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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>