path: root/doc/kdbus.message.xml

<?xml version='1.0'?> <!--*-nxml-*-->
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
        "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">

<refentry id="kdbus.message">

  <refentryinfo>
    <title>kdbus.message</title>
    <productname>kdbus.message</productname>
  </refentryinfo>

  <refmeta>
    <refentrytitle>kdbus.message</refentrytitle>
    <manvolnum>7</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>kdbus.message</refname>
    <refpurpose>kdbus message</refpurpose>
  </refnamediv>

  <refsect1>
    <title>Description</title>

    <para>
      Messages are passed to the kernel with the <varname>KDBUS_CMD_SEND</varname>
      ioctl. Depending on the destination address of the message, the kernel
      delivers the message to the specific destination connection, or to all
      connections on the same bus.
      Sending messages across buses is not possible. Messages are always queued in
      the memory pool of the destination connection (see below).
    </para>

    <para>
      The <varname>KDBUS_CMD_SEND</varname> ioctl uses a
      <varname>struct kdbus_cmd_send</varname> to describe the message
      transfer.
    </para>
    <programlisting>
struct kdbus_cmd_send {
  __u64 size;
  __u64 flags;
  __u64 kernel_flags;
  __u64 kernel_msg_flags;
  __u64 return_flags;
  __u64 msg_address;
  struct kdbus_msg_info reply;
  struct kdbus_item items[0];
};
    </programlisting>

    <para>The fields in this struct are described below.</para>

    <variablelist>
      <varlistentry>
        <term><varname>size</varname></term>
        <listitem><para>
          The overall size of the struct, including its items.
        </para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>flags</varname></term>
        <listitem><para>Flags for message delivery</para>
          <variablelist>
            <varlistentry>
              <term><varname>KDBUS_SEND_SYNC_REPLY</varname></term>
              <listitem>
                <para>
                  By default, all calls to kdbus are considered asynchronous,
                  non-blocking. However, as there are many use cases that need to wait
                  for a remote peer to answer a method call, there's a way to send a
                  message and wait for a reply in a synchronous fashion. This is what
                  the KDBUS_MSG_SYNC_REPLY controls. The KDBUS_CMD_SEND ioctl will block
                  until the reply has arrived, the timeout limit is reached, in case the
                  remote connection was shut down, or if interrupted by a signal before
                  any reply; see signal(7).

                  The offset of the reply message in the sender's pool is stored in in
                  'offset_reply' when the ioctl has returned without error. Hence, there
                  is no need for another KDBUS_CMD_RECV ioctl or anything else to receive
                  the reply.
                </para>
              </listitem>
            </varlistentry>
          </variablelist>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>kernel_flags</varname></term>
        <listitem><para>
          Valid flags for this command, returned by the kernel upon each call.
        </para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>return_flags</varname></term>
        <listitem><para>
          Flags returned by the kernel. Currently unused and always set to
          zero by the kernel.
        </para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>msg_address</varname></term>
        <listitem><para>
          In this field, userspace has to provide a pointer to a message
          (<varname>struct kdbus_msg</varname>) to send. See below for a
          detailed description.
        </para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>reply</varname></term>
        <listitem><para>
          Only used for synchronous replies. See description of
          <varname>struct kdbus_cmd_recv</varname> for more details.
        </para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>items</varname></term>
        <listitem>
          <para>
            The following items are currently recognized.
          </para>
          <variablelist>
            <varlistentry>
              <term><varname>KDBUS_ITEM_CANCEL_FD</varname></term>
              <listitem>
                <para>
                  When this optional item is passed in, and the call is
                  executed as SYNC call, the passed in file descriptor can be
                  used as alternative cancellation point. The kernel will call
                  poll()
                  <citerefentry><refentrytitle>poll</refentrytitle><manvolnum>2</manvolnum></citerefentry>
                  on this file descriptor, and if it reports any incoming
                  bytes, the blocking send operation will be canceled, and the
                  call will return <varname>-ECANCELED</varname>. Any type of
                  file descriptor on which
                  <citerefentry><refentrytitle>poll</refentrytitle><manvolnum>2</manvolnum></citerefentry>
                  can be called on can be used as payload to this item.
                  For asynchronous message sending, this item is allowed but
                  ignored.
                </para>
              </listitem>
            </varlistentry>
          </variablelist>
          <para>
            All other items are rejected, and the ioctl will fail with -EINVAL.
          </para>
        </listitem>
      </varlistentry>
    </variablelist>

