Systemd/man/sd_journal_get_fd.xml

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<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_get_events</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_get_events</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 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>. Use
                <function>sd_journal_get_events()</function> for an
                events mask to watch for. 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_get_events()</function>
                will return the <function>poll()</function> mask to
                wait for. This function will return a combination of
                <literal>POLLIN</literal> and
                <literal>POLLOUT</literal> and similar to fill into
                the <literal>.events</literal> field of
                <literal>struct pollfd</literal>.</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 <function>poll()</function> wake-up
                <function>sd_journal_process()</function> needs to be
                called to process events. 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_get_events()</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_get_events()</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_get_events()</function>
                returns a combination of <literal>POLLIN</literal>,
                <literal>POLLOUT</literal> and suchlike 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_get_events()</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 void *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, (const char*) 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(j);
        pollfd.events = sd_journal_get_events(j);
        poll(&amp;pollfd, 1, sd_journal_reliable_fd(j) &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>