systemd/man/sd_event_add_time.xml
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<?xml version='1.0'?>
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<!--
SPDX-License-Identifier: LGPL-2.1+
-->
<refentry id="sd_event_add_time" xmlns:xi="http://www.w3.org/2001/XInclude">
<refentryinfo>
<title>sd_event_add_time</title>
<productname>systemd</productname>
</refentryinfo>
<refmeta>
<refentrytitle>sd_event_add_time</refentrytitle>
<manvolnum>3</manvolnum>
</refmeta>
<refnamediv>
<refname>sd_event_add_time</refname>
<refname>sd_event_source_get_time</refname>
<refname>sd_event_source_set_time</refname>
<refname>sd_event_source_get_time_accuracy</refname>
<refname>sd_event_source_set_time_accuracy</refname>
<refname>sd_event_source_get_time_clock</refname>
<refname>sd_event_time_handler_t</refname>
<refpurpose>Add a timer event source to an event loop</refpurpose>
</refnamediv>
<refsynopsisdiv>
<funcsynopsis>
<funcsynopsisinfo>#include &lt;systemd/sd-event.h&gt;</funcsynopsisinfo>
<funcsynopsisinfo><token>typedef</token> struct sd_event_source sd_event_source;</funcsynopsisinfo>
<funcprototype>
<funcdef>typedef int (*<function>sd_event_time_handler_t</function>)</funcdef>
<paramdef>sd_event_source *<parameter>s</parameter></paramdef>
<paramdef>uint64_t <parameter>usec</parameter></paramdef>
<paramdef>void *<parameter>userdata</parameter></paramdef>
</funcprototype>
<funcprototype>
<funcdef>int <function>sd_event_add_time</function></funcdef>
<paramdef>sd_event *<parameter>event</parameter></paramdef>
<paramdef>sd_event_source **<parameter>source</parameter></paramdef>
<paramdef>clockid_t <parameter>clock</parameter></paramdef>
<paramdef>uint64_t <parameter>usec</parameter></paramdef>
<paramdef>uint64_t <parameter>accuracy</parameter></paramdef>
<paramdef>sd_event_time_handler_t <parameter>handler</parameter></paramdef>
<paramdef>void *<parameter>userdata</parameter></paramdef>
</funcprototype>
<funcprototype>
<funcdef>int <function>sd_event_source_get_time</function></funcdef>
<paramdef>sd_event_source *<parameter>source</parameter></paramdef>
<paramdef>uint64_t *<parameter>usec</parameter></paramdef>
</funcprototype>
<funcprototype>
<funcdef>int <function>sd_event_source_set_time</function></funcdef>
<paramdef>sd_event_source *<parameter>source</parameter></paramdef>
<paramdef>uint64_t <parameter>usec</parameter></paramdef>
</funcprototype>
<funcprototype>
<funcdef>int <function>sd_event_source_get_time_accuracy</function></funcdef>
<paramdef>sd_event_source *<parameter>source</parameter></paramdef>
<paramdef>uint64_t *<parameter>usec</parameter></paramdef>
</funcprototype>
<funcprototype>
<funcdef>int <function>sd_event_source_set_time_accuracy</function></funcdef>
<paramdef>sd_event_source *<parameter>source</parameter></paramdef>
<paramdef>uint64_t <parameter>usec</parameter></paramdef>
</funcprototype>
<funcprototype>
<funcdef>int <function>sd_event_source_get_time_clock</function></funcdef>
<paramdef>sd_event_source *<parameter>source</parameter></paramdef>
<paramdef>clockid_t *<parameter>clock</parameter></paramdef>
</funcprototype>
</funcsynopsis>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<para><function>sd_event_add_time()</function> adds a new timer event source to an event loop. The event loop
object is specified in the <parameter>event</parameter> parameter, the event source object is returned in the
<parameter>source</parameter> parameter. The <parameter>clock</parameter> parameter takes a clock identifier, one
of <constant>CLOCK_REALTIME</constant>, <constant>CLOCK_MONOTONIC</constant>, <constant>CLOCK_BOOTTIME</constant>,
<constant>CLOCK_REALTIME_ALARM</constant>, or <constant>CLOCK_BOOTTIME_ALARM</constant>. See
<citerefentry><refentrytitle>timerfd_create</refentrytitle><manvolnum>2</manvolnum></citerefentry> for details
regarding the various types of clocks. The <parameter>usec</parameter> parameter specifies the earliest time, in
microseconds (µs), relative to the clock's epoch, when the timer shall be triggered. If a time already in the past
is specified (including <constant>0</constant>), this timer source "fires" immediately and is ready to be
dispatched. If the parameter is specified as <constant>UINT64_MAX</constant> the timer event will never elapse,
which may be used as an alternative to explicitly disabling a timer event source with