    <para>
      The fields in this struct are described below.
      The message referenced the <varname>msg_address</varname> above has
      the following layout.
    </para>

    <programlisting>
struct kdbus_msg {
  __u64 size;
  __u64 flags;
  __s64 priority;
  __u64 dst_id;
  __u64 src_id;
  __u64 payload_type;
  __u64 cookie;
  __u64 timeout_ns;
  __u64 cookie_reply;
  struct kdbus_item items[0];
};
    </programlisting>

    <para>The fields in this struct are described below.</para>

    <variablelist>
      <varlistentry>
        <term><varname>size</varname></term>
        <listitem><para>
          The overall size of the struct, including its items.
        </para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>flags</varname></term>
        <listitem><para>Flags to describe message details.</para>
          <variablelist>
            <varlistentry>
              <term><varname>KDBUS_MSG_EXPECT_REPLY</varname></term>
              <listitem>
                <para>
                  Expect a reply to this message from the remote peer. With this bit set,
                  the timeout_ns field must be set to a non-zero number of nanoseconds in
                  which the receiving peer is expected to reply. If such a reply is not
                  received in time, the sender will be notified with a timeout message
                  (see below). The value must be an absolute value, in nanoseconds and
                  based on <varname>CLOCK_MONOTONIC</varname>.
                </para><para>
                  For a message to be accepted as reply, it must be a direct message to
                  the original sender (not a broadcast), and its kdbus_msg.reply_cookie
                  must match the previous message's kdbus_msg.cookie.
                </para><para>
                  Expected replies also temporarily open the policy of the sending
                  connection, so the other peer is allowed to respond within the given
                  time window.
                </para>
              </listitem>
            </varlistentry>

            <varlistentry>
              <term><varname>KDBUS_MSG_NO_AUTO_START</varname></term>
              <listitem>
                <para>
                  By default, when a message is sent to an activator connection, the
                  activator is notified and will start an implementer. This flag inhibits
                  that behavior. With this bit set, and the remote being an activator,
                  <varname>-EADDRNOTAVAIL</varname> is returned from the ioctl.
                </para>
              </listitem>
            </varlistentry>
          </variablelist>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>priority</varname></term>
        <listitem><para>
          The priority of this message. Receiving messages (see below) may
          optionally be constrained to messages of a minimal priority. This
          allows for use cases where timing critical data is interleaved with
          control data on the same connection. If unused, the priority field
          should be set to zero.
        </para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>dst_id</varname></term>
        <listitem><para>
          The numeric ID of the destination connection, or
          <varname>KDBUS_DST_ID_BROADCAST</varname>
          (~0ULL) to address every peer on the bus, or
          <varname>KDBUS_DST_ID_NAME</varname> (0) to look
          it up dynamically from the bus' name registry.
          In the latter case, an item of type
          <varname>KDBUS_ITEM_DST_NAME</varname> is mandatory.
        </para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>src_id</varname></term>
        <listitem><para>
          Upon return of the ioctl, this member will contain the sending
          connection's numerical ID. Should be 0 at send time.
        </para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>payload_type</varname></term>
        <listitem><para>
          Type of the payload in the actual data records. Currently, only
          <varname>KDBUS_PAYLOAD_DBUS</varname> is accepted as input value
          of this field. When receiving messages that are generated by the
          kernel (notifications), this field will contain
          <varname>KDBUS_PAYLOAD_KERNEL</varname>.
        </para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>cookie</varname></term>
        <listitem><para>
          Cookie of this message, for later recognition. Also, when replying
          to a message (see above), the <varname>cookie_reply</varname>
          field must match this value.
        </para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>timeout_ns</varname></term>
        <listitem><para>
          If the message sent requires a reply from the remote peer (see above),
          this field contains the timeout in absolute nanoseconds based on
          <varname>CLOCK_MONOTONIC</varname>. Also see
          <citerefentry><refentrytitle>clock_gettime</refentrytitle><manvolnum>2</manvolnum></citerefentry>.
        </para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>cookie_reply</varname></term>
        <listitem><para>
          If the message sent is a reply to another message, this field must
          match the cookie of the formerly received message.
        </para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>items</varname></term>
        <listitem>
          <para>
            A dynamically sized list of items to contain additional information.
            The following items are expected/valid:
          </para>
          <variablelist>
            <varlistentry>
              <term><varname>KDBUS_ITEM_PAYLOAD_VEC</varname></term>
              <term><varname>KDBUS_ITEM_PAYLOAD_MEMFD</varname></term>
              <term><varname>KDBUS_ITEM_FDS</varname></term>
              <listitem>
                <para>
                  Actual data records containing the payload. See section
                  "Passing of Payload Data".
                </para>
              </listitem>
            </varlistentry>

