Index: CMakeLists.txt
===================================================================
--- CMakeLists.txt
+++ CMakeLists.txt
@@ -58,6 +58,7 @@
 option(LIBCXX_ENABLE_STDIN "Build libc++ with support for stdin/std::cin." ON)
 option(LIBCXX_ENABLE_STDOUT "Build libc++ with support for stdout/std::cout." ON)
 option(LIBCXX_ENABLE_THREADS "Build libc++ with support for threads." ON)
+option(LIBCXX_ENABLE_THREAD_UNSAFE_C_FUNCTIONS "Build libc++ with support for thread-unsafe C functions" ON)
 option(LIBCXX_BUILD_32_BITS "Build 32 bit libc++" OFF)
 option(LIBCXX_ENABLE_MONOTONIC_CLOCK
   "Build libc++ with support for a monotonic clock.
@@ -283,6 +284,11 @@
                       " when LIBCXX_ENABLE_THREADS is also set to OFF.")
 endif()
 
+# LIBCXX_ENABLE_THREAD_UNSAFE_C_FUNCTIONS configuration
+if (NOT LIBCXX_ENABLE_THREAD_UNSAFE_C_FUNCTIONS)
+  add_definitions(-D_LIBCPP_HAS_NO_THREAD_UNSAFE_C_FUNCTIONS)
+endif()
+
 # Configure for sanitizers. If LIBCXX_BUILT_STANDALONE then we have to do
 # the flag translation ourselves. Othewise LLVM's CMakeList.txt will handle it.
 if (LIBCXX_BUILT_STANDALONE)
Index: include/__config
===================================================================
--- include/__config
+++ include/__config
@@ -751,4 +751,10 @@
 #define _LIBCPP_PROVIDES_DEFAULT_RUNE_TABLE
 #endif
 
+// Thread-unsafe functions such as strtok(), mbtowc() and localtime()
+// are not available.
+#ifdef __CloudABI__
+#define _LIBCPP_HAS_NO_THREAD_UNSAFE_C_FUNCTIONS
+#endif
+
 #endif  // _LIBCPP_CONFIG
Index: include/clocale
===================================================================
--- include/clocale
+++ include/clocale
@@ -45,7 +45,9 @@
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 using ::lconv;
+#ifndef _LIBCPP_HAS_NO_THREAD_UNSAFE_C_FUNCTIONS
 using ::setlocale;
+#endif
 using ::localeconv;
 
 _LIBCPP_END_NAMESPACE_STD
Index: include/cstdlib
===================================================================
--- include/cstdlib
+++ include/cstdlib
@@ -147,9 +147,11 @@
 #undef lldiv
 using ::lldiv;
 #endif // _LIBCPP_HAS_NO_LONG_LONG
+#ifndef _LIBCPP_HAS_NO_THREAD_UNSAFE_C_FUNCTIONS
 using ::mblen;
 using ::mbtowc;
 using ::wctomb;
+#endif
 using ::mbstowcs;
 using ::wcstombs;
 #ifdef _LIBCPP_HAS_QUICK_EXIT
Index: include/cstring
===================================================================
--- include/cstring
+++ include/cstring
@@ -102,7 +102,9 @@
 inline _LIBCPP_INLINE_VISIBILITY       char* strstr(      char* __s1, const char* __s2) {return ::strstr(__s1, __s2);}
 #endif
 
+#ifndef _LIBCPP_HAS_NO_THREAD_UNSAFE_C_FUNCTIONS
 using ::strtok;
+#endif
 using ::memset;
 using ::strerror;
 using ::strlen;
Index: include/ctime
===================================================================
--- include/ctime
+++ include/ctime
@@ -61,10 +61,12 @@
 using ::difftime;
 using ::mktime;
 using ::time;
+#ifndef _LIBCPP_HAS_NO_THREAD_UNSAFE_C_FUNCTIONS
 using ::asctime;
 using ::ctime;
 using ::gmtime;
 using ::localtime;
+#endif
 using ::strftime;
 
 _LIBCPP_END_NAMESPACE_STD
Index: src/locale.cpp
===================================================================
--- src/locale.cpp
+++ src/locale.cpp
@@ -575,8 +575,10 @@
     locale& g = __global();
     locale r = g;
     g = loc;
+#ifndef _LIBCPP_HAS_NO_THREAD_UNSAFE_C_FUNCTIONS
     if (g.name() != "*")
         setlocale(LC_ALL, g.name().c_str());
+#endif
     return r;
 }
 
