<page>
    <title>Stream Tags</title>
    <ns>0</ns>
    <id>3482</id>
    <revision>
      <id>6198</id>
      <parentid>6191</parentid>
      <timestamp>2019-09-30T20:17:37Z</timestamp>
      <contributor>
        <username>777arc</username>
        <id>632</id>
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      <text xml:space="preserve" bytes="12354">[[Category:Usage Manual]]
== 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 can lack for control and meta data.

Part of this is solved using the existing message passing interface, which
allows blocks to subscribe to messages published by any other block in
the flowgraph (see [[Message Passing]]). The main drawback to the
message passing system is that is works asynchronously, meaning that
there is no guarantee when a message may arrive relative to the data
stream.

Stream tags are an isosynchronous data stream that runs parallel to
the main data stream. A stream ''tag'' is generated by a block's work
function and from there on flows downstream alongside a particular sample,
until it reaches a sink or is forced to stop propagating by another
block.

Stream tags are defined for a specific item in the data stream and are
formed as a key:value pair. The ''key'' identifies what the ''value'' represents
while the value holds the data that the tag contains. Both ''key'' and
''value'' are [[Polymorphic Types (PMTs)]] where the ''key'' is a PMT symbol while
the ''value'' is any type of PMT and can therefore handle any data we wish
to pass. An additional part of the tag is the ''srcid'', which is a PMT
symbol and is used to identify the block that created the tag (which
is usually the block's alias).

== API Extensions to the gr::block ==

To enable the stream tags, we have extended the API of gr::block to
understand ''absolute'' item numbers. In the data stream model, each
block's work function is given a buffer in the data stream that is
referenced from 0 to N-1. This is a ''relative'' offset into the data
stream. The absolute reference starts from the beginning of the
flowgraph and continues to count up with every item. Each input stream
is associated with a concept of the 'number of items read' and each
output stream has a 'number of items written'. These are retrieved during 
runtime using the two API calls:

 unsigned long int nitems_read(unsigned int which_input);
 unsigned long int nitems_written(unsigned int which_output);

Each tag is associated with some item in this absolute time scale that
is calculated using these functions.

Like the rest of the data stream, the number of items read/written are
only updated once during the call to work. So in a work function,
nitems_read/written will refer to the state of the data stream at the
start of the work function. We must therefore add to this value the
current relative offset in the data stream. So if we are iterating ''i'' over all output items, we would write the stream tag to output ports
at nitems_written(0)+i for the 0th output port.

== Stream Tags API ==

The stream tags API is split into two parts: adding tags to a stream, 
and getting tags from a stream. 
Note that the functions described below are only meant to be accessed
within a call to general_work/work. While they can be called at other points
in time by a block, the behavior outside of work is undefined without
exact knowledge of the item counts in the buffers.

=== Adding a Tag to a Stream ===

We add a tag to a particular output stream of the block using:

* gr::block::add_item_tag: Adds an item tag to a particular output port using a gr::tag_t data type or by specifying the tag values.

We can output them to multiple output streams if we want, but to do so
means calling this function once for each port.  This function can be 
provided with a gr::tag_t data type, or each value of the tag can be
explicitly given.

Again, a tag is defined as:

# offset: The offset, in absolute item time, of the tag in the data stream.
# key: the PMT symbol identifying the type of tag.
# value: the PMT holding the data of the tag.
# srcid: (optional) the PMT symbol identifying the block which created the tag.

We can create a gr::tag_t structure to hold all of the above
information of a tag, which is probably the easiest/best way to do
it. The gr::tag_t struct is defined as having the same members as in
the above list. To add a gr::tag_t tag to a stream, use the function:

  void add_item_tag(unsigned int which_output, const tag_t &tag);

The secondary API allows us to create a tag by explicitly listing all
of the tag information in the function call:

  void add_item_tag(unsigned int which_output,
                    uint64_t abs_offset,
                    const pmt::pmt_t &key,
                    const pmt::pmt_t &value,
                    const pmt::pmt_t &srcid=pmt::PMT_F);

In Python, we can add a tag to a stream using one of the following:

 add_item_tag(which_output, abs_offset, key, value)
 add_item_tag(which_output, abs_offset, key, value, srcid)

Note that the key and value are both PMTs.  To create a string type PMT you can use pmt.intern("example_key")

Here is an example of a Python block that loops through samples and puts tags on random ones, and outputs the same signal:
<syntaxhighlight lang="python">
 import numpy as np
 from gnuradio import gr
 import pmt
 
 class blk(gr.sync_block):
     def __init__(self):
         gr.sync_block.__init__(
             self,
             name='Embedded Python Block',
             in_sig=[np.complex64],
             out_sig=[np.complex64]
         )
 
     def work(self, input_items, output_items):
         for indx, sample in enumerate(input_items[0]):
             if np.random.rand() > 0.95: # 5% chance this sample is chosen
                 key = pmt.intern("example_key")
                 value = pmt.intern("example_value")
                 self.add_item_tag(0, self.nitems_written(0) + indx, key, value)
                 # note: (self.nitems_written(0) + indx) is our current sample, in absolute time
         output_items[0][:] = input_items[0] # copy input to output
         return len(output_items[0])
</syntaxhighlight>

=== Getting tags from a Stream ===

To get tags from a particular input stream, we have two
functions we can use:

# gr::block::get_tags_in_range: Gets all tags from a particular input port between a certain range of items (in absolute item time).