            <varlistentry>
              <term><varname>KDBUS_ITEM_BLOOM_FILTER</varname></term>
              <listitem>
                <para>
                  Bloom filter for matches (see below).
                </para>
              </listitem>
            </varlistentry>

            <varlistentry>
              <term><varname>KDBUS_ITEM_DST_NAME</varname></term>
              <listitem>
                <para>
                  Well-known name to send this message to. Required if
                  <varname>dst_id</varname> is set to
                  <varname>KDBUS_DST_ID_NAME</varname>.
                  If a connection holding the given name can't be found,
                  <varname>-ESRCH</varname> is returned.
                </para>
                <para>
                  For messages to a unique name (ID), this item is optional. If present,
                  the kernel will make sure the name owner matches the given unique name.
                  This allows userspace tie the message sending to the condition that a
                  name is currently owned by a certain unique name.
                </para>
              </listitem>
            </varlistentry>
          </variablelist>
        </listitem>
      </varlistentry>
    </variablelist>

    <para>
      The message will be augmented by the requested metadata items when
      queued into the receiver's pool. See
      <citerefentry><refentrytitle>kdbus.connection</refentrytitle><manvolnum>7</manvolnum></citerefentry> and
      <citerefentry><refentrytitle>kdbus.item</refentrytitle><manvolnum>7</manvolnum></citerefentry>
      for more information on metadata.
    </para>
  </refsect1>

  <refsect1>
    <title>Message layout</title>

    <para>The layout of a message is shown below.</para>

    <programlisting>
  +-------------------------------------------------------------------------+
  | Message                                                                 |
  | +---------------------------------------------------------------------+ |
  | | Header                                                              | |
  | | size:          overall message size, including the data records     | |
  | | destination:   connection ID of the receiver                        | |
  | | source:        connection ID of the sender (set by kernel)          | |
  | | payload_type:  "DBusDBus" textual identifier stored as uint64_t     | |
  | +---------------------------------------------------------------------+ |
  | +---------------------------------------------------------------------+ |
  | | Data Record                                                         | |
  | | size:  overall record size (without padding)                        | |
  | | type:  type of data                                                 | |
  | | data:  reference to data (address or file descriptor)               | |
  | +---------------------------------------------------------------------+ |
  | +---------------------------------------------------------------------+ |
  | | padding bytes to the next 8 byte alignment                          | |
  | +---------------------------------------------------------------------+ |
  | +---------------------------------------------------------------------+ |
  | | Data Record                                                         | |
  | | size:  overall record size (without padding)                        | |
  | | ...                                                                 | |
  | +---------------------------------------------------------------------+ |
  | +---------------------------------------------------------------------+ |
  | | padding bytes to the next 8 byte alignment                          | |
  | +---------------------------------------------------------------------+ |
  | +---------------------------------------------------------------------+ |
  | | Data Record                                                         | |
  | | size:  overall record size                                          | |
  | | ...                                                                 | |
  | +---------------------------------------------------------------------+ |
  |   ... further data records ...                                          |
  +-------------------------------------------------------------------------+
    </programlisting>
  </refsect1>

