man/sd_event_add_signal.xml

<?xml version='1.0'?>
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
  "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->

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

  <refentryinfo>
    <title>sd_event_add_signal</title>
    <productname>systemd</productname>
  </refentryinfo>

  <refmeta>
    <refentrytitle>sd_event_add_signal</refentrytitle>
    <manvolnum>3</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>sd_event_add_signal</refname>
    <refname>sd_event_source_get_signal</refname>
    <refname>sd_event_signal_handler_t</refname>
    <refname>SD_EVENT_SIGNAL_PROCMASK</refname>

    <refpurpose>Add a UNIX process signal 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>

      <funcsynopsisinfo><constant>SD_EVENT_SIGNAL_PROCMASK</constant></funcsynopsisinfo>

      <funcprototype>
        <funcdef>typedef int (*<function>sd_event_signal_handler_t</function>)</funcdef>
        <paramdef>sd_event_source *<parameter>s</parameter></paramdef>
        <paramdef>const struct signalfd_siginfo *<parameter>si</parameter></paramdef>
        <paramdef>void *<parameter>userdata</parameter></paramdef>
      </funcprototype>

      <funcprototype>
        <funcdef>int <function>sd_event_add_signal</function></funcdef>
        <paramdef>sd_event *<parameter>event</parameter></paramdef>
        <paramdef>sd_event_source **<parameter>source</parameter></paramdef>
        <paramdef>int <parameter>signal</parameter></paramdef>
        <paramdef>sd_event_signal_handler_t <parameter>handler</parameter></paramdef>
        <paramdef>void *<parameter>userdata</parameter></paramdef>
      </funcprototype>

      <funcprototype>
        <funcdef>int <function>sd_event_source_get_signal</function></funcdef>
        <paramdef>sd_event_source *<parameter>source</parameter></paramdef>
      </funcprototype>
    </funcsynopsis>
  </refsynopsisdiv>

  <refsect1>
    <title>Description</title>

    <para><function>sd_event_add_signal()</function> adds a new UNIX process signal event source to an event
    loop. The event loop object is specified in the <parameter>event</parameter> parameter, and the event
    source object is returned in the <parameter>source</parameter> parameter. The
    <parameter>signal</parameter> parameter specifies the numeric signal to be handled (see <citerefentry
    project='man-pages'><refentrytitle>signal</refentrytitle><manvolnum>7</manvolnum></citerefentry>).</para>

    <para>The <parameter>handler</parameter> parameter is a function to call when the signal is received or
    <constant>NULL</constant>. The handler function will be passed the <parameter>userdata</parameter>
    pointer, which may be chosen freely by the caller. The handler also receives a pointer to a
    <structname>signalfd_siginfo</structname> structure containing information about the received signal. See
    <citerefentry
    project='man-pages'><refentrytitle>signalfd</refentrytitle><manvolnum>2</manvolnum></citerefentry> for
    further information. The handler may return negative to signal an error (see below), other return values
    are ignored. If <parameter>handler</parameter> is <constant>NULL</constant>, a default handler that calls
    <citerefentry><refentrytitle>sd_event_exit</refentrytitle><manvolnum>3</manvolnum></citerefentry> will be
    used.</para>

    <para>Only a single handler may be installed for a specific signal. The signal must be blocked in all
    threads before this function is called (using <citerefentry
    project='man-pages'><refentrytitle>sigprocmask</refentrytitle><manvolnum>2</manvolnum></citerefentry> or
    <citerefentry
    project='man-pages'><refentrytitle>pthread_sigmask</refentrytitle><manvolnum>3</manvolnum></citerefentry>). For
    convenience, if the special flag <constant>SD_EVENT_SIGNAL_PROCMASK</constant> is ORed into the specified
    signal the signal will be automatically masked as necessary, for the calling thread. Note that this only
    works reliably if the signal is already masked in all other threads of the process, or if there are no
    other threads at the moment of invocation.</para>

    <para>By default, the event source is enabled permanently (<constant>SD_EVENT_ON</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 either be disabled after the invocation,
    even if the <constant>SD_EVENT_ON</constant> mode was requested before, or it will cause the loop to
    terminate, see
    <citerefentry><refentrytitle>sd_event_source_set_exit_on_failure</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
    </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_signal()</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> parameter to <function>sd_event_add_signal()</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 passed as
    the exit code parameter to
    <citerefentry><refentrytitle>sd_event_exit</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para>

    <para><function>sd_event_source_get_signal()</function> returns
    the configured signal number of an event source created previously
    with <function>sd_event_add_signal()</function>. It takes the
    event source object as the <parameter>source</parameter>
    parameter.</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>

    <refsect2>
      <title>Errors</title>

      <para>Returned errors 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>-EBUSY</constant></term>

          <listitem><para>A handler is already installed for this
          signal or the signal was not blocked previously.</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>-EDOM</constant></term>

          <listitem><para>The passed event source is not a signal event source.</para></listitem>
        </varlistentry>

      </variablelist>
    </refsect2>
  </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_time</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_description</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_event_source_set_userdata</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_event_source_set_floating</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry project='man-pages'><refentrytitle>signal</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
      <citerefentry project='man-pages'><refentrytitle>signalfd</refentrytitle><manvolnum>2</manvolnum></citerefentry>,
      <citerefentry project='man-pages'><refentrytitle>sigprocmask</refentrytitle><manvolnum>2</manvolnum></citerefentry>,
      <citerefentry project='man-pages'><refentrytitle>pthread_sigmask</refentrytitle><manvolnum>3</manvolnum></citerefentry>
    </para>
  </refsect1>

</refentry>