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 specific thread may operate on a given object during its entire lifetime. It's safe to allocate multiple independent objects and use each from a specific thread in parallel. However, it's not safe to allocate such an object in one thread, and operate or free it from any other, even if locking is used to ensure these threads don't operate on it at the very same time.
These APIs are implemented as a shared library, which can be compiled and linked to with the libsystemd pkg-config(1) file.