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<refentry id="sd_event_wait" xmlns:xi="http://www.w3.org/2001/XInclude">

<refentryinfo>

<title>sd_event_wait</title>

<productname>systemd</productname>

<authorgroup>

<author>

<contrib>Developer</contrib>

<firstname>Tom</firstname>

<surname>Gundersen</surname>

<email>teg@jklm.no</email>

</author>

</authorgroup>

</refentryinfo>

<refmeta>

<refentrytitle>sd_event_wait</refentrytitle>

<manvolnum>3</manvolnum>

</refmeta>

<refnamediv>

<refname>sd_event_wait</refname>

<refname>sd_event_prepare</refname>

<refname>sd_event_dispatch</refname>

<refname>sd_event_get_state</refname>

<refname>SD_EVENT_INITIAL</refname>

<refname>SD_EVENT_PREPARING</refname>

<refname>SD_EVENT_ARMED</refname>

<refname>SD_EVENT_PENDING</refname>

<refname>SD_EVENT_RUNNING</refname>

<refname>SD_EVENT_EXITING</refname>

<refname>SD_EVENT_FINISHED</refname>

<refpurpose>Low-level event loop operations</refpurpose>

</refnamediv>

<refsynopsisdiv>

<funcsynopsis>

<funcsynopsisinfo>#include &lt;systemd/sd-event.h&gt;</funcsynopsisinfo>

<funcsynopsisinfo><token>enum</token> {

<constant>SD_EVENT_INITIAL</constant>,

<constant>SD_EVENT_PREPARING</constant>,

<constant>SD_EVENT_ARMED</constant>,

<constant>SD_EVENT_PENDING</constant>,

<constant>SD_EVENT_RUNNING</constant>,

<constant>SD_EVENT_EXITING</constant>,

<constant>SD_EVENT_FINISHED</constant>,

};</funcsynopsisinfo>

<funcprototype>

<funcdef>int <function>sd_event_prepare</function></funcdef>

<paramdef>sd_event *<parameter>event</parameter></paramdef>

</funcprototype>

<funcprototype>

<funcdef>int <function>sd_event_wait</function></funcdef>

<paramdef>sd_event *<parameter>event</parameter></paramdef>

<paramdef>uint64_t <parameter>usec</parameter></paramdef>

</funcprototype>

<funcprototype>

<funcdef>int <function>sd_event_dispatch</function></funcdef>

<paramdef>sd_event *<parameter>event</parameter></paramdef>

</funcprototype>

<funcprototype>

<funcdef>int <function>sd_event_get_state</function></funcdef>

<paramdef>sd_event *<parameter>event</parameter></paramdef>

</funcprototype>

</funcsynopsis>

</refsynopsisdiv>

<refsect1>

<title>Description</title>

<para>The low-level <function>sd_event_prepare()</function>,

<function>sd_event_wait()</function> and

<function>sd_event_dispatch()</function> functions may be used to

execute specific phases of an event loop. See

<citerefentry><refentrytitle>sd_event_run</refentrytitle><manvolnum>3</manvolnum></citerefentry>

and

<citerefentry><refentrytitle>sd_event_loop</refentrytitle><manvolnum>3</manvolnum></citerefentry>

for higher-level functions that execute individual but complete

iterations of an event loop or run it continuously.</para>

<para><function>sd_event_prepare()</function> checks for pending

events and arms necessary timers. If any events are ready to be

processed ("pending"), it returns a positive, non-zero value, and the caller

should process these events with

<function>sd_event_dispatch()</function>.</para>

<para><function>sd_event_dispatch()</function> dispatches the

highest priority event source that has a pending event. On

success, <function>sd_event_dispatch()</function> returns either

zero, which indicates that no further event sources may be

dispatched and exiting of the event loop was requested via

<citerefentry><refentrytitle>sd_event_exit</refentrytitle><manvolnum>3</manvolnum></citerefentry>;

