diff --git a/arm-software/embedded/arm-runtimes/CMakeLists.txt b/arm-software/embedded/arm-runtimes/CMakeLists.txt
index 26322e941d5d..2559fcb03543 100644
--- a/arm-software/embedded/arm-runtimes/CMakeLists.txt
+++ b/arm-software/embedded/arm-runtimes/CMakeLists.txt
@@ -729,6 +729,7 @@ if(ENABLE_CXX_LIBS)
             -DLIBCXXABI_ENABLE_STATIC_UNWINDER=${ENABLE_EXCEPTIONS}
             -DLIBCXX_ENABLE_EXCEPTIONS=${ENABLE_EXCEPTIONS}
             -DLIBCXX_ENABLE_RTTI=${ENABLE_RTTI}
+            -DLIBCXX_LIBC=picolibc
         )
         if(ENABLE_LIBCXX_TESTS)
             set(cxxlibs_lit_args "${LLVM_LIT_ARGS} --xunit-xml-output=results.junit.xml")
@@ -759,6 +760,7 @@ if(ENABLE_CXX_LIBS)
             -DLIBCXXABI_ENABLE_STATIC_UNWINDER=${ENABLE_EXCEPTIONS}
             -DLIBCXX_ENABLE_EXCEPTIONS=${ENABLE_EXCEPTIONS}
             -DLIBCXX_ENABLE_RTTI=${ENABLE_RTTI}
+            # -DLIBCXX_LIBC=llvm-libc # this breaks libcxxabi build below by adding -nostdlibinc in HandleLibC.cmake - needs a refactoring to enable
             )
             set(lib_compile_flags "${lib_compile_flags} -D_LIBCPP_PRINT=1")
     elseif(C_LIBRARY MATCHES "^newlib")
@@ -772,6 +774,7 @@ if(ENABLE_CXX_LIBS)
             -DLIBCXXABI_ENABLE_STATIC_UNWINDER=${ENABLE_EXCEPTIONS}
             -DLIBCXX_ENABLE_EXCEPTIONS=${ENABLE_EXCEPTIONS}
             -DLIBCXX_ENABLE_RTTI=${ENABLE_RTTI}
+            -DLIBCXX_LIBC=newlib
         )
 
         if(C_LIBRARY STREQUAL newlib-nano)
diff --git a/libcxx/CMakeLists.txt b/libcxx/CMakeLists.txt
index dffdd7a3c70a..2c311a74f3c8 100644
--- a/libcxx/CMakeLists.txt
+++ b/libcxx/CMakeLists.txt
@@ -227,7 +227,7 @@ option(LIBCXX_USE_COMPILER_RT "Use compiler-rt instead of libgcc" OFF)
 
 # C Library options -----------------------------------------------------------
 
-set(LIBCXX_SUPPORTED_C_LIBRARIES system llvm-libc)
+set(LIBCXX_SUPPORTED_C_LIBRARIES system llvm-libc picolibc newlib)
 set(LIBCXX_LIBC "system" CACHE STRING "Specify C library to use. Supported values are ${LIBCXX_SUPPORTED_C_LIBRARIES}.")
 if (NOT "${LIBCXX_LIBC}" IN_LIST LIBCXX_SUPPORTED_C_LIBRARIES)
   message(FATAL_ERROR "Unsupported C library: '${LIBCXX_LIBC}'. Supported values are ${LIBCXX_SUPPORTED_C_LIBRARIES}.")
diff --git a/libcxx/cmake/Modules/HandleLibC.cmake b/libcxx/cmake/Modules/HandleLibC.cmake
index 1b0564ae6fcc..656c5dc2427e 100644
--- a/libcxx/cmake/Modules/HandleLibC.cmake
+++ b/libcxx/cmake/Modules/HandleLibC.cmake
@@ -17,6 +17,7 @@ if (LIBCXX_LIBC STREQUAL "system")
 
 # Link against the in-tree LLVM libc
 elseif (LIBCXX_LIBC STREQUAL "llvm-libc")
+  set(LIBCXX_LIBC_LLVMLIBC 1)
   add_library(libcxx-libc-headers INTERFACE)
   target_link_libraries(libcxx-libc-headers INTERFACE libc-headers)
   if(CXX_SUPPORTS_NOSTDLIBINC_FLAG)
@@ -36,4 +37,13 @@ elseif (LIBCXX_LIBC STREQUAL "llvm-libc")
 
