#053: Adding llvm Snapshots for R Package Testing

Fri, 03 Oct 2025 17:09:00 CDT

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.