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feat: 为没有操作系统的机器构建 C++ 运行时
libc++ / libc++abi / libunwind 现在也为 riscv64-none-elf 建得出来。实测: 1431 个对象,ELF 64-bit LSB relocatable UCB RISC-V,__cxa_throw 与 __cxa_begin_catch 有定义。 llvm-generated/ 变成一个配置一个目录 —— 那正是 musl-generated/ 已有的形状, 理由相同:它们是 configure 产物,而一个没有 configure 步骤的包把它们提交进来。 freestanding 与 generic 的差别只有三位加两个开关: 线程 hosted 目标下 libc++ 自己认出系统并找到 pthread;不被认出的系统上 它停在 "No thread API" 而不是猜 —— 正确,因为不猜正是 configure 存在的理由。所以 freestanding 把它写出来。 文件系统 关掉,并且实现它的源文件随之不编 —— 与 random.cpp 同一条规则, 也是 libc++ 自己的构建用同一个开关做的事。 终端 关掉。 dladdr 关掉:没有动态加载器,它接口里那个类型不存在。 _GNU_SOURCE libc++abi 为线程标识去够 syscall,而 musl 只在这个宏下声明 那个名字。LLVM 自己的 runtimes 构建出于同样理由定义它 —— 而宿主构建从别处拿到了它,这个差别直到这个目标来问才可见。 ⚠️ 一并暴露的一处不完整:__assertion_handler 也是 configure 产物,而宿主那次 构建悄悄用了工具链载荷里的那份。现在它在 llvm-generated/ 里(与模板逐字相同, 所以是收进来而不是改写)。裸机这一试才让它现形 —— 宿主上它一直是绿的。 ⚠️ import std 在这种目标上仍被拒绝,而拒绝它的不是这个包: error: `import std;` is not available on 'riscv64-none-elf' --- a freestanding target has no hosted standard library. 那句话陈述的前提,这个包让它不成立了。解除它是构建工具的改动 —— 它问的是 「目标是不是 freestanding」,而该问的是「hosted 标准库在不在」。
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‎.mcpp/.xlings.json‎

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{
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"index_repos": [
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],
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"deps": ["xim:picolibc-riscv@1.8.12"],
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"lang": "en",
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"mirror": "auto"
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}

‎README.md‎

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@@ -53,6 +53,39 @@ With it at `1`, none. The second follows from openkal reporting `ENOSYS` for
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entropy: `std::random_device` is not built, and a program that names it is told
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by the linker.
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## Two configurations, and why there are two
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`llvm-generated/` carries one directory per configuration, which is the shape
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`musl-generated/` already has and for the same reason: these are configure
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products, and a package with no configure step commits them.
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| | `generic` | `freestanding` |
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| --- | --- | --- |
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| threads | libc++ recognises the system and finds pthread itself | stated, because an unrecognised system makes libc++ stop with `No thread API` rather than guess |
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| filesystem | yes | no — and the sources that implement it are then not compiled, which is what libc++'s own build does with the same switch |
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| terminal | yes | no |
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A target with no operating system also needs two switches a hosted one answers
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by recognising the system: the unwinder's use of `dladdr` (there is no dynamic
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loader, and the type its interface names does not exist), and `_GNU_SOURCE`
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(libc++abi reaches `syscall` for a thread identity, and musl declares that name
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only under it — LLVM's own runtimes build defines it for the same reason).
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**Measured 2026-08-22**: `mcpp build --target riscv64-none-elf` produces 1431
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objects, `ELF 64-bit LSB relocatable, UCB RISC-V`, with `__cxa_throw` and
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`__cxa_begin_catch` defined.
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⚠️ `import std;` on such a target is still refused, and not by this package:
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```
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error: `import std;` is not available on 'riscv64-none-elf'
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--- a freestanding target has no hosted standard library.
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```
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That message states a premise this package makes false. Lifting it is a change
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to the build tool, which asks whether the *target* is freestanding rather than
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whether a hosted standard library is *present*.
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## What a program gets
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`examples/cxx` asserts it, and every assertion is written so that it can fail:

