Function sd_journal_get_timeout 

pub unsafe extern "C" fn sd_journal_get_timeout(
    journal: *mut sd_journal,
    timeout: *mut u64,
) -> c_int

int sd_journal_get_timeout(sd_journal *j, uint64_t *timeout_usec);

https://www.freedesktop.org/software/systemd/man/sd_journal_get_fd.html#