/*! \page page_msg_passing Message Passing

\section msg_passing_introduction Introduction

GNU Radio was originally a streaming system with no other mechanism to
pass data between blocks. Streams of data are a model that work well
for samples, bits, etc., but are not really the right mechanism for
control data, metadata, and, often, packet structures (at least at
some point in the processing chain).

We solved part of this problem by introducing the tag stream (see \ref
page_stream_tags). This is a parallel stream to the data
streaming. The difference is that tags are designed to hold metadata
and control information. Tags are specifically associated with a
particular sample in the data stream and flow downstream alongside the
data. This model allows other blocks to identify that an event or
action has occurred or should occur on a particular item. The major
limitation is that the tag stream is really only accessible inside a
work function and only flows in one direction. Its benefit is that it
is isosynchronous with the data.

We want a more general message passing system for a couple of
reasons. The first is to allow blocks downstream to communicate back
to blocks upstream. The second is to allow an easier way for us to
communicate back and forth between external applications and GNU
Radio. The new message passing interface handles these cases, although
it does so on an asynchronous basis.

The message passing interface heavily relies on Polymorphic Types
(PMTs) in GNU Radio. For further information about these data
structures, see the page \ref page_pmt.

\section msg_passing_api Message Passing API

The message passing interface is designed into the gr::basic_block,
which is the parent class for all blocks in GNU Radio. Each block has
a set of message queues to hold incoming messages and can post
messages to the message queues of other blocks. The blocks also
distinguish between input and output ports.

A block has to declare its input and output message ports in its
constructor. The message ports are described by a name, which is in
practice a PMT symbol (<em>i.e.</em>, an interned string). The API calls
to register a new port are:

\code
  void message_port_register_in(pmt::pmt_t port_id)
  void message_port_register_out(pmt::pmt_t port_id)
\endcode

The ports are now identifiable by that port name. Other blocks who may
want to post or receive messages on a port must subscribe to it. When
a block has a message to send, they are published on a particular
port. The subscribe and publish API looks like:

\code
  void message_port_pub(pmt::pmt_t port_id,
                        pmt::pmt_t msg);
  void message_port_sub(pmt::pmt_t port_id,
                        pmt::pmt_t target);
  void message_port_unsub(pmt::pmt_t port_id,
                          pmt::pmt_t target);
\endcode

Any block that has a subscription to another block's output message
port will receive the message when it is published. Internally, when a
block publishes a message, it simply iterates through all blocks that
have subscribed and uses the gr::basic_block::_post method to send the
message to that block's message queue.


\subsection msg_passing_msg_handler Message Handler Functions

A subscriber block must declare a message handler function to process
the messages that are posted to it. After using the
gr::basic_block::message_port_register_in to declare a subscriber port, we
must then bind this port to the message handler. For this, we use
Boost's 'bind' function:

\code
  set_msg_handler(pmt::pmt_t port_id,
    boost::bind(&block_class::message_handler_function, this, _1));
\endcode

The 'port_id' is the same PMT as used when registering the input
port. The 'block_class::message_handler_function' is the member
function of the class designated to handle messages to this port. The
'this' and '_1' are standard ways of using the Boost bind function to
pass the 'this' pointer as the first argument to the class (standard
OOP practice) and the _1 is an indicator that the function expects 1
additional argument. The prototype for all message handling functions
is:

\code
  void block_class::message_handler_function(pmt::pmt_t msg);
\endcode

We give an example of using this below.


\subsection msg_passing_fg_connect Connecting Messages through the Flowgraph

From the flowgraph level, we have instrumented a gr::hier_block2::msg_connect
method to make it easy to subscribe blocks to other blocks'
messages. The message connection method looks like the following
code. Assume that the block \b src has an output message port named
\a pdus and the block \b dbg has an input port named \a print.

\code
    self.tb.msg_connect(src, "pdus", dbg, "print")
\endcode

All messages published by the \b src block on port \a pdus will be
received by \b dbg on port \a print. Note here how we are just using
strings to define the ports, not PMT symbols. This is a convenience to
the user to be able to more easily type in the port names (for
reference, you can create a PMT symbol in Python using the
pmt::intern function as pmt.intern("string")).

Users can also query blocks for the names of their input and output
ports using the following API calls:

\code
    pmt::pmt_t message_ports_in();
    pmt::pmt_t message_ports_out();
\endcode

The return value for these are a PMT vector filled with PMT symbols,
so PMT operators must be used to manipulate them.

