sd_journal_get_cursor man page
sd_journal_get_cursor, sd_journal_test_cursor — Get cursor string for or test cursor string against the current journal entry
int sd_journal_get_cursor(sd_journal *j, char **cursor);
int sd_journal_test_cursor(sd_journal *j, const char *cursor);
sd_journal_get_cursor() returns a cursor string for the current journal entry. A cursor is a serialization of the current journal position formatted as text. The string only contains printable characters and can be passed around in text form. The cursor identifies a journal entry globally and in a stable way and may be used to later seek to it via sd_journal_seek_cursor(3). The cursor string should be considered opaque and not be parsed by clients. Seeking to a cursor position without the specific entry being available locally will seek to the next closest (in terms of time) available entry. The call takes two arguments: a journal context object and a pointer to a string pointer where the cursor string will be placed. The string is allocated via libc malloc(3) and should be freed after use with free(3).
Note that sd_journal_get_cursor() will not work before sd_journal_next(3) (or related call) has been called at least once, in order to position the read pointer at a valid entry.
sd_journal_test_cursor() may be used to check whether the current position in the journal matches the specified cursor. This is useful since cursor strings do not uniquely identify an entry: the same entry might be referred to by multiple different cursor strings, and hence string comparing cursors is not possible. Use this call to verify after an invocation of sd_journal_seek_cursor(3) whether the entry being sought to was actually found in the journal or the next closest entry was used instead.
sd_journal_get_cursor() returns 0 on success or a negative errno-style error code. sd_journal_test_cursor() returns positive if the current entry matches the specified cursor, 0 if it does not match the specified cursor or a negative errno-style error code on failure.
All functions listed here are thread-agnostic and only a single thread may operate on a given sd_journal object.
The sd_journal_get_cursor() and sd_journal_test_cursor() interfaces are available as a shared library, which can be compiled and linked to with the libsystemd pkg-config(1) file.
systemd(1), sd-journal(3), sd_journal_open(3), sd_journal_seek_cursor(3)
sd-journal(3), sd_journal_next(3), sd_journal_open(3), sd_journal_seek_head(3), systemd.directives(7), systemd.index(7).
sd_journal_test_cursor(3) is an alias of sd_journal_get_cursor(3).