Fork of Original Code Base: anongit.freedesktop.org/systemd

This is the initial fork of the code base from freedsktop.org.
The code is provided here as a reference of the initial starting
point and for possible future checkouts after a large portion
of this code is removed.

Merge git://anongit.freedesktop.org/systemd/systemd

1 files changed, 272 insertions, 0 deletions

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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_get_fd">
+
+        <refentryinfo>
+                <title>sd_journal_get_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_get_fd</refentrytitle>
+                <manvolnum>3</manvolnum>
+        </refmeta>
+
+        <refnamediv>
+                <refname>sd_journal_get_fd</refname>
+                <refname>sd_journal_reliable_fd</refname>
+                <refname>sd_journal_process</refname>
+                <refname>sd_journal_wait</refname>
+                <refname>SD_JOURNAL_NOP</refname>
+                <refname>SD_JOURNAL_APPEND</refname>
+                <refname>SD_JOURNAL_INVALIDATE</refname>
+                <refpurpose>Journal change notification
+                interface</refpurpose>
+        </refnamediv>
+
+        <refsynopsisdiv>
+                <funcsynopsis>
+                        <funcsynopsisinfo>#include &lt;systemd/sd-journal.h&gt;</funcsynopsisinfo>
+
+                        <funcprototype>
+                                <funcdef>int <function>sd_journal_get_fd</function></funcdef>
+                                <paramdef>sd_journal* <parameter>j</parameter></paramdef>
+                        </funcprototype>
+
+                        <funcprototype>
+                                <funcdef>int <function>sd_journal_reliable_fd</function></funcdef>
+                                <paramdef>sd_journal* <parameter>j</parameter></paramdef>
+                        </funcprototype>
+
+                        <funcprototype>
+                                <funcdef>int <function>sd_journal_process</function></funcdef>
+                                <paramdef>sd_journal* <parameter>j</parameter></paramdef>
+                        </funcprototype>
+
+                        <funcprototype>
+                                <funcdef>int <function>sd_journal_wait</function></funcdef>
+                                <paramdef>sd_journal* <parameter>j</parameter></paramdef>
+                                <paramdef>uint64_t <parameter>timeout_usec</parameter></paramdef>
+                        </funcprototype>
+
+                </funcsynopsis>
+        </refsynopsisdiv>
+
+        <refsect1>
+                <title>Description</title>
+
+                <para><function>sd_journal_get_fd()</function> returns
+                a file descriptor that may be asynchronously polled in
+                an external event loop and is signaled readable as
+                soon as the journal changes, because new entries or
+                files were added, rotation took place, or files have
+                been deleted, and similar. The file descriptor is
+                suitable for usage in
+                <citerefentry><refentrytitle>poll</refentrytitle><manvolnum>2</manvolnum></citerefentry>
+                where it will yield POLLIN on changes. The call takes
+                one argument: the journal context object. Note that
+                not all file systems are capable of generating the
+                necessary events for wakeups from this file descriptor
+                to be enirely reliable. In particular network files
+                systems do not generate suitable file change events in
+                all cases. In such a case an application should not
+                rely alone on wake-ups from this file descriptor but
+                wake up and recheck the journal in regular time
+                intervals, for example every 2s. To detect
+                cases where this is necessary, use
+                <function>sd_journal_reliable_fd()</function>,
+                below.</para>
+
+                <para><function>sd_journal_reliable_fd()</function>
+                may be used to check whether the wakeup events from
+                the file descriptor returned by
+                <function>sd_journal_get_fd</function> are sufficient
+                to track changes to the journal. If this call returns
+                0, it is necessary to regularly recheck for journal
+                changes (suggestion: every 2s). If this call returns a
+                positive integer this is not necessary, and wakeups
+                from the file descriptor returned by
+                <function>sd_journal_get_fd()</function> are
+                sufficient as only source for wake-ups.</para>
+
+                <para>After each POLLIN wake-up
+                <function>sd_journal_process()</function> needs to be
+                called to process events and reset the readable state
+                of the file descriptor. This call will also indicate
+                what kind of change has been detected (see below; note
+                that spurious wake-ups are possible).</para>
+
+                <para>A synchronous alternative for using
+                <function>sd_journal_get_fd()</function>,
+                <function>sd_journal_reliable_fd()</function> and
+                <function>sd_journal_process()</function> is
+                <function>sd_journal_wait()</function>. It will
+                synchronously wait until the journal gets changed,
