::Atomic<size_t> finished;
+//     int data;
+//   };
+//   void *addr =
+//       LIBC_NAMESPACE::mmap(nullptr, sizeof(SharedData), PROT_READ |
+//       PROT_WRITE,
+//                            MAP_ANONYMOUS | MAP_SHARED, -1, 0);
+//   ASSERT_NE(addr, MAP_FAILED);
+//   auto *shared = reinterpret_cast<SharedData *>(addr);
+//   shared->data = 0;
+//   LIBC_NAMESPACE::RawMutex::init(&shared->mutex);
+//   // Avoid pull in our own implementation of pthread_t.
+// #ifdef SYS_fork
+//   long pid = LIBC_NAMESPACE::syscall_impl<long>(SYS_fork);
+// #elif defined(SYS_clone)
+//   long pid = LIBC_NAMESPACE::syscall_impl<long>(SYS_clone, SIGCHLD, 0);
+// #endif
+//   for (int i = 0; i < 10000; ++i) {
+//     shared->mutex.lock(LIBC_NAMESPACE::cpp::nullopt, true);
+//     shared->data++;
+//     shared->mutex.unlock(true);
+//   }
+//   // Mark the thread as finished.
+//   shared->finished.fetch_add(1);
+//   // let the child exit early to avoid output pollution
+//   if (pid == 0)
+//     LIBC_NAMESPACE::exit(0);
+//   while (shared->finished.load() != 2)
+//     LIBC_NAMESPACE::sleep_briefly();
+//   ASSERT_EQ(shared->data, 20000);
+//   LIBC_NAMESPACE::munmap(addr, sizeof(SharedData));
+// }
diff --git a/libc/test/src/__support/time/darwin/CMakeLists.txt b/libc/test/src/__support/time/darwin/CMakeLists.txt
new file mode 100644
index 0000000000000..ee1247b354173
--- /dev/null
+++ b/libc/test/src/__support/time/darwin/CMakeLists.txt
@@ -0,0 +1,8 @@
+add_libc_test(
+  clock_gettime
+  SUITE libc-support-time-tests
+  SRCS clock_gettime.cpp
+  DEPENDS
+    libc.src.__support.CPP.expected
+    libc.src.__support.time.darwin.clock_gettime
+)
diff --git a/libc/test/src/__support/time/darwin/clock_gettime.cpp b/libc/test/src/__support/time/darwin/clock_gettime.cpp
new file mode 100644
index 0000000000000..d593c5d02744a
--- /dev/null
+++ b/libc/test/src/__support/time/darwin/clock_gettime.cpp
@@ -0,0 +1,20 @@
+//===-- unit tests for darwin's time utilities --------------------------===//
+//
+// 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/__support/time/clock_gettime.h"
+#include "src/__support/CPP/expected.h"
+#include "test/UnitTest/Test.h"
+
+template <class T, class E>
+using expected = LIBC_NAMESPACE::cpp::expected<T, E>;
+
+TEST(LlvmLibcSupportDarwinClockGetTime, BasicGetTime) {
+  struct timespec ts;
+  auto result = LIBC_NAMESPACE::internal::clock_gettime(CLOCK_REALTIME, &ts);
+  ASSERT_TRUE(result.has_value());
+}
diff --git a/libc/test/src/stdlib/CMakeLists.txt b/libc/test/src/stdlib/CMakeLists.txt
index 0eb373c3fa061..9765e42095aa5 100644
--- a/libc/test/src/stdlib/CMakeLists.txt
+++ b/libc/test/src/stdlib/CMakeLists.txt
@@ -397,6 +397,19 @@ if(LLVM_LIBC_FULL_BUILD)
       libc-stdlib-tests
     SRCS
       _Exit_test.cpp
+    DEPENDS
+      libc.src.__support.OSUtil.osutil
+      libc.src.stdlib._Exit
+  )
+
+  add_libc_test(
+    exit_test
+    # The EXPECT_EXITS test is only availible for unit tests.
+    UNIT_TEST_ONLY
+    SUITE
+      libc-stdlib-tests
+    SRCS
+      exit_test.cpp
     DEPENDS
       libc.src.stdlib._Exit
       libc.src.stdlib.exit
diff --git a/libc/test/src/stdlib/_Exit_test.cpp b/libc/test/src/stdlib/_Exit_test.cpp
index 333277dc01dca..57c432828c2f3 100644
--- a/libc/test/src/stdlib/_Exit_test.cpp
+++ b/libc/test/src/stdlib/_Exit_test.cpp
@@ -7,13 +7,9 @@
 //===----------------------------------------------------------------------===//
 
 #include "src/stdlib/_Exit.h"
-#include "src/stdlib/exit.h"
 #include "test/UnitTest/Test.h"
 
 TEST(LlvmLibcStdlib, _Exit) {
   EXPECT_EXITS([] { LIBC_NAMESPACE::_Exit(1); }, 1);
   EXPECT_EXITS([] { LIBC_NAMESPACE::_Exit(65); }, 65);
-
-  EXPECT_EXITS([] { LIBC_NAMESPACE::exit(1); }, 1);
-  EXPECT_EXITS([] { LIBC_NAMESPACE::exit(65); }, 65);
 }
diff --git a/libc/test/src/stdlib/exit_test.cpp b/libc/test/src/stdlib/exit_test.cpp
new file mode 100644
index 0000000000000..5c82d8303036a
--- /dev/null
+++ b/libc/test/src/stdlib/exit_test.cpp
@@ -0,0 +1,15 @@
+//===-- Unittests for exit -----------------------------------------------===//
+//
+// 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/stdlib/exit.h"
+#include "test/UnitTest/Test.h"
+
+TEST(LlvmLibcStdlib, exit) {
+  EXPECT_EXITS([] { LIBC_NAMESPACE::exit(1); }, 1);
+  EXPECT_EXITS([] { LIBC_NAMESPACE::exit(65); }, 65);
+}