Index: test/CMakeLists.txt
===================================================================
--- test/CMakeLists.txt
+++ test/CMakeLists.txt
@@ -46,6 +46,7 @@
   pythonize_bool(LIBCXX_ENABLE_STDIN)
   pythonize_bool(LIBCXX_ENABLE_STDOUT)
   pythonize_bool(LIBCXX_ENABLE_THREADS)
+  pythonize_bool(LIBCXX_ENABLE_THREAD_UNSAFE_C_FUNCTIONS)
   pythonize_bool(LIBCXX_ENABLE_MONOTONIC_CLOCK)
   # The tests shouldn't link to any ABI library when it has been linked into
   # libc++ statically.
Index: test/libcxx/test/config.py
===================================================================
--- test/libcxx/test/config.py
+++ test/libcxx/test/config.py
@@ -365,6 +365,8 @@
         elif not enable_monotonic_clock:
             self.lit_config.fatal('enable_monotonic_clock cannot be false when'
                                   ' enable_threads is true.')
+        self.configure_compile_flags_no_thread_unsafe_c_functions()
+
         # Use verbose output for better errors
         self.cxx.flags += ['-v']
         sysroot = self.get_lit_conf('sysroot')
@@ -424,6 +426,15 @@
         self.cxx.compile_flags += ['-D_LIBCPP_HAS_NO_THREADS']
         self.config.available_features.add('libcpp-has-no-threads')
 
