# CxxWrap-based wrapper throws error in running finalizer: ReadOnlyMemoryError()
**URL:** https://discourse.julialang.org/t/cxxwrap-based-wrapper-throws-error-in-running-finalizer-readonlymemoryerror/67097
**Category:** General Usage
**Tags:** cxx, cxxwrap
**Created:** [August 27, 2021, 12:26pm UTC](https://discourse.julialang.org/t/cxxwrap-based-wrapper-throws-error-in-running-finalizer-readonlymemoryerror/67097 "2021-08-27T12:26:16Z")
**Posts on this page:** 2
**Page:** 1
### Author:  [@stemann](https://discourse.julialang.org/u/stemann)
#### Post date: [August 27, 2021, 12:26pm UTC](https://discourse.julialang.org/t/cxxwrap-based-wrapper-throws-error-in-running-finalizer-readonlymemoryerror/67097/1 "2021-08-27T12:26:16Z")
Hi
I am updating a CxxWrap-based wrapper for a C++ library from Julia 1.0 and CxxWrap 0.8.1 - with succesful unit tests - and a dev container - to Julia 1.6 (1.3+) and the latest released CxxWrap, 0.11.2: [https://github.com/IHPSystems/pylon\_julia\_wrapper](https://github.com/IHPSystems/pylon_julia_wrapper)
Now, I am getting some errors I do not quite know how to debug - it looks like some finalizers are accessing invalid memory (the errors do not crash the Julia process though):
```julia
Precompiling project...
✓ Libiconv_jll
✓ XML2_jll
✓ EzXML
✓ TestReports
✓ PylonWrapper
5 dependencies successfully precompiled in 7 seconds (8 already precompiled)
[ Info: Testing PylonWrapper
error in running finalizer: ReadOnlyMemoryError()
unknown function (ip: 0x19698a0)
error in running finalizer: ReadOnlyMemoryError()
unknown function (ip: 0x19698a0)
error in running finalizer: ReadOnlyMemoryError()
unknown function (ip: 0x19698a0)
```
On [this branch](https://github.com/IHPSystems/pylon_julia_wrapper/tree/feature/julia_1.6)
The wrapper is wrapping a couple of methods that [return](https://github.com/IHPSystems/pylon_julia_wrapper/blob/feature/julia_1.6/src/pylon_wrapper.cpp#L382) and [take](https://github.com/IHPSystems/pylon_julia_wrapper/blob/feature/julia_1.6/src/pylon_wrapper.cpp#L262) pointers to abstract C++ interfaces, `IPylonDevice*`, which gives off a warning [when wrapping the abstract C++ interface “data type”](https://github.com/IHPSystems/pylon_julia_wrapper/blob/feature/julia_1.6/src/pylon_wrapper.cpp#L252): I am wondering whether that may be the source of error:
```julia
[build] In file included from /root/.julia/artifacts/16e1de4679fb8520a8af4e6831c7c8e9893d18b4/include/jlcxx/jlcxx.hpp:14:
[build] /root/.julia/artifacts/16e1de4679fb8520a8af4e6831c7c8e9893d18b4/include/jlcxx/module.hpp:892:5: warning: delete called on 'Pylon::IDevice' that is abstract but has non-virtual destructor [-Wdelete-non-virtual-dtor]
[build] delete to_delete;
[build] ^
[build] /root/.julia/artifacts/16e1de4679fb8520a8af4e6831c7c8e9893d18b4/include/jlcxx/module.hpp:937:34: note: in instantiation of function template specialization 'jlcxx::detail::finalize' requested here
[build] mod.method("__delete", detail::finalize);
[build] ^
[build] /root/.julia/artifacts/16e1de4679fb8520a8af4e6831c7c8e9893d18b4/include/jlcxx/module.hpp:1183:5: note: in instantiation of function template specialization 'jlcxx::add_default_methods' requested here
[build] add_default_methods(*this);
[build] ^
[build] /root/.julia/artifacts/16e1de4679fb8520a8af4e6831c7c8e9893d18b4/include/jlcxx/module.hpp:1194:10: note: in instantiation of function template specialization 'jlcxx::Module::add_type_internal, _jl_datatype_t>' requested here
[build] return add_type_internal(name, super);
[build] ^
[build] /workspaces/pylon_julia_wrapper/src/pylon_wrapper.cpp:249:10: note: in instantiation of function template specialization 'jlcxx::Module::add_type, _jl_datatype_t>' requested here
