373 lines
11 KiB
HTML
373 lines
11 KiB
HTML
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<HTML><HEAD><TITLE>Man page of SD_EVENT_WAIT</TITLE>
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</HEAD><BODY>
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<H1>SD_EVENT_WAIT</H1>
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Section: sd_event_wait (3)<BR>Updated: <BR><A HREF="#index">Index</A>
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<A HREF="/cgi-bin/man/man2html">Return to Main Contents</A><HR>
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<A NAME="lbAB"> </A>
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<H2>NAME</H2>
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sd_event_wait, sd_event_prepare, sd_event_dispatch, sd_event_get_state, sd_event_get_iteration, SD_EVENT_INITIAL, SD_EVENT_PREPARING, SD_EVENT_ARMED, SD_EVENT_PENDING, SD_EVENT_RUNNING, SD_EVENT_EXITING, SD_EVENT_FINISHED - Low-level event loop operations
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<A NAME="lbAC"> </A>
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<H2>SYNOPSIS</H2>
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<P>
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<B>
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</B><PRE>
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#include <<A HREF="file:///usr/include/systemd/sd-event.h">systemd/sd-event.h</A>>
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</PRE>
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<P>
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<B>
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</B><PRE>
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enum {
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<B>SD_EVENT_INITIAL</B>,
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<B>SD_EVENT_PREPARING</B>,
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<B>SD_EVENT_ARMED</B>,
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<B>SD_EVENT_PENDING</B>,
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<B>SD_EVENT_RUNNING</B>,
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<B>SD_EVENT_EXITING</B>,
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<B>SD_EVENT_FINISHED</B>,
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};
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</PRE>
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<DL COMPACT>
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<DT id="1">
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<B>int sd_event_prepare(sd_event *</B><I>event</I><B>);</B>
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<DT id="2">
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<B>int sd_event_wait(sd_event *</B><I>event</I><B>, uint64_t </B><I>usec</I><B>);</B>
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<DT id="3">
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<B>int sd_event_dispatch(sd_event *</B><I>event</I><B>);</B>
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<DT id="4">
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<B>int sd_event_get_state(sd_event *</B><I>event</I><B>);</B>
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<DT id="5">
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<B>int sd_event_get_iteration(sd_event *</B><I>event</I><B>, uint64_t *</B><I>ret</I><B>);</B>
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</DL>
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<A NAME="lbAD"> </A>
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<H2>DESCRIPTION</H2>
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<P>
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<DD>The low-level
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<B>sd_event_prepare()</B>,
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<B>sd_event_wait()</B>
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and
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<B>sd_event_dispatch()</B>
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functions may be used to execute specific phases of an event loop. See
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<B><A HREF="/cgi-bin/man/man2html?3+sd_event_run">sd_event_run</A></B>(3)
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and
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<B><A HREF="/cgi-bin/man/man2html?3+sd_event_loop">sd_event_loop</A></B>(3)
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for higher-level functions that execute individual but complete iterations of an event loop or run it continuously.
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<P>
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<B>sd_event_prepare()</B>
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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
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<B>sd_event_dispatch()</B>.
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<P>
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<B>sd_event_dispatch()</B>
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dispatches the highest priority event source that has a pending event. On success,
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<B>sd_event_dispatch()</B>
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returns either zero, which indicates that no further event sources may be dispatched and exiting of the event loop was requested via
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<B><A HREF="/cgi-bin/man/man2html?3+sd_event_exit">sd_event_exit</A></B>(3); 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
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<B>sd_event_prepare()</B>
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again.
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<P>
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In case
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<B>sd_event_prepare()</B>
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returned zero,
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<B>sd_event_wait()</B>
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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
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<B>sd_event_dispatch()</B>. Otherwise, the event loop returned to its initial state and the next event loop iteration should be initiated by invoking
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<B>sd_event_prepare()</B>
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again.
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<P>
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<B>sd_event_get_state()</B>
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may be used to determine the state the event loop is currently in. It returns one of the states described below.
