# Return matrix of Cchar from Fortran library

**URL:** <https://discourse.julialang.org/t/return-matrix-of-cchar-from-fortran-library/80430>\
**Category:** New to Julia\
**Tags:** question, fortran\
**Created:** [May 3, 2022, 2:43pm UTC](https://discourse.julialang.org/t/return-matrix-of-cchar-from-fortran-library/80430 "2022-05-03T14:43:26Z")\
**Posts on this page:** 3\
**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:** [May 3, 2022, 2:43pm UTC](https://discourse.julialang.org/t/return-matrix-of-cchar-from-fortran-library/80430/1 "2022-05-03T14:43:26Z")

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I have been having difficulty getting a Fortran subroutine return a matrix of `Cchar` to Julia (contained in a struct; in practice there are other components in `s_t` below). On the Fortran side I have

```julia
module test

    use iso_c_binding

    type, bind(c) :: s_t
        character(kind = c_char) :: fmat(8, 2)
    end type

contains

    pure function copy_s2a(s) result(a)
        implicit none
        character(len = *), intent(in) :: s
        character(kind = c_char) :: a(len(s))
        integer :: i
        do i = 1, len(s)
            a(i) = s(i:i)
        end do
    end function copy_s2a

    subroutine c_matrixChar(s) bind(C, name = "C_MatrixChar")
        implicit none
        type(s_t), intent(out) :: s
        s%fmat(:, 1) = copy_s2a('mat1' // c_null_char)
        s%fmat(:, 2) = copy_s2a('mat2' // c_null_char)
    end subroutine c_matrixChar

end module

```

While on the Julia side I have

```julia
using StaticArrays

struct s_t
    fmat::SMatrix{8,2,Cchar}
end

function test()
    sref = Ref{s_t}()
    ccall((:C_MatrixChar, /path/to/lib.so), Cvoid, (Ref{s_t},), sref)
    return sref[]
end

s = test()
println("s.fmat")
println(s.fmat[:,1])

```

This results in a segmentation fault whenever I try to access `fmat`. (So this would occur in the final line of the code.)

Note that if I just had a vector of `Cchar`, e.g. I had `fvec::SVector(8)` and `character(kind = c_char) :: fvec(8)` then this would run as expected and I would be able to convert the `Cchar` array to a string.

What am I doing wrong here?

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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:** [May 3, 2022, 3:25pm UTC](https://discourse.julialang.org/t/return-matrix-of-cchar-from-fortran-library/80430/2 "2022-05-03T15:25:57Z")

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I wonder if it’s because `SMatrix{8, 2, Cchar}` isn’t concrete. This catches me a lot. There’s a fourth type parameter that needs to be set to the total number of elements, in this case 16. Try making the type of that field `SMatrix{8, 2, Cchar, 16}`.

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<div class="post-metadata">

**Author:** ![james3](https://avatars.discourse-cdn.com/v4/letter/j/b5e925/32.png) [@james3](https://discourse.julialang.org/u/james3)\
**Post date:** [May 3, 2022, 3:28pm UTC](https://discourse.julialang.org/t/return-matrix-of-cchar-from-fortran-library/80430/3 "2022-05-03T15:28:31Z")

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This works. Thanks a lot. This had been driving me crazy trying to figure this one out…