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<refentry id="sd_bus_message_get_cookie">

  <refentryinfo>
    <title>sd_bus_message_get_cookie</title>
    <productname>systemd</productname>

    <authorgroup>
      <author>
        <contrib>Developer</contrib>
        <firstname>Lennart</firstname>
        <surname>Poettering</surname>
        <email>lennart@poettering.net</email>
      </author>
    </authorgroup>
  </refentryinfo>

  <refmeta>
    <refentrytitle>sd_bus_message_get_cookie</refentrytitle>
    <manvolnum>3</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>sd_bus_message_get_cookie</refname>
    <refname>sd_bus_message_get_reply_cookie</refname>
    <refpurpose>Returns the transaction cookie of a message</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <funcsynopsis>
      <funcsynopsisinfo>#include &lt;systemd/sd-bus.h&gt;</funcsynopsisinfo>

      <funcprototype>
        <funcdef>int <function>sd_bus_message_get_cookie</function></funcdef>
        <paramdef>sd_bus_message *<parameter>message</parameter></paramdef>
        <paramdef>uint64_t *<parameter>cookie</parameter></paramdef>
      </funcprototype>

      <funcprototype>
        <funcdef>int <function>sd_bus_message_get_reply_cookie</function></funcdef>
        <paramdef>sd_bus_message *<parameter>message</parameter></paramdef>
        <paramdef>uint64_t *<parameter>cookie</parameter></paramdef>
      </funcprototype>
    </funcsynopsis>
  </refsynopsisdiv>

  <refsect1>
    <title>Description</title>

    <para><function>sd_bus_message_get_cookie()</function> returns the
    transaction cookie of a message. The cookie uniquely identifies a
    message within each bus peer, but is not globally unique. It is
    assigned when a message is sent.</para>

    <para><function>sd_bus_message_get_reply_cookie()</function>
    returns the transaction cookie of the message the specified
    message is a response to. When a reply message is generated for a
    method call message, its cookie is copied over into this field.
    Note that while every message that is transferred is identified by
    a cookie, only response messages carry a reply cookie
    field.</para>

    <para>Both functions take a message object as first parameter and
    a place to store the 64-bit cookie in.</para>
  </refsect1>

  <refsect1>
    <title>Return Value</title>

    <para>On success, these calls return 0 or a positive integer. On
    failure, these calls return a negative errno-style error
    code.</para>

    <para>On success, the cookie/reply cookie is returned in the
    specified 64-bit unsigned integer variable.</para>
  </refsect1>

  <refsect1>
    <title>Errors</title>

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

    <variablelist>
      <varlistentry>
        <term><constant>-EINVAL</constant></term>

        <listitem><para>A specified parameter
        is invalid.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><constant>-ENODATA</constant></term>

        <listitem><para>No cookie has been assigned to this message.
        This either indicates that the message has not been sent yet
        and hence has no cookie assigned, or that the message is not a
        method response message and hence carries a reply cookie
        field.</para></listitem>
      </varlistentry>
    </variablelist>
  </refsect1>

  <refsect1>
    <title>Notes</title>

    <para>The <function>sd_bus_message_get_cookie()</function> and
    <function>sd_bus_message_get_reply_cookie()</function> interfaces
    are available as a shared library, which can be compiled and
    linked to with the
    <constant>libsystemd</constant> <citerefentry project='die-net'><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>
    file.</para>
  </refsect1>

  <refsect1>
    <title>See Also</title>

    <para>
      <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd-bus</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_bus_new</refentrytitle><manvolnum>3</manvolnum></citerefentry>
    </para>
  </refsect1>

</refentry>