  <refsect1>
    <title>Passing of Payload Data</title>

    <para>
      When connecting to the bus, receivers request a memory pool of a given size,
      large enough to carry all backlog of data enqueued for the connection. The
      pool is internally backed by a shared memory file which can be mmap()ed by
      the receiver.
    </para>
    <para>
      See
      <citerefentry><refentrytitle>kdbus.pool</refentrytitle><manvolnum>7</manvolnum></citerefentry>
      for more information.
    </para>

    <para>
      The following items are used to carry the actual data payload of a message.
    </para>

    <variablelist>
      <varlistentry>
        <term><varname>KDBUS_MSG_PAYLOAD_VEC</varname></term>
        <listitem>
          <para>
            Messages are directly copied by the sending process into the receiver's pool.
            This way, two peers can exchange data by effectively doing a single-copy from
            one process to another; the kernel will not buffer the data anywhere else.
          </para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>KDBUS_MSG_PAYLOAD_MEMFD</varname></term>
        <listitem>
          <para>
            Messages can reference <emphasis>memfd</emphasis> files which contain the data.
            memfd files are tmpfs-backed files that allow sealing of the content of the
            file, which prevents all writable access to the file content.
          </para>
          <para>
            Only memfds that have
            <varname>(F_SEAL_SHRINK|F_SEAL_GROW|F_SEAL_WRITE|F_SEAL_SEAL)</varname>
            set are accepted as payload data, which enforces reliable passing of data.
            The receiver can assume that neither the sender nor anyone else can alter the
            content after the message is sent.
          </para>
          <para>
            Apart from the sender filling-in the content into memfd files, the data will
            be passed as zero-copy from one process to another, read-only, shared between
            the peers.
          </para>
        </listitem>
      </varlistentry>
    </variablelist>

    <para>
      The sender must not make any assumptions on the type in which data is received
      by the remote peer. The kernel is free to re-pack multiple VEC and MEMFD
      payloads. For instance, the kernel may decide to merge multiple VECs into a
      single VEC, inline MEMFD payloads into memory or merge all passed VECs into a
      single MEMFD. However, the kernel preserves the order of passed data. This
      means that the order of all VEC and MEMFD items is not changed in respect to
      each other.
    </para>
    <para>
      In other words: All passed VEC and MEMFD data payloads are treated as a single
      stream of data that may be received by the remote peer in a different set of
      hunks than it was sent as.
    </para>
  </refsect1>

  <refsect1>
    <title>Receiving messages</title>

    <para>
      Messages are received by the client with the KDBUS_CMD_RECV ioctl.
      The endpoint file of the bus supports <emphasis>poll/epoll/select</emphasis>
      to wake up the receiving process when new messages are queued up to be received.
    </para>

    <para>
      With the KDBUS_CMD_RECV ioctl, a <varname>struct kdbus_cmd_recv</varname> is used.
    </para>

    <programlisting>
struct kdbus_cmd_recv {
  __u64 size;
  __u64 flags;
  __u64 kernel_flags;
  __u64 return_flags;
  __s64 priority;
  __u64 dropped_msgs;
  struct kdbus_msg_info msg;
  struct kdbus_item items[0];
};
    </programlisting>

    <para>The fields in this struct are described below.</para>

    <variablelist>
      <varlistentry>
        <term><varname>size</varname></term>
        <listitem><para>
          The overall size of the struct, including its items.
        </para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>flags</varname></term>
        <listitem><para>Flags to control the receive command.</para>
          <variablelist>
            <varlistentry>
              <term><varname>KDBUS_RECV_PEEK</varname></term>
              <listitem>
                <para>
                  Just return the location of the next message. Do not install 
                  file descriptors or anything else. This is usually used to
                  determine the sender of the next queued message.
                </para>
              </listitem>
            </varlistentry>

