clang++: Usage, Options & Common CI Errors

clang++ compiles C++ with LLVM, optionally against libc++ instead of libstdc++.

clang++ is the C++ driver for clang. On Linux it defaults to libstdc++ but can target LLVM’s libc++ with -stdlib=libc++ - mixing the two across objects is the classic CI link failure.

What it does

clang++ compiles and links C++ using LLVM. It links a C++ standard library automatically (libstdc++ on Linux by default, libc++ with -stdlib=libc++) and supports the same sanitizers and standards flags as clang.

Common usage

clang++ -std=c++20 -Wall -O2 main.cpp -o app
clang++ -stdlib=libc++ -std=c++20 main.cpp -o app
clang++ -c util.cpp -o util.o
clang++ -fsanitize=address -g main.cpp -o app
clang++ main.cpp -o app -lpthread

Options

Common errors in CI

Linking objects built with -stdlib=libc++ against ones built with libstdc++ yields a flood of "undefined reference" errors because the std::__1 vs std:: symbol namespaces differ - build every object and dependency with the same -stdlib. "fatal error: ‘cstdio’ file not found" with -stdlib=libc++ means the libc++ dev headers are not installed (libc++-dev). For the GCC standard library the -std flag must match what dependencies expect.

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