sd_bus_send, sd_bus_send_to, sd_bus_message_send - Queue a D-Bus message for transfer
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);
sd_bus_send() queues the bus message object m for transfer. If bus is NULL, the bus that m is attached to is used. bus only needs to be set when the message is sent to a different bus than the one it's attached to, for example when forwarding messages. If the output parameter cookie is not NULL, it is set to the message identifier. This value can later be used to match incoming replies to their corresponding messages. If cookie is set to NULL and the message is not sealed, sd_bus_send() assumes the message m doesn't expect a reply and adds the necessary headers to indicate this.
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).
On success, these functions return a non-negative integer. On failure, they return a negative errno-style error code.
Returned errors may indicate the following problems:
Functions described here are available as a shared library, which can be compiled against and linked to with the libsystemd pkg-config(1) file.
The code described here uses getenv(3), which is declared to be not multi-thread-safe. This means that the code calling the functions described here must not call setenv(3) from a parallel thread. It is recommended to only do calls to setenv() from an early phase of the program when no other threads have been started.