+                possibly using a 2s time-out if this is necessary (see
+                above). In either way the maximum time this call
+                sleeps may be controlled with the
+                <parameter>timeout_usec</parameter> parameter. Pass
+                <literal>(uint64_t) -1</literal> to wait
+                indefinitely. Internally this call simply combines
+                <function>sd_journal_get_fd()</function>,
+                <function>sd_journal_reliable_fd()</function>,
+                <function>poll()</function> and
+                <function>sd_journal_process()</function> into
+                one.</para>
+
+        </refsect1>
+
+        <refsect1>
+                <title>Return Value</title>
+
+                <para><function>sd_journal_get_fd()</function> returns a valid file descriptor on success or a negative errno-style error
+                code.</para>
+
+                <para><function>sd_journal_reliable_fd()</function>
+                returns a positive integer if the file descriptor
+                returned by <function>sd_journal_get_fd()</function>
+                is sufficient as sole wake-up source for journal
+                change events. Returns 0 if it is not sufficient and
+                the journal needs to be checked manually in regular
+                time intervals for changes. Returns a negative
+                errno-style error code on failure.</para>
+
+                <para><function>sd_journal_process()</function> and
+                <function>sd_journal_wait()</function> return one of
+                <literal>SD_JOURNAL_NOP</literal>,
+                <literal>SD_JOURNAL_APPEND</literal> or
+                <literal>SD_JOURNAL_INVALIDATE</literal> on success or
+                a negative errno-style error code. If
+                <literal>SD_JOURNAL_NOP</literal> is returned the
+                journal didn't change since the last invocation. If
+                <literal>SD_JOURNAL_APPEND</literal> is returned new
+                entries have been appended to the end of the
+                journal. If <literal>SD_JOURNAL_INVALIDATE</literal>
+                journal files were added or removed (possibly due to
+                rotation). In the latter event live-view UIs should
+                probably refresh their entire display while in the
+                case of <literal>SD_JOURNAL_APPEND</literal> it is
+                sufficient to simply continue reading at the previous
+                end of the journal.</para>
+        </refsect1>
+
+        <refsect1>
+                <title>Notes</title>
+
+                <para>The <function>sd_journal_get_fd()</function>,
+                <function>sd_journal_reliable_fd()</function>,
+                <function>sd_journal_process()</function> and
+                <function>sd_journal_wait()</function> interfaces are
+                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>Iterating through the journal, in a live view tracking all changes:</para>
+
+                <programlisting>#include &lt;stdio.h&gt;
+#include &lt;string.h&gt;
+#include &lt;systemd/sd-journal.h&gt;
+
+int main(int argc, char *argv[]) {
+        int r;
+        sd_journal *j;
+        r = sd_journal_open(&amp;j, SD_JOURNAL_LOCAL_ONLY);
+        if (r &lt; 0) {
+                fprintf(stderr, "Failed to open journal: %s\n", strerror(-r));
+                return 1;
+        }
+        for (;;)  {
+                const char *d;
+                size_t l;
+                r = sd_journal_next(j);
+                if (r &lt; 0) {
+                        fprintf(stderr, "Failed to iterate to next entry: %s\n", strerror(-r));
+                        break;
+                }
+                if (r == 0) {
+                        /* Reached the end, let's wait for changes, and try again */
+                        r = sd_journal_wait(j, (uint64_t) -1);
+                        if (r &lt; 0) {
+                                fprintf(stderr, "Failed to wait for changes: %s\n", strerror(-r));
+                                break;
+                        }
+                        continue;
+                }
+                r = sd_journal_get_data(j, "MESSAGE", &amp;d, &amp;l);
+                if (r &lt; 0) {
+                        fprintf(stderr, "Failed to read message field: %s\n", strerror(-r));
+                        continue;
+                }
+                printf("%.*s\n", (int) l, d);
+        }
+        sd_journal_close(j);
+        return 0;
+}</programlisting>
+
+                <para>Waiting with <function>poll()</function> (this
+                example lacks all error checking for the sake of
+                simplicity):</para>
+
+                <programlisting>#include &lt;sys/poll.h&gt;
+#include &lt;systemd/sd-journal.h&gt;
+
+int wait_for_changes(sd_journal *j) {
+        struct pollfd pollfd;
+        pollfd.fd = sd_journal_get_fd();
+        pollfd.events = POLLIN;
+        poll(&amp;pollfd, 1, sd_journal_reliable_fd() &gt; 0 ? -1 : 2000);
+        return sd_journal_process(j);
+}
+                </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_journal_open</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+                        <citerefentry><refentrytitle>sd_journal_next</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+                        <citerefentry><refentrytitle>poll</refentrytitle><manvolnum>2</manvolnum></citerefentry>
+                </para>
+        </refsect1>
+
+</refentry>