# Problem with undefined symbol in new block created by gr\_modtool in 3.7.2 **URL:** \ **Category:** GNU Radio\ **Created:** [March 5, 2014, 8:59pm UTC](https://www.ruby-forum.com/t/problem-with-undefined-symbol-in-new-block-created-by-gr-modtool-in-3-7-2/239587 "2014-03-05T20:59:04Z")\ **Posts on this page:** 7\ **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:** [March 5, 2014, 8:59pm UTC](https://www.ruby-forum.com/t/problem-with-undefined-symbol-in-new-block-created-by-gr-modtool-in-3-7-2/239587/1 "2014-03-05T20:59:04Z")
Hello all, I am dealing with the problem of undefined symbol when trying to execute a python script for testing my new custom block created in GNU Radio 3.7.2. I used gr\_modtool for creating the new block and also followed the instructions posted at [http://gnuradio.org/redmine/projects/gnuradio/wiki/OutOfTreeModulesConfig](http://gnuradio.org/redmine/projects/gnuradio/wiki/OutOfTreeModulesConfig) for correctly configuring it but the problem still holds. Any ideas? Thanks, -George ---
**Author:** ![glsmyth](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/a9adbd/32.png) [@glsmyth](https://www.ruby-forum.com/u/glsmyth)\ **Post date:** [March 5, 2014, 9:23pm UTC](https://www.ruby-forum.com/t/problem-with-undefined-symbol-in-new-block-created-by-gr-modtool-in-3-7-2/239587/2 "2014-03-05T21:23:01Z")
-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA256 Hi George, This is a little unspecific. Most probably you have a problem similar to the fftw- related problem of activecat, but that’s only guessing… Please heed [http://gnuradio.org/redmine/projects/gnuradio/wiki/ReportingErrors](http://gnuradio.org/redmine/projects/gnuradio/wiki/ReportingErrors) when reporting errors. Greetings, Marcus On March 5, 2014 8:57:46 PM CET, George S. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > * * * > > * * * > > Discuss-gnuradio mailing list > [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) > [Discuss-gnuradio Info Page](https://lists.gnu.org/mailman/listinfo/discuss-gnuradio) * * * Sent from my Android device with K-9 Mail. Please excuse my brevity. -----BEGIN PGP SIGNATURE----- Version: APG v1.0.9 iQFABAEBCAAqBQJTF4c7IxxNYXJjdXMgTfxsbGVyIDxtYXJjdXNAaG9zdGFsaWEu ZGU+AAoJEBKNLRrpJvfojQoH/2a609iWLTsywtozQ+FcZkuQT6uCy13vX3n4b5k3 rTvy9QP3uHuHQkiRZHDbZoJ+mZ9kFOf6NeG20RGdf8i4yCB1AZ8F7uvvdi7CebXT sRQxoQE+8Vz5VS+faAStAOK3+tArdRGZOU2v3gTYXGCvATaeM3A+rxsNQ7LKPwM8 sI5KZ3xgvgikkObbFuo00MOaLA7tHzMttXxWjU5riXann1l1eOyGIzM1j0XO1gAD imofs0DfXHB4cE0Gi43E1gjjeqEW034dLZAS7w/c/Gq85Z0Ni+z1a35kS5UYvYhQ ndeEMqrLQv4ghCRnHR93ob+8YRlqMedVjDDeKQdjEh0D2jY= =27ny -----END PGP SIGNATURE----- ---
**Author:** ![glsmyth](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/a9adbd/32.png) [@glsmyth](https://www.ruby-forum.com/u/glsmyth)\ **Post date:** [March 5, 2014, 9:24pm UTC](https://www.ruby-forum.com/t/problem-with-undefined-symbol-in-new-block-created-by-gr-modtool-in-3-7-2/239587/3 "2014-03-05T21:24:13Z")
What is the “undefined symbol”? This could be something not linked properly inside the CMake files. Michael On Wed, Mar 5, 2014 at 11:57 AM, George S. \< ---
