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4afd3348c7
GLIB has recently started to officially support the gcc cleanup
attribute in its public API, hence let's do the same for our APIs.
With this patch we'll define an xyz_unrefp() call for each public
xyz_unref() call, to make it easy to use inside a
__attribute__((cleanup())) expression. Then, all code is ported over to
make use of this.
The new calls are also documented in the man pages, with examples how to
use them (well, I only added docs where the _unref() call itself already
had docs, and the examples, only cover sd_bus_unrefp() and
sd_event_unrefp()).
This also renames sd_lldp_free() to sd_lldp_unref(), since that's how we
tend to call our destructors these days.
Note that this defines no public macro that wraps gcc's attribute and
makes it easier to use. While I think it's our duty in the library to
make our stuff easy to use, I figure it's not our duty to make gcc's own
features easy to use on its own. Most likely, client code which wants to
make use of this should define its own:
#define _cleanup_(function) __attribute__((cleanup(function)))
Or similar, to make the gcc feature easier to use.
Making this logic public has the benefit that we can remove three header
files whose only purpose was to define these functions internally.
See #2008.
246 lines
10 KiB
XML
246 lines
10 KiB
XML
<?xml version='1.0'?> <!--*- Mode: nxml; nxml-child-indent: 2; indent-tabs-mode: nil -*-->
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<!--
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This file is part of systemd.
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Copyright 2014 Lennart Poettering
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systemd is free software; you can redistribute it and/or modify it
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under the terms of the GNU Lesser General Public License as published by
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the Free Software Foundation; either version 2.1 of the License, or
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(at your option) any later version.
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systemd is distributed in the hope that it will be useful, but
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WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public License
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along with systemd; If not, see <http://www.gnu.org/licenses/>.
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-->
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<refentry id="sd_event_new" xmlns:xi="http://www.w3.org/2001/XInclude">
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<refentryinfo>
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<title>sd_event_new</title>
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<productname>systemd</productname>
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<authorgroup>
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<author>
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<contrib>Developer</contrib>
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<firstname>Lennart</firstname>
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<surname>Poettering</surname>
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<email>lennart@poettering.net</email>
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</author>
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</authorgroup>
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</refentryinfo>
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<refmeta>
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<refentrytitle>sd_event_new</refentrytitle>
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<manvolnum>3</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>sd_event_new</refname>
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<refname>sd_event_default</refname>
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<refname>sd_event_ref</refname>
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<refname>sd_event_unref</refname>
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<refname>sd_event_unrefp</refname>
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<refname>sd_event_get_tid</refname>
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<refname>sd_event</refname>
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<refpurpose>Acquire and release an event loop object</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<funcsynopsis>
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<funcsynopsisinfo>#include <systemd/sd-event.h></funcsynopsisinfo>
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<funcsynopsisinfo><token>typedef</token> struct sd_event sd_event;</funcsynopsisinfo>
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<funcprototype>
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<funcdef>int <function>sd_event_new</function></funcdef>
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<paramdef>sd_event **<parameter>event</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_event_default</function></funcdef>
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<paramdef>sd_event **<parameter>event</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>sd_event *<function>sd_event_ref</function></funcdef>
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<paramdef>sd_event *<parameter>event</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>sd_event *<function>sd_event_unref</function></funcdef>
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<paramdef>sd_event *<parameter>event</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>void <function>sd_event_unrefp</function></funcdef>
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<paramdef>sd_event **<parameter>event</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_event_get_tid</function></funcdef>
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<paramdef>sd_event *<parameter>event</parameter></paramdef>
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<paramdef>pid_t *<parameter>tid</parameter></paramdef>
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</funcprototype>
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</funcsynopsis>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para><function>sd_event_new()</function> allocates a new event
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loop object. The event loop object is returned in the
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<parameter>event</parameter> parameter. After use, drop
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the returned reference with
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<function>sd_event_unref()</function>. When the last reference is
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dropped, the object is freed.</para>
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<para><function>sd_event_default()</function> acquires a reference
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to the default event loop object of the calling thread, possibly
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allocating a new object if no default event loop object has been
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allocated yet for the thread. After use, drop the returned
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reference with <function>sd_event_unref()</function>. When the
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last reference is dropped, the event loop is freed. If this
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function is called while the object returned from a previous call
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from the same thread is still referenced, the same object is
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returned again, but the reference is increased by one. It is
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recommended to use this call instead of
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<function>sd_event_new()</function> in order to share event loop
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objects between various components that are dispatched in the same
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thread. All threads have exactly either zero or one default event loop
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objects associated, but never more.</para>
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<para>After allocating an event loop object, add event sources to
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it with
