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<refentry id="sd_bus_negotiate_fds">

  <refentryinfo>
    <title>sd_bus_negotiate_fds</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_negotiate_fds</refentrytitle>
    <manvolnum>3</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>sd_bus_negotiate_fds</refname>
    <refname>sd_bus_negotiate_timestamp</refname>
    <refname>sd_bus_negotiate_creds</refname>

    <refpurpose>Control feature negotiation on bus connections</refpurpose>
  </refnamediv>

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

      <funcprototype>
        <funcdef>int <function>sd_bus_negotiate_fds</function></funcdef>
        <paramdef>sd_bus *<parameter>bus</parameter></paramdef>
        <paramdef>int <parameter>b</parameter></paramdef>
      </funcprototype>

      <funcprototype>
        <funcdef>int <function>sd_bus_negotiate_timestamp</function></funcdef>
        <paramdef>sd_bus *<parameter>bus</parameter></paramdef>
        <paramdef>int <parameter>b</parameter></paramdef>
      </funcprototype>

      <funcprototype>
        <funcdef>int <function>sd_bus_negotiate_creds</function></funcdef>
        <paramdef>sd_bus *<parameter>bus</parameter></paramdef>
        <paramdef>int <parameter>b</parameter></paramdef>
        <paramdef>uint64_t <parameter>mask</parameter></paramdef>
      </funcprototype>
    </funcsynopsis>
  </refsynopsisdiv>

  <refsect1>
    <title>Description</title>

    <para><function>sd_bus_negotiate_fds()</function> controls whether
    file descriptor passing shall be negotiated for the specified bus
    connection. It takes a bus object and a boolean, which, when true,
    enables file descriptor passing, and, when false, disables
    it. Note that not all transports and servers support file
    descriptor passing. In particular, networked transports generally
    do not support file descriptor passing. To find out whether file
    descriptor passing is available after negotiation, use
    <citerefentry><refentrytitle>sd_bus_can_send</refentrytitle><manvolnum>3</manvolnum></citerefentry>
    and pass <constant>SD_BUS_TYPE_UNIX_FD</constant>. Note that file
    descriptor passing is always enabled for both sending and
    receiving or for neither, but never only in one direction. By
    default, file descriptor passing is negotiated for all
    connections.</para>

    <para>Note that when bus activation is used, it is highly
    recommended to set the <option>AcceptFileDescriptors=</option>
    setting in the <filename>.busname</filename> unit file to the same
    setting as negotiated by the program ultimately activated. By
    default, file descriptor passing is enabled for both.</para>

    <para><function>sd_bus_negotiate_timestamps()</function> controls
    whether implicit sender timestamps shall be attached automatically
    to all incoming messages. Takes a bus object and a boolean, which,
    when true, enables timestamping, and, when false, disables it.
    Use
    <citerefentry><refentrytitle>sd_bus_message_get_monotonic_usec</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
    <citerefentry><refentrytitle>sd_bus_message_get_realtime_usec</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
    <citerefentry><refentrytitle>sd_bus_message_get_seqnum</refentrytitle><manvolnum>3</manvolnum></citerefentry>
    to query the timestamps of incoming messages. If negotiation is
    disabled or not supported, these calls will fail with
    <constant>-ENODATA</constant>. Note that not all transports
    support timestamping of messages. Specifically, timestamping is
    only available on the kdbus transport, but not on dbus1. The
    timestamping is applied by the kernel and cannot be manipulated by
    userspace. By default, message timestamping is not negotiated for
    connections.</para>

    <para><function>sd_bus_negotiate_creds()</function> controls
    whether and which implicit sender credentials shall be attached
    automatically to all incoming messages. Takes a bus object and a
    boolean indicating whether to enable or disable the credential
    parts encoded in the bit mask value argument. Note that not all
    transports support attaching sender credentials to messages, or do
    not support all types of sender credential parameters, or might
    suppress them under certain circumstances for individual
    messages. Specifically, implicit sender credentials on messages
    are only fully supported on kdbus transports, and dbus1 only
    supports <constant>SD_BUS_CREDS_UNIQUE_NAME</constant>. The sender
    credentials are attached by the kernel and cannot be manipulated
    by userspace, and are thus suitable for authorization
    decisions. By default, only
    <constant>SD_BUS_CREDS_WELL_KNOWN_NAMES</constant> and
    <constant>SD_BUS_CREDS_UNIQUE_NAME</constant> are enabled. In
    fact, these two credential fields are always sent along and cannot
    be turned off.</para>

    <para>The <function>sd_bus_negotiate_fds()</function> function may
    be called only before the connection has been started with
    <citerefentry><refentrytitle>sd_bus_start</refentrytitle><manvolnum>3</manvolnum></citerefentry>. Both
    <function>sd_bus_negotiate_timestamp()</function> and
    <function>sd_bus_negotiate_creds()</function> may also be called
    after a connection has been set up. Note that, when operating on a
    connection that is shared between multiple components of the same
    program (for example via
    <citerefentry><refentrytitle>sd_bus_default</refentrytitle><manvolnum>3</manvolnum></citerefentry>),
    it is highly recommended to only enable additional per message
    metadata fields, but never disable them again, in order not to
    disable functionality needed by other components.</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>
  </refsect1>

  <refsect1>
    <title>Errors</title>

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

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

        <listitem><para>The bus connection has already been started.</para></listitem>
      </varlistentry>
    </variablelist>
  </refsect1>

  <refsect1>
    <title>Notes</title>

    <para><function>sd_bus_negotiate_fds()</function> and the other
    functions described here 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_start</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_bus_message_can_send</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_bus_message_get_monotonic_usec</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_bus_message_get_realtime_usec</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_bus_message_get_seqnum</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_bus_message_get_creds</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd.busname</refentrytitle><manvolnum>5</manvolnum></citerefentry>
    </para>
  </refsect1>

</refentry>