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<P>
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<B>sd_event_get_iteration()</B>
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may be used to determine the current iteration of the event loop. It returns an unsigned 64bit integer containing a counter that increases monotonically with each iteration of the event loop, starting with 0. The counter is increased at the time of the
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<B>sd_event_prepare()</B>
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invocation.
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<P>
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All five functions take, as the first argument, the event loop object
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<I>event</I>
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that has been created with
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<B>sd_event_new()</B>. The timeout for
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<B>sd_event_wait()</B>
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is specified in
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<I>usec</I>
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in microseconds.
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<B>(uint64_t) -1</B>
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may be used to specify an infinite timeout.
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<A NAME="lbAE"> </A>
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<H2>STATE MACHINE</H2>
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<P>
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The event loop knows the following states, that may be queried with
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<B>sd_event_get_state()</B>.
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<P>
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<B>SD_EVENT_INITIAL</B>
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<DL COMPACT><DT id="6"><DD>
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The initial state the event loop is in, before each event loop iteration. Use
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<B>sd_event_prepare()</B>
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to transition the event loop into the
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<B>SD_EVENT_ARMED</B>
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or
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<B>SD_EVENT_PENDING</B>
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states.
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</DL>
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<P>
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<B>SD_EVENT_PREPARING</B>
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<DL COMPACT><DT id="7"><DD>
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An event source is currently being prepared, i.e. the preparation handler is currently being executed, as set with
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<B><A HREF="/cgi-bin/man/man2html?3+sd_event_source_set_prepare">sd_event_source_set_prepare</A></B>(3). This state is only seen in the event source preparation handler that is invoked from the
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<B>sd_event_prepare()</B>
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call and is immediately followed by
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<B>SD_EVENT_ARMED</B>
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or
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<B>SD_EVENT_PENDING</B>.
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</DL>
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<P>
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<B>SD_EVENT_ARMED</B>
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<DL COMPACT><DT id="8"><DD>
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<B>sd_event_prepare()</B>
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has been called and no event sources were ready to be dispatched. Use
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<B>sd_event_wait()</B>
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to wait for new events, and transition into
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<B>SD_EVENT_PENDING</B>
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or back into
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<B>SD_EVENT_INITIAL</B>.
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</DL>
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<P>
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<B>SD_EVENT_PENDING</B>
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<DL COMPACT><DT id="9"><DD>
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<B>sd_event_prepare()</B>
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or
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<B>sd_event_wait()</B>
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have been called and there were event sources with events pending. Use
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<B>sd_event_dispatch()</B>
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to dispatch the highest priority event source and transition back to
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<B>SD_EVENT_INITIAL</B>, or
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<B>SD_EVENT_FINISHED</B>.
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</DL>
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<P>
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<B>SD_EVENT_RUNNING</B>
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<DL COMPACT><DT id="10"><DD>
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A regular event source is currently being dispatched. This state is only seen in the event source handler that is invoked from the
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<B>sd_event_dispatch()</B>
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call, and is immediately followed by
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<B>SD_EVENT_INITIAL</B>
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or
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<B>SD_EVENT_FINISHED</B>
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as soon the event source handler returns. Note that during dispatching of exit event sources the
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<B>SD_EVENT_EXITING</B>
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state is seen instead.
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</DL>
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<P>
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<B>SD_EVENT_EXITING</B>
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<DL COMPACT><DT id="11"><DD>
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Similar to
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<B>SD_EVENT_RUNNING</B>
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but is the state in effect while dispatching exit event sources. It is followed by
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<B>SD_EVENT_INITIAL</B>
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or
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<B>SD_EVENT_FINISHED</B>
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as soon as the event handler returns.
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</DL>
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<P>
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<B>SD_EVENT_FINISHED</B>
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<DL COMPACT><DT id="12"><DD>
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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.
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</DL>
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<P>
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A simplified flow chart of the states and the calls to transition between them is shown below. Note that
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<B>SD_EVENT_PREPARING</B>,
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<B>SD_EVENT_RUNNING</B>
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and
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<B>SD_EVENT_EXITING</B>
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are not shown here.