   # TODO: There's no support for building LLVM libc as a shared library yet.
   add_library(libcxx-libc-shared INTERFACE)
+elseif (LIBCXX_LIBC STREQUAL "picolibc")
+  set(LIBCXX_LIBC_PICOLIBC 1)
+  # picolibc is derived from newlib and behaves the same in regards to libc++
+  # so setting both here:
+  # * LIBCXX_LIBC_NEWLIB is used now
+  # * LIBCXX_LIBC_PICOLIBC can be used for further customizations later
+  set(LIBCXX_LIBC_NEWLIB 1)
+elseif (LIBCXX_LIBC STREQUAL "newlib")
+  set(LIBCXX_LIBC_NEWLIB 1)
 endif()
diff --git a/libcxx/include/__config_site.in b/libcxx/include/__config_site.in
index fc01aaf2d874..f01400bd351e 100644
--- a/libcxx/include/__config_site.in
+++ b/libcxx/include/__config_site.in
@@ -42,6 +42,11 @@
 // Hardening.
 #cmakedefine _LIBCPP_HARDENING_MODE_DEFAULT @_LIBCPP_HARDENING_MODE_DEFAULT@
 
+// C libraries
+#cmakedefine LIBCXX_LIBC_LLVMLIBC
+#cmakedefine LIBCXX_LIBC_PICOLIBC
+#cmakedefine LIBCXX_LIBC_NEWLIB
+
 // __USE_MINGW_ANSI_STDIO gets redefined on MinGW
 #ifdef __clang__
 #  pragma clang diagnostic push
diff --git a/libcxx/include/__configuration/platform.h b/libcxx/include/__configuration/platform.h
index 0b9afb2c397f..8d0f8f63f521 100644
--- a/libcxx/include/__configuration/platform.h
+++ b/libcxx/include/__configuration/platform.h
@@ -42,18 +42,6 @@
 #  endif
 #endif
 
-// This is required in order for _NEWLIB_VERSION to be defined in places where we use it.
-// TODO: We shouldn't be including arbitrarily-named headers from libc++ since this can break valid
-//       user code. Move code paths that need _NEWLIB_VERSION to another customization mechanism.
-#if __has_include(<picolibc.h>)
-#  include <picolibc.h>
-#endif
-
-// Downstream issue: 378 (Include newlib.h to define _NEWLIB_VERSION)
-#if __has_include(<newlib.h>)
-#  include <newlib.h>
-#endif
-
 #ifndef __BYTE_ORDER__
 #  error                                                                                                               \
       "Your compiler doesn't seem to define __BYTE_ORDER__, which is required by libc++ to know the endianness of your target platform"
diff --git a/libcxx/include/__cxx03/__fwd/ios.h b/libcxx/include/__cxx03/__fwd/ios.h
index dc03e8c6bab2..5776ad90ea62 100644
--- a/libcxx/include/__cxx03/__fwd/ios.h
+++ b/libcxx/include/__cxx03/__fwd/ios.h
@@ -31,7 +31,7 @@ using wios = basic_ios<wchar_t>;
 template <class _CharT, class _Traits>
 class _LIBCPP_PREFERRED_NAME(ios) _LIBCPP_IF_WIDE_CHARACTERS(_LIBCPP_PREFERRED_NAME(wios)) basic_ios;
 