‎examples/import-std/.mcpp/.xlings.json‎

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{
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"index_repos": [
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],
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"deps": ["xim:picolibc-riscv@1.8.12"],
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"lang": "en",
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"mirror": "auto"
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}

‎llvm-generated/freestanding/__assertion_handler‎

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// -*- C++ -*-
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//===----------------------------------------------------------------------===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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#ifndef _LIBCPP___ASSERTION_HANDLER
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#define _LIBCPP___ASSERTION_HANDLER
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#if __cplusplus < 201103L && defined(_LIBCPP_USE_FROZEN_CXX03_HEADERS)
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# include <__cxx03/__config>
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# include <__cxx03/__verbose_abort>
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# include <__cxx03/__verbose_trap>
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#else
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# include <__config>
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# include <__log_hardening_failure>
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# include <__verbose_abort>
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# include <__verbose_trap>
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#endif
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#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
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# pragma GCC system_header
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#endif
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#if __cplusplus < 201103L && defined(_LIBCPP_USE_FROZEN_CXX03_HEADERS)
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// Keep the old implementation that doesn't support assertion semantics for backward compatibility with the frozen C++03
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// mode.
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# if _LIBCPP_HARDENING_MODE == _LIBCPP_HARDENING_MODE_DEBUG
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# define _LIBCPP_ASSERTION_HANDLER(message) _LIBCPP_VERBOSE_ABORT("%s", message)
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# else
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# define _LIBCPP_ASSERTION_HANDLER(message) _LIBCPP_VERBOSE_TRAP(message)
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# endif // _LIBCPP_HARDENING_MODE == _LIBCPP_HARDENING_MODE_DEBUG
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#else
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# if _LIBCPP_ASSERTION_SEMANTIC == _LIBCPP_ASSERTION_SEMANTIC_IGNORE
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# define _LIBCPP_ASSERTION_HANDLER(message) ((void)0)
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# elif _LIBCPP_ASSERTION_SEMANTIC == _LIBCPP_ASSERTION_SEMANTIC_OBSERVE
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# define _LIBCPP_ASSERTION_HANDLER(message) _LIBCPP_LOG_HARDENING_FAILURE(message)
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# elif _LIBCPP_ASSERTION_SEMANTIC == _LIBCPP_ASSERTION_SEMANTIC_QUICK_ENFORCE
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# define _LIBCPP_ASSERTION_HANDLER(message) _LIBCPP_VERBOSE_TRAP(message)
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# elif _LIBCPP_ASSERTION_SEMANTIC == _LIBCPP_ASSERTION_SEMANTIC_ENFORCE
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# define _LIBCPP_ASSERTION_HANDLER(message) _LIBCPP_VERBOSE_ABORT("%s", message)
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# else
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# error _LIBCPP_ASSERTION_SEMANTIC must be set to one of the following values: \
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_LIBCPP_ASSERTION_SEMANTIC_IGNORE, \
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_LIBCPP_ASSERTION_SEMANTIC_OBSERVE, \
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_LIBCPP_ASSERTION_SEMANTIC_QUICK_ENFORCE, \
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_LIBCPP_ASSERTION_SEMANTIC_ENFORCE
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# endif // _LIBCPP_ASSERTION_SEMANTIC == _LIBCPP_ASSERTION_SEMANTIC_IGNORE
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#endif // __cplusplus < 201103L && defined(_LIBCPP_USE_FROZEN_CXX03_HEADERS)
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#endif // _LIBCPP___ASSERTION_HANDLER