**Author:** ![glsmyth](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/a9adbd/32.png) [@glsmyth](https://www.ruby-forum.com/u/glsmyth)\ **Post date:** [March 5, 2014, 10:46pm UTC](https://www.ruby-forum.com/t/problem-with-undefined-symbol-in-new-block-created-by-gr-modtool-in-3-7-2/239587/4 "2014-03-05T22:46:13Z")
The first place I’d look is in your\_block.h. Did you remember to declare any public member functions to be pure virtual in that file, and then declare your (non-virtual) functions in your\_block\_impl.h? If you forgot to declare pure virtual functions, i.e., “virtual return\_type your\_block(args\_list) = 0;” in your\_block.h, it will compile but you will get the undefined symbol at runtime. This probably a common pitfall and it might be worth making a note in the wiki for OutOfTreeModules. I’ll add that. Architects of gr\_modtool… can you add an autogen comment in \*.h files reminding the user to declare pure virtual functions? I think users who don’t deeply understand OO and particularly its C++ syntax would benefit from this reminder. I think it’s a common paradigm to use other examples to fill in the “guts” of gr\_modtool generated C++ code, but things like this are easy to miss. Sean ---
**Author:** ![glsmyth](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/a9adbd/32.png) [@glsmyth](https://www.ruby-forum.com/u/glsmyth)\ **Post date:** [March 5, 2014, 11:03pm UTC](https://www.ruby-forum.com/t/problem-with-undefined-symbol-in-new-block-created-by-gr-modtool-in-3-7-2/239587/5 "2014-03-05T23:03:34Z")
Thanks everyone. In order to be more specific regarding the future readers of this post, I am using Ubuntu 12.04 and GNU Radio v3.7.2. I am using the embedded gr\_modtool in order to create a custom signal processing block from scratch. My block does not use FFT. Finally, the problem was solved by just ignoring the implementation header file and only include the public header file in the /include/nameofBlock . ## Thanks, -George George S. ---
**Author:** ![glsmyth](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/a9adbd/32.png) [@glsmyth](https://www.ruby-forum.com/u/glsmyth)\ **Post date:** [March 5, 2014, 10:51pm UTC](https://www.ruby-forum.com/t/problem-with-undefined-symbol-in-new-block-created-by-gr-modtool-in-3-7-2/239587/6 "2014-03-05T22:51:36Z")
Ah, nevermind about the wiki edit. The use of pure virtual functions is already discussed in [http://gnuradio.org/redmine/projects/gnuradio/wiki/BlocksCodingGuide](http://gnuradio.org/redmine/projects/gnuradio/wiki/BlocksCodingGuide) . Still, the gr\_modtool addition may be useful. ---
**Author:** ![glsmyth](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/a9adbd/32.png) [@glsmyth](https://www.ruby-forum.com/u/glsmyth)\ **Post date:** [March 6, 2014, 10:56am UTC](https://www.ruby-forum.com/t/problem-with-undefined-symbol-in-new-block-created-by-gr-modtool-in-3-7-2/239587/7 "2014-03-06T10:56:54Z")
On 05.03.2014 22:44, Nowlan, Sean wrote: > Architects of gr\_modtool… can you add an autogen comment in \*.h > files reminding the user to declare pure virtual functions? I think > users who don’t deeply understand OO and particularly its C++ syntax > would benefit from this reminder. I think it’s a common paradigm to > use other examples to fill in the “guts” of gr\_modtool generated C++ > code, but things like this are easy to miss. Disciples of gr\_modtool… When you create a block w/ modtool, it has zero public methods (except for make(), which is arguably special, and gets auto-created in the impl.cc file). So if I put in a reminder, it will actually remind people before there’s a reason to declare a function, which might confuse people. Perhaps a more general instruction on how to add public methods would be good, though. I’ll think about this. M