<citerefentry><refentrytitle>sd_event_source_set_enabled</refentrytitle><manvolnum>3</manvolnum></citerefentry>. The
<parameter>accuracy</parameter> parameter specifies an additional accuracy value in µs specifying how much the
timer event may be delayed. Use <constant>0</constant> to select the default accuracy (250ms). Use 1µs for maximum
accuracy. Consider specifying 60000000µs (1min) or larger for long-running events that may be delayed
substantially. Picking higher accuracy values allows the system to coalesce timer events more aggressively,
improving power efficiency. The <parameter>handler</parameter> parameter shall reference a function to call when
the timer elapses. The handler function will be passed the <parameter>userdata</parameter> pointer, which may be
chosen freely by the caller. The handler is also passed the configured trigger time, even if it is actually called
slightly later, subject to the specified accuracy value, the kernel timer slack (see
<citerefentry><refentrytitle>prctl</refentrytitle><manvolnum>2</manvolnum></citerefentry>), and additional
scheduling latencies. To query the actual time the handler was called use
<citerefentry><refentrytitle>sd_event_now</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para>
<para>By default, the timer will elapse once
(<constant>SD_EVENT_ONESHOT</constant>), but this may be changed
with
<citerefentry><refentrytitle>sd_event_source_set_enabled</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
If the handler function returns a negative error code, it will be
disabled after the invocation, even if the
<constant>SD_EVENT_ON</constant> mode was requested before. Note
that a timer event set to <constant>SD_EVENT_ON</constant> will
fire continuously unless its configured time is updated using
<function>sd_event_source_set_time()</function>.
</para>
<para>To destroy an event source object use
<citerefentry><refentrytitle>sd_event_source_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
but note that the event source is only removed from the event loop
when all references to the event source are dropped. To make sure
an event source does not fire anymore, even if it is still referenced,
disable the event source using
<citerefentry><refentrytitle>sd_event_source_set_enabled</refentrytitle><manvolnum>3</manvolnum></citerefentry>
with <constant>SD_EVENT_OFF</constant>.</para>
<para>If the second parameter of
<function>sd_event_add_time()</function> is
<constant>NULL</constant> no reference to the event source object
is returned. In this case the event source is considered
"floating", and will be destroyed implicitly when the event loop
itself is destroyed.</para>
<para>If the <parameter>handler</parameter> to
<function>sd_event_add_time()</function> is
<constant>NULL</constant>, and the event source fires, this will
be considered a request to exit the event loop. In this case, the
<parameter>userdata</parameter> parameter, cast to an integer, is
used for the exit code passed to
<citerefentry><refentrytitle>sd_event_exit</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para>
<para>Use <constant>CLOCK_BOOTTIME_ALARM</constant> and
<constant>CLOCK_REALTIME_ALARM</constant> to define event sources
that may wake up the system from suspend.</para>
<para>In order to set up relative timers (that is, relative to the
current time), retrieve the current time via
<citerefentry><refentrytitle>sd_event_now</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
add the desired timespan to it, and use the result as
the <parameter>usec</parameter> parameter to
<function>sd_event_add_time()</function>.</para>
<para>In order to set up repetitive timers (that is, timers that
are triggered in regular intervals), set up the timer normally,
for the first invocation. Each time the event handler is invoked,
update the timer's trigger time with
<citerefentry><refentrytitle>sd_event_source_set_time</refentrytitle><manvolnum>3</manvolnum></citerefentry> for the next timer
iteration, and reenable the timer using
<function>sd_event_source_set_enabled()</function>. To calculate
the next point in time to pass to
<function>sd_event_source_set_time()</function>, either use as
base the <parameter>usec</parameter> parameter passed to the timer
callback, or the timestamp returned by
