Similarly, sd_bus_message_get_realtime_usec() returns the realtime (wallclock) timestamp of the time the message was sent. This value is in microseconds since Jan 1st, 1970, i.e. in the CLOCK_REALTIME clock.
sd_bus_message_get_seqnum() returns the kernel-assigned sequence number of the message. The kernel assigns a global, monotonically increasing sequence number to all messages transmitted on the local system, at the time the message was sent. This sequence number is useful for determining message send order, even across different buses of the local system. The sequence number combined with the boot ID of the system (as returned by sd_id128_get_boot(3)) is a suitable globally unique identifier for bus messages.
Note that the sending order and receiving order of messages might differ, in particular for broadcast messages. This means that the sequence number and the timestamps of messages a client reads are not necessarily monotonically increasing.
These timestamps and the sequence number are attached to each message by the kernel and cannot be manipulated by the sender.
Note that these timestamps are only available on some bus transports, and only after support for them has been negotiated with the sd_bus_negotiate_timestamp(3) call.
On success, these calls return 0 or a positive integer. On failure, these calls return a negative errno-style error code.
Returned errors may indicate the following problems:
These APIs are implemented as a shared library, which can be compiled and linked to with the libsystemd pkg-config(1) file.