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9809a788e4
The meaning of the return value, the default handlers, and loop exiting are now described.
312 lines
16 KiB
XML
312 lines
16 KiB
XML
<?xml version='1.0'?>
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
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<refentry id="sd_event_add_io" xmlns:xi="http://www.w3.org/2001/XInclude">
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<refentryinfo>
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<title>sd_event_add_io</title>
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<productname>systemd</productname>
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</refentryinfo>
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<refmeta>
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<refentrytitle>sd_event_add_io</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_add_io</refname>
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<refname>sd_event_source_get_io_events</refname>
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<refname>sd_event_source_set_io_events</refname>
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<refname>sd_event_source_get_io_revents</refname>
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<refname>sd_event_source_get_io_fd</refname>
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<refname>sd_event_source_set_io_fd</refname>
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<refname>sd_event_source_get_io_fd_own</refname>
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<refname>sd_event_source_set_io_fd_own</refname>
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<refname>sd_event_source</refname>
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<refname>sd_event_io_handler_t</refname>
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<refpurpose>Add an I/O event source to an event loop</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_source sd_event_source;</funcsynopsisinfo>
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<funcprototype>
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<funcdef>typedef int (*<function>sd_event_io_handler_t</function>)</funcdef>
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<paramdef>sd_event_source *<parameter>s</parameter></paramdef>
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<paramdef>int <parameter>fd</parameter></paramdef>
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<paramdef>uint32_t <parameter>revents</parameter></paramdef>
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<paramdef>void *<parameter>userdata</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_event_add_io</function></funcdef>
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<paramdef>sd_event *<parameter>event</parameter></paramdef>
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<paramdef>sd_event_source **<parameter>source</parameter></paramdef>
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<paramdef>int <parameter>fd</parameter></paramdef>
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<paramdef>uint32_t <parameter>events</parameter></paramdef>
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<paramdef>sd_event_io_handler_t <parameter>handler</parameter></paramdef>
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<paramdef>void *<parameter>userdata</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_event_source_get_io_events</function></funcdef>
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<paramdef>sd_event_source *<parameter>source</parameter></paramdef>
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<paramdef>uint32_t *<parameter>events</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_event_source_set_io_events</function></funcdef>
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<paramdef>sd_event_source *<parameter>source</parameter></paramdef>
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<paramdef>uint32_t <parameter>events</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_event_source_get_io_revents</function></funcdef>
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<paramdef>sd_event_source *<parameter>source</parameter></paramdef>
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<paramdef>uint32_t *<parameter>revents</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_event_source_get_io_fd</function></funcdef>
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<paramdef>sd_event_source *<parameter>source</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_event_source_set_io_fd</function></funcdef>
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<paramdef>sd_event_source *<parameter>source</parameter></paramdef>
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<paramdef>int <parameter>fd</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_event_source_get_io_fd_own</function></funcdef>
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<paramdef>sd_event_source *<parameter>source</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_event_source_set_io_fd_own</function></funcdef>
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<paramdef>sd_event_source *<parameter>source</parameter></paramdef>
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<paramdef>int <parameter>b</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_add_io()</function> adds a new I/O event
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source to an event loop. The event loop object is specified in the
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<parameter>event</parameter> parameter, the event source object is
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returned in the <parameter>source</parameter> parameter. The
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<parameter>fd</parameter> parameter takes the UNIX file descriptor
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to watch, which may refer to a socket, a FIFO, a message queue, a
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serial connection, a character device, or any other file descriptor
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compatible with Linux
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<citerefentry project='man-pages'><refentrytitle>epoll</refentrytitle><manvolnum>7</manvolnum></citerefentry>. The
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<parameter>events</parameter> parameter takes a bit mask of events
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to watch for, a combination of the following event flags:
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<constant>EPOLLIN</constant>, <constant>EPOLLOUT</constant>,
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<constant>EPOLLRDHUP</constant>, <constant>EPOLLPRI</constant>,
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and <constant>EPOLLET</constant>, see
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<citerefentry project='man-pages'><refentrytitle>epoll_ctl</refentrytitle><manvolnum>2</manvolnum></citerefentry>
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for details.</para>
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<para>The <parameter>handler</parameter> is a function to call when the event source is triggered or
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<constant>NULL</constant>. The <parameter>userdata</parameter> pointer will be passed to the handler
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function, and may be chosen freely by the caller. The handler will also be passed the file descriptor the
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event was seen on, as well as the actual event flags. It's generally a subset of the events watched,
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however may additionally include <constant>EPOLLERR</constant> and <constant>EPOLLHUP</constant>. The
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handler may return negative to signal an error (see below), other return values are ignored. If
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<parameter>handler</parameter> is <constant>NULL</constant>, a default handler that calls
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<citerefentry><refentrytitle>sd_event_exit</refentrytitle><manvolnum>3</manvolnum></citerefentry> will be
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used.</para>
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<para>By default, an event source will stay enabled continuously (<constant>SD_EVENT_ON</constant>), but
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this may be changed with
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<citerefentry><refentrytitle>sd_event_source_set_enabled</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
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If the handler function returns a negative error code, it will either be disabled after the invocation,
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even if the <constant>SD_EVENT_ON</constant> mode was requested before, or it will cause the loop to
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terminate, see
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<citerefentry><refentrytitle>sd_event_source_set_exit_on_failure</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
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Note that an event source set to <constant>SD_EVENT_ON</constant> will fire continuously unless data is
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read from or written to the file descriptor to reset the mask of events seen.</para>
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<para>Setting the I/O event mask to watch for to 0 does not mean
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that the event source won't be triggered anymore, as
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<constant>EPOLLHUP</constant> and <constant>EPOLLERR</constant>
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may be triggered even with a zero event mask. To temporarily
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disable an I/O event source use
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<citerefentry><refentrytitle>sd_event_source_set_enabled</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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with <constant>SD_EVENT_OFF</constant> instead.</para>
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<para>To destroy an event source object use
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<citerefentry><refentrytitle>sd_event_source_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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but note that the event source is only removed from the event loop
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when all references to the event source are dropped. To make sure
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an event source does not fire anymore, even if it is still referenced,
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disable the event source using
