man/sd_journal_get_data.xml

   1 <?xml version='1.0'?> <!--*-nxml-*-->
   2 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
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   8   Copyright 2012 Lennart Poettering
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  23
  24 <refentry id="sd_journal_get_data">
  25
  26         <refentryinfo>
  27                 <title>sd_journal_get_data</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_get_data</refentrytitle>
  42                 <manvolnum>3</manvolnum>
  43         </refmeta>
  44
  45         <refnamediv>
  46                 <refname>sd_journal_get_data</refname>
  47                 <refname>sd_journal_enumerate_data</refname>
  48                 <refname>sd_journal_restart_data</refname>
  49                 <refname>SD_JOURNAL_FOREACH_DATA</refname>
  50                 <refname>sd_journal_set_data_threshold</refname>
  51                 <refname>sd_journal_get_data_threshold</refname>
  52                 <refpurpose>Read data fields from the current journal entry</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_get_data</function></funcdef>
  61                                 <paramdef>sd_journal *<parameter>j</parameter></paramdef>
  62                                 <paramdef>const char *<parameter>field</parameter></paramdef>
  63                                 <paramdef>const void **<parameter>data</parameter></paramdef>
  64                                 <paramdef>size_t *<parameter>length</parameter></paramdef>
  65                         </funcprototype>
  66
  67                         <funcprototype>
  68                                 <funcdef>int <function>sd_journal_enumerate_data</function></funcdef>
  69                                 <paramdef>sd_journal *<parameter>j</parameter></paramdef>
  70                                 <paramdef>const void **<parameter>data</parameter></paramdef>
  71                                 <paramdef>size_t *<parameter>length</parameter></paramdef>
  72                         </funcprototype>
  73
  74                         <funcprototype>
  75                                 <funcdef>void <function>sd_journal_restart_data</function></funcdef>
  76                                 <paramdef>sd_journal *<parameter>j</parameter></paramdef>
  77                         </funcprototype>
  78
  79                         <funcprototype>
  80                                 <funcdef><function>SD_JOURNAL_FOREACH_DATA</function></funcdef>
  81                                 <paramdef>sd_journal *<parameter>j</parameter></paramdef>
  82                                 <paramdef>const void *<parameter>data</parameter></paramdef>
  83                                 <paramdef>size_t <parameter>length</parameter></paramdef>
  84                         </funcprototype>
  85
  86                         <funcprototype>
  87                                 <funcdef>int <function>sd_journal_set_data_threshold</function></funcdef>
  88                                 <paramdef>sd_journal *<parameter>j</parameter></paramdef>
  89                                 <paramdef>size_t <parameter>sz</parameter></paramdef>
  90                         </funcprototype>
  91
  92                         <funcprototype>
  93                                 <funcdef>int <function>sd_journal_get_data_threshold</function></funcdef>
  94                                 <paramdef>sd_journal *<parameter>j</parameter></paramdef>
  95                                 <paramdef>size_t *<parameter>sz</parameter></paramdef>
  96                         </funcprototype>
  97                 </funcsynopsis>
  98         </refsynopsisdiv>
  99
 100         <refsect1>
 101                 <title>Description</title>
 102
 103                 <para><function>sd_journal_get_data()</function> gets
 104                 the data object associated with a specific field from
 105                 the current journal entry. It takes four arguments:
 106                 the journal context object, a string with the field
 107                 name to request, plus a pair of pointers to
 108                 pointer/size variables where the data object and its
 109                 size shall be stored in. The field name should be an
 110                 entry field name. Well-known field names are listed in
 111                 <citerefentry><refentrytitle>systemd.journal-fields</refentrytitle><manvolnum>7</manvolnum></citerefentry>. The
 112                 returned data is in a read-only memory map and is only
 113                 valid until the next invocation of
 114                 <function>sd_journal_get_data()</function> or
 115                 <function>sd_journal_enumerate_data()</function>, or
 116                 the read pointer is altered. Note that the data
 117                 returned will be prefixed with the field name and
 118                 '='. Also note that by default data fields larger than
 119                 64K might get truncated to 64K. This threshold may be
 120                 changed and turned off with
 121                 <function>sd_journal_set_data_threshold()</function> (see
 122                 below).</para>
 123
 124                 <para><function>sd_journal_enumerate_data()</function>
 125                 may be used to iterate through all fields of the
 126                 current entry. On each invocation the data for the
 127                 next field is returned. The order of these fields is
 128                 not defined. The data returned is in the same format
 129                 as with <function>sd_journal_get_data()</function> and
 130                 also follows the same life-time semantics.</para>
 131
 132                 <para><function>sd_journal_restart_data()</function>
 133                 resets the data enumeration index to the beginning of
 134                 the entry. The next invocation of
 135                 <function>sd_journal_enumerate_data()</function> will return the first
 136                 field of the entry again.</para>
 137
 138                 <para>Note that the
 139                 <function>SD_JOURNAL_FOREACH_DATA()</function> macro
