man/sd_journal_query_unique.xml

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  23
  24 <refentry id="sd_journal_query_unique">
  25
  26   <refentryinfo>
  27     <title>sd_journal_query_unique</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_query_unique</refentrytitle>
  42     <manvolnum>3</manvolnum>
  43   </refmeta>
  44
  45   <refnamediv>
  46     <refname>sd_journal_query_unique</refname>
  47     <refname>sd_journal_enumerate_unique</refname>
  48     <refname>sd_journal_restart_unique</refname>
  49     <refname>SD_JOURNAL_FOREACH_UNIQUE</refname>
  50     <refpurpose>Read unique data fields from the journal</refpurpose>
  51   </refnamediv>
  52
  53   <refsynopsisdiv>
  54     <funcsynopsis>
  55       <funcsynopsisinfo>#include &lt;systemd/sd-journal.h&gt;</funcsynopsisinfo>
  56
  57       <funcprototype>
  58         <funcdef>int <function>sd_journal_query_unique</function></funcdef>
  59         <paramdef>sd_journal *<parameter>j</parameter></paramdef>
  60         <paramdef>const char *<parameter>field</parameter></paramdef>
  61       </funcprototype>
  62
  63       <funcprototype>
  64         <funcdef>int <function>sd_journal_enumerate_unique</function></funcdef>
  65         <paramdef>sd_journal *<parameter>j</parameter></paramdef>
  66         <paramdef>const void **<parameter>data</parameter></paramdef>
  67         <paramdef>size_t *<parameter>length</parameter></paramdef>
  68       </funcprototype>
  69
  70       <funcprototype>
  71         <funcdef>void <function>sd_journal_restart_unique</function></funcdef>
  72         <paramdef>sd_journal *<parameter>j</parameter></paramdef>
  73       </funcprototype>
  74
  75       <funcprototype>
  76         <funcdef><function>SD_JOURNAL_FOREACH_UNIQUE</function></funcdef>
  77         <paramdef>sd_journal *<parameter>j</parameter></paramdef>
  78         <paramdef>const void *<parameter>data</parameter></paramdef>
  79         <paramdef>size_t <parameter>length</parameter></paramdef>
  80       </funcprototype>
  81
  82     </funcsynopsis>
  83   </refsynopsisdiv>
  84
  85   <refsect1>
  86     <title>Description</title>
  87
  88     <para><function>sd_journal_query_unique()</function> queries the
  89     journal for all unique values the specified field can take. It
  90     takes two arguments: the journal to query and the field name to
  91     look for. Well-known field names are listed on
  92     <citerefentry><refentrytitle>systemd.journal-fields</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
  93     Field names must be specified without a trailing '='. After this
  94     function has been executed successfully the field values may be
  95     queried using <function>sd_journal_enumerate_unique()</function>.
  96     Invoking this call a second time will change the field name being
  97     queried and reset the enumeration index to the first field value
  98     that matches.</para>
  99
 100     <para><function>sd_journal_enumerate_unique()</function> may be
 101     used to iterate through all data fields which match the previously
 102     selected field name as set with
 103     <function>sd_journal_query_unique()</function>. On each invocation
 104     the next field data matching the field name is returned. The order
 105     of the returned data fields is not defined. It takes three
 106     arguments: the journal context object, plus a pair of pointers to
 107     pointer/size variables where the data object and its size shall be
 108     stored in. The returned data is in a read-only memory map and is
 109     only valid until the next invocation of
 110     <function>sd_journal_enumerate_unique()</function>. Note that the
 111     data returned will be prefixed with the field name and '='. Note
 112     that this call is subject to the data field size threshold as
 113     controlled by
 114     <function>sd_journal_set_data_threshold()</function>.</para>
 115
 116     <para><function>sd_journal_restart_unique()</function> resets the
 117     data enumeration index to the beginning of the list. The next
 118     invocation of <function>sd_journal_enumerate_unique()</function>
 119     will return the first field data matching the field name
 120     again.</para>
 121
 122     <para>Note that the
 123     <function>SD_JOURNAL_FOREACH_UNIQUE()</function> macro may be used
 124     as a handy wrapper around
 125     <function>sd_journal_restart_unique()</function> and
 126     <function>sd_journal_enumerate_unique()</function>.</para>
 127
 128     <para>Note that these functions currently are not influenced by
 129     matches set with <function>sd_journal_add_match()</function> but
 130     this might change in a later version of this software.</para>
 131
 132     <para>To enumerate all field names currently in use (and thus all suitable field parameters for
 133     <function>sd_journal_query_unique()</function>), use the
 134     <citerefentry><refentrytitle>sd_journal_enumerate_fields</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 135     call.</para>
 136   </refsect1>
 137
 138   <refsect1>
 139     <title>Return Value</title>
 140
 141     <para><function>sd_journal_query_unique()</function> returns 0 on
 142     success or a negative errno-style error code.
