man: document sd_journal_stream_fd()

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+<?xml version='1.0'?> <!--*-nxml-*-->
+<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
+        "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
+
+<!--
+  This file is part of systemd.
+
+  Copyright 2012 Lennart Poettering
+
+  systemd is free software; you can redistribute it and/or modify it
+  under the terms of the GNU Lesser General Public License as published by
+  the Free Software Foundation; either version 2.1 of the License, or
+  (at your option) any later version.
+
+  systemd is distributed in the hope that it will be useful, but
+  WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+  Lesser General Public License for more details.
+
+  You should have received a copy of the GNU Lesser General Public License
+  along with systemd; If not, see <http://www.gnu.org/licenses/>.
+-->
+
+<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 STDOUT/STDERR 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 <literal>LOG_EMERG</literal>,
+                <literal>LOG_ALERT</literal>,
+                <literal>LOG_CRIT</literal>,
+                <literal>LOG_ERR</literal>,
+                <literal>LOG_WARNING</literal>,
+                <literal>LOG_NOTICE</literal>,
+                <literal>LOG_INFO</literal>,
+                <literal>LOG_DEBUG</literal>, as defined in
+                <filename>syslog.h</filename>, see
+                <citerefentry><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>
+                for details. The third argument is a boolean: if true
+                kernel-style log priority level prefixes (such as
+                <literal>SD_WARNING</literal>) 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
+                mesages 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>The <function>sd_journal_stream_fd()</function>
+                interface is available as shared library, which can
+                be compiled and linked to with the
+                <literal>libsystemd-journal</literal>
+                <citerefentry><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>
+                file.</para>
+        </refsect1>
+
+        <refsect1>
+                <title>Examples</title>
+
+                <para>Creating a log stream suitable for
+                <citerefentry><refentrytitle>fprintf</refentrytitle><manvolnum>3</manvolnum></citerefentry>:</para>
+
+                <programlisting>#include &lt;syslog.h&gt;
+#include &lt;stdio.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);
+}</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><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+                        <citerefentry><refentrytitle>fprintf</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+                        <citerefentry><refentrytitle>systemd.journal-fields</refentrytitle><manvolnum>7</manvolnum></citerefentry>
+                </para>
+        </refsect1>
+
+</refentry>