 140                 may be used as a handy wrapper around
 141                 <function>sd_journal_restart_data()</function> and
 142                 <function>sd_journal_enumerate_data()</function>.</para>
 143
 144                 <para>Note that these functions will not work before
 145                 <citerefentry><refentrytitle>sd_journal_next</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 146                 (or related call) has been called at least
 147                 once, in order to position the read pointer at a valid entry.</para>
 148
 149                 <para><function>sd_journal_set_data_threshold()</function>
 150                 may be used to change the data field size threshold
 151                 for data returned by
 152                 <function>sd_journal_get_data()</function>,
 153                 <function>sd_journal_enumerate_data()</function> and
 154                 <function>sd_journal_enumerate_unique()</function>. This
 155                 threshold is a hint only: it indicates that the client
 156                 program is interested only in the initial parts of the
 157                 data fields, up to the threshold in size -- but the
 158                 library might still return larger data objects. That
 159                 means applications should not rely exclusively on this
 160                 setting to limit the size of the data fields returned,
 161                 but need to apply a explicit size limit on the
 162                 returned data as well. This threshold defaults to 64K
 163                 by default. To retrieve the complete data fields this
 164                 threshold should be turned off by setting it to 0, so
 165                 that the library always returns the complete data
 166                 objects. It is recommended to set this threshold as
 167                 low as possible since this relieves the library from
 168                 having to decompress large compressed data objects in
 169                 full.</para>
 170
 171                 <para><function>sd_journal_get_data_threshold()</function>
 172                 returns the currently configured data field size
 173                 threshold.</para>
 174         </refsect1>
 175
 176         <refsect1>
 177                 <title>Return Value</title>
 178
 179                 <para><function>sd_journal_get_data()</function>
 180                 returns 0 on success or a negative errno-style error
 181                 code. If the current entry does not include the
 182                 specified field, -ENOENT is returned. If
 183                 <citerefentry><refentrytitle>sd_journal_next</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 184                 has not been called at least once, -EADDRNOTAVAIL is
 185                 returned. <function>sd_journal_enumerate_data()</function>
 186                 returns a positive integer if the next field has been
 187                 read, 0 when no more fields are known, or a negative
 188                 errno-style error
 189                 code. <function>sd_journal_restart_data()</function>
 190                 returns
 191                 nothing. <function>sd_journal_set_data_threshold()</function>
 192                 and <function>sd_journal_get_threshold()</function>
 193                 return 0 on success or a negative errno-style error
 194                 code.</para>
 195         </refsect1>
 196
 197         <refsect1>
 198                 <title>Notes</title>
 199
 200                 <para>The <function>sd_journal_get_data()</function>,
 201                 <function>sd_journal_enumerate_data()</function>,
 202                 <function>sd_journal_restart_data()</function>,
 203                 <function>sd_journal_set_data_threshold()</function>
 204                 and
 205                 <function>sd_journal_get_data_threshold()</function>
 206                 interfaces are available as a shared library, which can
 207                 be compiled and linked to with the
 208                 <constant>libsystemd</constant> <citerefentry project='die-net'><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 209                 file.</para>
 210         </refsect1>
 211
 212         <refsect1>
 213                 <title>Examples</title>
 214
 215                 <para>See
 216                 <citerefentry><refentrytitle>sd_journal_next</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 217                 for a complete example how to use
 218                 <function>sd_journal_get_data()</function>.</para>
 219
 220                 <para>Use the
 221                 <function>SD_JOURNAL_FOREACH_DATA</function> macro to
 222                 iterate through all fields of the current journal
 223                 entry:</para>
 224
 225                 <programlisting>...
 226 int print_fields(sd_journal *j) {
 227         const void *data;
 228         size_t length;
 229         SD_JOURNAL_FOREACH_DATA(j, data, length)
 230                 printf("%.*s\n", (int) length, data);
 231 }
 232 ...</programlisting>
 233
 234         </refsect1>
 235
 236         <refsect1>
 237                 <title>See Also</title>
 238
 239                 <para>
 240                         <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 241                         <citerefentry><refentrytitle>systemd.journal-fields</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 242                         <citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 243                         <citerefentry><refentrytitle>sd_journal_open</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 244                         <citerefentry><refentrytitle>sd_journal_next</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 245                         <citerefentry><refentrytitle>sd_journal_get_realtime_usec</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 246                         <citerefentry><refentrytitle>sd_journal_query_unique</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 247                 </para>
 248         </refsect1>
 249
 250 </refentry>