-#if defined(_NEWLIB_VERSION)
+#if defined(LIBCXX_LIBC_NEWLIB)
 // On newlib, off_t is 'long int'
 using streamoff = long int; // for char_traits in <string>
 #else
diff --git a/libcxx/include/__cxx03/__locale b/libcxx/include/__cxx03/__locale
index d5faa89b99fc..d32ac05d2818 100644
--- a/libcxx/include/__cxx03/__locale
+++ b/libcxx/include/__cxx03/__locale
@@ -384,7 +384,7 @@ public:
   static const mask xdigit       = _ISXDIGIT;
   static const mask blank        = _ISBLANK;
   static const mask __regex_word = 0x8000;
-#elif defined(_NEWLIB_VERSION)
+#elif defined(LIBCXX_LIBC_NEWLIB)
   // Same type as Newlib's _ctype_ array in newlib/libc/include/ctype.h.
   typedef char mask;
   // In case char is signed, static_cast is needed to avoid warning on
diff --git a/libcxx/include/__cxx03/__locale_dir/locale_base_api.h b/libcxx/include/__cxx03/__locale_dir/locale_base_api.h
index a20f0952f52c..ccb8201e8ab1 100644
--- a/libcxx/include/__cxx03/__locale_dir/locale_base_api.h
+++ b/libcxx/include/__cxx03/__locale_dir/locale_base_api.h
@@ -17,7 +17,7 @@
 #  include <__cxx03/__locale_dir/locale_base_api/android.h>
 #elif defined(__sun__)
 #  include <__cxx03/__locale_dir/locale_base_api/solaris.h>
-#elif defined(_NEWLIB_VERSION)
+#elif defined(LIBCXX_LIBC_NEWLIB)
 #  include <__cxx03/__locale_dir/locale_base_api/newlib.h>
 #elif defined(__OpenBSD__)
 #  include <__cxx03/__locale_dir/locale_base_api/openbsd.h>
diff --git a/libcxx/include/__cxx03/fstream b/libcxx/include/__cxx03/fstream
index 44bdabc4602b..f7290878bbc0 100644
--- a/libcxx/include/__cxx03/fstream
+++ b/libcxx/include/__cxx03/fstream
@@ -209,7 +209,7 @@ typedef basic_fstream<wchar_t> wfstream;
 _LIBCPP_PUSH_MACROS
 #include <__cxx03/__undef_macros>
 
-#if defined(_LIBCPP_MSVCRT) || defined(_NEWLIB_VERSION)
+#if defined(_LIBCPP_MSVCRT) || defined(LIBCXX_LIBC_NEWLIB)
 #  define _LIBCPP_HAS_NO_OFF_T_FUNCTIONS
 #endif
 
diff --git a/libcxx/include/__cxx03/locale b/libcxx/include/__cxx03/locale
index 64162f5a4ff2..0de138060185 100644
--- a/libcxx/include/__cxx03/locale
+++ b/libcxx/include/__cxx03/locale
@@ -220,7 +220,7 @@ template <class charT> class messages_byname;
 
 #  if defined(__unix__) || (defined(__APPLE__) && defined(__MACH__))
 // Most unix variants have catopen.  These are the specific ones that don't.
-#    if !defined(__BIONIC__) && !defined(_NEWLIB_VERSION) && !defined(__EMSCRIPTEN__)
+#    if !defined(__BIONIC__) && !defined(LIBCXX_LIBC_NEWLIB) && !defined(__EMSCRIPTEN__)
 #      define _LIBCPP_HAS_CATOPEN 1
 #      include <nl_types.h>
 #    endif
diff --git a/libcxx/include/__cxx03/regex b/libcxx/include/__cxx03/regex
index b96d59d3a252..7b3aaf52dc9f 100644
--- a/libcxx/include/__cxx03/regex
+++ b/libcxx/include/__cxx03/regex
@@ -984,7 +984,7 @@ public:
   typedef _CharT char_type;
   typedef basic_string<char_type> string_type;
   typedef locale locale_type;
-#if defined(__BIONIC__) || defined(_NEWLIB_VERSION)
+#if defined(__BIONIC__) || defined(LIBCXX_LIBC_NEWLIB)
   // Originally bionic's ctype_base used its own ctype masks because the
   // builtin ctype implementation wasn't in libc++ yet. Bionic's ctype mask
   // was only 8 bits wide and already saturated, so it used a wider type here
@@ -993,9 +993,7 @@ public:
   // implementation, but this was not updated to match. Since then Android has
   // needed to maintain a stable libc++ ABI, and this can't be changed without
   // an ABI break.
-  // We also need this workaround for newlib since _NEWLIB_VERSION is not
-  // defined yet inside __config, so we can't set the
-  // _LIBCPP_PROVIDES_DEFAULT_RUNE_TABLE macro. Additionally, newlib is
+  // We also need this workaround for newlib since, newlib is
   // often used for space constrained environments, so it makes sense not to
   // duplicate the ctype table.
   typedef uint16_t char_class_type;
diff --git a/libcxx/include/__fwd/ios.h b/libcxx/include/__fwd/ios.h
index 831624f4b1c5..4db2c5da3861 100644
--- a/libcxx/include/__fwd/ios.h
+++ b/libcxx/include/__fwd/ios.h
@@ -31,7 +31,7 @@ using wios = basic_ios<wchar_t>;
 template <class _CharT, class _Traits>
 class _LIBCPP_PREFERRED_NAME(ios) _LIBCPP_IF_WIDE_CHARACTERS(_LIBCPP_PREFERRED_NAME(wios)) basic_ios;
 
