An application developer who uses sd-journal in a heavily threaded application currently has to deal with the fact that its thread safety is undefined. It would be far more helpful if systemd could define precisely what level of thread safety it guarantees vis-à-vis the sd-journal interface.

It would be particularly nice if sd-journal could guarantee that messages can be safely submitted to the journal from multiple threads simultaneously.

In any case, whatever level of thread-safety is chosen, it needs to be clearly defined in the documentation.