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<html><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8"><title>sd_journal_get_fd</title><meta name="generator" content="DocBook XSL Stylesheets V1.78.1"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><style>
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</style><a href="index.html">Index </a>·
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<a href="../libudev/index.html">gudev </a><span style="float:right">systemd 204</span><hr><div class="refentry"><a name="sd_journal_get_fd"></a><div class="titlepage"></div><div class="refnamediv"><h2>Name</h2><p>sd_journal_get_fd, sd_journal_get_events, sd_journal_get_timeout, sd_journal_process, sd_journal_wait, sd_journal_reliable_fd, SD_JOURNAL_NOP, SD_JOURNAL_APPEND, SD_JOURNAL_INVALIDATE — Journal change notification
interface</p></div><div class="refsynopsisdiv"><h2>Synopsis</h2><div class="funcsynopsis"><pre class="funcsynopsisinfo">#include <systemd/sd-journal.h></pre><table border="0" class="funcprototype-table" summary="Function synopsis" style="cellspacing: 0; cellpadding: 0;"><tr><td><code class="funcdef">int <b class="fsfunc">sd_journal_get_fd</b>(</code></td><td>sd_journal* <var class="pdparam">j</var><code>)</code>;</td></tr></table><div class="funcprototype-spacer"> </div><table border="0" class="funcprototype-table" summary="Function synopsis" style="cellspacing: 0; cellpadding: 0;"><tr><td><code class="funcdef">int <b class="fsfunc">sd_journal_get_events</b>(</code></td><td>sd_journal* <var class="pdparam">j</var><code>)</code>;</td></tr></table><div class="funcprototype-spacer"> </div><table border="0" class="funcprototype-table" summary="Function synopsis" style="cellspacing: 0; cellpadding: 0;"><tr><td><code class="funcdef">int <b class="fsfunc">sd_journal_get_timeout</b>(</code></td><td>sd_journal* <var class="pdparam">j</var>, </td></tr><tr><td> </td><td>uint64_t* <var class="pdparam">timeout_usec</var><code>)</code>;</td></tr></table><div class="funcprototype-spacer"> </div><table border="0" class="funcprototype-table" summary="Function synopsis" style="cellspacing: 0; cellpadding: 0;"><tr><td><code class="funcdef">int <b class="fsfunc">sd_journal_process</b>(</code></td><td>sd_journal* <var class="pdparam">j</var><code>)</code>;</td></tr></table><div class="funcprototype-spacer"> </div><table border="0" class="funcprototype-table" summary="Function synopsis" style="cellspacing: 0; cellpadding: 0;"><tr><td><code class="funcdef">int <b class="fsfunc">sd_journal_wait</b>(</code></td><td>sd_journal* <var class="pdparam">j</var>, </td></tr><tr><td> </td><td>uint64_t <var class="pdparam">timeout_usec</var><code>)</code>;</td></tr></table><div class="funcprototype-spacer"> </div><table border="0" class="funcprototype-table" summary="Function synopsis" style="cellspacing: 0; cellpadding: 0;"><tr><td><code class="funcdef">int <b class="fsfunc">sd_journal_reliable_fd</b>(</code></td><td>sd_journal* <var class="pdparam">j</var><code>)</code>;</td></tr></table><div class="funcprototype-spacer"> </div></div></div><div class="refsect1"><a name="idm259780895680"></a><h2 id="Description">Description<a class="headerlink" title="Permalink to this headline" href="#Description">¶</a></h2><p><code class="function">sd_journal_get_fd()</code> 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
<a href="poll.html"><span class="citerefentry"><span class="refentrytitle">poll</span>(2)</span></a>. Use
<code class="function">sd_journal_get_events()</code> 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 for changes to
be noticed immediately. In particular network files
systems do not generate suitable file change events in
all cases. Cases like this can be detected with
<code class="function">sd_journal_reliable_fd()</code>,
below. <code class="function">sd_journal_get_timeout()</code>
will ensure in these cases that wake-ups happen
frequently enough for changes to be noticed, although
with a certain latency.</p><p><code class="function">sd_journal_get_events()</code>
