[libc++] Handle Newlib and picolibc using options

inglorion · inglorion · commit 77927318e78a · 2025-08-11T08:20:15.000Z
Previously, we relied on _NEWLIB_VERSION being defined to detect if we are building with Newlib or its derivative picolibc. This is somewhat unreliable, because whether or not _NEWLIB_VERSION is defined depends on if any headers from the C library have been included or not. This could lead to inconsistent definitions, for example as observed in bug #152763. The code contained a workaround that detected picolibc by looking for its header file, with a TODO to replace this logic with a different mechanism. This change addresses that TODO by handling Newlib and picolibc similarly to how we handle musl: by providing a LIBCXX_HAS_*LIBC option and a corresponding _LIBCPP_HAS_* macro to be used in code. Specifically: - For Newlibc, we provide LIBCXX_HAS_NEWLIB_LIBC and _LIBCPP_HAS_NEWLIB_LIBC. - For picolibc, we provide LIBCXX_HAS_PICOLIBC and _LIBCPP_HAS_PICOLIBC. Existing code has been rewritten so that instead of checking for defined(_NEWLIB_VERSION) it checks for _LIBCPP_HAS_NEWLIB_LIBC || _LIBCPP_HAS_PICOLIBC.
diff --git a/libcxx/CMakeLists.txt b/libcxx/CMakeLists.txt
@@ -285,6 +285,8 @@ endif()
 
 # Feature options -------------------------------------------------------------
 option(LIBCXX_HAS_MUSL_LIBC "Build libc++ with support for the Musl C library" OFF)
+option(LIBCXX_HAS_NEWLIB_LIBC "Build libc++ with support for the Newlib C library" OFF)
+option(LIBCXX_HAS_PICOLIBC "Build libc++ with support for the picolibc C library" OFF)
 option(LIBCXX_HAS_PTHREAD_API "Ignore auto-detection and force use of pthread API" OFF)
 option(LIBCXX_HAS_WIN32_THREAD_API "Ignore auto-detection and force use of win32 thread API" OFF)
 option(LIBCXX_HAS_EXTERNAL_THREAD_API
@@ -742,6 +744,8 @@ config_define(${LIBCXX_HAS_PTHREAD_API} _LIBCPP_HAS_THREAD_API_PTHREAD)
 config_define(${LIBCXX_HAS_EXTERNAL_THREAD_API} _LIBCPP_HAS_THREAD_API_EXTERNAL)
 config_define(${LIBCXX_HAS_WIN32_THREAD_API} _LIBCPP_HAS_THREAD_API_WIN32)
 config_define(${LIBCXX_HAS_MUSL_LIBC} _LIBCPP_HAS_MUSL_LIBC)
+config_define(${LIBCXX_HAS_NEWLIB_LIBC} _LIBCPP_HAS_NEWLIB_LIBC)
+config_define(${LIBCXX_HAS_PICOLIBC} _LIBCPP_HAS_PICOLIBC)
 config_define_if(LIBCXX_NO_VCRUNTIME _LIBCPP_NO_VCRUNTIME)
 config_define(${LIBCXX_ENABLE_FILESYSTEM} _LIBCPP_HAS_FILESYSTEM)
 config_define(${LIBCXX_ENABLE_RANDOM_DEVICE} _LIBCPP_HAS_RANDOM_DEVICE)
diff --git a/libcxx/include/__configuration/platform.h b/libcxx/include/__configuration/platform.h
@@ -42,13 +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
-
 #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
@@ -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 _LIBCPP_HAS_NEWLIB_LIBC || _LIBC_HAS_PICOLIBC
 // 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
@@ -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 _LIBCPP_HAS_NEWLIB_LIBC || _LIBC_HAS_PICOLIBC
   // 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
@@ -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 _LIBCPP_HAS_NEWLIB_LIBC || _LIBC_HAS_PICOLIBC
 #  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
@@ -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) || _LIBCPP_HAS_NEWLIB_LIBC || _LIBC_HAS_PICOLIBC
 #  define _LIBCPP_HAS_NO_OFF_T_FUNCTIONS
 #endif
 
