sd_bus_get_n_queued_read - Man Page
Get the number of pending bus messages in the read and write queues of a bus connection object
int sd_bus_get_n_queued_read(sd_bus *bus, uint64_t *ret);
int sd_bus_get_n_queued_write(sd_bus *bus, uint64_t *ret);
sd_bus_get_n_queued_read() may be used to query the number of bus messages in the read queue of a bus connection object. The read queue contains all messages read from the transport medium (e.g. network socket) but not yet processed locally. The function expects two arguments: the bus object to query, and a pointer to a 64bit counter variable to write the current queue size to. Use sd_bus_process() in order to process queued messages, i.e. to reduce the size of the read queue (as well as, in fact, the write queue, see below).
Similarly, sd_bus_get_n_queued_write() may be used to query the number of currently pending bus messages in the write queue of a bus connection object. The write queue contains all messages enqueued into the connection with a call such as sd_bus_send() but not yet written to the transport medium. The expected arguments are similar to sd_bus_get_n_queued_read(). Here too, use sd_bus_process() to reduce the size of the write queue. Alternatively, use sd_bus_flush() to synchronously write out any pending bus messages until the write queue is empty.
On success, these functions return 0 or a positive integer. On failure, they return a negative errno-style error code.
Returned errors may indicate the following problems:
The bus connection was created in a different process.
systemd(1), sd-bus(3), sd_bus_process(3), sd_bus_send(3), sd_bus_flush(3)
sd-bus(3), systemd.directives(7), systemd.index(7).
The man page sd_bus_get_n_queued_write(3) is an alias of sd_bus_get_n_queued_read(3).