Each block has internal methods to handle posting and receiving of
messages. The gr::basic_block::_post method takes in a message and
places it into its queue. The publishing model uses the
gr::basic_block::_post method of the blocks as the way to access the
message queue. So the message queue of the right name will have a new
message. Posting messages also has the benefit of waking up the
block's thread if it is in a wait state. So if idle, as soon as a
message is posted, it will wake up and and call the message handler.

The other side of the action in a block is in the message
handler. When a block has an input message port, it needs a callback
function to handle messages received on that port. We use a Boost bind
operator to bind the message port to the message handling
function. When a new message is pushed onto a port's message queue,
it is this function that is used to process the message.


\section msg_passing_python Message Passing in Python Blocks

ADD STUFF HERE


\section msg_passing_examples Code Examples

The following is snippets of code from blocks current in GNU Radio
that take advantage of message passing. We will be using
gr::blocks::message_debug and gr::blocks::tagged_stream_to_pdu below
to show setting up both input and output message passing capabilities.

The gr::blocks::message_debug block is used for debugging the message
passing system. It describes three input message ports: \a print, \a
store, and \a pdu_print. The \a print port simply prints out all
messages to standard out while the \a store port keeps a list of all
messages posted to it. The \a pdu_print port specially formats PDU
messages for printing to standard out. The \a store port works in
conjunction with a gr::blocks::message_debug::get_message(int i) call
that allows us to retrieve message \p i afterward.

The constructor of this block looks like this:

\code
{
  message_port_register_in(pmt::mp("print"));
  set_msg_handler(pmt::mp("print"),
    boost::bind(&message_debug_impl::print, this, _1));

  message_port_register_in(pmt::mp("store"));
  set_msg_handler(pmt::mp("store"),
    boost::bind(&message_debug_impl::store, this, _1));

  message_port_register_in(pmt::mp("print_pdu"));
  set_msg_handler(pmt::mp("print_pdu"),
    boost::bind(&message_debug_impl::print_pdu, this, _1));
}
\endcode

So the three ports are registered by their respective names. We then
use the gr::basic_block::set_msg_handler function to identify this
particular port name with a callback function. The Boost \a bind
function (<a target="_blank"
href="http://www.boost.org/doc/libs/1_52_0/libs/bind/bind.html">Boost::bind</a>)
here binds the callback to a function of this block's class. So now
the functions in the block's private implementation class,
gr::blocks::message_debug_impl::print,
gr::blocks::message_debug_impl::store, and
gr::blocks::message_debug_impl::print_pdu, are assigned to handle
messages passed to them. Below is the \a print function for reference.

\code
void
message_debug_impl::print(pmt::pmt_t msg)
{
  std::cout << "***** MESSAGE DEBUG PRINT ********\n";
  pmt::print(msg);
  std::cout << "**********************************\n";
}
\endcode

The function simply takes in the PMT message and prints it. The method
pmt::print is a function in the PMT library to print the
PMT in a friendly, (mostly) pretty manner.

The gr::blocks::tagged_stream_to_pdu block only defines a single
output message port. In this case, its constructor contains the line:

\code
{
  message_port_register_out(pdu_port_id);
}
\endcode

So we are only creating a single output port where \a pdu_port_id
is defined in the file pdu.h as \a pdus.

This blocks purpose is to take in a stream of samples along with
stream tags and construct a predefined PDU message from this. In GNU
Radio, we define a PDU as a PMT pair of (metadata, data). The metadata
describes the samples found in the data portion of the
pair. Specifically, the metadata can contain the length of the data
segment and any other information (sample rate, etc.). The PMT vectors
know their own length, so the length value is not actually necessary
unless useful for purposes down the line. The metadata is a PMT
dictionary while the data segment is a PMT uniform vector of either
bytes, floats, or complex values.

In the end, when a PDU message is ready, the block calls its
gr::blocks::tagged_stream_to_pdu_impl::send_message function that is
shown below.

