<refnamediv>
<refname>sd_journal_get_fd</refname>
- <refname>sd_journal_reliable_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>
</funcprototype>
<funcprototype>
- <funcdef>int <function>sd_journal_reliable_fd</function></funcdef>
+ <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>
<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>
<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
+ 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. 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
+ all cases. Cases like this can be detected with
<function>sd_journal_reliable_fd()</function>,
- below.</para>
+ 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, &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;
+}</programlisting>
- <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
+ <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 and reset the readable state
- of the file descriptor. This call will also indicate
+ 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_reliable_fd()</function> and
+ <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,
- 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>,
+ 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's 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_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>
- 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>
+ 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
- <literal>SD_JOURNAL_NOP</literal>,
- <literal>SD_JOURNAL_APPEND</literal> or
- <literal>SD_JOURNAL_INVALIDATE</literal> on success or
+ <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
- <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
+ <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 <literal>SD_JOURNAL_INVALIDATE</literal>
+ 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 <literal>SD_JOURNAL_APPEND</literal> it is
+ 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>
<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>
+ <constant>libsystemd-journal</constant> <citerefentry><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>
file.</para>
</refsect1>
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 = POLLIN;
- poll(&pollfd, 1, sd_journal_reliable_fd(j) > 0 ? -1 : 2000);
+ pollfd.events = sd_journal_get_events(j);
+ poll(&pollfd, 1, msec);
return sd_journal_process(j);
}
</programlisting>
</refsect1>
-
<refsect1>
<title>See Also</title>
<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>poll</refentrytitle><manvolnum>2</manvolnum></citerefentry>,
+ <citerefentry><refentrytitle>clock_gettime</refentrytitle><manvolnum>2</manvolnum></citerefentry>
</para>
</refsect1>
~ianmdlvl / elogind.git / blobdiff