man: document interaction of --root= and the user/group databases (#7344)

[thirdparty/systemd.git] / man / sd_journal_stream_fd.xml

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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
  "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">

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<refentry id="sd_journal_stream_fd">

  <refentryinfo>
    <title>sd_journal_stream_fd</title>
    <productname>systemd</productname>

    <authorgroup>
      <author>
        <contrib>Developer</contrib>
        <firstname>Lennart</firstname>
        <surname>Poettering</surname>
        <email>lennart@poettering.net</email>
      </author>
    </authorgroup>
  </refentryinfo>

  <refmeta>
    <refentrytitle>sd_journal_stream_fd</refentrytitle>
    <manvolnum>3</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>sd_journal_stream_fd</refname>
    <refpurpose>Create log stream file descriptor to the journal</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <funcsynopsis>
      <funcsynopsisinfo>#include &lt;systemd/sd-journal.h&gt;</funcsynopsisinfo>

      <funcprototype>
        <funcdef>int <function>sd_journal_stream_fd</function></funcdef>
        <paramdef>const char *<parameter>identifier</parameter></paramdef>
        <paramdef>int <parameter>priority</parameter></paramdef>
        <paramdef>int <parameter>level_prefix</parameter></paramdef>
      </funcprototype>

    </funcsynopsis>
  </refsynopsisdiv>

  <refsect1>
    <title>Description</title>

    <para><function>sd_journal_stream_fd()</function> may be used to
    create a log stream file descriptor. Log messages written to this
    file descriptor as simple newline-separated text strings are
    written to the journal. This file descriptor can be used
    internally by applications or be made standard output or standard
    error of other processes executed.</para>

    <para><function>sd_journal_stream_fd()</function> takes a short
    program identifier string as first argument, which will be written
    to the journal as _SYSLOG_IDENTIFIER= field for each log entry
    (see
    <citerefentry><refentrytitle>systemd.journal-fields</refentrytitle><manvolnum>7</manvolnum></citerefentry>
    for more information). The second argument shall be the default
    priority level for all messages. The priority level is one of
    <constant>LOG_EMERG</constant>, <constant>LOG_ALERT</constant>,
    <constant>LOG_CRIT</constant>, <constant>LOG_ERR</constant>,
    <constant>LOG_WARNING</constant>, <constant>LOG_NOTICE</constant>,
    <constant>LOG_INFO</constant>, <constant>LOG_DEBUG</constant>, as
    defined in <filename>syslog.h</filename>, see
    <citerefentry project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>
    for details. The third argument is a boolean: if true kernel-style
    log level prefixes (such as <constant>SD_WARNING</constant>) are
    interpreted, see
    <citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>
    for more information.</para>

    <para>It is recommended that applications log UTF-8 messages only
    with this API, but this is not enforced.</para>
  </refsect1>

  <refsect1>
    <title>Return Value</title>

    <para>The call returns a valid write-only file descriptor on
    success or a negative errno-style error code.</para>
  </refsect1>

  <refsect1>
    <title>Notes</title>

    <para>Function <function>sd_journal_stream_fd()</function> is thread-safe and may be called
    from multiple threads. All calls will return the same file descriptor, although temporarily
    multiple file descriptors may be open.</para>

    <para>The <function>sd_journal_stream_fd()</function> interface is
    available as a shared library, which can be compiled and linked to
    with the
    <constant>libsystemd</constant> <citerefentry project='die-net'><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>
    file.</para>
  </refsect1>

  <refsect1>
    <title>Examples</title>

    <para>Creating a log stream suitable for
    <citerefentry project='man-pages'><refentrytitle>fprintf</refentrytitle><manvolnum>3</manvolnum></citerefentry>:</para>

    <programlisting>#include &lt;syslog.h&gt;
#include &lt;stdio.h&gt;
#include &lt;string.h&gt;
#include &lt;unistd.h&gt;
#include &lt;systemd/sd-journal.h&gt;
#include &lt;systemd/sd-daemon.h&gt;

int main(int argc, char *argv[]) {
  int fd;
  FILE *log;
  fd = sd_journal_stream_fd("test", LOG_INFO, 1);
  if (fd &lt; 0) {
    fprintf(stderr, "Failed to create stream fd: %s\n", strerror(-fd));
    return 1;
  }
  log = fdopen(fd, "w");
  if (!log) {
    fprintf(stderr, "Failed to create file object: %m\n");
    close(fd);
    return 1;
  }
  fprintf(log, "Hello World!\n");
  fprintf(log, SD_WARNING "This is a warning!\n");
  fclose(log);
  return 0;
}</programlisting>

  </refsect1>

  <refsect1>
    <title>See Also</title>

    <para>
      <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_journal_print</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry project='man-pages'><refentrytitle>fprintf</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd.journal-fields</refentrytitle><manvolnum>7</manvolnum></citerefentry>
    </para>
  </refsect1>

</refentry>