Rack development blog

Development
8

As some of you know, in order to produce a Linux plugin build that works on all Linux distros supported by Rack, you must compile against glibc 2.23 and libstdc++ 5.4.0, which is what Ubuntu 16.04 uses. This is because the ABIs of the GNU implementations of the C and C++ standard library change every few years.

Previously the only way to correctly do this is to use Ubuntu 16.04 (and thus an old version of GCC) for building plugins, either in a virtual machine, Docker container, or installed on a system.

Thanks to wheybags’ glibc_version_header, you can add the following to your plugin’s Makefile and build on any Linux distro, even bleeding edge Arch with GCC 9 if you want.

FLAGS += -include force_link_glibc_2.23.h

This works by specifying each glibc symbol version explicitly instead of defaulting to the newest versions (which might be beyond 2.23). This header is not included by default because it only works in probably ~75% of cases. Your plugin will still fail to load in Ubuntu 16.04 if you

  • use certain libstdc++ code. Most of the C++ stdlib is header-only but not all of it. glibc_version_header only fixes glibc symbols, not libstdc++ symbols. Furthermore, libstdc++ links to glibc and will use newer symbols unless you compile libstdc++ yourself and statically link it (which is more complicated than it’s worth).
  • build dependencies without passing it that flag.
  • use dlopen, since libdl occassionally changes its ABI but doesn’t offer versioned symbols (at least in my distro’s libdl build).

So if you use glibc_version_header, be sure to give your binary to an Ubuntu 16.04 user to test that it loads. Or test by running

objdump -p plugin.so

and looking at the Version References: section for higher versions of GLIBC and GLIBCXX.

This post is only relevant if you are building your own Linux plugin binaries. None of the above is needed for

  • Windows, since Mingw-w64 uses Microsoft’s MSVCRT.DLL for its C stdlib and Rack bundles libstdc++6.dll
  • Mac, since Rack and plugins are built with Apple LLVM’s -mmacosx-version-min=10.7, which is a fantastic feature that tells clang to link to the libc ABI used by Mac 10.7.
  • Source code git repos submitted to https://github.com/VCVRack/library or private ZIP files emailed to VCV, since VCV’s build service uses an Ubuntu 16.04 Docker container for Linux builds.

(The length and complexity of this post is one of the reasons why Linux is often not targeted by commercial software vendors, such as VST developers.)

4 Likes