# Message passing(connect) in C++ **URL:** \ **Category:** GNU Radio\ **Created:** [April 30, 2015, 1:14pm UTC](https://www.ruby-forum.com/t/message-passing-connect-in-c/241788 "2015-04-30T13:14:29Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![Detlef\_R](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/58f4c7/32.png) [@Detlef\_R](https://www.ruby-forum.com/u/Detlef_R)\ **Post date:** [April 30, 2015, 1:14pm UTC](https://www.ruby-forum.com/t/message-passing-connect-in-c/241788/1 "2015-04-30T13:14:29Z")
Hi all, I have another question. How can I connect the input and output port of different blocks,from inside a C++ block? In the documentation I’ve only seen the msg\_connect() for python Thanks, marco ---
**Author:** ![marco\_Ribero](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/e9c0ed/32.png) [@marco\_Ribero](https://www.ruby-forum.com/u/marco_Ribero)\ **Post date:** [April 30, 2015, 1:46pm UTC](https://www.ruby-forum.com/t/message-passing-connect-in-c/241788/2 "2015-04-30T13:46:37Z")
Hi Marco, The top\_block class has a member named: connect. You can use it in the main() to connect different blocks. However, if you want to dis/connect blocks within C++ blocks, you must pass top\_block pointer to the class in some fashion. Once I did and I got errors because I didn’t have accessed to the “connect” from within the block. I prefer to change your topology to refuse the need you’ve mentioned. Otherwise describe your problem with more details. Best, Mostafa On Thu, Apr 30, 2015 at 3:43 PM, marco Ribero [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) > [Discuss-gnuradio Info Page](https://lists.gnu.org/mailman/listinfo/discuss-gnuradio) – * * * Department of Electrical Engineering Aboureyhan Building MMWCL LAB Amirkabir University Of Technology Tehran IRAN Tel: +98 (919) 158-7730 LAB: [http://ele.aut.ac.ir/~mmwcl/?page\_id=411](http://ele.aut.ac.ir/~mmwcl/?page_id=411) Homepage: [http://ele.aut.ac.ir/~alizadeh/](http://ele.aut.ac.ir/~alizadeh/) * * * ---
**Author:** ![marco\_Ribero](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/e9c0ed/32.png) [@marco\_Ribero](https://www.ruby-forum.com/u/marco_Ribero)\ **Post date:** [April 30, 2015, 1:50pm UTC](https://www.ruby-forum.com/t/message-passing-connect-in-c/241788/3 "2015-04-30T13:50:08Z")
excuse me, I’m seeing the new API 3.7.7 of GNURadio which is changed. The “connect” is a member of gr::flowgraph which connects both blocks’ ports and message ports: > **[GNU Radio Manual and C++ API Reference: gr::flowgraph Class Reference](https://www.gnuradio.org/doc/doxygen/classgr_1_1flowgraph.html)** Best, On Thu, Apr 30, 2015 at 4:15 PM, Mostafa A. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > > I have another question. How can I connect the input and output port of > > IRAN > Tel: +98 (919) 158-7730 > LAB: [http://ele.aut.ac.ir/~mmwcl/?page\_id=411](http://ele.aut.ac.ir/~mmwcl/?page_id=411) > Homepage: [http://ele.aut.ac.ir/~alizadeh/](http://ele.aut.ac.ir/~alizadeh/) > > * * * – * * * Department of Electrical Engineering Aboureyhan Building MMWCL LAB Amirkabir University Of Technology Tehran IRAN Tel: +98 (919) 158-7730 LAB: [http://ele.aut.ac.ir/~mmwcl/?page\_id=411](http://ele.aut.ac.ir/~mmwcl/?page_id=411) Homepage: [http://ele.aut.ac.ir/~alizadeh/](http://ele.aut.ac.ir/~alizadeh/) * * * ---
**Author:** ![marco\_Ribero](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/e9c0ed/32.png) [@marco\_Ribero](https://www.ruby-forum.com/u/marco_Ribero)\ **Post date:** [April 30, 2015, 2:57pm UTC](https://www.ruby-forum.com/t/message-passing-connect-in-c/241788/4 "2015-04-30T14:57:19Z")
I want to exchange data between adjacent blocks which are already connected through the GUI, without ask to the user to write additional code. So my approach would be: the upper block send down its name(through tags) and the lower block configure the communication, so I thought to take advantage from message passing. If I won’t be able to use this approach, I will use some memory shared mechanism. Just a question,in order to use the gr::flowgraph:connect(),I need to get the object flowgraph from the XML file situated in GRC,isn’t it? Il giorno gio 30 apr 2015 alle ore 13:48 Mostafa A. \< removed\_email\_address@domain.invalid\> ha scritto: ---
