sd_journal_get_seqnum — Read sequence number from the current journal entry
#include <systemd/sd-journal.h>
int sd_journal_get_seqnum( | sd_journal *j, |
| uint64_t *ret_seqnum, | |
sd_id128_t *ret_seqnum_id); |
sd_journal_get_seqnum() returns the sequence number of the current journal
entry. It takes three arguments: the journal context object, a pointer to a 64-bit unsigned integer to
store the sequence number in, and a buffer to return the 128-bit sequence number ID in.
When writing journal entries to disk each systemd-journald instance will number
them sequentially, starting from 1 for the first entry written after subsystem initialization. Each such
series of sequence numbers is associated with a 128-bit sequence number ID which is initialized randomly,
once at systemd-journal initialization. Thus, while multiple instances of
systemd-journald will assign the same sequence numbers to their written journal
entries, they will have a distinct sequence number IDs. The sequence number is assigned at the moment of
writing the entry to disk. If log entries are rewritten (for example because the volatile logs from
/run/log/ are flushed to /var/log/ via
systemd-journald-flush.service) they will get new sequence numbers assigned.
Sequence numbers may be used to order entries (entries associated with the same sequence number ID
and lower sequence numbers should be ordered chronologically before those with higher sequence numbers),
and to detect lost entries. Note that journal service instances typically write to multiple journal files
in parallel (for example because SplitMode= is used), in which case each journal file
will only contain a subset of the sequence numbers. To recover the full stream of journal entries the
files must be combined ("interleaved"), a process that primarily relies on the sequence numbers. When
journal files are rotated (due to size or time limits), the series of sequence numbers is continued in
the replacement files. All journal files generated from the same journal instance will carry the same
sequence number ID.
As the sequence numbers are assigned at the moment of writing the journal entries to disk they do
not exist if storage is disabled via SplitMode=.
The ret_seqnum and ret_seqnum_id parameters may be specified
as NULL in which case the relevant data is not returned (but the call will otherwise
succeed).
Note that these functions 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.
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.
Functions described here are available as a shared
library, which can be compiled against and linked to with the
libsystemd pkg-config(1)
file.