will return the <code class="function">poll()</code> mask to
wait for. This function will return a combination of
<code class="literal">POLLIN</code> and
<code class="literal">POLLOUT</code> and similar to fill into
the <code class="literal">.events</code> field of
<code class="literal">struct pollfd</code>.</p><p><code class="function">sd_journal_get_timeout()</code>
will return a timeout value for usage in <code class="function">poll()</code>. This returns a value in microseconds since the epoch of CLOCK_MONOTONIC for timing out <code class="function">poll()</code> in <code class="literal">timeout_usec</code>. See
<a href="clock_gettime.html"><span class="citerefentry"><span class="refentrytitle">clock_gettime</span>(2)</span></a>
for details about
<code class="literal">CLOCK_MONOTONIC</code>. If there's no
timeout to wait for this will fill in
<code class="literal">(uint64_t) -1</code> instead. Note that
<code class="function">poll()</code> takes a relative timeout
in milliseconds rather than an absolute timeout in
microseconds. To convert the absolute 'us' timeout into
relative 'ms', use code like the following:</p><pre class="programlisting">uint64_t t;
int msec;
sd_journal_get_timeout(m, &t);
if (t == (uint64_t) -1)
msec = -1;
else {
struct timespec ts;
uint64_t n;
clock_getttime(CLOCK_MONOTONIC, &ts);
n = (uint64_t) ts.tv_sec * 1000000 + ts.tv_nsec / 1000;
msec = t > n ? (int) ((t - n + 999) / 1000) : 0;
}</pre><p>The code above does not do any error checking
for brevity's sake. The calculated <code class="literal">msec</code>
integer can be passed directly as
<code class="function">poll()</code>'s timeout
parameter.</p><p>After each <code class="function">poll()</code> wake-up
<code class="function">sd_journal_process()</code> 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).</p><p>A synchronous alternative for using
<code class="function">sd_journal_get_fd()</code>,
<code class="function">sd_journal_get_events()</code>,
<code class="function">sd_journal_get_timeout()</code> and
<code class="function">sd_journal_process()</code> is
<code class="function">sd_journal_wait()</code>. It will
synchronously wait until the journal gets changed. The
maximum time this call sleeps may be controlled with
the <em class="parameter"><code>timeout_usec</code></em>
parameter. Pass <code class="literal">(uint64_t) -1</code> to
wait indefinitely. Internally this call simply
combines <code class="function">sd_journal_get_fd()</code>,
<code class="function">sd_journal_get_events()</code>,
<code class="function">sd_journal_get_timeout()</code>,
<code class="function">poll()</code> and
<code class="function">sd_journal_process()</code> into
one.</p><p><code class="function">sd_journal_reliable_fd()</code>
may be used to check whether the wakeup events from
the file descriptor returned by
<code class="function">sd_journal_get_fd()</code> are known to
be immediately triggered. On certain file systems
where file change events from the OS are not available
(such as NFS) changes need to be polled for
repeatedly, and hence are detected only with a certain
latency. This call will return a positive value if the
journal changes are detected immediately and zero when
they need to be polled for and hence might be noticed
only with a certain latency. Note that there's usually
no need to invoke this function directly as
<code class="function">sd_journal_get_timeout()</code> on these
file systems will ask for timeouts explicitly
anyway.</p></div><div class="refsect1"><a name="idm259780854192"></a><h2 id="Return Value">Return Value<a class="headerlink" title="Permalink to this headline" href="#Return%20Value">¶</a></h2><p><code class="function">sd_journal_get_fd()</code> returns
a valid file descriptor on success or a negative
errno-style error code.</p><p><code class="function">sd_journal_get_events()</code>
returns a combination of <code class="literal">POLLIN</code>,
<code class="literal">POLLOUT</code> and suchlike on success or
a negative errno-style error code.</p><p><code class="function">sd_journal_reliable_fd()</code>
