# Fortran to Julia with a nested struct

**URL:** https://discourse.julialang.org/t/fortran-to-julia-with-a-nested-struct/79099
**Category:** New to Julia
**Tags:** fortran, struct
**Created:** [April 6, 2022, 1:55pm UTC](https://discourse.julialang.org/t/fortran-to-julia-with-a-nested-struct/79099 "2022-04-06T13:55:13Z")
**Posts on this page:** 2
**Page:** 1

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### Author: ![james3](https://avatars.discourse-cdn.com/v4/letter/j/b5e925/32.png) [@james3](https://discourse.julialang.org/u/james3)
#### Post date: [April 6, 2022, 1:55pm UTC](https://discourse.julialang.org/t/fortran-to-julia-with-a-nested-struct/79099/1 "2022-04-06T13:55:13Z")

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This is following on from a question I had here [https://discourse.julialang.org/t/fortran-to-julia-struct/79081](https://discourse.julialang.org/t/fortran-to-julia-struct/79081).

I’m still trying to get Julia speaking to a Fortran library. My problem now is dealing with nested structs / derived types. An example (building on my previous MWE) is as follows.

Fortran code:

```julia
module fortranjulia
  
  use, intrinsic :: iso_c_binding
  implicit none

  type, bind(c) :: n_t
    integer(c_int) :: b1
  end type n_t

  type, bind(c) :: m_t
    integer(c_int) :: a1
    real(c_double) :: a2(3)
    type(n_t) :: a3
    type(n_t) :: a4(2)
  end type m_t

contains

  subroutine print_mytype(m) bind(C, name = "print_mytype")
    type(m_t), intent(in) :: m
    print *, m%a1
    print *, m%a2
    print *, m%a3%b1
    print *, m%a4(1)%b1
    print *, m%a4(2)%b1
  end subroutine print_mytype

end module

```

The Julia code is

```julia
using StaticArrays
using Parameters

@with_kw mutable struct n_t
    b1::Cint = 99
end

@with_kw mutable struct m_t
    a1::Cint = 123456
    a2::SVector{3, Cdouble} = [1.0, 2.0, 3.0]
    a3::n_t = n_t()
    a4::SVector{2, n_t} = @SVector [n_t() for _ = 1:2]
end

m = m_t()
println("Julia")
println(m.a1)
println(m.a2)
println(m.a3.b1)
println(m.a4[1].b1)
println(m.a4[2].b1)

println("Fortran")
ccall((:print_mytype, "/path/to/fortran_julia"), Cvoid, (Ref{m_t},), m)

```

All the Fortran output from the nested struct is garbage.

```julia
Julia
123456
[1.0, 2.0, 3.0]
99
99
99
Fortran
      123456
   1.00000000000000 2.00000000000000 3.00000000000000     
  1594405776
       32554
  1594405792

```

Any help with this is appreciated. Thank you.

---

<div class="post-metadata">

### Author: ![stevengj](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stevengj/32/71_2.png) [@stevengj](https://discourse.julialang.org/u/stevengj)
#### Post date: [April 6, 2022, 2:01pm UTC](https://discourse.julialang.org/t/fortran-to-julia-with-a-nested-struct/79099/2 "2022-04-06T14:01:51Z")

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> [@james3](#):
>
> All the Fortran output from the nested struct is garbage.

Don’t use `mutable struct`, use `struct`. The problem is that `a3::n_t` for a mutable struct is (loosely) analogous to `struct nt *a3` in C.

The reason is that if you do something like:

```julia
m = m_t()
x = m.a3 # a "pointer" to the same object as m.a3, not a copy!
x.b1 = 2

```

then you need to get `m.a3.b1 == 2` — this is what it means to have a mutable object, and it requires mutable-struct fields to be references (“pointers”), not stored inline as in a nested C struct or nested immutable struct.

Then, when you call the Fortran routine, construct an object `mref = Ref{m_t}()` to pass to the `ccall`, after which you can run `m = mref[]` to extract the struct data.