‎llvm-generated/freestanding/__config_site‎

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//===----------------------------------------------------------------------===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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#ifndef _LIBCPP___CONFIG_SITE
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#define _LIBCPP___CONFIG_SITE
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#define _LIBCPP_ABI_VERSION 1
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#define _LIBCPP_ABI_NAMESPACE __1
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#define _LIBCPP_ABI_FORCE_ITANIUM 0
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#define _LIBCPP_ABI_FORCE_MICROSOFT 0
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#define _LIBCPP_HAS_THREADS 1
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#define _LIBCPP_HAS_MONOTONIC_CLOCK 1
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#define _LIBCPP_HAS_TERMINAL 0
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#define _LIBCPP_HAS_MUSL_LIBC 1
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#define _LIBCPP_HAS_THREAD_API_PTHREAD 1
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#define _LIBCPP_HAS_THREAD_API_EXTERNAL 0
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#define _LIBCPP_HAS_THREAD_API_WIN32 0
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#define _LIBCPP_HAS_THREAD_API_C11 0 // FIXME: Is this guarding dead code?
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/* #undef _LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS */
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#define _LIBCPP_HAS_VENDOR_AVAILABILITY_ANNOTATIONS 0
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/* #undef _LIBCPP_NO_VCRUNTIME */
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/* #undef _LIBCPP_TYPEINFO_COMPARISON_IMPLEMENTATION */
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#define _LIBCPP_HAS_FILESYSTEM 0
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#define _LIBCPP_HAS_RANDOM_DEVICE 0
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#define _LIBCPP_HAS_LOCALIZATION 1
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#define _LIBCPP_HAS_UNICODE 1
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#define _LIBCPP_HAS_WIDE_CHARACTERS 1
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#define _LIBCPP_HAS_TIME_ZONE_DATABASE 1
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#define _LIBCPP_INSTRUMENTED_WITH_ASAN 0
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// PSTL backends
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/* #undef _LIBCPP_PSTL_BACKEND_SERIAL */
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#define _LIBCPP_PSTL_BACKEND_STD_THREAD
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/* #undef _LIBCPP_PSTL_BACKEND_LIBDISPATCH */
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// Hardening.
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#define _LIBCPP_HARDENING_MODE_DEFAULT 2
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#define _LIBCPP_ASSERTION_SEMANTIC_DEFAULT 2
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// C libraries
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#define _LIBCPP_LIBC_PICOLIBC 0
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#define _LIBCPP_LIBC_NEWLIB 0
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// __USE_MINGW_ANSI_STDIO gets redefined on MinGW
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#ifdef __clang__
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# pragma clang diagnostic push
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# pragma clang diagnostic ignored "-Wmacro-redefined"
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#endif
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#ifdef __clang__
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# pragma clang diagnostic pop
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#endif
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#endif // _LIBCPP___CONFIG_SITE

‎llvm-generated/generic/__assertion_handler‎

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// -*- C++ -*-
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//===----------------------------------------------------------------------===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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#ifndef _LIBCPP___ASSERTION_HANDLER
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#define _LIBCPP___ASSERTION_HANDLER
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#if __cplusplus < 201103L && defined(_LIBCPP_USE_FROZEN_CXX03_HEADERS)
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# include <__cxx03/__config>
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# include <__cxx03/__verbose_abort>
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# include <__cxx03/__verbose_trap>
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#else
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# include <__config>
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# include <__log_hardening_failure>
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# include <__verbose_abort>
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# include <__verbose_trap>
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#endif
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#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
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# pragma GCC system_header
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#endif
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#if __cplusplus < 201103L && defined(_LIBCPP_USE_FROZEN_CXX03_HEADERS)
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// Keep the old implementation that doesn't support assertion semantics for backward compatibility with the frozen C++03
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// mode.
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# if _LIBCPP_HARDENING_MODE == _LIBCPP_HARDENING_MODE_DEBUG
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# define _LIBCPP_ASSERTION_HANDLER(message) _LIBCPP_VERBOSE_ABORT("%s", message)
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# else
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# define _LIBCPP_ASSERTION_HANDLER(message) _LIBCPP_VERBOSE_TRAP(message)
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# endif // _LIBCPP_HARDENING_MODE == _LIBCPP_HARDENING_MODE_DEBUG
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#else
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# if _LIBCPP_ASSERTION_SEMANTIC == _LIBCPP_ASSERTION_SEMANTIC_IGNORE
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# define _LIBCPP_ASSERTION_HANDLER(message) ((void)0)
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# elif _LIBCPP_ASSERTION_SEMANTIC == _LIBCPP_ASSERTION_SEMANTIC_OBSERVE
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# define _LIBCPP_ASSERTION_HANDLER(message) _LIBCPP_LOG_HARDENING_FAILURE(message)
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# elif _LIBCPP_ASSERTION_SEMANTIC == _LIBCPP_ASSERTION_SEMANTIC_QUICK_ENFORCE
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# define _LIBCPP_ASSERTION_HANDLER(message) _LIBCPP_VERBOSE_TRAP(message)
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# elif _LIBCPP_ASSERTION_SEMANTIC == _LIBCPP_ASSERTION_SEMANTIC_ENFORCE
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# define _LIBCPP_ASSERTION_HANDLER(message) _LIBCPP_VERBOSE_ABORT("%s", message)
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# else
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# error _LIBCPP_ASSERTION_SEMANTIC must be set to one of the following values: \
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_LIBCPP_ASSERTION_SEMANTIC_IGNORE, \
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_LIBCPP_ASSERTION_SEMANTIC_OBSERVE, \
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_LIBCPP_ASSERTION_SEMANTIC_QUICK_ENFORCE, \
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_LIBCPP_ASSERTION_SEMANTIC_ENFORCE
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# endif // _LIBCPP_ASSERTION_SEMANTIC == _LIBCPP_ASSERTION_SEMANTIC_IGNORE
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#endif // __cplusplus < 201103L && defined(_LIBCPP_USE_FROZEN_CXX03_HEADERS)
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#endif // _LIBCPP___ASSERTION_HANDLER