-#if defined(_NEWLIB_VERSION)
+#if defined(LIBCXX_LIBC_NEWLIB)
 // On newlib, off_t is 'long int'
 using streamoff = long int; // for char_traits in <string>
 #else
diff --git a/libcxx/include/__locale b/libcxx/include/__locale
index 92e45e2531c2..b0c7e4ca5814 100644
--- a/libcxx/include/__locale
+++ b/libcxx/include/__locale
@@ -389,7 +389,7 @@ public:
   static const mask xdigit       = _ISXDIGIT;
   static const mask blank        = _ISBLANK;
   static const mask __regex_word = 0x8000;
-#  elif defined(_NEWLIB_VERSION)
+#  elif defined(LIBCXX_LIBC_NEWLIB)
   // Same type as Newlib's _ctype_ array in newlib/libc/include/ctype.h.
   typedef char mask;
   // In case char is signed, static_cast is needed to avoid warning on
diff --git a/libcxx/include/__locale_dir/messages.h b/libcxx/include/__locale_dir/messages.h
index c04bf04025ff..4ebedc9de6d4 100644
--- a/libcxx/include/__locale_dir/messages.h
+++ b/libcxx/include/__locale_dir/messages.h
@@ -22,7 +22,7 @@
 
 #  if defined(__unix__) || (defined(__APPLE__) && defined(__MACH__))
 // Most unix variants have catopen.  These are the specific ones that don't.
-#    if !defined(__BIONIC__) && !defined(_NEWLIB_VERSION) && !defined(__EMSCRIPTEN__)
+#    if !defined(__BIONIC__) && !defined(LIBCXX_LIBC_NEWLIB) && !defined(__EMSCRIPTEN__)
 #      define _LIBCPP_HAS_CATOPEN 1
 #      include <nl_types.h>
 #    else
diff --git a/libcxx/include/fstream b/libcxx/include/fstream
index a62c085db23d..44715dc5b775 100644
--- a/libcxx/include/fstream
+++ b/libcxx/include/fstream
@@ -192,7 +192,7 @@ typedef basic_fstream<wchar_t> wfstream;
 #  include <__config>
 
 // Downstream issue: #375 (Enable fstream independently of filesystem)
-#  if _LIBCPP_HAS_FILESYSTEM && _LIBCPP_HAS_LOCALIZATION || defined(_NEWLIB_VERSION)
+#  if _LIBCPP_HAS_FILESYSTEM && _LIBCPP_HAS_LOCALIZATION || defined(LIBCXX_LIBC_NEWLIB)
 