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<citerefentry><refentrytitle>sd_event_add_io</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_event_add_time</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_event_add_signal</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_event_add_child</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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or
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<citerefentry><refentrytitle>sd_event_add_defer</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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and then execute the event loop using
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<citerefentry><refentrytitle>sd_event_run</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para>
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<para><function>sd_event_ref()</function> increases the reference
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count of the specified event loop object by one.</para>
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<para><function>sd_event_unref()</function> decreases the
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reference count of the specified event loop object by one. If
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the count hits zero, the object is freed. Note that it
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is freed regardless of whether it is the default event loop object for a
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thread or not. This means that allocating an event loop with
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<function>sd_event_default()</function>, then releasing it, and
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then acquiring a new one with
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<function>sd_event_default()</function> will result in two
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distinct objects. Note that, in order to free an event loop object,
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all remaining event sources of the event loop also need to be
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freed as each keeps a reference to it.</para>
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<para><function>sd_event_unrefp()</function> is similar to
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<function>sd_event_unref()</function> but takes a pointer to a
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pointer to an <type>sd_event</type> object. This call is useful in
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conjunction with GCC's and LLVM's <ulink
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url="https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html">Clean-up
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Variable Attribute</ulink>. Note that this function is defined as
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inline function. Use a declaration like the following,
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in order to allocate an event loop object that is freed
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automatically as the code block is left:</para>
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<programlisting>{
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__attribute__((cleanup(sd_event_unrefp)) sd_event *event = NULL;
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int r;
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…
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r = sd_event_default(&event);
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if (r < 0)
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fprintf(stderr, "Failed to allocate event loop: %s\n", strerror(-r));
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…
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}</programlisting>
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<para><function>sd_event_ref()</function>,
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<function>sd_event_unref()</function> and
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<function>sd_event_unrefp()</function> execute no operation if the
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passed in event loop object is <constant>NULL</constant>.</para>
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<para><function>sd_event_get_tid()</function> retrieves the thread
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identifier ("TID") of the thread the specified event loop object
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is associated with. This call is only supported for event loops
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allocated with <function>sd_event_default()</function>, and
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returns the identifier for the thread the event loop is the
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default event loop of. See <citerefentry
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project='man-pages'><refentrytitle>gettid</refentrytitle><manvolnum>2</manvolnum></citerefentry>
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for more information on thread identifiers.</para>
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</refsect1>
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<refsect1>
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<title>Return Value</title>
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<para>On success, <function>sd_event_new()</function> and
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<function>sd_event_default()</function> return 0 or a positive
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integer. On failure, they return a negative errno-style error
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code. <function>sd_event_ref()</function> always returns a pointer
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to the event loop object passed
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in. <function>sd_event_unref()</function> always returns
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<constant>NULL</constant>.</para>
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</refsect1>
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<refsect1>
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<title>Errors</title>
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<para>Returned errors may indicate the following problems:</para>
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<variablelist>
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<varlistentry>
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<term><constant>-ENOMEM</constant></term>
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<listitem><para>Not enough memory to allocate the object.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>-EMFILE</constant></term>
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<listitem><para>The maximum number of event loops has been allocated.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>-ENXIO</constant></term>
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<listitem><para><function>sd_event_get_tid()</function> was
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invoked on an event loop object that was not allocated with
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<function>sd_event_default()</function>.</para></listitem>
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</varlistentry>
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</variablelist>
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</refsect1>
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<xi:include href="libsystemd-pkgconfig.xml" />
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<refsect1>
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<title>See Also</title>
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<para>
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd-event</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_event_add_io</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_event_add_time</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_event_add_signal</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_event_add_child</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_event_add_defer</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_event_add_post</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_event_add_exit</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_event_run</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry project='man-pages'><refentrytitle>gettid</refentrytitle><manvolnum>2</manvolnum></citerefentry>
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</para>
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</refsect1>
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</refentry>
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