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<a href="../libudev/index.html">gudev </a><span style="float:right">systemd 219</span><hr><div class="refentry"><a name="sd_journal_next"></a><div class="titlepage"></div><div class="refnamediv"><h2>Name</h2><p>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</p></div><div class="refsynopsisdiv"><h2>Synopsis</h2><div class="funcsynopsis"><pre class="funcsynopsisinfo">#include <systemd/sd-journal.h></pre><table border="0" class="funcprototype-table" summary="Function synopsis" style="cellspacing: 0; cellpadding: 0;"><tr><td><code class="funcdef">int <b class="fsfunc">sd_journal_next</b>(</code></td><td>sd_journal *<var class="pdparam">j</var><code>)</code>;</td></tr></table><div class="funcprototype-spacer"> </div><table border="0" class="funcprototype-table" summary="Function synopsis" style="cellspacing: 0; cellpadding: 0;"><tr><td><code class="funcdef">int <b class="fsfunc">sd_journal_previous</b>(</code></td><td>sd_journal *<var class="pdparam">j</var><code>)</code>;</td></tr></table><div class="funcprototype-spacer"> </div><table border="0" class="funcprototype-table" summary="Function synopsis" style="cellspacing: 0; cellpadding: 0;"><tr><td><code class="funcdef">int <b class="fsfunc">sd_journal_next_skip</b>(</code></td><td>sd_journal *<var class="pdparam">j</var>, </td></tr><tr><td> </td><td>uint64_t <var class="pdparam">skip</var><code>)</code>;</td></tr></table><div class="funcprototype-spacer"> </div><table border="0" class="funcprototype-table" summary="Function synopsis" style="cellspacing: 0; cellpadding: 0;"><tr><td><code class="funcdef">int <b class="fsfunc">sd_journal_previous_skip</b>(</code></td><td>sd_journal *<var class="pdparam">j</var>, </td></tr><tr><td> </td><td>uint64_t <var class="pdparam">skip</var><code>)</code>;</td></tr></table><div class="funcprototype-spacer"> </div><table border="0" class="funcprototype-table" summary="Function synopsis" style="cellspacing: 0; cellpadding: 0;"><tr><td><code class="funcdef"><b class="fsfunc">SD_JOURNAL_FOREACH</b>(</code></td><td>sd_journal *<var class="pdparam">j</var><code>)</code>;</td></tr></table><div class="funcprototype-spacer"> </div><table border="0" class="funcprototype-table" summary="Function synopsis" style="cellspacing: 0; cellpadding: 0;"><tr><td><code class="funcdef"><b class="fsfunc">SD_JOURNAL_FOREACH_BACKWARDS</b>(</code></td><td>sd_journal *<var class="pdparam">j</var><code>)</code>;</td></tr></table><div class="funcprototype-spacer"> </div></div></div><div class="refsect1"><a name="idm140058934083792"></a><h2 id="Description">Description<a class="headerlink" title="Permalink to this headline" href="#Description">¶</a></h2><p><code class="function">sd_journal_next()</code> advances the read
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pointer into the journal by one entry. The only argument taken is
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a journal context object as allocated via
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<a href="sd_journal_open.html"><span class="citerefentry"><span class="refentrytitle">sd_journal_open</span>(3)</span></a>.
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After successful invocation the entry may be read with functions
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such as
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<a href="sd_journal_get_data.html"><span class="citerefentry"><span class="refentrytitle">sd_journal_get_data</span>(3)</span></a>.</p><p>Similarly, <code class="function">sd_journal_previous()</code> sets
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the read pointer back one entry.</p><p><code class="function">sd_journal_next_skip()</code> and
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<code class="function">sd_journal_previous_skip()</code> advance/set back
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the read pointer by multiple entries at once, as specified in the
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<code class="varname">skip</code> parameter.</p><p>The journal is strictly ordered by reception time, and hence
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advancing to the next entry guarantees that the entry then
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pointing to is later in time than then previous one, or has the
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same timestamp.</p><p>Note that
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<a href="sd_journal_get_data.html"><span class="citerefentry"><span class="refentrytitle">sd_journal_get_data</span>(3)</span></a>
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and related calls will fail unless
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<code class="function">sd_journal_next()</code> has been invoked at least
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once in order to position the read pointer on a journal
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entry.</p><p>Note that the <code class="function">SD_JOURNAL_FOREACH()</code>
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macro may be used as a wrapper around
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<a href="sd_journal_seek_head.html"><span class="citerefentry"><span class="refentrytitle">sd_journal_seek_head</span>(3)</span></a>
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and <code class="function">sd_journal_next()</code> in order to make
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iterating through the journal easier. See below for an example.
