sd_journal_next(3) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | NOTES | EXAMPLES | SEE ALSO | COLOPHON

SD_JOURNAL_NEXT(3)           sd_journal_next          SD_JOURNAL_NEXT(3)

NAME         top

       sd_journal_next, sd_journal_previous, sd_journal_next_skip,
       sd_journal_previous_skip, SD_JOURNAL_FOREACH,
       SD_JOURNAL_FOREACH_BACKWARDS - Advance or set back the read
       pointer in the journal

SYNOPSIS         top

       #include <systemd/sd-journal.h>

       int sd_journal_next(sd_journal *j);

       int sd_journal_previous(sd_journal *j);

       int sd_journal_next_skip(sd_journal *j, uint64_t skip);

       int sd_journal_previous_skip(sd_journal *j, uint64_t skip);

       SD_JOURNAL_FOREACH(sd_journal *j);

       SD_JOURNAL_FOREACH_BACKWARDS(sd_journal *j);

DESCRIPTION         top

       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_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 (2^31-1).

       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         top

       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         top

       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.

EXAMPLES         top

       Iterating through the journal:

           #include <stdio.h>
           #include <string.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) {
               fprintf(stderr, "Failed to open journal: %s\n", strerror(-r));
               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) {
                 fprintf(stderr, "Failed to read message field: %s\n", strerror(-r));
                 continue;
               }

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

SEE ALSO         top

       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)

COLOPHON         top

       This page is part of the systemd (systemd system and service
       manager) project.  Information about the project can be found at
       ⟨http://www.freedesktop.org/wiki/Software/systemd⟩.  If you have
       a bug report for this manual page, see
       ⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩.
       This page was obtained from the project's upstream Git repository
       ⟨https://github.com/systemd/systemd.git⟩ on 2021-04-01.  (At that
       time, the date of the most recent commit that was found in the
       repository was 2021-04-01.)  If you discover any rendering
       problems in this HTML version of the page, or you believe there
       is a better or more up-to-date source for the page, or you have
       corrections or improvements to the information in this COLOPHON
       (which is not part of the original manual page), send a mail to
       man-pages@man7.org

systemd 248                                           SD_JOURNAL_NEXT(3)

Pages that refer to this page: sd-journal(3)sd_journal_add_match(3)sd_journal_get_catalog(3)sd_journal_get_cursor(3)sd_journal_get_data(3)sd_journal_get_fd(3)sd_journal_get_realtime_usec(3)sd_journal_open(3)sd_journal_query_unique(3)sd_journal_seek_head(3)systemd.directives(7)systemd.index(7)