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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | NOTES | EXAMPLES | SEE ALSO | COLOPHON |
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SD_JOURNAL_NEXT(3) sd_journal_next SD_JOURNAL_NEXT(3)
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
#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);
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.
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.
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.
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;
}
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)
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 2023-06-23. (At that
time, the date of the most recent commit that was found in the
repository was 2023-06-23.) 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 253 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_get_seqnum(3), sd_journal_open(3), sd_journal_query_unique(3), sd_journal_seek_head(3), systemd.directives(7), systemd.index(7)
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HTML rendering created 2023-06-24 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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