NAME

       sd_journal_next, sd_journal_previous, sd_journal_step_one, 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

       #include <systemd/sd-journal.h>

       int sd_journal_next(sd_journal *j);

       int sd_journal_previous(sd_journal *j);

       int sd_journal_step_one(sd_journal *j, int advanced);

       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

       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) {
               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;
           }

HISTORY

       sd_journal_next(), sd_journal_previous(), sd_journal_next_skip(),
       sd_journal_previous_skip(), SD_JOURNAL_FOREACH(), and SD_JOURNAL_FOREACH_BACKWARDS() were
       added in version 187.

       sd_journal_step_one() was added in version 254.

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)