typedef int (*sd_bus_message_handler_t)(sd_bus_message *m, void *userdata, sd_bus_error *ret_error);
int sd_bus_call(sd_bus *bus, sd_bus_message *m, uint64_t usec, sd_bus_error *ret_error, sd_bus_message **reply);
int sd_bus_call_async(sd_bus *bus, sd_bus_slot **slot, sd_bus_message *m, sd_bus_message_handler_t callback, void *userdata, uint64_t usec);
sd_bus_call_async() is like sd_bus_call() but works asynchronously. The callback indicates the function to call when the response arrives. The userdata pointer will be passed to the callback function, and may be chosen freely by the caller. If slot is not NULL and sd_bus_call_async() succeeds, slot is set to a slot object which can be used to cancel the method call at a later time using sd_bus_slot_unref(3). If slot is NULL, the lifetime of the method call is bound to the lifetime of the bus object itself, and it cannot be cancelled independently. See sd_bus_slot_set_floating(3) for details. callback is called when a reply arrives with the reply, userdata and an sd_bus_error output parameter as its arguments. Unlike sd_bus_call(), the sd_bus_error output parameter passed to the callback will be empty. To determine whether the method call succeeded, use sd_bus_message_is_method_error(3) on the reply message passed to the callback instead. If the callback returns zero and the sd_bus_error output parameter is still empty when the callback finishes, other handlers registered with functions such as sd_bus_add_filter(3) or sd_bus_add_match(3) are given a chance to process the message. If the callback returns a non-zero value or the sd_bus_error output parameter is not empty when the callback finishes, no further processing of the message is done. Generally, you want to return zero from the callback to give other registered handlers a chance to process the reply as well. (Note that the sd_bus_error parameter is an output parameter of the callback function, not an input parameter; it can be used to propagate errors from the callback handler, it will not receive any error that was received as method reply.)
If usec is zero, the default D-Bus method call timeout is used. See sd_bus_get_method_call_timeout(3).