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Starting with release 3.8, YAML replaces XML as the file format for GNU Radio Companion. This is triggered by switching from Cheetah to Mako as the templating engine, since Cheetah does not support Python 3. Specifically, this will impact .grc files, block descriptions and block tree files. This article won’t focus on the .grc files, because they aren’t meant for manual editing.

The most notable change is of course the absence of XML’s angle brackets in favour of YAML’s colon-separated keys and values, and the change in file names for blocks. The latter is important for GRC to recognise the file. Namely, the “.xml” ending has been replaced with “.block.yml” for block descriptions and the underscore in block tree files has been replaced with a dot. (For example, “qtgui_tree.xml” becomes “qtgui.tree.yml”)

Block Descriptions

The content of the block descriptions is still the same, although it has been shuffled around a bit. The parts are elaborated below, in the order they should appear in the files.


id: blocks_multiply_const_vxx
label: Multiply Const

-   id: type
    label: IO Type

The ID is unique for each block and is used to identify it.


id: blocks_multiply_const_vxx
label: Multiply Const

-   id: type
    label: IO Type

The label is simply the human-readable name of the block, and will be visible from within GRC. It will not appear in the generated code.

Flags (optional)

id: blocks_throttle
label: Throttle
flags: throttle

-   id: type
    label: Type

The flags indicate special attributes of the block. The only current example of this is the throttle flag, which is used in the Throttle and hardware blocks. For more information on throttling, see the Guided Tutorial: [1]


-   id: tr_chan
    label: Trigger Channel
    category: Trigger
    dtype: int
    default: '0'
    hide: part

This part describes the parameters to display to the user. A number of keywords is used there:

A unique name for the parameter, it is not displayed to the user but can be used to reference the parameter inside this file: ${<id>}
Human readable name to display to the user.
Type of the data handled by the parameter
This can have many values
raw, complex, real, float, int, hex, bool
Vectors of numbers
complex_vector, real_vector, float_vector, int_vector
string, file_open, file_save, _multiline _mutiline_python_external
Other special types
gui_hint, import, id, stream_id, name and enum
A default value for the parameter
Used to organise a large number of parameters. If set, a new tab will be created and named as the value of the keyword with the parameter inside.
If the value is part, the parameter will not be shown on the block representation inside GRC.
If it's all, it will not be shown at all, even inside the properties window.
Inherit properties from another parameter. The value given is the id of that parameter.

Inputs and Outputs (optional)

id: qtgui_freq_sink_x
    default: '1.0'
    hide: ${ ('part' if int(nconnections) >= 10 else 'all') }

-   domain: stream
    dtype: ${ type.t }
    multiplicity: ${ (0 if (type == 'msg_complex' or type == 'msg_float') else nconnections) }
    optional: true
-   domain: message
    id: freq
    optional: true
    hide: ${ showports }

-   domain: message

This describes the input ports. domain can be either stream or message. Stream ports need a type, which usually is specified as a parameter. This is true for our example, the type is specified in type.t. The multiplicity tells us how many "copies" of this port we want. (Yes, this can be zero!) Finally, the optional flag tells us whether this port must be connected or not. (GRC won't generate the flowgraph if a non-optional port isn't connected)

Message ports[2] don't have a specified type here, but they have IDs. This message port can also be hidden, using the "Show message ports" option in the parameters.

The output ports work similarly.

Asserts (optional)

id: blocks_throttle
    dtype: ${ type }
    vlen: ${ vlen }

- ${ vlen > 0 }

    imports: from gnuradio import blocks
    make: blocks.throttle(${type.size}*${vlen}, ${samples_per_second},${ignoretag})

Asserts (previously known as "checks" for the XML blocks) are expressions that need to be true, otherwise GRC won't let you generate the flowgraph.


id: blocks_message_strobe_random
    optional: true

    imports: |-
        from gnuradio import blocks
        import pmt
    make: blocks.message_strobe_random(${msg}, ${dist}, ${mean}, ${std})
    - set_msg(${msg})
    - set_dist(${dist})

The templates describe the code that is created when GRC generates the flowgraph. This part consists of the imports, the make/initialization statements and the callbacks. The values of these keys often happen to span over multiple lines, in which case you're likely to see the "|-" symbols. This is YAML syntax for a literal block scalar[3] and the hyphen means that the line break at the end is omitted.

Your block probably utilizes parts of GNU Radio or other modules that need to be imported. These are specified as imports.

make holds the initialization code, and often depends on several of the parameters. Some of the more involved blocks may also use the "%" symbol, denoting the use of a YAML directive[4]. This can occur both in imports and make. The snippet below may be helpful:

id: rational_resampler_xxx
    make: |-
        % if taps:
        % else:
        % endif

Please note that the "${}" is not valid syntax in a directive.


id: variable_rrc_filter_taps
documentation: |-
    This is a convenience wrapper for calling firdes.root_raised_cosine(...).

file_format: 1

documentation simply contains information about the block. This information is displayed in the block's Documentation tab in GRC.

File Format


Variable Make

Variable Value

Block Tree Files

The block tree files are fairly straightforward, and tells GRC how to divide the block tree into categories. The following example snippet from gr-digital (which is part of core GNU Radio) describes two categories, Coding and Equalizers. The blocks are specified using their ids, which should equal their file names without ".block.yml".

- Coding:
  - digital_additive_scrambler_bb
  - digital_descrambler_bb
  - digital_scrambler_bb
- Equalizers:
  - digital_cma_equalizer_cc
  - digital_lms_dd_equalizer_cc