            <varlistentry>
              <term><varname>KDBUS_RECV_DROP</varname></term>
              <listitem>
                <para>
                  Drop the next message without doing anything else with it,
                  and free the pool slice. This a short-cut for
                  <varname>KDBUS_RECV_PEEK</varname> and
                  <varname>KDBUS_CMD_FREE</varname>.
                </para>
              </listitem>
            </varlistentry>

            <varlistentry>
              <term><varname>KDBUS_RECV_USE_PRIORITY</varname></term>
              <listitem>
                <para>
                  Dequeue the messages ordered by their priority, and filtering
                  them with the priority field (see below).
                </para>
              </listitem>
            </varlistentry>
          </variablelist>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>kernel_flags</varname></term>
        <listitem><para>
          Valid flags for this command, returned by the kernel upon each call.
        </para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>return_flags</varname></term>
        <listitem><para>
          Flags returned by the kernel. Currently unused and always set to
          zero by the kernel.
        </para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>priority</varname></term>
        <listitem><para>
          With <varname>KDBUS_RECV_USE_PRIORITY</varname> set in
          <varname>flags</varname>, messages will be dequeued ordered by their
          priority, starting with the highest value. Also, messages will be
          filtered by the value given in this field, so the returned message
          will at least have the requested priority. If no such message is
          waiting in the queue, <varname>-ENOMSG</varname> is returned.
        </para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>dropped_msgs</varname></term>
        <listitem><para>
          If the <varname>CMD_RECV</varname> ioctl fails with 
          <varname>-EOVERFLOW</varname>, this field is filled by
          the kernel with the number of messages that couldn't be transmitted
          to this receiving connection. In that case, the
          <varname>offset</varname> member must not be accessed.
        </para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>msg</varname></term>
        <listitem><para>
          Embedded struct containing information on the received message when
          this command succeeded (see below).
        </para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>items</varname></term>
        <listitem><para>
          Items to specify further details for the receive command.
          Currently unused, and all items will be rejected with
          <varname>-EINVAL</varname>,
        </para></listitem>
      </varlistentry>
    </variablelist>

    <para>
      Both <varname>struct kdbus_cmd_recv</varname> and 
      <varname>struct kdbus_cmd_send</varname> embed 
      <varname>struct kdbus_msg_info</varname>.
      For the <varname>KDBUS_CMD_SEND</varname> ioctl, it is used to catch
      synchronous replies, if one was requested, and is unused otherwise.
    </para>

    <programlisting>
struct kdbus_msg_info {
  __u64 offset;
  __u64 msg_size;
  __u64 return_flags;
};
    </programlisting>

    <para>The fields in this struct are described below.</para>

    <variablelist>
      <varlistentry>
        <term><varname>offset</varname></term>
        <listitem><para>
          Upon return of the ioctl, this field contains the offset in the receiver's
          memory pool. The memory must be freed with
          <varname>KDBUS_CMD_FREE</varname>. See
          <citerefentry><refentrytitle>kdbus.pool</refentrytitle><manvolnum>7</manvolnum></citerefentry>
          for further details.
        </para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>msg_size</varname></term>
        <listitem><para>
          Upon successful return of the ioctl, this field contains the size of the
          allocated slice at offset <varname>offset</varname>.
          It is the combination of the size of the stored
          <varname>struct kdbus_msg</varname> object plus all appended VECs.
          You can use it in combination with <varname>offset</varname> to map
          a single message, instead of mapping the entire pool. See
          <citerefentry><refentrytitle>kdbus.pool</refentrytitle><manvolnum>7</manvolnum></citerefentry>
          for further details.
        </para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>return_flags</varname></term>
        <listitem>
          <para>
            Kernel-provided return flags. Currently, the following flags are defined.
          </para>
          <variablelist>
            <varlistentry>
              <term><varname>KDBUS_RECV_RETURN_INCOMPLETE_FDS</varname></term>
              <listitem>
                <para>
                  The message contained file descriptors which couldn't be installed
                  into the receiver's task. Most probably that happened because the
                  maximum number of file descriptors for that task were exceeded.
                  The message is still delivered, so this is not a fatal condition.
                  File descriptors inside the <varname>KDBUS_ITEM_FDS</varname>
                  item that could not be installed will be set to -1.
                </para>
              </listitem>
            </varlistentry>
          </variablelist>
        </listitem>
      </varlistentry>
    </variablelist>