diff --git 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__) && !_LIBCPP_HAS_NEWLIB_LIBC && !_LIBCPP_HAS_PICOLIBC && !defined(__EMSCRIPTEN__)
 #      define _LIBCPP_HAS_CATOPEN 1
 #      include <nl_types.h>
 #    endif
diff --git 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__) || _LIBCPP_HAS_NEWLIB_LIBC || _LIBC_HAS_PICOLIBC
   // 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
@@ -993,11 +993,10 @@ 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
-  // often used for space constrained environments, so it makes sense not to
-  // duplicate the ctype table.
+  //
+  // We also use this logic for Newlib and picolibc. These are often used
+  // for space constrained environments, so it makes sense no to duplicate
+  // the ctype table.
   typedef uint16_t char_class_type;
 #else
   typedef ctype_base::mask char_class_type;
diff --git 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 _LIBCPP_HAS_NEWLIB_LIBC || _LIBC_HAS_PICOLIBC
 // 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
@@ -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 _LIBCPP_HAS_NEWLIB_LIBC || _LIBC_HAS_PICOLIBC
   // 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
@@ -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__) && !_LIBCPP_HAS_NEWLIB_LIBC && !_LIBC_HAS_PICOLIBC && !defined(__EMSCRIPTEN__)
 #      define _LIBCPP_HAS_CATOPEN 1
 #      include <nl_types.h>
 #    else
diff --git a/libcxx/include/fstream b/libcxx/include/fstream
@@ -977,7 +977,7 @@ 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 _LIBCPP_HAS_NEWLIB_LIBC || _LIBC_HAS_PICOLIBC
   return fseek(__file, __offset, __whence);
 #    else
   return ::fseeko(__file, __offset, __whence);
@@ -988,7 +988,7 @@ 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 _LIBCPP_HAS_NEWLIB_LIBC || _LIBC_HAS_PICOLIBC
   return ftell(__file);
 #    else
   return ftello(__file);
diff --git 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__) || _LIBCPP_HAS_NEWLIB_LIBC || _LIBC_HAS_PICOLIBC
   // 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
@@ -1013,11 +1013,10 @@ 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
-  // often used for space constrained environments, so it makes sense not to
-  // duplicate the ctype table.
+  //
+  // We also use this logic for Newlib and picolibc. These are often used
+  // for space constrained environments, so it makes sense no to duplicate
+  // the ctype table.
   typedef uint16_t char_class_type;
 #    else
   typedef ctype_base::mask char_class_type;
diff --git 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 _LIBCPP_HAS_NEWLIB_LIBC || _LIBC_HAS_PICOLIBC
 #  define _LIBCPP_ELAST __ELASTERROR
 #elif defined(__NuttX__)
 // No _LIBCPP_ELAST needed on NuttX
diff --git a/libcxx/src/locale.cpp b/libcxx/src/locale.cpp
@@ -933,7 +933,7 @@ const ctype<char>::mask* ctype<char>::classic_table() noexcept {
   return __pctype_func();
 #  elif defined(__EMSCRIPTEN__)
   return *__ctype_b_loc();
-#  elif defined(_NEWLIB_VERSION)
+#  elif _LIBCPP_HAS_NEWLIB_LIBC || _LIBC_HAS_PICOLIBC
   // 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/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__) && !_LIBCPP_HAS_NEWLIB_LIBC && !_LIBCPP_HAS_PICOLIBC
 #  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
+#if !_LIBCPP_HAS_NEWLIB_LIBC && !_LIBC_HAS_PICOLIBC
 # 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
@@ -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 _LIBCPP_HAS_NEWLIB_LIBC || _LIBC_HAS_PICOLIBC
         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
@@ -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 _LIBCPP_HAS_NEWLIB_LIBC || _LIBC_HAS_PICOLIBC
     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
@@ -48,7 +48,7 @@
 # include <string.h> // strverscmp
 #endif
 
-#if defined(_NEWLIB_VERSION) && defined(__STRICT_ANSI__)
+#if (_LIBCPP_HAS_NEWLIB_LIBC || _LIBC_HAS_PICOLIBC) && defined(__STRICT_ANSI__)
 // Newlib provides this, but in the header it's under __STRICT_ANSI__
 extern "C" {
   int mkstemp(char*);