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<P>
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<DL COMPACT><DT id="13"><DD>
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<PRE>
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INITIAL -<---<---<---<---<---<---<---<---<---<---<---<---\
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| ^
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v ret == 0 |
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sd_event_prepare() >--->--->--->--->- ARMED |
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| ret > 0 | |
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v v ret == 0 |
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PENDING <---<---<---<---<---< sd_event_wait() >--->--->--+
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| ret > 0 ^
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v |
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sd_event_dispatch() >--->--->--->--->--->--->--->--->--->--->/
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| ret > 0
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| ret == 0
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v
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FINISHED
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</PRE>
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</DL>
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<A NAME="lbAF"> </A>
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<H2>RETURN VALUE</H2>
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<P>
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On success, these functions return 0 or a positive integer. On failure, they return a negative errno-style error code. In case of
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<B>sd_event_prepare()</B>
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and
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<B>sd_event_wait()</B>, 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
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<B>sd_event_dispatch()</B>, a positive, non-zero return code indicates that the event loop returned to its initial state and zero indicates the event loop has exited.
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<B>sd_event_get_state()</B>
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returns a positive or zero state on success.
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<A NAME="lbAG"> </A>
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<H3>Errors</H3>
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<P>
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Returned errors may indicate the following problems:
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<P>
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<B>-EINVAL</B>
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<DL COMPACT><DT id="14"><DD>
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The
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<I>event</I>
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parameter is invalid or
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<B>NULL</B>.
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</DL>
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<P>
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<B>-EBUSY</B>
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<DL COMPACT><DT id="15"><DD>
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The event loop object is not in the right state.
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</DL>
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<P>
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<B>-ESTALE</B>
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<DL COMPACT><DT id="16"><DD>
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The event loop is already terminated.
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</DL>
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<P>
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<B>-ECHILD</B>
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<DL COMPACT><DT id="17"><DD>
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The event loop has been created in a different process.
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</DL>
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<P>
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Other errors are possible, too.
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<A NAME="lbAH"> </A>
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<H2>NOTES</H2>
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<P>
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These APIs are implemented as a shared library, which can be compiled and linked to with the
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<B>libsystemd</B> <B><A HREF="/cgi-bin/man/man2html?1+pkg-config">pkg-config</A></B>(1)
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file.
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<A NAME="lbAI"> </A>
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<H2>SEE ALSO</H2>
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<P>
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<B><A HREF="/cgi-bin/man/man2html?1+systemd">systemd</A></B>(1),
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<B><A HREF="/cgi-bin/man/man2html?3+sd_event_new">sd_event_new</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+sd_event_add_io">sd_event_add_io</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+sd_event_add_time">sd_event_add_time</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+sd_event_add_signal">sd_event_add_signal</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+sd_event_add_child">sd_event_add_child</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+sd_event_add_inotify">sd_event_add_inotify</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+sd_event_add_defer">sd_event_add_defer</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+sd_event_run">sd_event_run</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+sd_event_get_fd">sd_event_get_fd</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+sd_event_source_set_prepare">sd_event_source_set_prepare</A></B>(3)
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<P>
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<HR>
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<A NAME="index"> </A><H2>Index</H2>
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<DL>
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<DT id="18"><A HREF="#lbAB">NAME</A><DD>
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<DT id="19"><A HREF="#lbAC">SYNOPSIS</A><DD>
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<DT id="20"><A HREF="#lbAD">DESCRIPTION</A><DD>
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<DT id="21"><A HREF="#lbAE">STATE MACHINE</A><DD>
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<DT id="22"><A HREF="#lbAF">RETURN VALUE</A><DD>
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<DL>
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<DT id="23"><A HREF="#lbAG">Errors</A><DD>
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</DL>
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<DT id="24"><A HREF="#lbAH">NOTES</A><DD>
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<DT id="25"><A HREF="#lbAI">SEE ALSO</A><DD>
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</DL>
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<HR>
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This document was created by
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<A HREF="/cgi-bin/man/man2html">man2html</A>,
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using the manual pages.<BR>
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Time: 00:05:55 GMT, March 31, 2021
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</BODY>
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</HTML>
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