man/sd_journal_next.xml

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  11   Copyright 2012 Lennart Poettering
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  26
  27 <refentry id="sd_journal_next">
  28
  29   <refentryinfo>
  30     <title>sd_journal_next</title>
  31     <productname>systemd</productname>
  32
  33     <authorgroup>
  34       <author>
  35         <contrib>Developer</contrib>
  36         <firstname>Lennart</firstname>
  37         <surname>Poettering</surname>
  38         <email>lennart@poettering.net</email>
  39       </author>
  40     </authorgroup>
  41   </refentryinfo>
  42
  43   <refmeta>
  44     <refentrytitle>sd_journal_next</refentrytitle>
  45     <manvolnum>3</manvolnum>
  46   </refmeta>
  47
  48   <refnamediv>
  49     <refname>sd_journal_next</refname>
  50     <refname>sd_journal_previous</refname>
  51     <refname>sd_journal_next_skip</refname>
  52     <refname>sd_journal_previous_skip</refname>
  53     <refname>SD_JOURNAL_FOREACH</refname>
  54     <refname>SD_JOURNAL_FOREACH_BACKWARDS</refname>
  55     <refpurpose>Advance or set back the read pointer in the journal</refpurpose>
  56   </refnamediv>
  57
  58   <refsynopsisdiv>
  59     <funcsynopsis>
  60       <funcsynopsisinfo>#include &lt;systemd/sd-journal.h&gt;</funcsynopsisinfo>
  61
  62       <funcprototype>
  63         <funcdef>int <function>sd_journal_next</function></funcdef>
  64         <paramdef>sd_journal *<parameter>j</parameter></paramdef>
  65       </funcprototype>
  66
  67       <funcprototype>
  68         <funcdef>int <function>sd_journal_previous</function></funcdef>
  69         <paramdef>sd_journal *<parameter>j</parameter></paramdef>
  70       </funcprototype>
  71
  72       <funcprototype>
  73         <funcdef>int <function>sd_journal_next_skip</function></funcdef>
  74         <paramdef>sd_journal *<parameter>j</parameter></paramdef>
  75         <paramdef>uint64_t <parameter>skip</parameter></paramdef>
  76       </funcprototype>
  77
  78       <funcprototype>
  79         <funcdef>int <function>sd_journal_previous_skip</function></funcdef>
  80         <paramdef>sd_journal *<parameter>j</parameter></paramdef>
  81         <paramdef>uint64_t <parameter>skip</parameter></paramdef>
  82       </funcprototype>
  83
  84       <funcprototype>
  85         <funcdef><function>SD_JOURNAL_FOREACH</function></funcdef>
  86         <paramdef>sd_journal *<parameter>j</parameter></paramdef>
  87       </funcprototype>
  88
  89       <funcprototype>
  90         <funcdef><function>SD_JOURNAL_FOREACH_BACKWARDS</function></funcdef>
  91         <paramdef>sd_journal *<parameter>j</parameter></paramdef>
  92       </funcprototype>
  93     </funcsynopsis>
  94   </refsynopsisdiv>
  95
  96   <refsect1>
  97     <title>Description</title>
  98
  99     <para><function>sd_journal_next()</function> advances the read
 100     pointer into the journal by one entry. The only argument taken is
 101     a journal context object as allocated via
 102     <citerefentry><refentrytitle>sd_journal_open</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
 103     After successful invocation the entry may be read with functions
 104     such as
 105     <citerefentry><refentrytitle>sd_journal_get_data</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para>
 106
 107     <para>Similarly, <function>sd_journal_previous()</function> sets
 108     the read pointer back one entry.</para>
 109
 110     <para><function>sd_journal_next_skip()</function> and
 111     <function>sd_journal_previous_skip()</function> advance/set back
 112     the read pointer by multiple entries at once, as specified in the
 113     <varname>skip</varname> parameter.</para>
 114
 115     <para>The journal is strictly ordered by reception time, and hence
 116     advancing to the next entry guarantees that the entry then
 117     pointing to is later in time than then previous one, or has the
 118     same timestamp.</para>
 119
 120     <para>Note that
 121     <citerefentry><refentrytitle>sd_journal_get_data</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 122     and related calls will fail unless
 123     <function>sd_journal_next()</function> has been invoked at least
 124     once in order to position the read pointer on a journal
 125     entry.</para>
 126
 127     <para>Note that the <function>SD_JOURNAL_FOREACH()</function>
 128     macro may be used as a wrapper around
 129     <citerefentry><refentrytitle>sd_journal_seek_head</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 130     and <function>sd_journal_next()</function> in order to make
 131     iterating through the journal easier. See below for an example.
 132     Similarly, <function>SD_JOURNAL_FOREACH_BACKWARDS()</function> may
 133     be used for iterating the journal in reverse order.</para>
 134   </refsect1>
 135
 136   <refsect1>
 137     <title>Return Value</title>
 138
 139     <para>The four calls return the number of entries advanced/set
 140     back on success or a negative errno-style error code. When the end
 141     or beginning of the journal is reached, a number smaller than
 142     requested is returned. More specifically, if
 143     <function>sd_journal_next()</function> or
 144     <function>sd_journal_previous()</function> reach the end/beginning
 145     of the journal they will return 0, instead of 1 when they are
 146     successful. This should be considered an EOF marker.</para>
 147   </refsect1>
 148
 149   <refsect1>
 150     <title>Notes</title>
 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>