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[libc++] Add support for picolibc and newlib in RUNTIMES_USE_LIBC #147956

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[libc++] Add support for picolibc and newlib in RUNTIMES_USE_LIBC #147956

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[libc++] Add support for picolibc and newlib in RUNTIMES_USE_LIBC
voltur01 Jul 10, 2025
a876988
Provide a warning if RUNTIMES_USE_LIBC is not used
voltur01 Jul 15, 2025
dad9c9a
Address review comments
voltur01 Jul 16, 2025
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15 changes: 15 additions & 0 deletions libcxx/CMakeLists.txt
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Expand Up @@ -481,6 +481,21 @@ include(config-ix)
include(HandleLibC) # Setup the C library flags
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General comment on the whole patch: Let's use _LIBCPP_foo instead of LIBCXX_foo for the macro names. That's what we do consistently everywhere, and we must use a reserved identifier.

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Indeed, done.

include(HandleLibCXXABI) # Setup the ABI library flags

# Set C library in use to define respective macro in __config_site
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I would suggest we do this instead (around line 753 where we handle other __config_site variables):

if (RUNTIMES_USE_LIBC STREQUAL "llvm-libc")
  config_define(_LIBCPP_LIBC_LLVMLIBC 1)
elseif (RUNTIMES_USE_LIBC STREQUAL "picolibc")
  config_define(_LIBCPP_LIBC_PICOLIBC 1)
elseif (RUNTIMES_USE_LIBC STREQUAL "newlib")
  config_define(_LIBCPP_LIBC_NEWLIB 1)
endif()

That's more consistent with what we do for the other __config_site macros.

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Thank you for review!

Good point, moved and made use of config_define() for consistency.

# RUNTIMES_USE_LIBC was checked in HandleLibC to be one of accepted values
if (RUNTIMES_USE_LIBC STREQUAL "llvm-libc")
set(LIBCXX_LIBC_LLVMLIBC 1)
elseif (RUNTIMES_USE_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)
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Do you absolutely need this right now (if so, why)? I think it would be cleaner to have a 1-1 mapping between the macros we define and the libc we're using, otherwise things become confusing when reading the source.

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It was for completeness only - removed now.

elseif (RUNTIMES_USE_LIBC STREQUAL "newlib")
set(LIBCXX_LIBC_NEWLIB 1)
endif()

# FIXME(EricWF): See the FIXME on LIBCXX_ENABLE_PEDANTIC.
# Remove the -pedantic flag and -Wno-pedantic and -pedantic-errors
# so they don't get transformed into -Wno and -errors respectively.
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5 changes: 5 additions & 0 deletions libcxx/include/__config_site.in
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Expand Up @@ -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
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I'd use #cmakedefine01 here and then I'd check #if _LIBCPP_LIBC_NEWLIB instead of #if defined(_LIBCPP_LIBC_NEWLIB). This can prevent mistakes when coupled with -Wundef.

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Very good suggestion, done - the checks are much cleaner now.


// __USE_MINGW_ANSI_STDIO gets redefined on MinGW
#ifdef __clang__
# pragma clang diagnostic push
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7 changes: 3 additions & 4 deletions libcxx/include/__configuration/platform.h
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Expand Up @@ -42,11 +42,10 @@
# 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.
// TODO: Remove this deprecated behavior after LLVM 22 release
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Since this is something that vendors should change, I'm not sure there's a benefit from deprecating this before we remove it.

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Agreed, I do not expect many people using it anyway - removed.

