Short: I want to output PMTs in my block, but GNU Radio won't let me.
Generally I write OOT blocks for different applications on GNU Radio. Here I am trying to write a block for outputting a file as a message type.
The thing is if the output of our block if of type numpy.float32, we declare the same using:
def __init__(self):
gr.sync_block.__init__(self,name="<anything_you_want>",out_sig=[(numpy.float32)])
Now I have a similar problem where I want the output to be of type PMT of GNU Radio. What should out_sig be in that case?
PMT (polymorphic types) are variable sized, portable containers.
They are used for message passing and tags, but not for streaming data.
If you want to output a message, then defined an empty out_sig, i.e.
..., out_sig=[], in_sig=[])
and use the message_port_register_out method to register an output port, and the message_port_pub method to send a message.
Message passing is documented in the doxygen documentation.
However, I'd recommend you take the Guided Tutorials, and read chapters 1-5, which explain the difference between item streams and messages. Since you can already write OOTs, this should be a quick read for you :)
If you want a minimal block that already uses the aforementioned methods, try my minimal variable_to_msg block.
Also, there's already a block that opens a file, and extracts messages: It's part of Tim O'Shea's gr-pyqt, and is called file_message_source.