    <para>
      Unless <varname>KDBUS_RECV_DROP</varname> was passed after a successful+
      <varname>KDBUS_CMD_RECV</varname> ioctl, the <varname>offset</varname>
      field contains the location of the new message inside the receiver's
      pool. The message is stored as <varname>struct kdbus_msg</varname> at
      this offset, and can be interpreted with the semantics described above.
    </para>
    <para>
      Also, if the connection allowed for file descriptor to be passed
      (<varname>KDBUS_HELLO_ACCEPT_FD</varname>), and if the message contained
      any, they will be installed into the receiving process when the
      <varname>KDBUS_CMD_RECV</varname> ioctl is called.
      The receiving task is obliged to close all of them appropriately. If
      <varname>KDBUS_RECV_PEEK</varname> is set, no file descriptors are
      installed. This allows for peeking at a message and dropping it via
      <varname>KDBUS_RECV_DROP</varname>, without installing the passed file
      descriptors into the receiving process.
    </para>
    <para>
      The caller is obliged to call the <varname>KDBUS_CMD_FREE</varname>
      ioctl with the returned offset when the memory is no longer needed.
    </para>
  </refsect1>

  <refsect1>
    <title>Notifications</title>
    <para>
      A kernel notification is a regular kdbus message with the following
      details.
    </para>

    <itemizedlist mark='opencircle'>
      <listitem><para>kdbus_msg.src_id == <varname>KDBUS_SRC_ID_KERNEL</varname></para></listitem>
      <listitem><para>kdbus_msg.dst_id == <varname>KDBUS_DST_ID_BROADCAST</varname></para></listitem>
      <listitem><para>kdbus_msg.payload_type == <varname>KDBUS_PAYLOAD_KERNEL</varname></para></listitem>
      <listitem><para>Has exactly one of the aforementioned items attached</para></listitem>
    </itemizedlist>

    <para>
      Kernel notifications have an item of type KDBUS_ITEM_TIMESTAMP attached.
    </para>

    <para>
      The kernel will notify its users of the following events.
    </para>

    <itemizedlist mark='opencircle'>
      <listitem><para>
        When connection A is terminated while connection B is waiting for a
        reply from it, connection B is notified with a message with an item of
        type <varname>KDBUS_ITEM_REPLY_DEAD</varname>.
      </para></listitem>

      <listitem><para>
        When connection A does not receive a reply from connection B within the
        specified timeout window, connection A will receive a message with an
        item of type <varname>KDBUS_ITEM_REPLY_TIMEOUT</varname>.
      </para></listitem>

      <listitem><para>
        When an ordinary connection (not a monitor) is created on or removed
        from a bus, messages with an item of type
        <varname>KDBUS_ITEM_ID_ADD</varname> or
        <varname>KDBUS_ITEM_ID_REMOVE</varname>, respectively, are delivered
        to all bus members that match these messages through their match
        database. Eavesdroppers (monitor connections) do not cause such
        notifications to be sent. They are invisible on the bus.
      </para></listitem>

      <listitem><para>
        When a connection gains or loses ownership of a name, messages with an
        item of type <varname>KDBUS_ITEM_NAME_ADD</varname>,
        <varname>KDBUS_ITEM_NAME_REMOVE</varname> or
        <varname>KDBUS_ITEM_NAME_CHANGE</varname> are delivered to all bus
        members that match these messages through their match database.
      </para></listitem>
    </itemizedlist>
  </refsect1>

  <refsect1>
    <title>Return value</title>
    <para>
      On success, all metioned ioctl commands return 0.
    </para>

    <refsect2>
      <title><varname>KDBUS_CMD_SEND</varname> may return the following errors</title>

