### Author:  [@gpxl](https://discourse.julialang.org/u/gpxl)
#### Post date: [February 5, 2022, 12:35am UTC](https://discourse.julialang.org/t/using-ccall-to-modify-fields-of-julia-struct/75835/1 "2022-02-05T00:35:27Z")
I am totally new to Julia in the last week, and I have decided to dive in and try to write a wrapper for a library I frequently use in my research. This library has structs which get initialized by passing them to a function which sets the values appropriately. Here is some simplified C code which reflects this (not from the actual library as that would be too long to include a MWE here):
```julia
#include
typedef char Msg[2048];
struct myStruct1{
double field1;
double * field2;
};
struct myStruct2{
double field3;
};
int init_myStruct(struct myStruct1 * s1, struct myStruct2 * s2, Msg m){
s1->field1 = 5.;
s1->field2 = &(s1->field1);
s2->field3 = *(s1->field2);
const char* tmp = "Initialized";
strcpy(m, tmp);
return 1;
}
```
I am trying to wrap this in Julia. The goal here is that I can compute what I need using this library, and once that is done I can do the rest of my analysis in Julia. To do so, I have create analogues of these structs in Julia, and have I think initialized them to be empty and filled later. Here is my Julia code:
```julia
struct JStruct1
field1::Cdouble
field2::Ptr{Cdouble}
JStruct1() = new()
end
struct JStruct2
field3::Cdouble
JStruct2() = new()
end
struct MyJuliaStruct
s1::JStruct1
s2::JStruct2
m::NTuple{2048, Cchar}
function MyJuliaStruct()
s1 = JStruct1()
s2 = JStruct2()
message=ntuple(x->convert(Cchar, 0*x), 2048)
new(s1, s2, message)
end
end
function main()
s = MyJuliaStruct()
ccall((:init_myStruct, "libtest.so"), Cint, (Ref{JStruct1}, Ref{JStruct2}, NTuple{2048, Cchar}), Ref(s.s1), Ref(s.s2), s.m)
println(s.s1.field1)
println(s.m)
end
main()
```
When I run this however, I get as output:
```julia
0.0
(0, 0, 0, 0, [2048 zeroes here that I won't repeat], 0)
```
The ccall seems to have worked fine but my values are not appropriately modified. I thought that by passing the Julia structs as `Ref(s.s1)`, the memory could be managed by both C and Julia and that this would work. Can anyone point me in the right direction? I have looked at some similar topics but haven’t been able to figure it out. Thanks for reading!
---
### Author:  [@lucifer1004](https://discourse.julialang.org/u/lucifer1004)
#### Post date: [February 5, 2022, 12:48am UTC](https://discourse.julialang.org/t/using-ccall-to-modify-fields-of-julia-struct/75835/2 "2022-02-05T00:48:24Z")
Have you tried mutable struct?
---
### Author:  [@Gnimuc](https://discourse.julialang.org/u/Gnimuc)
#### Post date: [February 5, 2022, 3:32am UTC](https://discourse.julialang.org/t/using-ccall-to-modify-fields-of-julia-struct/75835/3 "2022-02-05T03:32:02Z")
> [@gpxl](#):
>
> I thought that by passing the Julia structs as `Ref(s.s1)` , the memory could be managed by both C and Julia and that this would work.
This is not right. It’s basically wrong if you simply translate `&x` to `Ref(x)` in `ccall`. It should be something like:
```julia
x_ref = Ref(x)
ret = ccall(..., x_ref)
x = x_ref[]
ret
```
> [@gpxl](#):
>
> `ccall((:init_myStruct, "libtest.so"), Cint, (Ref{JStruct1}, Ref{JStruct2}, NTuple{2048, Cchar}), Ref(s.s1), Ref(s.s2), s.m)`
`NTuple{2048, Cchar}` → `Ptr{Cchar}`: In C, array types in the function proto should decay to corresponding pointer types.
`struct MyJuliaStruct` → `mutable struct MyJuliaStruct`: This should be mutable if you’re gonna change the fields. if you’d like to keep it immutable, you could use it in this way:
```julia
s = MyJuliaStruct()
s1_ref = Ref(s.s1)
s2_ref = Ref(s.s2)
m_ref = Ref(s.m)
ccall((:init_myStruct, "libtest.so"), Cint, (Ptr{JStruct1}, Ptr{JStruct2}, Ptr{Cchar}), s1_ref, s2_ref, m_ref)
s = MyJuliaStruct(s1_ref[], s2_ref[], m_ref[])
```
Or you could simply do:
```julia
s1_ref = Ref{JStruct1}()
s2_ref = Ref{JStruct2}()
m_ref = Ref{NTuple{2048, Cchar}}()
ccall((:init_myStruct, "libtest.so"), Cint, (Ptr{JStruct1}, Ptr{JStruct2}, Ptr{Cchar}), s1_ref, s2_ref, m_ref)
s = MyJuliaStruct(s1_ref[], s2_ref[], m_ref[])
```
---
### Author:  [@gpxl](https://discourse.julialang.org/u/gpxl)
#### Post date: [February 5, 2022, 11:43pm UTC](https://discourse.julialang.org/t/using-ccall-to-modify-fields-of-julia-struct/75835/5 "2022-02-05T23:43:45Z")
I somehow deleted my previous post, but that is OK as I want to modify it anyway. Thank you very much for your suggestions, they have been very helpful. I have implemented them as you suggest, so that my main() function is now:
```julia
function main()
s1_ref = Ref{JStruct1}()
s2_ref = Ref{JStruct2}()
m_ref = Ref{NTuple{2048, Cchar}}()
ccall((:init_myStruct, "libtest.so"), Cint, (Ptr{JStruct1}, Ptr{JStruct2}, Ptr{Cchar}), s1_ref, s2_ref, m_ref)
s = MyJuliaStruct(s1_ref[], s2_ref[], m_ref[])
println(s.s1.field1)
println(s.m)
end
main()
```
Running this gives the following error:
```julia
LoadError: UndefRefError: access to undefined reference
Stacktrace:
[1] getproperty
@ ./Base.jl:42 [inlined]
[2] unsafe_convert
@ ./refvalue.jl:42 [inlined]
[3] main()
@ Main ~/projects/misc/wrapping/cwrap.jl:28
[4] top-level scope
@ ~/projects/misc/wrapping/cwrap.jl:34
```
Do you have any idea why this might be happening? It is coming from the ccall itself because I get the same error even when commenting out lines after that.
---
### Author:  [@Gnimuc](https://discourse.julialang.org/u/Gnimuc)
#### Post date: [February 6, 2022, 4:46am UTC](https://discourse.julialang.org/t/using-ccall-to-modify-fields-of-julia-struct/75835/6 "2022-02-06T04:46:50Z")
> [@gpxl](#):
>
> ```julia
> struct JStruct2
> field3::Cdouble
> JStruct2() = new()
> end
>
> ```
You don’t need to define `JStruct2() = new()` for isbitstypes, but you could also use `Ref(JStruct1())` instead, if you’d like to keep it.