typedef int (*sd_bus_message_handler_t)(sd_bus_message *m, void *userdata, sd_bus_error *ret_error);
int sd_bus_add_match(sd_bus *bus, sd_bus_slot **slot, const char *match, sd_bus_message_handler_t callback, void *userdata);
int sd_bus_add_match_async(sd_bus *bus, sd_bus_slot **slot, const char *match, sd_bus_message_handler_t callback, sd_bus_message_handler_t install_callback, void *userdata);
int sd_bus_match_signal(sd_bus *bus, sd_bus_slot **slot, const char *sender, const char *path, const char *interface, const char *member, sd_bus_message_handler_t callback, void *userdata);
int sd_bus_match_signal_async(sd_bus *bus, sd_bus_slot **slot, const char *sender, const char *path, const char *interface, const char *member, sd_bus_message_handler_t callback, sd_bus_message_handler_t install_callback, void *userdata);
sd_bus_add_match_async() operates very similar to sd_bus_add_match(), however it installs the match asynchronously, in a non-blocking fashion: a request is sent to the broker, but the call does not wait for a response. The install_callback function is called when the response is later received, with the response message from the broker as parameter. If this function is specified as NULL a default implementation is used that terminates the bus connection should installing the match fail.
sd_bus_match_signal() is very similar to sd_bus_add_match(), but only matches signals, and instead of a match expression accepts four parameters: sender (the service name of the sender), path (the object path of the emitting object), interface (the interface the signal belongs to), member (the signal name), from which the match string is internally generated. Optionally, these parameters may be specified as NULL in which case the relevant field of incoming signals is not tested.
sd_bus_match_signal_async() combines the signal matching logic of sd_bus_match_signal() with the asynchronous behaviour of sd_bus_add_match_async().
On success, and if non-NULL, the slot return parameter will be set to a slot object that may be used as a reference to the installed match, and may be utilized to remove it again at a later time with sd_bus_slot_unref(3). If specified as NULL the lifetime of the match is bound to the lifetime of the bus object itself, and the match is generally not removed independently. See sd_bus_slot_set_floating(3) for details.
The message m passed to the callback is only borrowed, that is, the callback should not call sd_bus_message_unref(3) on it. If the callback wants to hold on to the message beyond the lifetime of the callback, it needs to call sd_bus_message_ref(3) to create a new reference.
If an error occurs during the callback invocation, the callback should return a negative error number (optionally, a more precise error may be returned in ret_error, as well). If it wants other callbacks that match the same rule to be called, it should return 0. Otherwise it should return a positive integer.
If the bus refers to a direct connection (i.e. not a bus connection, as set with sd_bus_set_bus_client(3)) the match is only installed on the client side, and the synchronous and asynchronous functions operate the same.
- D-Bus Specification