Description¶

sd_journal_next() advances the read pointer into the journal by one entry. The only argument taken is a journal context object as allocated via sd_journal_open(3). After successful invocation the entry may be read with functions such as sd_journal_get_data(3).

Similarly, sd_journal_previous() sets the read pointer back one entry.

sd_journal_step_one() also moves the read pointer. If the current location is the head of the journal, e.g. when this is called following sd_journal_seek_head(), then this is equivalent to sd_journal_next(), and the argument advanced will be ignored. Similarly, if the current location is the tail of the journal, e.g. when this is called following sd_journal_seek_tail(), then this is equivalent to sd_journal_previous(), and advanced will be ignored. Otherwise, this is equivalent to sd_journal_next() when advanced is non-zero, and sd_journal_previous() when advanced is zero.

sd_journal_next_skip() and sd_journal_previous_skip() advance/set back the read pointer by multiple entries at once, as specified in the skip parameter. The skip parameter must be less than or equal to 2147483647 (2³¹-1).

The journal is strictly ordered by reception time, and hence advancing to the next entry guarantees that the entry then pointing to is later in time than then previous one, or has the same timestamp.

Note that sd_journal_get_data(3) and related calls will fail unless sd_journal_next() has been invoked at least once in order to position the read pointer on a journal entry.

Note that the SD_JOURNAL_FOREACH() macro may be used as a wrapper around sd_journal_seek_head(3) and sd_journal_next() in order to make iterating through the journal easier. See below for an example. Similarly, SD_JOURNAL_FOREACH_BACKWARDS() may be used for iterating the journal in reverse order.

Return Value¶

The four calls return the number of entries advanced/set back on success or a negative errno-style error code. When the end or beginning of the journal is reached, a number smaller than requested is returned. More specifically, if sd_journal_next() or sd_journal_previous() reach the end/beginning of the journal they will return 0, instead of 1 when they are successful. This should be considered an EOF marker.

Notes¶

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.

Examples¶

Iterating through the journal:

/* SPDX-License-Identifier: MIT-0 */

#include <errno.h>
#include <stdio.h>
#include <systemd/sd-journal.h>

int main(int argc, char *argv[]) {
  int r;
  sd_journal *j;
  r = sd_journal_open(&j, SD_JOURNAL_LOCAL_ONLY);
  if (r < 0) {
    errno = -r;
    fprintf(stderr, "Failed to open journal: %m\n");
    return 1;
  }
  SD_JOURNAL_FOREACH(j) {
    const char *d;
    size_t l;

    r = sd_journal_get_data(j, "MESSAGE", (const void **)&d, &l);
    if (r < 0) {
      errno = -r;
      fprintf(stderr, "Failed to read message field: %m\n");
      continue;
    }

    printf("%.*s\n", (int) l, d);
  }
  sd_journal_close(j);
  return 0;
}

See Also¶

systemd(1), sd-journal(3), sd_journal_open(3), sd_journal_get_data(3), sd_journal_get_realtime_usec(3), sd_journal_get_cursor(3)