Index: libc/src/threads/linux/thread_utils.h
===================================================================
--- libc/src/threads/linux/thread_utils.h
+++ libc/src/threads/linux/thread_utils.h
@@ -9,9 +9,14 @@
 #ifndef LLVM_LIBC_SRC_THREADS_LINUX_THREAD_UTILS_H
 #define LLVM_LIBC_SRC_THREADS_LINUX_THREAD_UTILS_H
 
+#include "include/threads.h"
+
+#include <stdatomic.h>
 #include <stdint.h>
 
-using FutexData = _Atomic uint32_t;
+using FutexData = atomic_uint_least32_t;
+static_assert(sizeof(FutexData) == sizeof(thrd_t::__clear_tid),
+              "FutexData must be the same size as thrd_t::__clear_tid");
 
 struct ThreadParams {
   static constexpr uintptr_t DefaultStackSize = 1 << 15; // 32 KB