# Difficulties putting code in module (on an M1 machine) (pre-compilation, cfunctions, global variables)

**URL:** <https://discourse.julialang.org/t/difficulties-putting-code-in-module-on-an-m1-machine-pre-compilation-cfunctions-global-variables/84726>\
**Category:** General Usage\
**Tags:** module, precompilation, modules\
**Created:** [July 24, 2022, 9:42pm UTC](https://discourse.julialang.org/t/difficulties-putting-code-in-module-on-an-m1-machine-pre-compilation-cfunctions-global-variables/84726 "2022-07-24T21:42:00Z")\
**Posts on this page:** 5\
**Page:** 1

<div class="post-metadata">

**Author:** ![f.ij](https://avatars.discourse-cdn.com/v4/letter/f/8491ac/32.png) [@f.ij](https://discourse.julialang.org/u/f.ij)\
**Post date:** [July 24, 2022, 9:42pm UTC](https://discourse.julialang.org/t/difficulties-putting-code-in-module-on-an-m1-machine-pre-compilation-cfunctions-global-variables/84726/1 "2022-07-24T21:42:01Z")

</div>

I’m building an interactive simulation of a Spin Ising model and I’ve been experiencing some difficulties. I’m using [QML](https://github.com/barche/QML.jl/tree/qt6) to build an interface. Before, I had everything in a large script with a lot of global variables, and I’ve been trying to get everything neatly into a module, with all parameters in a struct `Sim`, which can be initialised when the module is used. The struct also can be called as a function to start the simulation interface. One problem I’m facing is that the QML packages (at least the way I’m using it) needs a c function as a method to update an image that is displayed. This c function is initialised as follows:

```julia
function showlatest(buffer::Array{UInt32, 1}, width32::Int32, height32::Int32)
    buffer = reshape(buffer, size(img[]))
    buffer = reinterpret(RGB, buffer)
    buffer .= img[]
    return
end

showlatest_cfunction = CxxWrap.@safe_cfunction(showlatest, Cvoid, 
                                               (Array{UInt32,1}, Int32, Int32))

```

The CxxWrap.@safe\_cfunction is defined as

```julia
macro safe_cfunction(f, rt, args)
  return esc(:($(@ __MODULE__ ).SafeCFunction(@cfunction($f, $rt, $args), $rt, [$(args.args...)])))
end

```

where

```julia
struct SafeCFunction
  fptr::Union{Ptr{Cvoid},Base.CFunction}
  return_type::Type
  argtypes::Array{Type,1}
end

```

In the showlatest function, there is a variable named `img`, which should be a Ref to a matrix. Ideally, I would store this variable only in my struct `Sim`, and define the SafeCFunction after a user has initialised an instance of the `Sim` struct, which should then have a local copy of the img reference. However, the macro CxxWrap.@safe\_cfunction, is of course run at compile time, so doing something like

```julia
function create_showlatest_cfunc(sim::Sim)
          img = sim.img
  function showlatest
    ...
  end
  
  showlatest_cfunction = CxxWrap.@safe_cfunction(showlatest, Cvoid, 
                                                 (Array{UInt32,1}, Int32, Int32))

end

```

gives the error that showlatest is not defined. I tried interpolating showlatest into the macro, e.g.:

```julia
...
showlatest_cfunction = CxxWrap.@safe_cfunction($showlatest, Cvoid, 
                                                 (Array{UInt32,1}, Int32, Int32))

```

but as it turns out, since I’m on an M1 machine, closures aren’t supported on the apple arm architecture, so this won’t work.

Before I had everything in a module, I would just have a global const variable called `img` and everything worked out.

What I tried now is just having a `const img` in the global scope of my module, and storing the same ref inside my struct. This way Sim should only be instantiated once, but that’s fine for my purposes. At first this seemed to be working, but at the time, my module wasn’t compiling. Now it’s compiling, but as soon as I run my code (I think the moment the cfunction is called actually, but it’s hard to confirm this), I get an error and my REPL in vscode crashes and disappears before I can read it. Screen recording it reveals this error:

```julia
signal (10): Bus error: 10
in expression starting at none:0
unknown function (ip: 0x1757b46b0
...

This happens when I make my module files visible using `push!(LOAD_PATH, pwd())` and then `using IsingSim` in my main file. When I instead import the file into my main file, and `using .IsingSim`, I get a different error:

libc++abi: terminating with uncaught exception of type std::runtime_error: Could not find Module QML when looking up function get_julia_call

```

signal (6): Abort trap: 6  
in expression starting at none:0  
\_\_pthread\_kill at /usr/lib/system/libststem\_kernel.dylib (unknown line)  
…