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Similarly, <code class="function">SD_JOURNAL_FOREACH_BACKWARDS()</code> may
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be used for iterating the journal in reverse order.</p></div><div class="refsect1"><a name="idm140058934070880"></a><h2 id="Return Value">Return Value<a class="headerlink" title="Permalink to this headline" href="#Return%20Value">¶</a></h2><p>The four calls return the number of entries advanced/set
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back on success or a negative errno-style error code. When the end
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or beginning of the journal is reached, a number smaller than
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requested is returned. More specifically, if
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<code class="function">sd_journal_next()</code> or
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<code class="function">sd_journal_previous()</code> reach the end/beginning
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of the journal they will return 0, instead of 1 when they are
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successful. This should be considered an EOF marker.</p></div><div class="refsect1"><a name="idm140058934060672"></a><h2 id="Notes">Notes<a class="headerlink" title="Permalink to this headline" href="#Notes">¶</a></h2><p>The <code class="function">sd_journal_next()</code>,
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<code class="function">sd_journal_previous()</code>,
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<code class="function">sd_journal_next_skip()</code> and
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<code class="function">sd_journal_previous_skip()</code> interfaces are
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available as a shared library, which can be compiled and linked to
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with the
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<code class="constant">libsystemd</code> <a href="http://linux.die.net/man/1/pkg-config"><span class="citerefentry"><span class="refentrytitle">pkg-config</span>(1)</span></a>
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file.</p></div><div class="refsect1"><a name="idm140058934054864"></a><h2 id="Examples">Examples<a class="headerlink" title="Permalink to this headline" href="#Examples">¶</a></h2><p>Iterating through the journal:</p><pre class="programlisting">#include <stdio.h>
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#include <string.h>
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#include <systemd/sd-journal.h>
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int main(int argc, char *argv[]) {
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int r;
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sd_journal *j;
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r = sd_journal_open(&j, SD_JOURNAL_LOCAL_ONLY);
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if (r < 0) {
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fprintf(stderr, "Failed to open journal: %s\n", strerror(-r));
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return 1;
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}
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SD_JOURNAL_FOREACH(j) {
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const char *d;
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size_t l;
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r = sd_journal_get_data(j, "MESSAGE", (const void **)&d, &l);
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if (r < 0) {
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fprintf(stderr, "Failed to read message field: %s\n", strerror(-r));
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continue;
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}
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printf("%.*s\n", (int) l, d);
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}
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sd_journal_close(j);
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return 0;
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}</pre></div><div class="refsect1"><a name="idm140058934052128"></a><h2 id="See Also">See Also<a class="headerlink" title="Permalink to this headline" href="#See%20Also">¶</a></h2><p>
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<a href="systemd.html"><span class="citerefentry"><span class="refentrytitle">systemd</span>(1)</span></a>,
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<a href="sd-journal.html"><span class="citerefentry"><span class="refentrytitle">sd-journal</span>(3)</span></a>,
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<a href="sd_journal_open.html"><span class="citerefentry"><span class="refentrytitle">sd_journal_open</span>(3)</span></a>,
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<a href="sd_journal_get_data.html"><span class="citerefentry"><span class="refentrytitle">sd_journal_get_data</span>(3)</span></a>,
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<a href="sd_journal_get_realtime_usec.html"><span class="citerefentry"><span class="refentrytitle">sd_journal_get_realtime_usec</span>(3)</span></a>,
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<a href="sd_journal_get_cursor.html"><span class="citerefentry"><span class="refentrytitle">sd_journal_get_cursor</span>(3)</span></a>
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</p></div></div></body></html>
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