 143     <function>sd_journal_enumerate_unique()</function> returns a
 144     positive integer if the next field data has been read, 0 when no
 145     more fields are known, or a negative errno-style error code.
 146     <function>sd_journal_restart_unique()</function> returns
 147     nothing.</para>
 148   </refsect1>
 149
 150   <refsect1>
 151     <title>Notes</title>
 152
 153     <para>All functions listed here are thread-agnostic and only a single thread may operate
 154     on a given <structname>sd_journal</structname> object.</para>
 155
 156     <para>The <function>sd_journal_query_unique()</function>,
 157     <function>sd_journal_enumerate_unique()</function> and
 158     <function>sd_journal_restart_unique()</function> interfaces are
 159     available as a shared library, which can be compiled and linked to
 160     with the
 161     <constant>libsystemd</constant> <citerefentry project='die-net'><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 162     file.</para>
 163   </refsect1>
 164
 165   <refsect1>
 166     <title>Examples</title>
 167
 168     <para>Use the <function>SD_JOURNAL_FOREACH_UNIQUE</function> macro
 169     to iterate through all values a field of the journal can take. The
 170     following example lists all unit names referenced in the
 171     journal:</para>
 172
 173     <programlisting>#include &lt;stdio.h&gt;
 174 #include &lt;string.h&gt;
 175 #include &lt;systemd/sd-journal.h&gt;
 176
 177 int main(int argc, char *argv[]) {
 178         sd_journal *j;
 179         const void *d;
 180         size_t l;
 181         int r;
 182
 183         r = sd_journal_open(&amp;j, SD_JOURNAL_LOCAL_ONLY);
 184         if (r &lt; 0) {
 185                 fprintf(stderr, "Failed to open journal: %s\n", strerror(-r));
 186                 return 1;
 187         }
 188         r = sd_journal_query_unique(j, "_SYSTEMD_UNIT");
 189         if (r &lt; 0) {
 190                 fprintf(stderr, "Failed to query journal: %s\n", strerror(-r));
 191                 return 1;
 192         }
 193         SD_JOURNAL_FOREACH_UNIQUE(j, d, l)
 194                 printf("%.*s\n", (int) l, (const char*) d);
 195         sd_journal_close(j);
 196         return 0;
 197 }</programlisting>
 198
 199   </refsect1>
 200
 201   <refsect1>
 202     <title>See Also</title>
 203
 204     <para>
 205       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 206       <citerefentry><refentrytitle>systemd.journal-fields</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 207       <citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 208       <citerefentry><refentrytitle>sd_journal_open</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 209       <citerefentry><refentrytitle>sd_journal_enumerate_fields</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 210       <citerefentry><refentrytitle>sd_journal_get_data</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 211       <citerefentry><refentrytitle>sd_journal_add_match</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 212     </para>
 213   </refsect1>
 214
 215 </refentry>