      <variablelist>
        <varlistentry>
          <term><varname>-EOPNOTSUPP</varname></term>
          <listitem><para>
            The connection is not an ordinary connection, or the passed
            file descriptors are either kdbus handles or unix domain
            sockets. Both are currently unsupported.
          </para></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>-EINVAL</varname></term>
          <listitem><para>
            The submitted payload type is <varname>KDBUS_PAYLOAD_KERNEL</varname>,
            <varname>KDBUS_MSG_EXPECT_REPLY</varname> was set without timeout or
            cookie values, <varname>KDBUS_SEND_SYNC_REPLY</varname> was set
            without <varname>KDBUS_MSG_EXPECT_REPLY</varname>, an invalid item
            was supplied, <varname>src_id</varname> was non-zero and was
            different from the current connection's ID, a supplied memfd had a
            size of 0, or a string was not properly null-terminated.
          </para></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>-ENOTUNIQ</varname></term>
          <listitem><para>
            The supplied destination is <varname>KDBUS_DST_ID_BROADCAST</varname>,
            a file descriptor was passed, <varname>KDBUS_MSG_EXPECT_REPLY</varname>
            was set, or a timeout was given for a broadcast message.
          </para></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>-E2BIG</varname></term>
          <listitem><para>
            Too many items
          </para></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>-EMSGSIZE</varname></term>
          <listitem><para>
            The size of the message header and items or the payload vector
            is excessive.
          </para></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>-EEXIST</varname></term>
          <listitem><para>
            Multiple <varname>KDBUS_ITEM_FDS</varname>,
            <varname>KDBUS_ITEM_BLOOM_FILTER</varname> or
            <varname>KDBUS_ITEM_DST_NAME</varname> items were supplied.
          </para></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>-EBADF</varname></term>
          <listitem><para>
            The supplied <varname>KDBUS_ITEM_FDS</varname> or
            <varname>KDBUS_ITEM_PAYLOAD_MEMFD</varname> items
            contained an illegal file descriptor.
          </para></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>-EMEDIUMTYPE</varname></term>
          <listitem><para>
            The supplied memfd is not a sealed kdbus memfd.
          </para></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>-EMFILE</varname></term>
          <listitem><para>
            Too many file descriptors inside a <varname>KDBUS_ITEM_FDS</varname>.
          </para></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>-EBADMSG</varname></term>
          <listitem><para>
            An item had illegal size, both a <varname>dst_id</varname> and a
            <varname>KDBUS_ITEM_DST_NAME</varname> was given, or both a name
            and a bloom filter was given.
          </para></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>-ETXTBSY</varname></term>
          <listitem><para>
            The supplied kdbus memfd file cannot be sealed or the seal
            was removed, because it is shared with other processes or
            still mapped with
            <citerefentry><refentrytitle>mmap</refentrytitle><manvolnum>2</manvolnum></citerefentry>.
          </para></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>-ECOMM</varname></term>
          <listitem><para>
            A peer does not accept the file descriptors addressed to it.
          </para></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>-EFAULT</varname></term>
          <listitem><para>
            The supplied bloom filter size was not 64-bit aligned or memory
            supplied by userspace could not be accessed by the kernel.
          </para></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>-EDOM</varname></term>
          <listitem><para>
            The supplied bloom filter size did not match the bloom filter
            size of the bus.
          </para></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>-EDESTADDRREQ</varname></term>
          <listitem><para>
            <varname>dst_id</varname> was set to
            <varname>KDBUS_DST_ID_NAME</varname>, but no
            <varname>KDBUS_ITEM_DST_NAME</varname> was attached.
          </para></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>-ESRCH</varname></term>
          <listitem><para>
            The name to look up was not found in the name registry.
          </para></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>-EADDRNOTAVAIL</varname></term>
          <listitem><para>
            <varname>KDBUS_MSG_NO_AUTO_START</varname> was given but the
            destination connection is an activator.
          </para></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>-ENXIO</varname></term>
          <listitem><para>
            The passed numeric destination connection ID couldn't be found,
            or is not connected.
          </para></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>-ECONNRESET</varname></term>
          <listitem><para>
            The destination connection is no longer active.
          </para></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>-ETIMEDOUT</varname></term>
          <listitem><para>
            Timeout while synchronously waiting for a reply.
          </para></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>-EINTR</varname></term>
          <listitem><para>
            Interrupted system call  while synchronously waiting for a reply.
          </para></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>-EPIPE</varname></term>
          <listitem><para>
            When sending a message, a synchronous reply from the receiving
            connection was expected but the connection died before answering.
          </para></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>-ENOBUFS</varname></term>
          <listitem><para>
            Too many pending messages on the receiver side.
          </para></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>-EREMCHG</varname></term>
          <listitem><para>
            Both a well-known name and a unique name (ID) was given, but
            the name is not currently owned by that connection.
          </para></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>-EXFULL</varname></term>
          <listitem><para>
            The memory pool of the receiver is full.
          </para></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>-EREMOTEIO</varname></term>
          <listitem><para>
            While synchronously waiting for a reply, the remote peer
            failed with an I/O error.
          </para></listitem>
        </varlistentry>
      </variablelist>
    </refsect2>

