Incorrect per-target runtime directory

[Ignition]

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Environment and version details

• Operating System+version: Linux Ubuntu 24.04.3 LTS
• Compiler+version: Clang 20.1.7 (built from source with LLVM_ENABLE_PER_TARGET_RUNTIME_DIR=ON, the LLVM
  default since LLVM 16)
• Shell: Bash
• B2 Version: B2 5.5.0 (OS=LINUX, jobs=24)

Brief problem description

clang.jam init-flags-cross sets vendor = pc ; sys = linux for Linux, producing --target=x86_64-pc-linux.
Clang's actual default triple is x86_64-unknown-linux-gnu. When clang uses the per-target runtime directory layout, b2 looks for lib/x86_64-pc-linux/libclang_rt.asan.a which doesn't exist, the actual path is lib/x86_64-unknown-linux-gnu/libclang_rt.asan.a.
Any link needing the sanitizer runtime fails, including ac library detection tests.

Steps to reproduce the issue

reproduce.sh

Actual behavior summary

With architecture=x86 address-model=64 (triggers init-flags-cross):
notice: loading B2 from /home/gareth/checkout/b2/src/build-system.jam
notice: Searching '/etc' '/home/gareth' '/home/gareth/checkout/b2' '/home/gareth/checkout/b2/src/util' '/home/gareth/checkout/b2/src/tools' '/home/gareth/checkout/b2/src/options' '/home/gareth/checkout/b2/src/engine' '/home/gareth/checkout/b2/src/contrib' '/home/gareth/checkout/b2/src/build' '/home/gareth/checkout/b2/src' for site-config configuration file 'site-config.jam'.
notice: Configuration file 'site-config.jam' not found in '/etc' '/home/gareth' '/home/gareth/checkout/b2' '/home/gareth/checkout/b2/src/util' '/home/gareth/checkout/b2/src/tools' '/home/gareth/checkout/b2/src/options' '/home/gareth/checkout/b2/src/engine' '/home/gareth/checkout/b2/src/contrib' '/home/gareth/checkout/b2/src/build' '/home/gareth/checkout/b2/src'.
notice: Loading explicitly specified user configuration file:
    /tmp/tmp.pL3WqbZhbU/user-config.jam
notice: Searching '/tmp/tmp.pL3WqbZhbU' for user-config configuration file 'user-config.jam'.
notice: Loading user-config configuration file 'user-config.jam' from '/tmp/tmp.pL3WqbZhbU'.
notice: will use '/opt/toolchain-v7/bin/clang' for clang-linux, condition <toolset>clang-linux-20
warning: toolset clang-linux initialization: can not find tool windres
warning: initialized from /tmp/tmp.pL3WqbZhbU/user-config.jam:5
notice: using rc compiler :: <toolset>clang-linux-20 :: as
...found 10 targets...
...updating 8 targets...
common.mkdir bin

        mkdir -p "bin"
    
common.mkdir bin/clang-linux-20

        mkdir -p "bin/clang-linux-20"
    
common.mkdir bin/clang-linux-20/debug

        mkdir -p "bin/clang-linux-20/debug"
    
common.mkdir bin/clang-linux-20/debug/address-model-64

        mkdir -p "bin/clang-linux-20/debug/address-model-64"
    
common.mkdir bin/clang-linux-20/debug/address-model-64/architecture-x86

        mkdir -p "bin/clang-linux-20/debug/address-model-64/architecture-x86"
    
common.mkdir bin/clang-linux-20/debug/address-model-64/architecture-x86/link-static

        mkdir -p "bin/clang-linux-20/debug/address-model-64/architecture-x86/link-static"
    
clang-linux.compile.c++ bin/clang-linux-20/debug/address-model-64/architecture-x86/link-static/test.o

    "/opt/toolchain-v7/bin/clang"   -fsanitize=address -O0 -fno-inline -Wall -g -m64 --target=x86_64-pc-linux       -c -o "bin/clang-linux-20/debug/address-model-64/architecture-x86/link-static/test.o" "test.cpp"

clang-linux.link bin/clang-linux-20/debug/address-model-64/architecture-x86/link-static/test

    "/opt/toolchain-v7/bin/clang"    -o "bin/clang-linux-20/debug/address-model-64/architecture-x86/link-static/test" -Wl,--start-group "bin/clang-linux-20/debug/address-model-64/architecture-x86/link-static/test.o"  -Wl,-Bstatic  -Wl,-Bdynamic  -Wl,--end-group -g -m64 -fsanitize=address --target=x86_64-pc-linux 

/opt/toolchain-v7/bin/ld: cannot find /opt/toolchain-v7/lib/clang/20/lib/x86_64-pc-linux/libclang_rt.asan_static.a: No such file or directory
/opt/toolchain-v7/bin/ld: cannot find /opt/toolchain-v7/lib/clang/20/lib/x86_64-pc-linux/libclang_rt.asan.a: No such file or directory
clang: error: linker command failed with exit code 1 (use -v to see invocation)
...failed clang-linux.link bin/clang-linux-20/debug/address-model-64/architecture-x86/link-static/test...

...updated 7 targets...

...failed updating 1 target...
   clang-linux.link bin/clang-linux-20/debug/address-model-64/architecture-x86/link-static/test

My project is using workaround at the moment of not using ASAN when building boost.

Expected behavior summary

The correct lib path should be used.

[grisumbras]

As far as I can see, this is a duplicate of #449. Add to your -config.jam

using clang : 20 : : <triple>x86_64-unknown-linux-gnu ; # or <triple>none if you're not cross-compiling

[Ignition]

@grisumbras can this not portably be fixed within b2, for example by asking the compiler directly? Or is it the something every downstream project will need to do?

$clang -dumpmachine
x86_64-pc-linux-gnu

$ /opt/toolchain-v7/bin/clang -dumpmachine 
x86_64-unknown-linux-gnu

For context: to fix I'd have to do something in conan recipe or within conan in order to generate the correct jam.

[Ignition]

Actually digging deeper, maybe its https://github.com/boostorg/build/blob/85252c9d6a3817daca23ddae94ce16c7be4d2c12/src/tools/clang.jam than needs updating.

[grisumbras]

I think, the only way to solve this is to not attempt to guess the target on clang 20+ at all.

[Ignition]

conan-io/conan-center-index#29733
I see I'm not the only person trying to figuring this out.

Feel free to close this issue, if nothing can be done from b2 side.

• There is a UX argument that native build statically guesses x86_64-pc-linux because that work for many distributions.
• Also a UX argument that uses tool collaboration to tell you the right triple
• Cross compilation, all bets are off? Requires explicitly being communicated what triple we are targetting.

[grisumbras]

We can attempt all sorts of things, but apparently the advice from Clang people is 1) don't specify the target when not cross-compiling 2) figure out somehow what is the correct target for the intended platform (this depends on how Clang was configured).