// To build libc++ with picolibc provide RUNTIMES_USE_LIBC=picolibc
#if __has_include(<picolibc.h>)
# include <picolibc.h>
# define LIBCXX_LIBC_NEWLIB
#endif

#ifndef __BYTE_ORDER__
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2 changes: 1 addition & 1 deletion libcxx/include/__cxx03/__fwd/ios.h
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Expand Up @@ -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
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2 changes: 1 addition & 1 deletion libcxx/include/__cxx03/__locale
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Expand Up @@ -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
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2 changes: 1 addition & 1 deletion libcxx/include/__cxx03/__locale_dir/locale_base_api.h
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Expand Up @@ -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>
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2 changes: 1 addition & 1 deletion libcxx/include/__cxx03/fstream
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Expand Up @@ -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

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2 changes: 1 addition & 1 deletion libcxx/include/__cxx03/locale
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Expand Up @@ -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
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6 changes: 2 additions & 4 deletions libcxx/include/__cxx03/regex
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Expand Up @@ -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
Expand All @@ -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;
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2 changes: 1 addition & 1 deletion libcxx/include/__fwd/ios.h
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Expand Up @@ -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
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2 changes: 1 addition & 1 deletion libcxx/include/__locale
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Expand Up @@ -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
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2 changes: 1 addition & 1 deletion libcxx/include/__locale_dir/messages.h
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Expand Up @@ -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
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4 changes: 2 additions & 2 deletions libcxx/include/fstream
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Expand Up @@ -964,7 +964,7 @@ 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);
Expand All @@ -975,7 +975,7 @@ 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);
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6 changes: 2 additions & 4 deletions libcxx/include/regex
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Expand Up @@ -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
Expand All @@ -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;
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2 changes: 1 addition & 1 deletion libcxx/src/include/config_elast.h
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Expand Up @@ -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
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2 changes: 1 addition & 1 deletion libcxx/src/locale.cpp
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Expand Up @@ -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 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)
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4 changes: 2 additions & 2 deletions libcxx/test/libcxx/system_reserved_names.gen.py
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Expand Up @@ -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
Expand Down Expand Up @@ -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
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2 changes: 1 addition & 1 deletion ...test/std/diagnostics/syserr/syserr.errcat/syserr.errcat.objects/generic_category.pass.cpp
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Expand Up @@ -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;
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2 changes: 1 addition & 1 deletion .../test/std/diagnostics/syserr/syserr.errcat/syserr.errcat.objects/system_category.pass.cpp
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Expand Up @@ -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 defined(LIBCXX_LIBC_NEWLIB)
const bool is_newlib = true;
#else
const bool is_newlib = false;
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2 changes: 1 addition & 1 deletion libcxx/test/support/platform_support.h
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Expand Up @@ -48,7 +48,7 @@
# 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*);
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1 change: 1 addition & 0 deletions libcxx/utils/ci/run-buildbot
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Expand Up @@ -230,6 +230,7 @@ function test-armv7m-picolibc() {
-DLIBUNWIND_TEST_CONFIG="armv7m-picolibc-libunwind.cfg.in" \
-DCMAKE_C_FLAGS="${flags}" \
-DCMAKE_CXX_FLAGS="${flags}" \
-DRUNTIMES_USE_LIBC=picolibc \
"${@}"

step "Installing compiler-rt"
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4 changes: 2 additions & 2 deletions runtimes/cmake/Modules/HandleLibC.cmake
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Expand Up @@ -10,10 +10,10 @@

include_guard(GLOBAL)

set(RUNTIMES_SUPPORTED_C_LIBRARIES system llvm-libc)
set(RUNTIMES_SUPPORTED_C_LIBRARIES system llvm-libc picolibc newlib)
set(RUNTIMES_USE_LIBC "system" CACHE STRING "Specify C library to use. Supported values are ${RUNTIMES_SUPPORTED_C_LIBRARIES}.")
if (NOT "${RUNTIMES_USE_LIBC}" IN_LIST RUNTIMES_SUPPORTED_C_LIBRARIES)
message(FATAL_ERROR "Unsupported C library: '${RUNTIMES_CXX_ABI}'. Supported values are ${RUNTIMES_SUPPORTED_C_LIBRARIES}.")
message(FATAL_ERROR "Unsupported C library: '${RUNTIMES_USE_LIBC}'. Supported values are ${RUNTIMES_SUPPORTED_C_LIBRARIES}.")
endif()

# Link against a system-provided libc
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