Dirk Eddelbuettel

Fri, 03 Oct 2025

#053: Adding llvm Snapshots for R Package Testing

Welcome to post 53 in the R4 series.

Continuing with posts #51 from Tuesday and #52 from Wednesday and their stated intent of posting some more … here is another quick one. Earlier today I helped another package developer who came to the r-package-devel list asking for help with a build error on the Fedora machine at CRAN running recent / development clang. In such situations, the best first step is often to replicate the issue. As I pointed out on the list, the LLVM team behind clang maintains an apt repo at apt.llvm.org/ making it a good resource to add to Debian-based container such as Rocker r-base or the offical r-base (the two are in fact interchangeable, and I take care of both).

A small pothole, however, is that the documentation at the top of apt.llvm.org site is a bit stale and behind two aspects that changed on current Debian systems (i.e. unstable/testing as used for r-base). First, apt now prefers files ending in .sources (in a nicer format) and second, it now really requires a key (which is good practice). As it took me a few minutes to regather how to meet both requirements, I reckoned I might as well script this.

Et voilà the following script does that:

  • it can update and upgrade the container (currently commented-out)
  • it fetches the repository key in ascii form from the llvm.org site
  • it creates the sources entry for, here tagged for llvm ‘current’ (22 at time of writing)
  • it sets up the required ~/.R/Makevars to use that compiler
  • it installs clang-22 (and clang++-22) (still using the g++ C++ library)
#!/bin/sh

## Update does not hurt but is not strictly needed
#apt update --quiet --quiet
#apt upgrade --yes

## wget -qO- https://apt.llvm.org/llvm-snapshot.gpg.key | sudo tee /etc/apt/trusted.gpg.d/apt.llvm.org.asc
## or as we are root in container
wget -qO- https://apt.llvm.org/llvm-snapshot.gpg.key > /etc/apt/trusted.gpg.d/apt.llvm.org.asc

cat <<EOF >/etc/apt/sources.list.d/llvm-dev.sources
Types: deb
URIs: http://apt.llvm.org/unstable/
# for clang-21 
#   Suites: llvm-toolchain-21
# for current clang
Suites: llvm-toolchain
Components: main
Signed-By: /etc/apt/trusted.gpg.d/apt.llvm.org.asc
EOF

test -d ~/.R || mkdir ~/.R
cat <<EOF >~/.R/Makevars
CLANGVER=-22
# CLANGLIB=-stdlib=libc++
CXX=clang++\$(CLANGVER) \$(CLANGLIB)
CXX11=clang++\$(CLANGVER) \$(CLANGLIB)
CXX14=clang++\$(CLANGVER) \$(CLANGLIB)
CXX17=clang++\$(CLANGVER) \$(CLANGLIB)
CXX20=clang++\$(CLANGVER) \$(CLANGLIB)
CC=clang\$(CLANGVER)
SHLIB_CXXLD=clang++\$(CLANGVER) \$(CLANGLIB)
EOF

apt update
apt install --yes clang-22

Once the script is run, one can test a package (or set of packages) against clang-22 and clang++-22. This may help R package developers. The script is also generic enough for other development communities who can ignore (or comment-out / delete) the bit about ~/.R/Makevars and deploy the compiler differently. Updating the softlink as apt-preferences does is one way and done in many GitHub Actions recipes. As we only need wget here a basic Debian container should work, possibly with the addition of wget. For R users r-base hits a decent sweet spot.

This post by Dirk Eddelbuettel originated on his Thinking inside the box blog. If you like this or other open-source work I do, you can now sponsor me at GitHub.

/code/r4 | permanent link