libc++.modules.json references non-existent path within build tree

[kamrann]

The std module json manifest, in my LLVM build tree at lib/x86_64-unknown-linux-gnu/libc++.modules.json, contains the following entry:

"source-path": "../../share/libc++/v1/std.cppm"

However, while the build tree has a /share folder, there is no libc++ folder within it.

If I run CMake install on my LLVM build then the resulting installation contains the std library module sources at the appropriate path, but if I attempt to use clang direct from the build tree, my build tool will fail. Presumably it should be possible to use in this form, not only installed?

Fedora 41 via WSL
clang version 21.0.0git (https://github.com/llvm/llvm-project.git 9312d5b)
Target: x86_64-unknown-linux-gnu

Configured with -DLLVM_ENABLE_PROJECTS=clang -DLLVM_ENABLE_RUNTIMES=libcxx

[ChuanqiXu9]

I don't think this is a valid use case. Installation is needed. Generally, in my workflow, when I don't want to install my build really, I will do:

cmake  -DCMAKE_INSTALL_PREFIX=<build-path>/installed ...
make -j
make install -j
use compile from  <build-path>/installed/bin

CC @mordante @ldionne for double check.

[kamrann]

I don't really have any particular reason to avoid install, but I've always been in the habit of using direct from the build tree during development and up to now it's worked fine, so I assumed this was due to some oversight. Parts of the LLVM getting started documentation also give the impression to me that install is considered optional.

[ChuanqiXu9]

I am not super sure too. Maybe you can send this as a question to discourse to get more qualified answer.

[ldionne]

What @ChuanqiXu9 said is correct: you need to perform an installation step. Installing the library is not optional, it's necessary for these things to work. CMake also adjusts install-names of dylibs upon installation, we put generated headers in the right location, etc.