 #    include <__algorithm/max.h>
 #    include <__assert>
@@ -965,7 +965,7 @@ template <class _CharT, class _Traits>
 int basic_filebuf<_CharT, _Traits>::__fseek(FILE* __file, pos_type __offset, int __whence) {
 #    if defined(_LIBCPP_MSVCRT_LIKE)
   return _fseeki64(__file, __offset, __whence);
-#    elif defined(_NEWLIB_VERSION)
+#    elif defined(LIBCXX_LIBC_NEWLIB)
   return fseek(__file, __offset, __whence);
 #    else
   return ::fseeko(__file, __offset, __whence);
@@ -976,7 +976,7 @@ template <class _CharT, class _Traits>
 typename basic_filebuf<_CharT, _Traits>::pos_type basic_filebuf<_CharT, _Traits>::__ftell(FILE* __file) {
 #    if defined(_LIBCPP_MSVCRT_LIKE)
   return _ftelli64(__file);
-#    elif defined(_NEWLIB_VERSION)
+#    elif defined(LIBCXX_LIBC_NEWLIB)
   return ftell(__file);
 #    else
   return ftello(__file);
diff --git a/libcxx/include/regex b/libcxx/include/regex
index bbc21e244dd1..29ba9f6a0672 100644
--- a/libcxx/include/regex
+++ b/libcxx/include/regex
@@ -1004,7 +1004,7 @@ public:
   typedef _CharT char_type;
   typedef basic_string<char_type> string_type;
   typedef locale locale_type;
-#    if defined(__BIONIC__) || defined(_NEWLIB_VERSION)
+#    if defined(__BIONIC__) || defined(LIBCXX_LIBC_NEWLIB)
   // Originally bionic's ctype_base used its own ctype masks because the
   // builtin ctype implementation wasn't in libc++ yet. Bionic's ctype mask
   // was only 8 bits wide and already saturated, so it used a wider type here
@@ -1013,9 +1013,7 @@ public:
   // implementation, but this was not updated to match. Since then Android has
   // needed to maintain a stable libc++ ABI, and this can't be changed without
   // an ABI break.
-  // We also need this workaround for newlib since _NEWLIB_VERSION is not
-  // defined yet inside __config, so we can't set the
-  // _LIBCPP_PROVIDES_DEFAULT_RUNE_TABLE macro. Additionally, newlib is
+  // We also need this workaround for newlib since newlib is
   // often used for space constrained environments, so it makes sense not to
   // duplicate the ctype table.
   typedef uint16_t char_class_type;
diff --git a/libcxx/src/include/config_elast.h b/libcxx/src/include/config_elast.h
index 7edff2d9375d..388286e41330 100644
--- a/libcxx/src/include/config_elast.h
+++ b/libcxx/src/include/config_elast.h
@@ -23,7 +23,7 @@
 #  define _LIBCPP_ELAST ELAST
 #elif defined(__LLVM_LIBC__)
 // No _LIBCPP_ELAST needed for LLVM libc
-#elif defined(_NEWLIB_VERSION)
+#elif defined(LIBCXX_LIBC_NEWLIB)
 #  define _LIBCPP_ELAST __ELASTERROR
 #elif defined(__NuttX__)
 // No _LIBCPP_ELAST needed on NuttX
diff --git a/libcxx/src/ios.instantiations.cpp b/libcxx/src/ios.instantiations.cpp
index 24106d0a225e..542a6387609a 100644
--- a/libcxx/src/ios.instantiations.cpp
+++ b/libcxx/src/ios.instantiations.cpp
@@ -38,7 +38,7 @@ template class _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS basic_ostringstream<char
 template class _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS basic_istringstream<char>;
 
 // Downstream issue: #375 (Enable fstream independently of filesystem)
-#if _LIBCPP_HAS_FILESYSTEM || defined(_NEWLIB_VERSION)
+#if _LIBCPP_HAS_FILESYSTEM || defined(LIBCXX_LIBC_NEWLIB)
 template class _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS basic_ifstream<char>;
 template class _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS basic_ofstream<char>;
 template class _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS basic_filebuf<char>;
diff --git a/libcxx/src/locale.cpp b/libcxx/src/locale.cpp
index 30a7a54e1c01..0481d3bb1da1 100644
--- a/libcxx/src/locale.cpp
+++ b/libcxx/src/locale.cpp
@@ -988,7 +988,7 @@ const ctype<char>::mask* ctype<char>::classic_table() noexcept {
   return __pctype_func();
 #  elif defined(__EMSCRIPTEN__)
   return *__ctype_b_loc();
-#  elif defined(_NEWLIB_VERSION)
+#  elif defined(LIBCXX_LIBC_NEWLIB)
   // Newlib has a 257-entry table in ctype_.c, where (char)0 starts at [1].
   return _ctype_ + 1;
 #  elif defined(_AIX)
diff --git a/libcxx/src/ostream.cpp b/libcxx/src/ostream.cpp
index 1cec2db0c45e..a323e38dea0c 100644
--- a/libcxx/src/ostream.cpp
+++ b/libcxx/src/ostream.cpp
@@ -9,7 +9,7 @@
 #include <__config>
 
 // Downstream issue: #375 (Enable fstream independently of filesystem)
-#if _LIBCPP_HAS_FILESYSTEM || defined(_NEWLIB_VERSION)
+#if _LIBCPP_HAS_FILESYSTEM || defined(LIBCXX_LIBC_NEWLIB)
 #  include <fstream>
 #endif
 #include <ostream>
diff --git a/libcxx/test/libcxx/system_reserved_names.gen.py b/libcxx/test/libcxx/system_reserved_names.gen.py
index a4f2928eda33..5e4809d20fd8 100644
--- a/libcxx/test/libcxx/system_reserved_names.gen.py
+++ b/libcxx/test/libcxx/system_reserved_names.gen.py
@@ -78,7 +78,7 @@
 