<function>sd_event_now()</function>. In the former case timer
events will be regular, while in the latter case the scheduling
latency will keep accumulating on the timer.</para>
<para><function>sd_event_source_get_time()</function> retrieves
the configured time value of an event source created
previously with <function>sd_event_add_time()</function>. It takes
the event source object and a pointer to a variable to store the
time in, relative to the selected clock's epoch, in µs.</para>
<para><function>sd_event_source_set_time()</function> changes the
time of an event source created previously with
<function>sd_event_add_time()</function>. It takes the event
source object and a time relative to the selected clock's epoch,
in µs.</para>
<para><function>sd_event_source_get_time_accuracy()</function>
retrieves the configured accuracy value of an event source
created previously with <function>sd_event_add_time()</function>. It
takes the event source object and a pointer to a variable to store
the accuracy in. The accuracy is specified in µs.</para>
<para><function>sd_event_source_set_time_accuracy()</function>
changes the configured accuracy of a timer event source created
previously with <function>sd_event_add_time()</function>. It takes
the event source object and accuracy, in µs.</para>
<para><function>sd_event_source_get_time_clock()</function>
retrieves the configured clock of an event source created
previously with <function>sd_event_add_time()</function>. It takes
the event source object and a pointer to a variable to store the
clock identifier in.</para>
</refsect1>
<refsect1>
<title>Return Value</title>
<para>On success, these functions return 0 or a positive
integer. On failure, they return a negative errno-style error
code. </para>
</refsect1>
<refsect1>
<title>Errors</title>
<para>Returned values may indicate the following problems:</para>
<variablelist>
<varlistentry>
<term><constant>-ENOMEM</constant></term>
<listitem><para>Not enough memory to allocate an object.</para></listitem>
</varlistentry>
<varlistentry>
<term><constant>-EINVAL</constant></term>
<listitem><para>An invalid argument has been passed.</para></listitem>
</varlistentry>
<varlistentry>
<term><constant>-ESTALE</constant></term>
<listitem><para>The event loop is already terminated.</para></listitem>
</varlistentry>
<varlistentry>
<term><constant>-ECHILD</constant></term>
<listitem><para>The event loop has been created in a different process.</para></listitem>
</varlistentry>
<varlistentry>
<term><constant>-EOPNOTSUPP</constant></term>
<listitem><para>The selected clock is not supported by the event loop implementation.</para></listitem>
</varlistentry>
<varlistentry>
<term><constant>-EDOM</constant></term>
<listitem><para>The passed event source is not a timer event source.</para></listitem>
</varlistentry>
</variablelist>
</refsect1>
<xi:include href="libsystemd-pkgconfig.xml" />
<refsect1>
<title>See Also</title>
<para>
<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry><refentrytitle>sd-event</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>sd_event_new</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>sd_event_now</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>sd_event_add_io</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>sd_event_add_signal</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>sd_event_add_child</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>sd_event_add_inotify</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>sd_event_add_defer</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>sd_event_source_set_enabled</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>sd_event_source_set_priority</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>sd_event_source_set_userdata</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>sd_event_source_set_description</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry project='man-pages'><refentrytitle>clock_gettime</refentrytitle><manvolnum>2</manvolnum></citerefentry>,
<citerefentry project='man-pages'><refentrytitle>timerfd_create</refentrytitle><manvolnum>2</manvolnum></citerefentry>,
<citerefentry project='man-pages'><refentrytitle>prctl</refentrytitle><manvolnum>2</manvolnum></citerefentry>
</para>
</refsect1>
</refentry>