\code
void
tagged_stream_to_pdu_impl::send_message()
{
  if(pmt::length(d_pdu_vector) != d_pdu_length) {
    throw std::runtime_error("msg length not correct");
  }

  pmt::pmt_t msg = pmt::cons(d_pdu_meta,
                             d_pdu_vector);
  message_port_pub(pdu_port_id, msg);

  d_pdu_meta = pmt::PMT_NIL;
  d_pdu_vector = pmt::PMT_NIL;
  d_pdu_length = 0;
  d_pdu_remain = 0;
  d_inpdu = false;
}
\endcode

This function does a bit of checking to make sure the PDU is ok as
well as some cleanup in the end. But it is the line where the message
is published that is important to this discussion. Here, the block
posts the PDU message to any subscribers by calling
gr::basic_block::message_port_pub publishing method.

There is similarly a gr::blocks::pdu_to_tagged_stream block that essentially
does the opposite. It acts as a source to a flowgraph and waits for
PDU messages to be posted to it on its input port \a pdus. It extracts
the metadata and data and processes them. The metadata dictionary is
split up into key:value pairs and stream tags are created out of
them. The data is then converted into an output stream of items and
passed along. The next section describes how PDUs can be passed into a
flowgraph using the gr::blocks::pdu_to_tagged_stream block.

\section msg_passing_posting Posting from External Sources

The last feature of the message passing architecture to discuss here
is how it can be used to take in messages from an external source. We
can call a block's gr::basic_block::_post method directly and pass it a
message. So any block with an input message port can receive messages
from the outside in this way.

The following example uses a gr::blocks::pdu_to_tagged_stream block
as the source block to a flowgraph. Its purpose is to wait for
messages as PDUs posted to it and convert them to a normal stream. The
payload will be sent on as a normal stream while the meta data will be
decoded into tags and sent on the tagged stream.

So if we have created a \b src block as a PDU to stream, it has a \a
pdus input port, which is how we will inject PDU messages to the
flowgraph. These PDUs could come from another block or flowgraph, but
here, we will create and insert them by hand.

\code
  port = pmt.intern("pdus")
  msg = pmt.cons(pmt.PMT_NIL,
                 pmt.make_u8vector(16, 0xFF))
  src.to_basic_block()._post(port, msg)
\endcode

The PDU's metadata section is empty, hence the pmt::PMT_NIL
object. The payload is now just a simple vector of 16 bytes of all
1's. To post the message, we have to access the block's gr::basic_block
class, which we do using the gr::basic_block::to_basic_block method and
then call the gr::basic_block::_post method to pass the PDU to the
right port.

All of these mechanisms are explored and tested in the QA code of the
file qa_pdu.py.

There are some examples of using the message passing infrastructure
through GRC in gr-blocks/examples/msg_passing.

\section msg_passing_commands Using messages as commands

Messages can be used to send commands to blocks. Examples for this include:

- gr::qtgui::freq_sink_c: The scaling of the frequency axis can be changed by messages
- gr::uhd::usrp_source and gr::uhd::usrp_sink: Many transceiver-related settings can
  be manipulated through command messages, such as frequency, gain and LO offset
- gr::digital::header_payload_demux, which receives an acknowledgement from a header parser
  block on how many payload items there are to process

There is no special PMT type to encode commands, however, it is strongly recommended
to use one of the following formats:

- pmt::cons(KEY, VALUE): This format is useful for commands that take a single value.
  Think of KEY and VALUE as the argument name and value, respectively. For the case of
  the QT GUI Frequency Sink, KEY would be "freq" and VALUE would be the new center frequency
  in Hz.
- pmt::dict((KEY1: VALUE1), (KEY2: VALUE2), ...): This is basically the same as the
  previous format, but you can provide multiple key/value pairs. This is particularly
  useful when a single command takes multiple arguments which can't be broken into
  multiple command messages (e.g., the USRP blocks might have both a timestamp and a
  center frequency in a command message, which are closely associated).

In both cases, all KEYs should be pmt::symbols (i.e. strings). VALUEs can be
whatever the block requires.

It might be tempting to deviate from this format, e.g. the QT Frequency sink could
simply take a float value as a command message, and it would still work fine.
However, there are some very good reasons to stick to this format:

- Interoperability: The more people use the standard format, the more likely it
  is that blocks from different sources can work together
- Inspectability: A message debug block will display more useful information about
  a message if its containing both a value and a key
- Intuition: This format is pretty versatile and unlikely to create situations
  where it is not sufficient (especially considering that values are PMTs themselves).
  As a counterexample, using positional arguments (something like "the first argument
  is the frequency, the second the gain") is easily forgotten, or changed in one place
  and not another, etc.

*/
