systemd/man/sd_journal_get_fd.xml
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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">
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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_get_timeout</refname>
<refname>sd_journal_process</refname>
<refname>sd_journal_wait</refname>
<refname>sd_journal_reliable_fd</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_get_timeout</function></funcdef>
<paramdef>sd_journal *<parameter>j</parameter></paramdef>
<paramdef>uint64_t *<parameter>timeout_usec</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>
<funcprototype>
<funcdef>int <function>sd_journal_reliable_fd</function></funcdef>
<paramdef>sd_journal *<parameter>j</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 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
<function>sd_journal_reliable_fd()</function>, below.
<function>sd_journal_get_timeout()</function> will ensure in these
cases that wake-ups happen frequently enough for changes to be
noticed, although with a certain latency.</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 <constant>POLLIN</constant> and
<constant>POLLOUT</constant> and similar to fill into the
<literal>.events</literal> field of <varname>struct
pollfd</varname>.</para>
<para><function>sd_journal_get_timeout()</function> will return a
timeout value for usage in <function>poll()</function>. This
returns a value in microseconds since the epoch of
<constant>CLOCK_MONOTONIC</constant> for timing out
<function>poll()</function> in <varname>timeout_usec</varname>.
See
<citerefentry><refentrytitle>clock_gettime</refentrytitle><manvolnum>2</manvolnum></citerefentry>
for details about <constant>CLOCK_MONOTONIC</constant>. If there
is no timeout to wait for, this will fill in <constant>(uint64_t)
-1</constant> instead. Note that <function>poll()</function> 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:</para>
<programlisting>uint64_t t;
int msec;
sd_journal_get_timeout(m, &amp;t);
if (t == (uint64_t) -1)
msec = -1;
else {
struct timespec ts;
uint64_t n;
clock_gettime(CLOCK_MONOTONIC, &amp;ts);
n = (uint64_t) ts.tv_sec * 1000000 + ts.tv_nsec / 1000;
msec = t > n ? (int) ((t - n + 999) / 1000) : 0;
}</programlisting>
<para>The code above does not do any error checking for brevity's
sake. The calculated <varname>msec</varname> integer can be passed
directly as <function>poll()</function>'s timeout
parameter.</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_get_timeout()</function> and
<function>sd_journal_process()</function> is
<function>sd_journal_wait()</function>. It will synchronously wait
until the journal gets changed. The maximum time this call sleeps
may be controlled with the <parameter>timeout_usec</parameter>
parameter. Pass <constant>(uint64_t) -1</constant> to wait
indefinitely. Internally this call simply combines
<function>sd_journal_get_fd()</function>,
<function>sd_journal_get_events()</function>,
<function>sd_journal_get_timeout()</function>,
<function>poll()</function> and
<function>sd_journal_process()</function> into one.</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 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 is usually no need to invoke this function
directly as <function>sd_journal_get_timeout()</function> on these
file systems will ask for timeouts explicitly anyway.</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 <constant>POLLIN</constant>,
<constant>POLLOUT</constant> 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> will generate wake-ups
immediately for all journal changes. Returns 0 if there might be a
latency involved.</para>
<para><function>sd_journal_process()</function> and
<function>sd_journal_wait()</function> return one of
<constant>SD_JOURNAL_NOP</constant>,
<constant>SD_JOURNAL_APPEND</constant> or
<constant>SD_JOURNAL_INVALIDATE</constant> on success or a
negative errno-style error code. If
<constant>SD_JOURNAL_NOP</constant> is returned, the journal did
not change since the last invocation. If
<constant>SD_JOURNAL_APPEND</constant> is returned, new entries
have been appended to the end of the journal. If
<constant>SD_JOURNAL_INVALIDATE</constant>, 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 <constant>SD_JOURNAL_APPEND</constant>, 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
a shared library, which can be compiled and linked to with the
<constant>libsystemd</constant> <citerefentry project='die-net'><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;poll.h&gt;
#include &lt;systemd/sd-journal.h&gt;
int wait_for_changes(sd_journal *j) {
struct pollfd pollfd;
int msec;
sd_journal_get_timeout(m, &amp;t);
if (t == (uint64_t) -1)
msec = -1;
else {
struct timespec ts;
uint64_t n;
clock_gettime(CLOCK_MONOTONIC, &amp;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(&amp;pollfd, 1, msec);
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>,
<citerefentry><refentrytitle>clock_gettime</refentrytitle><manvolnum>2</manvolnum></citerefentry>
</para>
</refsect1>
</refentry>