int sd_bus_send(sd_bus *bus, sd_bus_message *m, uint64_t *cookie);
int sd_bus_send_to(sd_bus *bus, sd_bus_message *m, const char *destination, uint64_t *cookie);
int sd_bus_message_send(sd_bus_message *m);
Note that in most scenarios, sd_bus_send() should not be called directly. Instead, use higher level functions such as sd_bus_call_method(3) and sd_bus_reply_method_return(3) which call sd_bus_send() internally.
sd_bus_send_to() is a shorthand for sending a message to a specific destination. It's main use case is to simplify sending unicast signal messages (signals that only have a single receiver). It's behavior is similar to calling sd_bus_message_set_destination(3) followed by calling sd_bus_send().
sd_bus_send()/sd_bus_send_to() will write the message directly to the underlying transport (e.g. kernel socket buffer) if possible. If the connection is not set up fully yet the message is queued locally. If the transport buffers are congested any unwritten message data is queued locally, too. If the connection has been closed or is currently being closed the call fails. sd_bus_process(3) should be invoked to write out any queued message data to the transport.
sd_bus_message_send() is the same as sd_bus_send() but without the first and last argument. sd_bus_message_send(m) is equivalent to sd_bus_send(sd_bus_message_get_bus(m), m, NULL).