+    def configure_compile_flags_no_thread_unsafe_c_functions(self):
+        enable_thread_unsafe_c_functions = self.get_lit_bool(
+            'enable_thread_unsafe_c_functions', True)
+        if not enable_thread_unsafe_c_functions:
+            self.cxx.compile_flags += [
+                '-D_LIBCPP_HAS_NO_THREAD_UNSAFE_C_FUNCTIONS']
+            self.config.available_features.add(
+                'libcpp-has-no-thread-unsafe-c-functions')
+
     def configure_compile_flags_no_monotonic_clock(self):
         self.cxx.compile_flags += ['-D_LIBCPP_HAS_NO_MONOTONIC_CLOCK']
         self.config.available_features.add('libcpp-has-no-monotonic-clock')
Index: test/lit.site.cfg.in
===================================================================
--- test/lit.site.cfg.in
+++ test/lit.site.cfg.in
@@ -12,6 +12,7 @@
 config.enable_stdin             = "@LIBCXX_ENABLE_STDIN@"
 config.enable_stdout            = "@LIBCXX_ENABLE_STDOUT@"
 config.enable_threads           = "@LIBCXX_ENABLE_THREADS@"
+config.enable_thread_unsafe_c_functions = "@LIBCXX_ENABLE_THREAD_UNSAFE_C_FUNCTIONS@"
 config.enable_monotonic_clock   = "@LIBCXX_ENABLE_MONOTONIC_CLOCK@"
 config.cxx_abi                  = "@LIBCXX_CXX_ABI_LIBNAME@"
 config.use_sanitizer            = "@LLVM_USE_SANITIZER@"
Index: test/std/depr/depr.c.headers/stdlib_h.pass.cpp
===================================================================
--- test/std/depr/depr.c.headers/stdlib_h.pass.cpp
+++ test/std/depr/depr.c.headers/stdlib_h.pass.cpp
@@ -71,12 +71,14 @@
     static_assert((std::is_same<decltype(div(0,0)), div_t>::value), "");
     static_assert((std::is_same<decltype(ldiv(0L,0L)), ldiv_t>::value), "");
     static_assert((std::is_same<decltype(lldiv(0LL,0LL)), lldiv_t>::value), "");
-    static_assert((std::is_same<decltype(mblen("",0)), int>::value), "");
     wchar_t* pw = 0;
     const wchar_t* pwc = 0;
     char* pc = 0;
+#ifndef _LIBCPP_HAS_NO_THREAD_UNSAFE_C_FUNCTIONS
+    static_assert((std::is_same<decltype(mblen("",0)), int>::value), "");
     static_assert((std::is_same<decltype(mbtowc(pw,"",0)), int>::value), "");
     static_assert((std::is_same<decltype(wctomb(pc,L' ')), int>::value), "");
+#endif
     static_assert((std::is_same<decltype(mbstowcs(pw,"",0)), size_t>::value), "");
     static_assert((std::is_same<decltype(wcstombs(pc,pwc,0)), size_t>::value), "");
 }
Index: test/std/depr/depr.c.headers/string_h.pass.cpp
===================================================================
--- test/std/depr/depr.c.headers/string_h.pass.cpp
+++ test/std/depr/depr.c.headers/string_h.pass.cpp
@@ -41,7 +41,9 @@
     static_assert((std::is_same<decltype(strrchr(cp, 0)), char*>::value), "");
     static_assert((std::is_same<decltype(strspn(cpc, cpc)), size_t>::value), "");
     static_assert((std::is_same<decltype(strstr(cp, cpc)), char*>::value), "");
+#ifndef _LIBCPP_HAS_NO_THREAD_UNSAFE_C_FUNCTIONS
     static_assert((std::is_same<decltype(strtok(cp, cpc)), char*>::value), "");
+#endif
     static_assert((std::is_same<decltype(memset(vp, 0, s)), void*>::value), "");
     static_assert((std::is_same<decltype(strerror(0)), char*>::value), "");
     static_assert((std::is_same<decltype(strlen(cpc)), size_t>::value), "");
Index: test/std/language.support/support.runtime/cstdlib.pass.cpp
===================================================================
--- test/std/language.support/support.runtime/cstdlib.pass.cpp
+++ test/std/language.support/support.runtime/cstdlib.pass.cpp
@@ -75,12 +75,14 @@
     static_assert((std::is_same<decltype(std::div(0LL,0LL)), std::lldiv_t>::value), "");
     static_assert((std::is_same<decltype(std::ldiv(0L,0L)), std::ldiv_t>::value), "");
     static_assert((std::is_same<decltype(std::lldiv(0LL,0LL)), std::lldiv_t>::value), "");
-    static_assert((std::is_same<decltype(std::mblen("",0)), int>::value), "");
     wchar_t* pw = 0;
     const wchar_t* pwc = 0;
     char* pc = 0;
+#ifndef _LIBCPP_HAS_NO_THREAD_UNSAFE_C_FUNCTIONS
+    static_assert((std::is_same<decltype(std::mblen("",0)), int>::value), "");
     static_assert((std::is_same<decltype(std::mbtowc(pw,"",0)), int>::value), "");
     static_assert((std::is_same<decltype(std::wctomb(pc,L' ')), int>::value), "");
+#endif
     static_assert((std::is_same<decltype(std::mbstowcs(pw,"",0)), std::size_t>::value), "");
     static_assert((std::is_same<decltype(std::wcstombs(pc,pwc,0)), std::size_t>::value), "");
 }
Index: test/std/language.support/support.runtime/ctime.pass.cpp
===================================================================
--- test/std/language.support/support.runtime/ctime.pass.cpp
+++ test/std/language.support/support.runtime/ctime.pass.cpp
@@ -30,10 +30,12 @@
     static_assert((std::is_same<decltype(std::difftime(t,t)), double>::value), "");
     static_assert((std::is_same<decltype(std::mktime(&tm)), std::time_t>::value), "");
     static_assert((std::is_same<decltype(std::time(&t)), std::time_t>::value), "");
+#ifndef _LIBCPP_HAS_NO_THREAD_UNSAFE_C_FUNCTIONS
     static_assert((std::is_same<decltype(std::asctime(&tm)), char*>::value), "");
     static_assert((std::is_same<decltype(std::ctime(&t)), char*>::value), "");
     static_assert((std::is_same<decltype(std::gmtime(&t)), std::tm*>::value), "");
     static_assert((std::is_same<decltype(std::localtime(&t)), std::tm*>::value), "");
+#endif
     char* c1 = 0;
     const char* c2 = 0;
     static_assert((std::is_same<decltype(std::strftime(c1,s,c2,&tm)), std::size_t>::value), "");
Index: test/std/localization/c.locales/clocale.pass.cpp
===================================================================
--- test/std/localization/c.locales/clocale.pass.cpp
+++ test/std/localization/c.locales/clocale.pass.cpp
@@ -12,6 +12,8 @@
 #include <clocale>
 #include <type_traits>
 
+#ifndef _LIBCPP_HAS_NO_THREAD_UNSAFE_C_FUNCTIONS
+
 #ifndef LC_ALL
 #error LC_ALL not defined
 #endif
@@ -36,6 +38,8 @@
 #error LC_TIME not defined
 #endif
 
