how to select a particular gcc-toolchain in clang? - c++

Question

Clang automatically selects the gcc-version with the highest version:
$ clang++ -v main.cpp
clang version 3.8.1-12
(tags/RELEASE_381/final)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/4.9
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/4.9.4
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/5.4.1
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/6.2.0
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/7.0.1
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.9
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.9.4
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/5.4.1
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/6.2.0
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/7.0.1
Selected GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/7.0.1
how can i force clang to use a different gcc installation, say 5.4.1 ?
i tried to call clang with --gcc-toolchain="/usr/lib/gcc/x86_64-linux-gnu/5.4.1" but without success.

Answer 1

An valid path for --gcc-toolchain is apparently "/usr" as clang seem to look for gcc in
$PREFIX/{include|lib}/gcc/$PLATFORM/$VERSION/*
so as a workaround you can trick clang to use a particular version by creating a filesystem with overlay-fs or symlinking a folder-structure containing only one folder
mkdir $MYTOOLCHAIN
cd $MYTOOLCHAIN
ln -s /usr/include include #for headerfiles
ln -s /usr/bin bin #for tools like ld
mkdir -p lib/gcc/x86_64-linux-gnu/ #clang will deduce what to select
cd lib/gcc/x86_64-linux-gnu/
#link the toolchain we want here
ln -s /usr/lib/gcc/x86_64-linux-gnu/$VERSION $VERSION
#usage: clang++ --gcc-toolchain=$MYTOOLCHAIN main.cpp
however maybe there is a better way by instructing clang to pick the version via a flag...