# Fortran to Julia structs with strings

**URL:** <https://discourse.julialang.org/t/fortran-to-julia-structs-with-strings/6662>\
**Category:** General Usage\
**Tags:** fortran\
**Created:** [October 24, 2017, 6:16pm UTC](https://discourse.julialang.org/t/fortran-to-julia-structs-with-strings/6662 "2017-10-24T18:16:23Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![raminammour](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/raminammour/32/13572_2.png) [@raminammour](https://discourse.julialang.org/u/raminammour)\
**Post date:** [October 24, 2017, 6:16pm UTC](https://discourse.julialang.org/t/fortran-to-julia-structs-with-strings/6662/1 "2017-10-24T18:16:23Z")

</div>

Hello all,

I am having trouble interfacing `structs` between Julia and Fortran. After reading around, I figured that the best way is to use `iso_c_binding` and that Fortran strings are not portable (is that true?), so I went with arrays of `c_char`.

I tried to define the array of chars as `Ptr{UInt8}` or `Vector{UInt8}` but got many segfaults, I have had more success with `NTulpe{N,UInt8}` but it is still not behaving.

Below is a minimal working example:

Fortran side:

```fortran
module m_struct
  use,intrinsic :: iso_c_binding
  implicit none

  type,bind(C) :: t_struct

  integer :: a
  real :: b
  complex :: c
  logical :: d
  character(C_CHAR),dimension(16) :: e

  end type t_struct

contains

  subroutine init_struct(struct,a,b,c,d,e) bind(C,name="init_struct")
    type(t_struct), intent(inout) :: struct
    integer, intent(in) :: a
    real, intent(in) :: b
    complex, intent(in) :: c
    logical, intent(in) :: d
    character(C_CHAR),dimension(16),intent(in) :: e

    struct%a=a
    struct%b=b
    struct%c=c
    struct%d=d
    struct%e=e
   

  end subroutine init_struct

  subroutine print_struct(struct) bind(C,name="print_struct")
    type(t_struct),intent(in) :: struct

    print *,'a=',struct%a
    print *,'b=',struct%b
    print *,'c=',struct%c
    print *,'d=',struct%d
    print *,'e=',struct%e

  end subroutine print_struct

end module m_struct

```

Julia side:

```julia

const structlib="./libstruct_iso_c.so"
const init_struct=(:init_struct,structlib)
const print_struct=(:print_struct,structlib)

mutable struct j2f_struct
           a::Int32
           b::Float32
           c::Complex64
           d::Bool
	   e::NTuple{16,UInt8}
end

```

```julia

julia> aa=(ntuple(i->convert(UInt8,' '),16))
(0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20)

julia> bb=map(i->convert(UInt8,i),('h','e','l','l','o'))
(0x68, 0x65, 0x6c, 0x6c, 0x6f)

julia> bb=(bb...,ntuple(i->convert(UInt8,' '),11)...)
(0x68, 0x65, 0x6c, 0x6c, 0x6f, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20)

julia> my_jstruct= j2f_struct(0,0,0,false,aa);

julia> my_jstruct2= j2f_struct(0,0,0,false,bb);

julia> ccall(print_struct,Void,(Ptr{j2f_struct},),Ref(my_jstruct))
 a= 0
 b= 0.0000000E+00
 c= (0.0000000E+00,0.0000000E+00)
 d= F
 e=             

julia> ccall(print_struct,Void,(Ptr{j2f_struct},),Ref(my_jstruct2))
 a= 0
 b= 0.0000000E+00
 c= (0.0000000E+00,0.0000000E+00)
 d= F
 e=lo ## NOTE MISSING 3 CHARACTERS

ccall(init_struct,Void,(Ref{j2f_struct},Ref{Int32},Ref{Float32},Ref{Complex64},Ref{Bool},Ref{NTuple{16,UInt8}}),Ref(my_jstruct),1,2.,3.2+5.*im,true,Ref(bb))

julia> ccall(print_struct,Void,(Ptr{j2f_struct},),Ref(my_jstruct))
 a= 1
 b= 2.000000    
 c= (3.200000,5.000000)
 d= T
 e=hello #PRINTS FINE

my_jstruct.e
(0x2b, 0x00, 0x00, 0x68, 0x65, 0x6c, 0x6c, 0x6f, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20)

bb
(0x68, 0x65, 0x6c, 0x6c, 0x6f, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20)

```

Can someone explain to me what is happening with the first 3 characters, or how to fix this? More generally, what is the way to interface with Fortran strings.

Thank you for your assistance 🙂

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<div class="post-metadata">

**Author:** ![danielmatz](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/danielmatz/32/2285_2.png) [@danielmatz](https://discourse.julialang.org/u/danielmatz)\
**Post date:** [October 24, 2017, 9:13pm UTC](https://discourse.julialang.org/t/fortran-to-julia-structs-with-strings/6662/2 "2017-10-24T21:13:28Z")

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I think it’s because Julia’s `Bool` isn’t the same size as Fortran’s `logical`. Check out `sizeof(Bool)` in Julia. I think you want to use the type `Cint` instead.

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<div class="post-metadata">

**Author:** ![raminammour](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/raminammour/32/13572_2.png) [@raminammour](https://discourse.julialang.org/u/raminammour)\
**Post date:** [October 24, 2017, 9:22pm UTC](https://discourse.julialang.org/t/fortran-to-julia-structs-with-strings/6662/3 "2017-10-24T21:22:18Z")

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Thank you, that solves it!

When I had tested the `Bool` without the `char` array it was the last field, so it “behaved correctly” though it was wrong.