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>All functions listed here are thread-agnostic and only a single thread may operate
 150     on a given <structname>sd_journal</structname> object.</para>
 151
 152     <para>The <function>sd_journal_next()</function>,
 153     <function>sd_journal_previous()</function>,
 154     <function>sd_journal_next_skip()</function> and
 155     <function>sd_journal_previous_skip()</function> interfaces are
 156     available as a shared library, which can be compiled and linked to
 157     with the
 158     <constant>libsystemd</constant> <citerefentry project='die-net'><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 159     file.</para>
 160   </refsect1>
 161
 162   <refsect1>
 163     <title>Examples</title>
 164
 165     <para>Iterating through the journal:</para>
 166
 167     <programlisting>#include &lt;stdio.h&gt;
 168 #include &lt;string.h&gt;
 169 #include &lt;systemd/sd-journal.h&gt;
 170
 171 int main(int argc, char *argv[]) {
 172   int r;
 173   sd_journal *j;
 174   r = sd_journal_open(&amp;j, SD_JOURNAL_LOCAL_ONLY);
 175   if (r &lt; 0) {
 176     fprintf(stderr, "Failed to open journal: %s\n", strerror(-r));
 177     return 1;
 178   }
 179   SD_JOURNAL_FOREACH(j) {
 180     const char *d;
 181     size_t l;
 182
 183     r = sd_journal_get_data(j, "MESSAGE", (const void **)&amp;d, &amp;l);
 184     if (r &lt; 0) {
 185       fprintf(stderr, "Failed to read message field: %s\n", strerror(-r));
 186       continue;
 187     }
 188
 189     printf("%.*s\n", (int) l, d);
 190   }
 191   sd_journal_close(j);
 192   return 0;
 193 }</programlisting>
 194
 195   </refsect1>
 196
 197   <refsect1>
 198     <title>See Also</title>
 199
 200     <para>
 201       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 202       <citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 203       <citerefentry><refentrytitle>sd_journal_open</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 204       <citerefentry><refentrytitle>sd_journal_get_data</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 205       <citerefentry><refentrytitle>sd_journal_get_realtime_usec</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 206       <citerefentry><refentrytitle>sd_journal_get_cursor</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 207     </para>
 208   </refsect1>
 209
 210 </refentry>