+#endif // !_LIBCPP_HAS_NO_THREAD_UNSAFE_C_FUNCTIONS
+
 #ifndef NULL
 #error NULL not defined
 #endif
@@ -43,6 +47,8 @@
 int main()
 {
     std::lconv lc;
+#ifndef _LIBCPP_HAS_NO_THREAD_UNSAFE_C_FUNCTIONS
     static_assert((std::is_same<decltype(std::setlocale(0, "")), char*>::value), "");
+#endif
     static_assert((std::is_same<decltype(std::localeconv()), std::lconv*>::value), "");
 }
Index: test/std/strings/c.strings/cstring.pass.cpp
===================================================================
--- test/std/strings/c.strings/cstring.pass.cpp
+++ test/std/strings/c.strings/cstring.pass.cpp
@@ -46,7 +46,9 @@
     static_assert((std::is_same<decltype(std::strspn(cpc, cpc)), std::size_t>::value), "");
 //    static_assert((std::is_same<decltype(std::strstr(cpc, cpc)), const char*>::value), "");
     static_assert((std::is_same<decltype(std::strstr(cp, cpc)), char*>::value), "");
+#ifndef _LIBCPP_HAS_NO_THREAD_UNSAFE_C_FUNCTIONS
     static_assert((std::is_same<decltype(std::strtok(cp, cpc)), char*>::value), "");
+#endif
     static_assert((std::is_same<decltype(std::memset(vp, 0, s)), void*>::value), "");
     static_assert((std::is_same<decltype(std::strerror(0)), char*>::value), "");
     static_assert((std::is_same<decltype(std::strlen(cpc)), std::size_t>::value), "");
Index: test/std/utilities/date.time/no.thread.unsafe.c.functions/asctime.fail.cpp
===================================================================
--- test/std/utilities/date.time/no.thread.unsafe.c.functions/asctime.fail.cpp
+++ test/std/utilities/date.time/no.thread.unsafe.c.functions/asctime.fail.cpp
@@ -0,0 +1,16 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include <ctime>
+
+int main() {
+    // asctime is not thread-safe.
+    std::time_t t = 0;
+    std::asctime(&t);
+}
Index: test/std/utilities/date.time/no.thread.unsafe.c.functions/ctime.fail.cpp
===================================================================
--- test/std/utilities/date.time/no.thread.unsafe.c.functions/ctime.fail.cpp
+++ test/std/utilities/date.time/no.thread.unsafe.c.functions/ctime.fail.cpp
@@ -0,0 +1,16 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include <ctime>
+
+int main() {
+    // ctime is not thread-safe.
+    std::time_t t = 0;
+    std::ctime(&t);
+}
Index: test/std/utilities/date.time/no.thread.unsafe.c.functions/gmtime.fail.cpp
===================================================================
--- test/std/utilities/date.time/no.thread.unsafe.c.functions/gmtime.fail.cpp
+++ test/std/utilities/date.time/no.thread.unsafe.c.functions/gmtime.fail.cpp
@@ -0,0 +1,16 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include <ctime>
+
+int main() {
+    // gmtime is not thread-safe.
+    std::time_t t = 0;
+    std::gmtime(&t);
+}
Index: test/std/utilities/date.time/no.thread.unsafe.c.functions/lit.local.cfg
===================================================================
--- test/std/utilities/date.time/no.thread.unsafe.c.functions/lit.local.cfg
+++ test/std/utilities/date.time/no.thread.unsafe.c.functions/lit.local.cfg
@@ -0,0 +1,2 @@
+if 'libcpp-has-no-thread-unsafe-c-functions' not in config.available_features:
+    config.unsupported = True
Index: test/std/utilities/date.time/no.thread.unsafe.c.functions/localtime.fail.cpp
===================================================================
--- test/std/utilities/date.time/no.thread.unsafe.c.functions/localtime.fail.cpp
+++ test/std/utilities/date.time/no.thread.unsafe.c.functions/localtime.fail.cpp
@@ -0,0 +1,16 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include <ctime>
+
+int main() {
+    // localtime is not thread-safe.
+    std::time_t t = 0;
+    std::localtime(&t);
+}
Index: test/std/utilities/date.time/tested_elsewhere.pass.cpp
===================================================================
--- test/std/utilities/date.time/tested_elsewhere.pass.cpp
+++ test/std/utilities/date.time/tested_elsewhere.pass.cpp
@@ -30,9 +30,11 @@
     static_assert((std::is_same<decltype(std::difftime(t,t)), double>::value), "");
     static_assert((std::is_same<decltype(std::mktime(&tm)), std::time_t>::value), "");
     static_assert((std::is_same<decltype(std::time(&t)), std::time_t>::value), "");
+#ifndef _LIBCPP_HAS_NO_THREAD_UNSAFE_C_FUNCTIONS
     static_assert((std::is_same<decltype(std::asctime(&tm)), char*>::value), "");
     static_assert((std::is_same<decltype(std::ctime(&t)), char*>::value), "");
     static_assert((std::is_same<decltype(std::gmtime(&t)), std::tm*>::value), "");
     static_assert((std::is_same<decltype(std::localtime(&t)), std::tm*>::value), "");
+#endif
     static_assert((std::is_same<decltype(std::strftime(str,s,"",&tm)), std::size_t>::value), "");
 }