20 Enabling GObject Introspection Support

GObject introspection is the standard mechanism for accessing GObject-based software from runtime environments. GObject is a feature of the GLib library that provides an object framework for the GNOME desktop and related software. GObject Introspection adds information to GObject that allows objects created within it to be represented across different programming languages. If you want to construct GStreamer pipelines using Python, or control UPnP infrastructure using Javascript and GUPnP, GObject introspection is the only way to do it.

This section describes the Yocto Project support for generating and packaging GObject introspection data. GObject introspection data is a description of the API provided by libraries built on top of the GLib framework, and, in particular, that framework’s GObject mechanism. GObject Introspection Repository (GIR) files go to -dev packages, typelib files go to main packages as they are packaged together with libraries that are introspected.

The data is generated when building such a library, by linking the library with a small executable binary that asks the library to describe itself, and then executing the binary and processing its output.

Generating this data in a cross-compilation environment is difficult because the library is produced for the target architecture, but its code needs to be executed on the build host. This problem is solved with the OpenEmbedded build system by running the code through QEMU, which allows precisely that. Unfortunately, QEMU does not always work perfectly as mentioned in the “Known Issues” section.

20.1 Enabling the Generation of Introspection Data

Enabling the generation of introspection data (GIR files) in your library package involves the following:

Inherit the gobject-introspection.bbclass class.
Make sure introspection is not disabled anywhere in the recipe or from anything the recipe includes. Also, make sure that “gobject-introspection-data” is not in DISTRO_FEATURES_BACKFILL_CONSIDERED and that “qemu-usermode” is not in MACHINE_FEATURES_BACKFILL_CONSIDERED. In either of these conditions, nothing will happen.
Try to build the recipe. If you encounter build errors that look like something is unable to find .so libraries, check where these libraries are located in the source tree and add the following to the recipe:
```
GIR_EXTRA_LIBS_PATH = "${B}/something/.libs"
```
Note

See recipes in the oe-core repository that use that GIR_EXTRA_LIBS_PATH variable as an example.
Look for any other errors, which probably mean that introspection support in a package is not entirely standard, and thus breaks down in a cross-compilation environment. For such cases, custom-made fixes are needed. A good place to ask and receive help in these cases is the Yocto Project mailing lists.

Note

Using a library that no longer builds against the latest Yocto Project release and prints introspection related errors is a good candidate for the previous procedure.

20.2 Disabling the Generation of Introspection Data

You might find that you do not want to generate introspection data. Or, perhaps QEMU does not work on your build host and target architecture combination. If so, you can use either of the following methods to disable GIR file generations:

Add the following to your distro configuration:
```
DISTRO_FEATURES_BACKFILL_CONSIDERED = "gobject-introspection-data"
```
Adding this statement disables generating introspection data using QEMU but will still enable building introspection tools and libraries (i.e. building them does not require the use of QEMU).
Add the following to your machine configuration:
```
MACHINE_FEATURES_BACKFILL_CONSIDERED = "qemu-usermode"
```
Adding this statement disables the use of QEMU when building packages for your machine. Currently, this feature is used only by introspection recipes and has the same effect as the previously described option.

Note

Future releases of the Yocto Project might have other features affected by this option.

If you disable introspection data, you can still obtain it through other means such as copying the data from a suitable sysroot, or by generating it on the target hardware. The OpenEmbedded build system does not currently provide specific support for these techniques.

20.3 Testing that Introspection Works in an Image

Use the following procedure to test if generating introspection data is working in an image:

Make sure that “gobject-introspection-data” is not in DISTRO_FEATURES_BACKFILL_CONSIDERED and that “qemu-usermode” is not in MACHINE_FEATURES_BACKFILL_CONSIDERED.
Build core-image-sato.
Launch a Terminal and then start Python in the terminal.

Enter the following in the terminal:

>>> from gi.repository import GLib
>>> GLib.get_host_name()

For something a little more advanced, enter the following see: https://python-gtk-3-tutorial.readthedocs.io/en/latest/introduction.html

20.4 Known Issues

Here are know issues in GObject Introspection Support:

qemu-ppc64 immediately crashes. Consequently, you cannot build introspection data on that architecture.
x32 is not supported by QEMU. Consequently, introspection data is disabled.
musl causes transient GLib binaries to crash on assertion failures. Consequently, generating introspection data is disabled.
Because QEMU is not able to run the binaries correctly, introspection is disabled for some specific packages under specific architectures (e.g. gcr, libsecret, and webkit).
QEMU usermode might not work properly when running 64-bit binaries under 32-bit host machines. In particular, “qemumips64” is known to not work under i686.