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<citerefentry><refentrytitle>sd_event_source_set_enabled</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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with <constant>SD_EVENT_OFF</constant>.</para>
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<para>If the second parameter of
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<function>sd_event_add_io()</function> is
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<constant>NULL</constant> no reference to the event source object
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is returned. In this case the event source is considered
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"floating", and will be destroyed implicitly when the event loop
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itself is destroyed.</para>
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<para>If the <parameter>handler</parameter> to <function>sd_event_add_io()</function> is
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<constant>NULL</constant>, and the event source fires, this will be considered a request to exit the
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event loop. In this case, the <parameter>userdata</parameter> parameter, cast to an integer, is passed as
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the exit code parameter to
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<citerefentry><refentrytitle>sd_event_exit</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para>
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<para>Note that this call does not take possession of the file descriptor passed in, ownership (and thus
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the duty to close it when it is no longer needed) remains with the caller. However, with the
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<function>sd_event_source_set_io_fd_own()</function> call (see below) the event source may optionally
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take ownership of the file descriptor after the event source has been created. In that case the file
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descriptor is closed automatically as soon as the event source is released.</para>
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<para>It is recommended to use
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<function>sd_event_add_io()</function> only in conjunction with
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file descriptors that have <constant>O_NONBLOCK</constant> set, to
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ensure that all I/O operations from invoked handlers are properly
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asynchronous and non-blocking. Using file descriptors without
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<constant>O_NONBLOCK</constant> might result in unexpected
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starvation of other event sources. See
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<citerefentry project='man-pages'><refentrytitle>fcntl</refentrytitle><manvolnum>2</manvolnum></citerefentry>
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for details on enabling <constant>O_NONBLOCK</constant> mode.</para>
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<para><function>sd_event_source_get_io_events()</function> retrieves
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the configured mask of watched I/O events of an event source created
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previously with <function>sd_event_add_io()</function>. It takes
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the event source object and a pointer to a variable to store the
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mask in.</para>
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<para><function>sd_event_source_set_io_events()</function>
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configures the mask of watched I/O events of an event source created
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previously with <function>sd_event_add_io()</function>. It takes the
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event source object and the new event mask.</para>
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<para><function>sd_event_source_get_io_revents()</function>
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retrieves the I/O event mask of currently seen but undispatched
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events from an event source created previously with
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<function>sd_event_add_io()</function>. It takes the event source
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object and a pointer to a variable to store the event mask
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in. When called from a handler function on the handler's event
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source object this will return the same mask as passed to the
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handler's <parameter>revents</parameter> parameter. This call is
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primarily useful to check for undispatched events of an event
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source from the handler of an unrelated (possibly higher priority)
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event source. Note the relation between
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<function>sd_event_source_get_pending()</function> and
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<function>sd_event_source_get_io_revents()</function>: both
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functions will report non-zero results when there's an event
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pending for the event source, but the former applies to all event
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source types, the latter only to I/O event sources.</para>
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<para><function>sd_event_source_get_io_fd()</function> retrieves
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the UNIX file descriptor of an event source created previously
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with <function>sd_event_add_io()</function>. It takes the event
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source object and returns the non-negative file descriptor
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or a negative error number on error (see below).</para>
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<para><function>sd_event_source_set_io_fd()</function>
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changes the UNIX file descriptor of an I/O event source created
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previously with <function>sd_event_add_io()</function>. It takes
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the event source object and the new file descriptor.</para>
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<para><function>sd_event_source_set_io_fd_own()</function> controls whether the file descriptor of the event source
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shall be closed automatically when the event source is freed, i.e. whether it shall be considered 'owned' by the
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event source object. By default it is not closed automatically, and the application has to do this on its own. The
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<parameter>b</parameter> parameter is a boolean parameter: if zero, the file descriptor is not closed automatically
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when the event source is freed, otherwise it is closed.</para>
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<para><function>sd_event_source_get_io_fd_own()</function> may be used to query the current setting of the file
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descriptor ownership boolean flag as set with <function>sd_event_source_set_io_fd_own()</function>. It returns
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positive if the file descriptor is closed automatically when the event source is destroyed, zero if not, and
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negative on error.</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, these functions return 0 or a positive
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integer. On failure, they return a negative errno-style error
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code.</para>
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<refsect2>
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<title>Errors</title>
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<para>Returned values 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 an object.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>-EINVAL</constant></term>
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<listitem><para>An invalid argument has been passed.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>-ESTALE</constant></term>
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<listitem><para>The event loop is already terminated.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>-ECHILD</constant></term>
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<listitem><para>The event loop has been created in a different process.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>-EDOM</constant></term>
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<listitem><para>The passed event source is not an I/O event source.</para></listitem>
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</varlistentry>
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</variablelist>
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</refsect2>
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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_new</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_event_now</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_inotify</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_source_set_enabled</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_event_source_set_priority</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_event_source_set_userdata</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_event_source_set_description</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_event_source_get_pending</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_event_source_set_floating</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry project='man-pages'><refentrytitle>epoll_ctl</refentrytitle><manvolnum>2</manvolnum></citerefentry>,
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<citerefentry project='man-pages'><refentrytitle>epoll</refentrytitle><manvolnum>7</manvolnum></citerefentry>
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</para>
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</refsect1>
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</refentry>
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