int sd_journal_seek_head(sd_journal *j);
int sd_journal_seek_tail(sd_journal *j);
int sd_journal_seek_monotonic_usec(sd_journal *j, sd_id128_t boot_id, uint64_t usec);
int sd_journal_seek_realtime_usec(sd_journal *j, uint64_t usec);
int sd_journal_seek_cursor(sd_journal *j, const char *cursor);
Similarly, sd_journal_seek_tail() may be used to seek to the end of the journal, i.e. the position after the most recent available entry.
sd_journal_seek_monotonic_usec() seeks to a position with the specified monotonic timestamp, i.e. CLOCK_MONOTONIC. Since monotonic time restarts on every reboot a boot ID needs to be specified as well.
sd_journal_seek_realtime_usec() seeks to a position with the specified realtime (wallclock) timestamp, i.e. CLOCK_REALTIME. Note that the realtime clock is not necessarily monotonic. If a realtime timestamp is ambiguous, it is not defined which position is sought to.
sd_journal_seek_cursor() seeks to the position at the specified cursor string. For details on cursors, see sd_journal_get_cursor(3). If no entry matching the specified cursor is found the call will seek to the next closest entry (in terms of time) instead. To verify whether the newly selected entry actually matches the cursor, use sd_journal_test_cursor(3).
Note that these calls do not actually make any entry the new current entry, this needs to be done in a separate step with a subsequent sd_journal_next(3) invocation (or a similar call). Only then, entry data may be retrieved via sd_journal_get_data(3) or an entry cursor be retrieved via sd_journal_get_cursor(3). If no entry exists that matches exactly the specified seek address, the next closest is sought to. If sd_journal_next(3) is used, the closest following entry will be sought to, if sd_journal_previous(3) is used the closest preceding entry is sought to.
These APIs are implemented as a shared library, which can be compiled and linked to with the libsystemd pkg-config(1) file.