[build] module.add_type("IDevice")
[build] ^
[build] In file included from /workspaces/pylon_julia_wrapper/src/pylon_wrapper.cpp:2:
[build] In file included from /root/.julia/artifacts/16e1de4679fb8520a8af4e6831c7c8e9893d18b4/include/jlcxx/jlcxx.hpp:14:
[build] /root/.julia/artifacts/16e1de4679fb8520a8af4e6831c7c8e9893d18b4/include/jlcxx/module.hpp:892:5: warning: delete called on 'Pylon::IPylonDevice' that is abstract but has non-virtual destructor [-Wdelete-non-virtual-dtor]
[build] delete to_delete;
[build] ^
[build] /root/.julia/artifacts/16e1de4679fb8520a8af4e6831c7c8e9893d18b4/include/jlcxx/module.hpp:937:34: note: in instantiation of function template specialization 'jlcxx::detail::finalize' requested here
[build] mod.method("__delete", detail::finalize);
[build] ^
[build] /root/.julia/artifacts/16e1de4679fb8520a8af4e6831c7c8e9893d18b4/include/jlcxx/module.hpp:1183:5: note: in instantiation of function template specialization 'jlcxx::add_default_methods' requested here
[build] add_default_methods(*this);
[build] ^
[build] /root/.julia/artifacts/16e1de4679fb8520a8af4e6831c7c8e9893d18b4/include/jlcxx/module.hpp:1194:10: note: in instantiation of function template specialization 'jlcxx::Module::add_type_internal, _jl_datatype_t>' requested here
[build] return add_type_internal(name, super);
[build] ^
[build] /workspaces/pylon_julia_wrapper/src/pylon_wrapper.cpp:252:10: note: in instantiation of function template specialization 'jlcxx::Module::add_type, _jl_datatype_t>' requested here
[build] module.add_type("IPylonDevice", jlcxx::julia_base_type());
[build] ^
[build] 6 warnings generated.
```
But I also get these finalizer errors [when I just pass `void*`](https://github.com/IHPSystems/pylon_julia_wrapper/blob/feature/julia_1.6_voidptr/src/pylon_wrapper.cpp#L262) (6 errors) instead of `IPylonDevice*` (3 errors):
```julia
[ Info: Testing PylonWrapper
error in running finalizer: ReadOnlyMemoryError()
unknown function (ip: 0x2191870)
error in running finalizer: ReadOnlyMemoryError()
unknown function (ip: 0x1ef91c0)
error in running finalizer: ReadOnlyMemoryError()
unknown function (ip: 0x1ef91c0)
error in running finalizer: ReadOnlyMemoryError()
unknown function (ip: 0x1ef91c0)
error in running finalizer: ReadOnlyMemoryError()
unknown function (ip: 0x24a0ff0)
error in running finalizer: ReadOnlyMemoryError()
unknown function (ip: 0x2317300)
error in running finalizer: ReadOnlyMemoryError()
unknown function (ip: 0x18dfd50)
```
The wrapper has to work around [a blocking C++ call](https://github.com/IHPSystems/pylon_julia_wrapper/blob/master/src/pylon_wrapper.cpp#L41), in order to get/wait for frames from a camera without blocking the Julia thread, so [it spins up](https://github.com/IHPSystems/pylon_julia_wrapper/blob/master/src/pylon_wrapper.cpp#L255) a C++ `std::thread`, but this C++ thread is only interacting with the Julia runtime by calling `uv_async_send`: [https://github.com/IHPSystems/pylon\_julia\_wrapper/blob/master/src/pylon\_wrapper.cpp#L51](https://github.com/IHPSystems/pylon_julia_wrapper/blob/master/src/pylon_wrapper.cpp#L51)
Any ideas for resolving warnings or getting rid of the errors?
---
### Author:  [@stemann](https://discourse.julialang.org/u/stemann)
#### Post date: [August 29, 2021, 8:58am UTC](https://discourse.julialang.org/t/cxxwrap-based-wrapper-throws-error-in-running-finalizer-readonlymemoryerror/67097/2 "2021-08-29T08:58:49Z")
Don’t spend time on this - I am quite sure the cause for the finalizer errors is due to the wrapped library and neither Julia, nor CxxWrap: The wrapped type destructors and thus the finalizers are failing due to the wrapped type trying to release the same resource more than once.