03.02.2019

Cross compilation support for gobject typelibs

Thanks to the work of maxice8 we’re pleased to announce cross compiling support for gobject-introspection and packages that depend on this tool. This is a big step forward to support more packages through cross compiling and stay future proof for glib development.

maxice8 put together a comprehensive post about his work. This article will be higher level overview of our work in this field.

What are gobject and typelibs?

gobject is an abstract description of types defined in glib. Glib is used on a wide range of application and libraries, most notably gtk. Glib itself is written in C but it’s possible to provide dynamic language bindings to other languages such as python.

This mechanism is not builtin into glib, but instead is provided by the gobject-introspection tool. This tool scans both the source and the binary blob of a library and creates a metadata file that is used to provide type information for other language bindings. For a deeper dive into gobject-introspection please consider the docs at readthedocs.io

The problem

When it comes to Void, we have high demands on portability of applications. Our armv6l, armv7l and aarch64 platforms a completely and exclusively cross compiled, meaning they are built on an x86_64 host. This leads to an interesting problem with gobject-introspection which by design needs to run on the same platform as the produced binaries are targeted.

Unfortunately, as already stated this has been a design decision of this software and we can’t do anything to change this constraint itself.

Furthermore, during the last few years the Gnome community drifted towards extensively using gobject-introspection in their products. This led to the breakage of more and more applications in the gnome-ecosystem.

The solution

During the last few months we considered different solutions to this problem. This first proposal to not support gir through cross compilation and only allow it through native build became more and more unsatisfying as the number of packages using this mechanism increased over time.

The second approach which was the followup to the one described above was to add native builders to the system and build those packages on those native builders. The issue was here that this would require a lot of infrastructure work and added a lot of complexity.

The third solution and the one we chose was to use the userspace emulation features of Qemu to emulate the target system only for those calls that needed to be on the target platform. The yocto project is already using this approach to cross compile Gnome Applications to other platform. We reused many of their tooling for void-packages. Nontheless, we hit many porting issues - our tooling must run on musl, too - , new bugs were triggered in void-packages - see for example these two posts at maxice8’s blog, and many other small issues needed to be resolved.

Basic support for cross compilation has already landed in void-packages, the musl port is not yet finished, but we expect it to work soon(tm).

maxice8 also prepared a blog post about his porting work on libgusb.