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The definitions for SettingsBackend are included in gio/gsettingsbackend.h while gio-2.0.vapi contains:

[CCode (cheader_filename = "gio/gio.h", type_id = "g_settings_backend_get_type ()")]
public abstract class SettingsBackend : GLib.Object {
...
}

gio/gio.h does not cause gio/gsettingsbackend.h to be included. With the current bindings the C compiler outputs warnings like implicit declaration of function ‘g_keyfile_settings_backend_new’; did you mean ‘some_other_function’? and generated code causes segfaults. Replacing all gio/gio.h occurences with gio/gsettingsbackend.h here fixes this issue. Also see https://developer.gnome.org/gio/stable/GSettingsBackend.html

edit: just replacing gio/gio.h occurences with gio/gsettingsbackend.h for SettingsBackend isn't a good solution as gio/gsettingsbackend.h then also gets included even when none of the functions of SettingsBackend get used.