I tried using @eval to define img in local scope

```julia
function showlatesteval(sim)
    img = sim.img
    function showlatest(buffer::Array{UInt32, 1}, width32::Int32, height32::Int32)
        buffer = reshape(buffer, size(img))
        buffer = reinterpret(ARGB32, buffer)
        buffer .= img
        return
    end

    @eval $:(CxxWrap.@safe_cfunction($showlatest, Cvoid, (Array{UInt32,1}, Int32, Int32)))
end

startSim(sim)
  showlatest_cfunction = showlatesteval(sim)
  ...
end

```

This gets me a bit further, and doesn’t throw the same error. I even see an image displayed, at first. However, my image should be (500,500), and I soon get one of two errors

```julia
UndefRefError: access to undefined reference

```

```julia
DimensionMismatch: new dimensions (10761626400, 4390640200) must be consistent with array size 250000

```

So it seems that my reference is getting invalidated somehow, though I don’t understand how.

I’m kind of at a loss right now, and don’t know how to continue. Any help would greatly be appreciated.

My full code can be found [here](https://github.com/rug-minds/Interactive-Spin-Ising-MCMC/tree/QT6_Module)

---

<div class="post-metadata">

**Author:** ![rdeits](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rdeits/32/286_2.png) [@rdeits](https://discourse.julialang.org/u/rdeits)\
**Post date:** [July 25, 2022, 1:05am UTC](https://discourse.julialang.org/t/difficulties-putting-code-in-module-on-an-m1-machine-pre-compilation-cfunctions-global-variables/84726/2 "2022-07-25T01:05:04Z")

</div>

Without getting into too much detail, it sounds like you need to move your initialization-time code into the ` __init__ ()` function, as described here: [Modules · The Julia Language](https://docs.julialang.org/en/v1/manual/modules/#Module-initialization-and-precompilation)

---

<div class="post-metadata">

**Author:** ![f.ij](https://avatars.discourse-cdn.com/v4/letter/f/8491ac/32.png) [@f.ij](https://discourse.julialang.org/u/f.ij)\
**Post date:** [July 26, 2022, 9:06am UTC](https://discourse.julialang.org/t/difficulties-putting-code-in-module-on-an-m1-machine-pre-compilation-cfunctions-global-variables/84726/3 "2022-07-26T09:06:04Z")

</div>

Thanks a lot! Putting the showlatest\_cfunction in the init function, keeping everything else in the global scope of my module, works!

I’m still not sure why the @eval trick didn’t work though.

---

<div class="post-metadata">

**Author:** ![f.ij](https://avatars.discourse-cdn.com/v4/letter/f/8491ac/32.png) [@f.ij](https://discourse.julialang.org/u/f.ij)\
**Post date:** [August 10, 2022, 10:43am UTC](https://discourse.julialang.org/t/difficulties-putting-code-in-module-on-an-m1-machine-pre-compilation-cfunctions-global-variables/84726/4 "2022-08-10T10:43:12Z")

</div>

Actually, coming back to this, there still appears to be a weird problem. I found this out because I got a new machine on which I did a clean install. I was getting the following problem again:

```julia
libc++abi: terminating with uncaught exception of type std::runtime_error: Could not find module QML when looking up function get_julia_call

signal(6): Abort trap: 6
In expression starting at none:0
__pthread_kill at /usr/lib/system/libststem_kernel.dylib (unknown line)
…

```

I was using the exact same code as on my old machine, on which the code executed without problem. The only difference was that I didn’t make a sysimage yet. Making a sysimage on the new machine fixes the problem. I found out that adding the line `using QML` in the main file where I include my own modules also fixed the problem (I already use QML in the module as well, of course). Naturally I want my module to be self contained, so this isn’t a great solution. What am I doing wrong?

---

<div class="post-metadata">

**Author:** ![f.ij](https://avatars.discourse-cdn.com/v4/letter/f/8491ac/32.png) [@f.ij](https://discourse.julialang.org/u/f.ij)\
**Post date:** [August 12, 2022, 10:12am UTC](https://discourse.julialang.org/t/difficulties-putting-code-in-module-on-an-m1-machine-pre-compilation-cfunctions-global-variables/84726/5 "2022-08-12T10:12:42Z")

</div>

I think that the problem is, that the external QML library (which is a c++ library) expects some module QML to be defined in the Main module, with the function get\_julia\_call to be there. However, since I’ve wrapped everything in my own module, the QML module is of course not defined in the Main module anymore.

I tried adding the line export QML in my own module and this seems to be doing the trick. It seems hacky though, so is there a better solution to do this?