summaryrefslogtreecommitdiff
path: root/src/libsystemd/sd_journal_next.xml
blob: 115fe26661cc1b003be1ef081a15f1d5827b9e28 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
<?xml version='1.0'?> <!--*-nxml-*-->
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
  "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">

<!--
  This file is part of systemd.

  Copyright 2012 Lennart Poettering

  systemd is free software; you can redistribute it and/or modify it
  under the terms of the GNU Lesser General Public License as published by
  the Free Software Foundation; either version 2.1 of the License, or
  (at your option) any later version.

  systemd is distributed in the hope that it will be useful, but
  WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
  Lesser General Public License for more details.

  You should have received a copy of the GNU Lesser General Public License
  along with systemd; If not, see <http://www.gnu.org/licenses/>.
-->

<refentry id="sd_journal_next">

  <refentryinfo>
    <title>sd_journal_next</title>
    <productname>systemd</productname>

    <authorgroup>
      <author>
        <contrib>Developer</contrib>
        <firstname>Lennart</firstname>
        <surname>Poettering</surname>
        <email>lennart@poettering.net</email>
      </author>
    </authorgroup>
  </refentryinfo>

  <refmeta>
    <refentrytitle>sd_journal_next</refentrytitle>
    <manvolnum>3</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>sd_journal_next</refname>
    <refname>sd_journal_previous</refname>
    <refname>sd_journal_next_skip</refname>
    <refname>sd_journal_previous_skip</refname>
    <refname>SD_JOURNAL_FOREACH</refname>
    <refname>SD_JOURNAL_FOREACH_BACKWARDS</refname>
    <refpurpose>Advance or set back the read pointer in the journal</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <funcsynopsis>
      <funcsynopsisinfo>#include &lt;systemd/sd-journal.h&gt;</funcsynopsisinfo>

      <funcprototype>
        <funcdef>int <function>sd_journal_next</function></funcdef>
        <paramdef>sd_journal *<parameter>j</parameter></paramdef>
      </funcprototype>

      <funcprototype>
        <funcdef>int <function>sd_journal_previous</function></funcdef>
        <paramdef>sd_journal *<parameter>j</parameter></paramdef>
      </funcprototype>

      <funcprototype>
        <funcdef>int <function>sd_journal_next_skip</function></funcdef>
        <paramdef>sd_journal *<parameter>j</parameter></paramdef>
        <paramdef>uint64_t <parameter>skip</parameter></paramdef>
      </funcprototype>

      <funcprototype>
        <funcdef>int <function>sd_journal_previous_skip</function></funcdef>
        <paramdef>sd_journal *<parameter>j</parameter></paramdef>
        <paramdef>uint64_t <parameter>skip</parameter></paramdef>
      </funcprototype>

      <funcprototype>
        <funcdef><function>SD_JOURNAL_FOREACH</function></funcdef>
        <paramdef>sd_journal *<parameter>j</parameter></paramdef>
      </funcprototype>

      <funcprototype>
        <funcdef><function>SD_JOURNAL_FOREACH_BACKWARDS</function></funcdef>
        <paramdef>sd_journal *<parameter>j</parameter></paramdef>
      </funcprototype>
    </funcsynopsis>
  </refsynopsisdiv>

  <refsect1>
    <title>Description</title>

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

    <para>Similarly, <function>sd_journal_previous()</function> sets
    the read pointer back one entry.</para>

    <para><function>sd_journal_next_skip()</function> and
    <function>sd_journal_previous_skip()</function> advance/set back
    the read pointer by multiple entries at once, as specified in the
    <varname>skip</varname> parameter.</para>

    <para>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.</para>

    <para>Note that
    <citerefentry><refentrytitle>sd_journal_get_data</refentrytitle><manvolnum>3</manvolnum></citerefentry>
    and related calls will fail unless
    <function>sd_journal_next()</function> has been invoked at least
    once in order to position the read pointer on a journal
    entry.</para>

    <para>Note that the <function>SD_JOURNAL_FOREACH()</function>
    macro may be used as a wrapper around
    <citerefentry><refentrytitle>sd_journal_seek_head</refentrytitle><manvolnum>3</manvolnum></citerefentry>
    and <function>sd_journal_next()</function> in order to make
    iterating through the journal easier. See below for an example.
    Similarly, <function>SD_JOURNAL_FOREACH_BACKWARDS()</function> may
    be used for iterating the journal in reverse order.</para>
  </refsect1>

  <refsect1>
    <title>Return Value</title>

    <para>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
    <function>sd_journal_next()</function> or
    <function>sd_journal_previous()</function> 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.</para>
  </refsect1>

  <refsect1>
    <title>Notes</title>

    <para>The <function>sd_journal_next()</function>,
    <function>sd_journal_previous()</function>,
    <function>sd_journal_next_skip()</function> and
    <function>sd_journal_previous_skip()</function> interfaces are
    available as a shared library, which can be compiled and linked to
    with the
    <constant>libsystemd</constant> <citerefentry project='die-net'><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>
    file.</para>
  </refsect1>

  <refsect1>
    <title>Examples</title>

    <para>Iterating through the journal:</para>

    <programlisting>#include &lt;stdio.h&gt;
#include &lt;string.h&gt;
#include &lt;systemd/sd-journal.h&gt;

int main(int argc, char *argv[]) {
  int r;
  sd_journal *j;
  r = sd_journal_open(&amp;j, SD_JOURNAL_LOCAL_ONLY);
  if (r &lt; 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 **)&amp;d, &amp;l);
    if (r &lt; 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;
}</programlisting>

  </refsect1>

  <refsect1>
    <title>See Also</title>

    <para>
      <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_journal_open</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_journal_get_data</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_journal_get_realtime_usec</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_journal_get_cursor</refentrytitle><manvolnum>3</manvolnum></citerefentry>
    </para>
  </refsect1>

</refentry>