or a positive non-zero value, which means that an event source was

dispatched and the loop returned to its initial state, and the

caller should initiate the next event loop iteration by invoking

<function>sd_event_prepare()</function> again.</para>

<para>In case <function>sd_event_prepare()</function> returned

zero, <function>sd_event_wait()</function> should be called to

wait for further events or a timeout. If any events are ready to

be processed, it returns a positive, non-zero value, and the

events should be dispatched with

<function>sd_event_dispatch()</function>. Otherwise, the event

loop returned to its initial state and the next event loop

iteration should be initiated by invoking

<function>sd_event_prepare()</function> again.</para>

<para><function>sd_event_get_state()</function> may be used to

determine the state the event loop is currently in. It returns one

of the states described below.</para>

<para>All four functions take, as the first argument, the event

loop object <parameter>event</parameter> that has been created

with <function>sd_event_new()</function>. The timeout for

<function>sd_event_wait()</function> is specified in

<parameter>usec</parameter> in milliseconds. <constant>(uint64_t)

-1</constant> may be used to specify an infinite timeout.</para>

</refsect1>

<refsect1>

<title>State Machine</title>

<para>The event loop knows the following states, that may be

queried with <function>sd_event_get_state()</function>.</para>

<variablelist>

<varlistentry>

<term><constant>SD_EVENT_INITIAL</constant></term>

<listitem><para>The initial state the event loop is in,

before each event loop iteration. Use

<function>sd_event_prepare()</function> to transition the

event loop into the <constant>SD_EVENT_ARMED</constant> or

<constant>SD_EVENT_PENDING</constant> states.</para></listitem>

</varlistentry>

<varlistentry>

<term><constant>SD_EVENT_PREPARING</constant></term>

<listitem><para>An event source is currently being prepared,

i.e. the preparation handler is currently being executed, as

set with

<citerefentry><refentrytitle>sd_event_set_prepare</refentrytitle><manvolnum>3</manvolnum></citerefentry>. This