‎mcpp.toml‎

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# build if it reaches a <ctype.h> that is not its own. The generated
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# configuration precedes both, because every header of the library includes it.
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include_dirs = [
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"llvm-generated/generic",
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"llvm/libcxx/include",
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"llvm/libcxx/src",
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"llvm/libcxxabi/include",
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"llvm/libc",
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]
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# ⚠️ THE CONFIGURATION IS PER TARGET, AND THAT IS WHAT `llvm-generated' IS FOR.
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#
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# A hosted target lets libc++ work out for itself that the thread API is
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# pthread's, because it recognises the system. A target with no operating system
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# is not recognised, and libc++ stops with `No thread API' rather than guessing
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# --- correctly, because guessing is what a configure step exists to avoid.
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#
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# So the freestanding configuration states it, and states the two facilities
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# that environment does not have. Nothing else differs: the C library beneath is
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# the same one.
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[target.'cfg(not(os = "none"))'.build]
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include_dirs = ["llvm-generated/generic"]
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[target.'cfg(os = "none")'.build]
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include_dirs = ["llvm-generated/freestanding"]
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# Two switches that a hosted target answers by recognising the system and this
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# one cannot.
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#
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# the unwinder's use of dladdr --- there is no dynamic loader here, and the
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# type its interface names does not exist;
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# _GNU_SOURCE --- libc++abi reaches `syscall' for a thread identity, and musl
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# declares that name only under this macro. LLVM's own runtimes build defines
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# it for the same reason; a hosted build here gets it from elsewhere and the
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# difference was invisible until this target asked.
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cxxflags = ["-D_LIBUNWIND_USE_DLADDR=0", "-D_GNU_SOURCE"]
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# The configuration turns filesystem off, so the sources that implement it must
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# not be compiled --- the same rule as random.cpp above, and the same one libc++'s
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# own build applies with the same switch.
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sources = ["!llvm/libcxx/src/filesystem/*.cpp"]
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[build]
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cxxflags = [
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"-nostdinc++",
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"-D_LIBCPP_BUILDING_LIBRARY",
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"-D_LIBCXXABI_USE_LLVM_UNWINDER=1",
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"-DLIBCXXABI_USE_LLVM_UNWINDER=1",
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"-DNDEBUG",
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# libc++abi implements exceptions; it cannot be compiled without them.
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# libc++abi IMPLEMENTS exceptions and run-time type information; it cannot be
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# compiled without them, and a freestanding target turns both off for the whole
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# graph (mcpp docs/13 records why: a BMI carries the setting, so a dependency
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# compiled with exceptions cannot be imported by a unit without them). The
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# target's flags precede a package's, so stating both here is what restores
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# them --- for this package, which is the one that supplies them, and for
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# nothing else.
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"-fexceptions",
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"-frtti",
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"-funwind-tables",
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"-w",
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# As with the C library beneath: a program links what it reaches and nothing

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