使用Bazel构建Carbon时Clang/LLVM报错的解决求助

问题描述 #

尝试构建Carbon-Lang，已完成准备步骤且Clang/LLVM可正常使用，但执行命令bazel run //explorer -- ./explorer/testdata/print/format_only.carbon时链接external/m4_v1.4.18/bin/m4失败，报错ld.lld找不到-lc++库与libunwind.a库。使用Arch Linux，曾将LLVM路径/usr/include/llvm添加到环境变量但无效，llvm-config --version可正常执行但which llvm无结果。

报错详情：

[bret@archxbret carbon-lang]$ bazel run //explorer -- ./explorer/testdata/print/format_only.carbon 
Starting local Bazel server and connecting to it... 
INFO: Invocation ID: 6fb0bb2c-cc78-4fb0-8e35-90759b75109d 
INFO: Analyzed target //explorer:explorer (67 packages loaded, 1567 targets configured). 
INFO: Found 1 target... 
ERROR: /home/bret/.cache/bazel/_bazel_bret/e6835bf0f6859c4bbbb42f026d02cce5/external/m4_v1.4.18/bin/BUILD.bazel:2:10: Linking external/m4_v1.4.18/bin/m4 (for host) failed: (Exit 1): 
clang++ failed: error executing command /usr/bin/clang++ @bazel-out/host/bin/external/m4_v1.4.18/bin/m4-2.params

Use --sandbox_debug to see verbose messages from the sandbox and retain the sandbox build root for debugging 
ld.lld: error: unable to find library -lc++ 
ld.lld: error: unable to find library -l:libunwind.a  
ld.lld: error: unable to find library -l:libunwind.a
clang++: error: linker command failed with exit code 1 (use -v to see invocation)
Target //explorer:explorer failed to build
Use --verbose_failures to see the command lines of failed build steps. 
INFO: Elapsed time: 29.584s, Critical Path: 0.68s 
INFO: 2 processes: 2 internal. 
FAILED: Build did NOT complete successfully 
FAILED: Build did NOT complete successfully

解决方案 #

1. 安装缺失的依赖包 #

Arch Linux默认未预装Clang专用的C++标准库libc++和 unwind 库libunwind，执行以下命令安装：

sudo pacman -S libc++ libunwind

2. 配置Bazel使用libc++标准库 #

Carbon的Bazel构建配置默认依赖libc++，但Arch Linux系统默认C++标准库是libstdc++，需要显式指定构建参数：

• 临时生效：直接在构建命令中追加参数

  bazel run //explorer --cxxopt=-stdlib=libc++ --linkopt=-stdlib=libc++ --linkopt=-lunwind -- ./explorer/testdata/print/format_only.carbon
  

• 永久生效：在Carbon-Lang项目根目录创建.bazelrc文件，添加以下内容，后续构建无需重复输入参数：

  build --action_env=CC=/usr/bin/clang
  build --action_env=CXX=/usr/bin/clang++
  build --cxxopt=-stdlib=libc++
  build --linkopt=-stdlib=libc++
  build --linkopt=-lunwind
  

3. 修正环境变量的错误配置 #

你之前将/usr/include/llvm添加到PATH是错误操作：PATH是可执行文件的搜索路径，LLVM的可执行命令（如clang、llvm-config）都在系统默认的/usr/bin路径下，无需额外添加。which llvm无结果是因为LLVM没有名为llvm的可执行命令，属于正常现象，无需处理。

内容的提问来源于stack exchange，提问作者Bretwalda