state is only seen in the event source preparation handler

that is invoked from the

<function>sd_event_prepare()</function> call and is

immediately followed by <constant>SD_EVENT_ARMED</constant> or

<constant>SD_EVENT_PENDING</constant>.</para></listitem>

</varlistentry>

<varlistentry>

<term><constant>SD_EVENT_ARMED</constant></term>

<listitem><para><function>sd_event_prepare()</function> has

been called and no event sources were ready to be

dispatched. Use <function>sd_event_wait()</function> to wait

for new events, and transition into

<constant>SD_EVENT_PENDING</constant> or back into

<constant>SD_EVENT_INITIAL</constant>.</para></listitem>

</varlistentry>

<varlistentry>

<term><constant>SD_EVENT_PENDING</constant></term>

<listitem><para><function>sd_event_prepare()</function> or

<function>sd_event_wait()</function> have been called and

there were event sources with events pending. Use

<function>sd_event_dispatch()</function> to dispatch the

highest priority event source and transition back to

<constant>SD_EVENT_INITIAL</constant>, or

<constant>SD_EVENT_FINISHED</constant>.</para></listitem>

</varlistentry>

<varlistentry>

<term><constant>SD_EVENT_RUNNING</constant></term>

<listitem><para>A regular event source is currently being

dispatched. This state is only seen in the event source

handler that is invoked from the

<function>sd_event_dispatch()</function> call, and is

immediately followed by <constant>SD_EVENT_INITIAL</constant>

or <constant>SD_EVENT_FINISHED</constant> as soon the event

source handler returns. Note that during dispatching of exit

event sources the <constant>SD_EVENT_EXITING</constant> state

is seen instead.</para></listitem>

</varlistentry>

<varlistentry>

<term><constant>SD_EVENT_EXITING</constant></term>

<listitem><para>Similar to

<constant>SD_EVENT_RUNNING</constant> but is the state in

effect while dispatching exit event sources. It is followed by

<constant>SD_EVENT_INITIAL</constant> or

<constant>SD_EVENT_FINISHED</constant> as soon as the event

handler returns.</para></listitem>

</varlistentry>

<varlistentry>

<term><constant>SD_EVENT_FINISHED</constant></term>

<listitem><para>The event loop has exited. All exit event

sources have run. If the event loop is in this state it serves

no purpose anymore, and should be freed.</para></listitem>

</varlistentry>

</variablelist>

<para>A simplified flow chart of the states and the calls to

transition between them is shown below. Note that

<constant>SD_EVENT_PREPARING</constant>,

<constant>SD_EVENT_RUNNING</constant> and

<constant>SD_EVENT_EXITING</constant> are not shown here.</para>

<programlisting>

INITIAL -&lt;---&lt;---&lt;---&lt;---&lt;---&lt;---&lt;---&lt;---&lt;---&lt;---&lt;---&lt;---\

| |

| ^

| |

v ret == 0 |

sd_event_prepare() &gt;---&gt;---&gt;---&gt;---&gt;- ARMED |

| | ^

| ret > 0 | |

| | |

v v ret == 0 |

PENDING &lt;---&lt;---&lt;---&lt;---&lt;---&lt; sd_event_wait() &gt;---&gt;---&gt;--+

| ret > 0 ^

| |

| |

v |

sd_event_dispatch() &gt;---&gt;---&gt;---&gt;---&gt;---&gt;---&gt;---&gt;---&gt;---&gt;---&gt;/

| ret > 0

| ret == 0

|

v

FINISHED

</programlisting>

</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. In case

of <function>sd_event_prepare()</function> and

<function>sd_event_wait()</function>, a positive, non-zero return

code indicates that events are ready to be processed and zero

indicates that no events are ready. In case of

<function>sd_event_dispatch()</function>, a positive, non-zero

return code indicates that the event loop returned to its initial

state and zero indicates the event loop has

exited. <function>sd_event_get_state()</function> returns a

positive or zero state on success.</para>

</refsect1>

<refsect1>

<title>Errors</title>

<para>Returned errors may indicate the following problems:</para>

<variablelist>

<varlistentry>

<term><constant>-EINVAL</constant></term>

<listitem><para>The <parameter>event</parameter> parameter is

invalid or <constant>NULL</constant>.</para></listitem>

</varlistentry>

<varlistentry>

<term><constant>-EBUSY</constant></term>

<listitem><para>The event loop object is not in the right

state.</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>

</variablelist>

<para>Other errors are possible, too.</para>

</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_new</refentrytitle><manvolnum>3</manvolnum></citerefentry>,

<citerefentry><refentrytitle>sd_event_add_io</refentrytitle><manvolnum>3</manvolnum></citerefentry>,

<citerefentry><refentrytitle>sd_event_add_time</refentrytitle><manvolnum>3</manvolnum></citerefentry>,

<citerefentry><refentrytitle>sd_event_add_signal</refentrytitle><manvolnum>3</manvolnum></citerefentry>,

<citerefentry><refentrytitle>sd_event_add_defer</refentrytitle><manvolnum>3</manvolnum></citerefentry>,

<citerefentry><refentrytitle>sd_event_add_exit</refentrytitle><manvolnum>3</manvolnum></citerefentry>,

<citerefentry><refentrytitle>sd_event_add_post</refentrytitle><manvolnum>3</manvolnum></citerefentry>,

<citerefentry><refentrytitle>sd_event_run</refentrytitle><manvolnum>3</manvolnum></citerefentry>,

<citerefentry><refentrytitle>sd_event_get_fd</refentrytitle><manvolnum>3</manvolnum></citerefentry>,

<citerefentry><refentrytitle>sd_event_source_set_prepare</refentrytitle><manvolnum>3</manvolnum></citerefentry>

</para>

</refsect1>

</refentry>