# gr::block::get_tags_in_window: Gets all tags from a particular input port between a certain range of items (in relative item time within the work function).

The difference between these functions is working in absolute item
time versus relative item time. Both of these pass back vectors of
gr::tag_t, and they both allow
specifying a particular key (as a PMT symbol) to filter against 
(or the fifth argument can be left out to search for all keys). 
Filtering for a certain key reduces the effort inside the work function
for getting the right tag's data.

For example, this call just returns any tags between the given range of items:

  void get_tags_in_range(std::vector<tag_t> &v,
                         unsigned int which_input,
                         uint64_t abs_start,
                         uint64_t abs_end);

Adding a fifth argument to this function allows us to filter on the
key ''key''.

  void get_tags_in_range(std::vector<tag_t> &v,
                         unsigned int which_input,
                         uint64_t abs_start,
                         uint64_t abs_end,
                         const pmt::pmt_t &key);

In Python, the main difference from the C++ function is that instead of having the first
argument be a vector where the tags are stored, the Python version
just returns a list of tags. We would use it like this:

<syntaxhighlight lang="python">
 def work(self, input_items, output_items):
     ....
     tags = self.get_tags_in_window(which_input, rel_start, rel_end)
     ....
</syntaxhighlight>

If you want to grab all tags on the samples currently being processed by work(), on input port 0, here's a minimal example of doing that:

<syntaxhighlight lang="python">
 import numpy as np
 from gnuradio import gr
 import pmt
 
 class blk(gr.sync_block):
     def __init__(self):
         gr.sync_block.__init__(self,name='Read Tags', in_sig=[np.float32], out_sig=None)
 
     def work(self, input_items, output_items):
         tags = self.get_tags_in_window(0, 0, len(input_items[0]))
         for tag in tags:
             key = pmt.to_python(tag.key) # convert from PMT to python string
             value = pmt.to_python(tag.value) # Note that the type(value) can be several things, it depends what PMT type it was
             print 'key:', key
             print 'value:', value, type(value)
             print ''
         return len(input_items[0])
</syntaxhighlight>

== Tag Propagation ==

Tags are propagated downstream from block to block like the normal
data streams. How tags are actually moved depends on a specific
propagation policy. We defined three types of policies:

# All-to-All: all tags from any input port are replicated to all output ports
# One-to-One: tags from input port ''i'' are only copied to output port ''i'' (depends on num inputs = num outputs).
# Dont: Does not propagate tags. Tags are either stopped here or the work function recreates them in some manner.

The default behavior of a block is the 'All-to-All' method of
propagation.

To set a different propagation policy, use the function:

  void set_tag_propagation_policy(tag_propagation_policy_t p);

See the gr::block::tag_propagation_policy_t documentation for details
on this enum type.

=== Tag Propagation through Rate Changes ===

When a tag is propagated through a block that has a rate change, the
item's offset in the data stream will change. The scheduler uses the
block's gr::block::relative_rate concept to perform the update on the
tag's offset value. The relative rate of a block determines the
relationship between the input rate and output rate. Decimators that
decimate by a factor of D have a relative rate of 1/D.

Synchronous blocks (gr::sync_block), decimators (gr::sync_decimator),
and interpolators (gr::sync_interpolator) all have pre-defined and
well-understood relative rates. A standard gr::block has a default
relative rate of 1.0, but this must be set if it does not work this
way. Often, we use a gr::block because we have no pre-conceived notion
of the number of input to output items. If it is important to pass
tags through these blocks that respect the change in item value, we
would have to use the TPP_DONT tag propagation policy and handle the
propagation internally.

In no case is the value of the tag modified when propagating through a
block. This becomes relevant when using [[Tagged Stream Blocks]].

== Notes on How to Use Tags ==

Tags can be very useful to an application, and their use is
spreading. USRP sources generate tag information on the time, sample
rate, and frequency of the board if anything changes. We have a meta
data file source/sink (see [[Metadata]]) that use tags to store
information about the data stream. But there are things to think about
when using tags in a block.

First, when tags are not being used, there is almost no effect on the
scheduler. However, when we use tags, we add overhead by getting and
extracting tags from a data stream. We also use overhead in
propagating the tags. For each tag, each block must copy a vector of
tags from the output port(s) of one block to the input port(s) of the
next block(s). These copy operations can add up.

The key is to minimize the use of tags. Use them when and only when
necessary and try to provide some control over how tags are generated
to control their frequency. A good example is the USRP source, which
generates a time tag. If it generated a tag with every sample, we
would have thousands of tags per second, which would add a significant
amount of overhead. This is because if we started at time t0 at
sample rate sr, then after N samples, we know that
we are now at time t0 + N/sr. So continuously producing new
tags adds no information.

The main issue we need to deal with in the above situation is when
there is a discontinuity in the packets received from the USRP. Since
we have no way of knowing in the flowgraph how many samples were
potentially lost, we have lost track of the timing information. The
USRP driver recognizes when packets have been dropped and uses this to
queue another tag, which allows us to resync. Likewise, any point the
sample rate or frequency changes, a new tag is issued.</text>
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