**Author:** ![marco\_Ribero](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/e9c0ed/32.png) [@marco\_Ribero](https://www.ruby-forum.com/u/marco_Ribero)\ **Post date:** [April 30, 2015, 10:45pm UTC](https://www.ruby-forum.com/t/message-passing-connect-in-c/241788/5 "2015-04-30T22:45:17Z")
Marcus, thank you very much for your deep answer. Your proposal about the usage of your\_block::detail()-\>input(0)-\>buffer()-\>link seem very interesting, so I shoulld evaluate if use this mechanism or only some memory allocation created with malloc. I’d prefer to simplify the life of user as much as possible…I’ll put my blocks on a repository, hoping that some user will use these blocks. Your proposal of a hierarcly approach is interesting,but in my case could be more problematic: user can have a DAG or graph flowgraph,and the reserch of the next block,performed by each involved block,could add some complexity, considering that I need to open connection only between adjacent blocks. I summarize what I’m doing: as thesis ,I’m trying to make a partial porting over CUDA…so I’m re-implementing blocks with CUDA. Each block allocate a circular buffer of device pages,passing these pointer with other info to the next block(using a tag with a pointer to host memory). Now I want to establish an initial handshake, because the following block could have preferences(usually not mandatory,because often the first step of a block is to copy data from global to shared memory,allowing some degree of freedom) about incoming data(e.g. min-max dimension of each page,min\_multiple for input data of FFT,…). Each block receive data throught device memory,elaborate all data and return a single useless tap,in order to wake up other blocks…at the end of chain,data will be copied into host memory. Marco ---
**Author:** ![marco\_Ribero](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/e9c0ed/32.png) [@marco\_Ribero](https://www.ruby-forum.com/u/marco_Ribero)\ **Post date:** [April 30, 2015, 4:38pm UTC](https://www.ruby-forum.com/t/message-passing-connect-in-c/241788/6 "2015-04-30T16:38:58Z")
Hi Marco, ok, I think it’s best if I describe a bit of what happens behind the curtains: GRC is really just a graphical generator for python code. As soon as you hit the “generate” button (or the run button, where generating is done inherently), python code is written that has no connection, knowledge or dependency to the XML that was used to generate it. From a user-friendliness point of view, I can fully understand your approach to take the burden to explicitly connect up- and downstream blocks off your user’s shoulders. You can actually hack together something like that even without the use of tags, by just implementing a start() method in your block. You’d pass a top\_block::sptr to the constructor, and then, in your\_block::start() get the upstream block doing your\_block::detail()-\>input(0)-\>buffer()-\>link(), check that block for being your other block type, and if not, continue the search through the flow graph in the same matter. After you’ve found your other block, ou use your stored top\_block::sptr to -\>msg\_connect(other\_block, this\_block). However: that breaks a lot of assumptions I would made when looking at a flow graph, mainly the assumption that downstream blocks don’t invisibly influence behaviour of upstream blocks. Honestly, letting your user connect your two blocks with msg\_connect doesn’t sound like a bad idea in the long run. Or you come up with a hier-block-based approach: – are sample streams, ## message passing ``` .----------------------------------------------. | your_hier_block | |----------------------------------------------| | | | .------------------------. | | | your_upstream_block | | | |------------------------| | | --->| |---------------->to ``` user blocks | | | | | | | ‘------------------------’ | ---------------\>|— ^ | | # | | # | | .------------------------. | | | your\_downstream\_block | | | |------------------------| | | | |\<----------------from user blocks | | | | | ‘------------------------’ | | | | ‘--------------------|-------------------------’ ------------------------------\>to further processing Best regards, Marcus