man/sd_journal_next.xml

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  23
  24 <refentry id="sd_journal_next">
  25
  26   <refentryinfo>
  27     <title>sd_journal_next</title>
  28     <productname>systemd</productname>
  29
  30     <authorgroup>
  31       <author>
  32         <contrib>Developer</contrib>
  33         <firstname>Lennart</firstname>
  34         <surname>Poettering</surname>
  35         <email>lennart@poettering.net</email>
  36       </author>
  37     </authorgroup>
  38   </refentryinfo>
  39
  40   <refmeta>
  41     <refentrytitle>sd_journal_next</refentrytitle>
  42     <manvolnum>3</manvolnum>
  43   </refmeta>
  44
  45   <refnamediv>
  46     <refname>sd_journal_next</refname>
  47     <refname>sd_journal_previous</refname>
  48     <refname>sd_journal_next_skip</refname>
  49     <refname>sd_journal_previous_skip</refname>
  50     <refname>SD_JOURNAL_FOREACH</refname>
  51     <refname>SD_JOURNAL_FOREACH_BACKWARDS</refname>
  52     <refpurpose>Advance or set back the read pointer in the journal</refpurpose>
  53   </refnamediv>
  54
  55   <refsynopsisdiv>
  56     <funcsynopsis>
  57       <funcsynopsisinfo>#include &lt;systemd/sd-journal.h&gt;</funcsynopsisinfo>
  58
  59       <funcprototype>
  60         <funcdef>int <function>sd_journal_next</function></funcdef>
  61         <paramdef>sd_journal *<parameter>j</parameter></paramdef>
  62       </funcprototype>
  63
  64       <funcprototype>
  65         <funcdef>int <function>sd_journal_previous</function></funcdef>
  66         <paramdef>sd_journal *<parameter>j</parameter></paramdef>
  67       </funcprototype>
  68
  69       <funcprototype>
  70         <funcdef>int <function>sd_journal_next_skip</function></funcdef>
  71         <paramdef>sd_journal *<parameter>j</parameter></paramdef>
  72         <paramdef>uint64_t <parameter>skip</parameter></paramdef>
  73       </funcprototype>
  74
  75       <funcprototype>
  76         <funcdef>int <function>sd_journal_previous_skip</function></funcdef>
  77         <paramdef>sd_journal *<parameter>j</parameter></paramdef>
  78         <paramdef>uint64_t <parameter>skip</parameter></paramdef>
  79       </funcprototype>
  80
  81       <funcprototype>
  82         <funcdef><function>SD_JOURNAL_FOREACH</function></funcdef>
  83         <paramdef>sd_journal *<parameter>j</parameter></paramdef>
  84       </funcprototype>
  85
  86       <funcprototype>
  87         <funcdef><function>SD_JOURNAL_FOREACH_BACKWARDS</function></funcdef>
  88         <paramdef>sd_journal *<parameter>j</parameter></paramdef>
  89       </funcprototype>
  90     </funcsynopsis>
  91   </refsynopsisdiv>
  92
  93   <refsect1>
  94     <title>Description</title>
  95
  96     <para><function>sd_journal_next()</function> advances the read
  97     pointer into the journal by one entry. The only argument taken is
  98     a journal context object as allocated via
  99     <citerefentry><refentrytitle>sd_journal_open</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
 100     After successful invocation the entry may be read with functions
 101     such as
 102     <citerefentry><refentrytitle>sd_journal_get_data</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para>
 103
 104     <para>Similarly, <function>sd_journal_previous()</function> sets
 105     the read pointer back one entry.</para>
 106
 107     <para><function>sd_journal_next_skip()</function> and
 108     <function>sd_journal_previous_skip()</function> advance/set back
 109     the read pointer by multiple entries at once, as specified in the
 110     <varname>skip</varname> parameter.</para>
 111
 112     <para>The journal is strictly ordered by reception time, and hence
 113     advancing to the next entry guarantees that the entry then
 114     pointing to is later in time than then previous one, or has the
 115     same timestamp.</para>
 116
 117     <para>Note that
 118     <citerefentry><refentrytitle>sd_journal_get_data</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 119     and related calls will fail unless
 120     <function>sd_journal_next()</function> has been invoked at least
 121     once in order to position the read pointer on a journal
 122     entry.</para>
 123
 124     <para>Note that the <function>SD_JOURNAL_FOREACH()</function>
 125     macro may be used as a wrapper around
 126     <citerefentry><refentrytitle>sd_journal_seek_head</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 127     and <function>sd_journal_next()</function> in order to make
 128     iterating through the journal easier. See below for an example.
 129     Similarly, <function>SD_JOURNAL_FOREACH_BACKWARDS()</function> may
 130     be used for iterating the journal in reverse order.</para>
 131   </refsect1>
 132
 133   <refsect1>
 134     <title>Return Value</title>
 135
 136     <para>The four calls return the number of entries advanced/set
 137     back on success or a negative errno-style error code. When the end
 138     or beginning of the journal is reached, a number smaller than
 139     requested is returned. More specifically, if
 140     <function>sd_journal_next()</function> or
 141     <function>sd_journal_previous()</function> reach the end/beginning
 142     of the journal they will return 0, instead of 1 when they are
 143     successful. This should be considered an EOF marker.</para>
 144   </refsect1>
 145
 146   <refsect1>
 147     <title>Notes</title>
 148
 149     <para>The <function>sd_journal_next()</function>,
 150     <function>sd_journal_previous()</function>,
 151     <function>sd_journal_next_skip()</function> and
 152     <function>sd_journal_previous_skip()</function> interfaces are
 153     available as a shared library, which can be compiled and linked to
 154     with the
 155     <constant>libsystemd</constant> <citerefentry project='die-net'><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 156     file.</para>
 157   </refsect1>
 158
 159   <refsect1>
 160     <title>Examples</title>
 161
 162     <para>Iterating through the journal:</para>
 163
 164     <programlisting>#include &lt;stdio.h&gt;
 165 #include &lt;string.h&gt;
 166 #include &lt;systemd/sd-journal.h&gt;
 167
 168 int main(int argc, char *argv[]) {
 169   int r;
 170   sd_journal *j;
 171   r = sd_journal_open(&amp;j, SD_JOURNAL_LOCAL_ONLY);
 172   if (r &lt; 0) {
 173     fprintf(stderr, "Failed to open journal: %s\n", strerror(-r));
 174     return 1;
 175   }
 176   SD_JOURNAL_FOREACH(j) {
 177     const char *d;
 178     size_t l;
 179
 180     r = sd_journal_get_data(j, "MESSAGE", (const void **)&amp;d, &amp;l);
 181     if (r &lt; 0) {
 182       fprintf(stderr, "Failed to read message field: %s\n", strerror(-r));
 183       continue;
 184     }
 185
 186     printf("%.*s\n", (int) l, d);
 187   }
 188   sd_journal_close(j);
 189   return 0;
 190 }</programlisting>
 191
 192   </refsect1>
 193
 194   <refsect1>
 195     <title>See Also</title>
 196
 197     <para>
 198       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 199       <citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 200       <citerefentry><refentrytitle>sd_journal_open</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 201       <citerefentry><refentrytitle>sd_journal_get_data</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 202       <citerefentry><refentrytitle>sd_journal_get_realtime_usec</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 203       <citerefentry><refentrytitle>sd_journal_get_cursor</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 204     </para>
 205   </refsect1>
 206
 207 </refentry>