# Wrapping c code and packaging the wrapper
**URL:** https://discourse.julialang.org/t/wrapping-c-code-and-packaging-the-wrapper/32363
**Category:** New to Julia
**Created:** [December 17, 2019, 12:40am UTC](https://discourse.julialang.org/t/wrapping-c-code-and-packaging-the-wrapper/32363 "2019-12-17T00:40:20Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@AlfTetzlaff](https://discourse.julialang.org/u/AlfTetzlaff)
#### Post date: [December 17, 2019, 12:40am UTC](https://discourse.julialang.org/t/wrapping-c-code-and-packaging-the-wrapper/32363/1 "2019-12-17T00:40:20Z")
Hey everybody!
I want to wrap a .so library file which provides access to the cameras of a certain vendor (ZWO ASI). For a start I need it for home use with possible distribution later. However, I don’t manage to create a package which I can import to other projects. I took a look into BinDeps, BinaryBuilder, BinaryProvider and what not and my head is already dizzy - I think this is all way over the top, as my .so file resides on my disk.
My current folder structure looks like this:
```julia
LibASICamera (this is the package i want to have)
|--src
| LibASICamera.jl
|
|--gen (contents auto-generated by Clang.jl)
ctypes.jl
LibASICamera_api.jl
LibASICamera_common.jl
ASICamera2 (this is where the c code resides)
|--include
| ASICamera2.h
|
|--lib/x64
libASICamera2.so
```
Here an example function from LibASICamera\_api.jl:
```julia
function ASIGetNumOfConnectedCameras()
ccall((:ASIGetNumOfConnectedCameras, libASICamera2), Cint, ())
end
```
One can see that the variable libASICamera2 is undefined at this point. So I thought it is best to point Libdl to the library folder and define the variable in LibASICamera.jl:
```julia
module LibASICamera
import Libdl
using CEnum
push!(Libdl.DL_LOAD_PATH, "/home/.../ASICamera2/lib/x64")
libASICamera2 = "libASICamera2"
include(joinpath(@ __DIR__ , "..", "gen", "ctypes.jl"))
include(joinpath(@ __DIR__ , "..", "gen", "LibASICamera_common.jl"))
include(joinpath(@ __DIR__ , "..", "gen", "LibASICamera_api.jl"))
export ...
end # module
```
This does not work. When I load the package by “using LibASICamera” and call the example function from above I get an error:
```julia
TypeError: in ccall: first argument not a pointer or valid constant expression, expected Ptr, got Tuple{Symbol,String} ASIGetNumOfConnectedCameras() at LibASICamera_api.jl:6
```
On the contrary, when I use the REPL and interactively push the load path and call the function, everything works as expected. Therefore I assume that the library was not loaded correctly and/or that I have a misconception about how loading libraries works.
What do I have to change to make it work? 🙂
---
### Author:  [@GunnarFarneback](https://discourse.julialang.org/u/GunnarFarneback)
#### Post date: [December 17, 2019, 6:43am UTC](https://discourse.julialang.org/t/wrapping-c-code-and-packaging-the-wrapper/32363/2 "2019-12-17T06:43:05Z")
I would start with
```julia
const libASICamera2 = "libASICamera2"
```
Then I would skip the `DL_LOAD_PATH` and use a full path directly for `libASICamera2`.
---
### Author:  [@AlfTetzlaff](https://discourse.julialang.org/u/AlfTetzlaff)
#### Post date: [December 17, 2019, 2:12pm UTC](https://discourse.julialang.org/t/wrapping-c-code-and-packaging-the-wrapper/32363/3 "2019-12-17T14:12:09Z")
Thank you! For some reason I don’t understand declaring the variable const did the trick.
---
### Author:  [@GunnarFarneback](https://discourse.julialang.org/u/GunnarFarneback)
#### Post date: [December 19, 2019, 1:43pm UTC](https://discourse.julialang.org/t/wrapping-c-code-and-packaging-the-wrapper/32363/4 "2019-12-19T13:43:52Z")
`ccall` is a part of the low-level infrastructure of Julia and despite the looks not an ordinary function. The need for `const` here is a consequence of that.