    <refsect2>
      <title><varname>KDBUS_CMD_RECV</varname> may return the following errors</title>

      <variablelist>
        <varlistentry>
          <term><varname>-EOPNOTSUPP</varname></term>
          <listitem><para>
            The connection is not an ordinary connection, or the passed
            file descriptors are either kdbus handles or unix domain
            sockets. Both are currently unsupported.
          </para></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>-EINVAL</varname></term>
          <listitem><para>
            Invalid flags or offset.
          </para></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>-EAGAIN</varname></term>
          <listitem><para>
            No message found in the queue
          </para></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>-ENOMSG</varname></term>
          <listitem><para>
            No message of the requested priority found.
          </para></listitem>
        </varlistentry>

        <varlistentry>
          <term><varname>-EOVERFLOW</varname></term>
          <listitem><para>
            Some messages have been lost.
          </para></listitem>
        </varlistentry>
      </variablelist>
    </refsect2>
  </refsect1>

  <refsect1>
    <title>See Also</title>
    <simplelist type="inline">
      <member><citerefentry><refentrytitle>kdbus</refentrytitle><manvolnum>7</manvolnum></citerefentry></member>
      <member><citerefentry><refentrytitle>kdbus.bus</refentrytitle><manvolnum>7</manvolnum></citerefentry></member>
      <member><citerefentry><refentrytitle>kdbus.connection</refentrytitle><manvolnum>7</manvolnum></citerefentry></member>
      <member><citerefentry><refentrytitle>kdbus.endpoint</refentrytitle><manvolnum>7</manvolnum></citerefentry></member>
      <member><citerefentry><refentrytitle>kdbus.fs</refentrytitle><manvolnum>7</manvolnum></citerefentry></member>
      <member><citerefentry><refentrytitle>kdbus.items</refentrytitle><manvolnum>7</manvolnum></citerefentry></member>
      <member><citerefentry><refentrytitle>kdbus.message</refentrytitle><manvolnum>7</manvolnum></citerefentry></member>
      <member><citerefentry><refentrytitle>kdbus.names</refentrytitle><manvolnum>7</manvolnum></citerefentry></member>
      <member><citerefentry><refentrytitle>kdbus.pool</refentrytitle><manvolnum>7</manvolnum></citerefentry></member>
      <member><citerefentry><refentrytitle>clock_gettime</refentrytitle><manvolnum>2</manvolnum></citerefentry></member>
      <member><citerefentry><refentrytitle>ioctl</refentrytitle><manvolnum>2</manvolnum></citerefentry></member>
      <member><citerefentry><refentrytitle>poll</refentrytitle><manvolnum>2</manvolnum></citerefentry></member>
      <member><citerefentry><refentrytitle>select</refentrytitle><manvolnum>2</manvolnum></citerefentry></member>
      <member><citerefentry><refentrytitle>epoll</refentrytitle><manvolnum>7</manvolnum></citerefentry></member>
    </simplelist>
  </refsect1>
</refentry>