sauparna · April 28, 2025 16:35
diff --git a/ubuntu-llvm-clang.txt b/ubuntu-llvm-clang.txt
 The message to share is that, on Ubuntu 24.04, to enable LLVM/Clang, run apt install llvm clang g++-14. If you want to set up a custom toolchain using a particular version of LLVM, it will require much more work. The reasoning are in a gist, if at all it's worth the reader's time:

 1. On Linux, using an LLVM version of one's choice is fairly straightforward. Download the appropriate tarball from https://releases.llvm.org, extract it, (the extracted directory contains the sub-directories `lib` and `bin`),  place the directory somewhere, and add the path to the directory's `bin` directory to your `PATH` variable. The LLVM tarball comes with Clang.

 2. That `bin` directory contains the tool `llvm-config` (or `llvm-19-config`, say), which when run, will tell you what flags to use to compile and link your C/C++ programs correctly.  At this point, if you are using `clang++` to compile C++ program, and linking it to LLVM's C++ standard-library in this way `clang++ -stdlib=libc++ f.cpp`, it will work, but running the executable will complain about the missing shared library `libc++.so.1`.  The LLVM tarball does not come with the standard C++ library binaries, so that leaves more work for you to do. You will have to fetch the libc++ source (which probably entails fetching `libcxx`, `libcxxabi`, `libunwind` if I am correct) and build them from source. An alternative is to use `apt install libc++` to obtain the C++ standard-libraries, but they will be routed to your system's standard locations and will not match the particular version you are trying to work with.

 3. On Ubuntu 24.04, if I wish to install LLVM/Clang from the standard distribution packages, using `apt install llvm`, it installs version 18. If you want version 19, you could run `apt install llvm-19`, but that will not install `clang`. `/usr/lib/llvm-19/bin/` will have the binary `clang-19` but not `clang`. I haven't tried thinking up a work-around, but whatever you could do about that will lead to a non-standard solution, defeating the purpose of having your distribution lay out the toolchain for you. Try `sudo apt install --dry-run llvm*` to get a sense of all the LLVM versions your distribution offers. `apt install libc++` will fetch you the version 18 C++ standard-libraries. 

 4. On Ubuntu 24.04, after you have installed LLVM/Clang, and not having touched the C++ libraries, compiling a C++ program using `clang++` will complain that the use of the compiler flag `-lstdc++` fails to find the standard C++ libraries. This is because since Linux has the GCC toolchain, clang choses to use GCC's standard libraries automatically. If your run `clang -v` it will tell you that it has found GCC versions 13 and 14, and that it has chosen GCC version 14. Check for yourself that there are two directories, `13` and `14`, in `/usr/lib/gcc/x86_64-linux-gnu/`. Oddly, the GCC 14 directory does not have the C++ libraries. So you install it yourself `apt install g++-14`.  This should complete setting up a the LLVM/Clang toolchain.
	The message to share is that, on Ubuntu 24.04, to enable LLVM/Clang, run apt install llvm clang g++-14. If you want to set up a custom toolchain using a particular version of LLVM, it will require much more work. The reasoning are in a gist, if at all it's worth the reader's time:

	1. On Linux, using an LLVM version of one's choice is fairly straightforward. Download the appropriate tarball from https://releases.llvm.org, extract it, (the extracted directory contains the sub-directories `lib` and `bin`), place the directory somewhere, and add the path to the directory's `bin` directory to your `PATH` variable. The LLVM tarball comes with Clang.

	2. That `bin` directory contains the tool `llvm-config` (or `llvm-19-config`, say), which when run, will tell you what flags to use to compile and link your C/C++ programs correctly. At this point, if you are using `clang++` to compile C++ program, and linking it to LLVM's C++ standard-library in this way `clang++ -stdlib=libc++ f.cpp`, it will work, but running the executable will complain about the missing shared library `libc++.so.1`. The LLVM tarball does not come with the standard C++ library binaries, so that leaves more work for you to do. You will have to fetch the libc++ source (which probably entails fetching `libcxx`, `libcxxabi`, `libunwind` if I am correct) and build them from source. An alternative is to use `apt install libc++` to obtain the C++ standard-libraries, but they will be routed to your system's standard locations and will not match the particular version you are trying to work with.

	3. On Ubuntu 24.04, if I wish to install LLVM/Clang from the standard distribution packages, using `apt install llvm`, it installs version 18. If you want version 19, you could run `apt install llvm-19`, but that will not install `clang`. `/usr/lib/llvm-19/bin/` will have the binary `clang-19` but not `clang`. I haven't tried thinking up a work-around, but whatever you could do about that will lead to a non-standard solution, defeating the purpose of having your distribution lay out the toolchain for you. Try `sudo apt install --dry-run llvm*` to get a sense of all the LLVM versions your distribution offers. `apt install libc++` will fetch you the version 18 C++ standard-libraries.

	4. On Ubuntu 24.04, after you have installed LLVM/Clang, and not having touched the C++ libraries, compiling a C++ program using `clang++` will complain that the use of the compiler flag `-lstdc++` fails to find the standard C++ libraries. This is because since Linux has the GCC toolchain, clang choses to use GCC's standard libraries automatically. If your run `clang -v` it will tell you that it has found GCC versions 13 and 14, and that it has chosen GCC version 14. Check for yourself that there are two directories, `13` and `14`, in `/usr/lib/gcc/x86_64-linux-gnu/`. Oddly, the GCC 14 directory does not have the C++ libraries. So you install it yourself `apt install g++-14`. This should complete setting up a the LLVM/Clang toolchain.