returns a positive integer if the file descriptor
returned by <code class="function">sd_journal_get_fd()</code>
will generate wake-ups immediately for all journal
changes. Returns 0 if there might be a latency
involved.</p><p><code class="function">sd_journal_process()</code> and
<code class="function">sd_journal_wait()</code> return one of
<code class="literal">SD_JOURNAL_NOP</code>,
<code class="literal">SD_JOURNAL_APPEND</code> or
<code class="literal">SD_JOURNAL_INVALIDATE</code> on success or
a negative errno-style error code. If
<code class="literal">SD_JOURNAL_NOP</code> is returned the
journal didn't change since the last invocation. If
<code class="literal">SD_JOURNAL_APPEND</code> is returned new
entries have been appended to the end of the
journal. If <code class="literal">SD_JOURNAL_INVALIDATE</code>
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 <code class="literal">SD_JOURNAL_APPEND</code> it is
sufficient to simply continue reading at the previous
end of the journal.</p></div><div class="refsect1"><a name="idm259775927152"></a><h2 id="Notes">Notes<a class="headerlink" title="Permalink to this headline" href="#Notes">¶</a></h2><p>The <code class="function">sd_journal_get_fd()</code>,
<code class="function">sd_journal_get_events()</code>,
<code class="function">sd_journal_reliable_fd()</code>,
<code class="function">sd_journal_process()</code> and
<code class="function">sd_journal_wait()</code> interfaces are
available as shared library, which can be compiled and
linked to with the
<code class="literal">libsystemd-journal</code>
<a href="pkg-config.html"><span class="citerefentry"><span class="refentrytitle">pkg-config</span>(1)</span></a>
file.</p></div><div class="refsect1"><a name="idm259775921328"></a><h2 id="Examples">Examples<a class="headerlink" title="Permalink to this headline" href="#Examples">¶</a></h2><p>Iterating through the journal, in a live view tracking all changes:</p><pre class="programlisting">#include <stdio.h>
#include <string.h>
#include <systemd/sd-journal.h>
int main(int argc, char *argv[]) {
int r;
sd_journal *j;
r = sd_journal_open(&j, SD_JOURNAL_LOCAL_ONLY);
if (r < 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 < 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 < 0) {
fprintf(stderr, "Failed to wait for changes: %s\n", strerror(-r));
break;
}
continue;
}
r = sd_journal_get_data(j, "MESSAGE", &d, &l);
if (r < 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;
}</pre><p>Waiting with <code class="function">poll()</code> (this
example lacks all error checking for the sake of
simplicity):</p><pre class="programlisting">#include <sys/poll.h>
#include <systemd/sd-journal.h>
int wait_for_changes(sd_journal *j) {
struct pollfd pollfd;
int msec;
sd_journal_get_timeout(m, &t);
if (t == (uint64_t) -1)
msec = -1;
else {
struct timespec ts;
uint64_t n;
clock_getttime(CLOCK_MONOTONIC, &ts);
n = (uint64_t) ts.tv_sec * 1000000 + ts.tv_nsec / 1000;
msec = t > n ? (int) ((t - n + 999) / 1000) : 0;
}
pollfd.fd = sd_journal_get_fd(j);
pollfd.events = sd_journal_get_events(j);
poll(&pollfd, 1, msec);
return sd_journal_process(j);
}
</pre></div><div class="refsect1"><a name="idm259775914704"></a><h2 id="See Also">See Also<a class="headerlink" title="Permalink to this headline" href="#See%20Also">¶</a></h2><p>
<a href="systemd.html"><span class="citerefentry"><span class="refentrytitle">systemd</span>(1)</span></a>,
<a href="sd-journal.html"><span class="citerefentry"><span class="refentrytitle">sd-journal</span>(3)</span></a>,
<a href="sd_journal_open.html"><span class="citerefentry"><span class="refentrytitle">sd_journal_open</span>(3)</span></a>,
<a href="sd_journal_next.html"><span class="citerefentry"><span class="refentrytitle">sd_journal_next</span>(3)</span></a>,
<a href="poll.html"><span class="citerefentry"><span class="refentrytitle">poll</span>(2)</span></a>,
<a href="clock_gettime.html"><span class="citerefentry"><span class="refentrytitle">clock_gettime</span>(2)</span></a>
</p></div></div></body></html>
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