Integration / D Bus Interview Questions
How do you subscribe to D-Bus signals using match rules?
To receive signals, a client registers a match rule with the bus daemon by calling AddMatch, describing which signals it wants delivered by filtering on fields like sender, interface, member (signal name), and path.
dbus-send --system --type=method_call \ --dest=org.freedesktop.DBus \ /org/freedesktop/DBus \ org.freedesktop.DBus.AddMatch \ string:"type='signal',interface='org.freedesktop.NetworkManager',member='StateChanged'"
Once the match is registered, the bus daemon starts forwarding any signal matching those criteria to that connection, which the client then reads from its message queue, typically through a client library's higher-level signal-subscription API rather than raw AddMatch calls in application code.
Match rules can be as broad or narrow as needed: an empty type='signal' rule receives every signal on the bus, while adding sender, interface, and path filters narrows it down to exactly the events an application actually cares about, keeping traffic manageable.
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