 // Test that libc++ doesn't use names that collide with FreeBSD system macros.
 // newlib and picolibc also define these macros
-#if !defined(__FreeBSD__) && !defined(_NEWLIB_VERSION)
+#if !defined(__FreeBSD__) && !defined(LIBCXX_LIBC_NEWLIB)
 #  define __null_sentinel SYSTEM_RESERVED_NAME
 #  define __generic SYSTEM_RESERVED_NAME
 #endif
@@ -117,7 +117,7 @@
 #endif
 
 // Newlib & picolibc use __input as a parameter name of a64l & l64a
-#ifndef _NEWLIB_VERSION
+#ifndef LIBCXX_LIBC_NEWLIB
 # define __input SYSTEM_RESERVED_NAME
 #endif
 #define __output SYSTEM_RESERVED_NAME
diff --git a/libcxx/test/std/diagnostics/syserr/syserr.errcat/syserr.errcat.objects/generic_category.pass.cpp b/libcxx/test/std/diagnostics/syserr/syserr.errcat/syserr.errcat.objects/generic_category.pass.cpp
index 542520330401..b4fa377b7f10 100644
--- a/libcxx/test/std/diagnostics/syserr/syserr.errcat/syserr.errcat.objects/generic_category.pass.cpp
+++ b/libcxx/test/std/diagnostics/syserr/syserr.errcat/syserr.errcat.objects/generic_category.pass.cpp
@@ -48,7 +48,7 @@ int main(int, char**)
         // responds with an empty message, which we probably want to
         // treat as a failure code otherwise, but we can detect that
         // with the preprocessor.
-#if defined(_NEWLIB_VERSION)
+#if defined(LIBCXX_LIBC_NEWLIB)
         const bool is_newlib = true;
 #else
         const bool is_newlib = false;
diff --git a/libcxx/test/std/diagnostics/syserr/syserr.errcat/syserr.errcat.objects/system_category.pass.cpp b/libcxx/test/std/diagnostics/syserr/syserr.errcat/syserr.errcat.objects/system_category.pass.cpp
index 255cbe75e2fa..b888a9a447e6 100644
--- a/libcxx/test/std/diagnostics/syserr/syserr.errcat/syserr.errcat.objects/system_category.pass.cpp
+++ b/libcxx/test/std/diagnostics/syserr/syserr.errcat/syserr.errcat.objects/system_category.pass.cpp
@@ -59,7 +59,7 @@ int main(int, char**) {
     // responds with an empty message, which we probably want to
     // treat as a failure code otherwise, but we can detect that
     // with the preprocessor.
-#if defined(_NEWLIB_VERSION)
+#if definedLIBCXX_LIBC_NEWLIB)
     const bool is_newlib = true;
 #else
     const bool is_newlib = false;
diff --git a/libcxx/test/support/platform_support.h b/libcxx/test/support/platform_support.h
index 031c434053b8..13fff18b59c9 100644
--- a/libcxx/test/support/platform_support.h
+++ b/libcxx/test/support/platform_support.h
@@ -41,7 +41,7 @@
 #   include <fcntl.h> // _O_EXCL, ...
 #   include <sys/stat.h> // _S_IREAD, ...
 // Downstream issue: #375 (Enable fstream independently of filesystem)
-#elif defined(_NEWLIB_VERSION)
+#elif defined(LIBCXX_LIBC_NEWLIB)
 // No need to include extra headers for the get_temp_file_name() implementation
 // below: tmpnam() is defined in <stdio.h>
 #elif __has_include(<unistd.h>)
@@ -52,7 +52,7 @@
 # include <string.h> // strverscmp
 #endif
 
-#if defined(_NEWLIB_VERSION) && defined(__STRICT_ANSI__)
+#if defined(LIBCXX_LIBC_NEWLIB) && defined(__STRICT_ANSI__)
 // Newlib provides this, but in the header it's under __STRICT_ANSI__
 extern "C" {
   int mkstemp(char*);
@@ -75,7 +75,7 @@ inline std::string get_temp_file_name() {
     abort();
   }
 // Downstream issue: #375 (Enable fstream independently of filesystem)
-#elif defined(_NEWLIB_VERSION)
+#elif defined(LIBCXX_LIBC_NEWLIB)
   char tmp_name[L_tmpnam];
   char *ret = tmpnam(tmp_name);
   if (ret == NULL) {