# Fortran calling convention when passing an array with unknown size
**URL:** https://discourse.julialang.org/t/fortran-calling-convention-when-passing-an-array-with-unknown-size/107057
**Category:** General Usage
**Tags:** ccall, fortran
**Created:** [December 2, 2023, 8:20pm UTC](https://discourse.julialang.org/t/fortran-calling-convention-when-passing-an-array-with-unknown-size/107057 "2023-12-02T20:20:23Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@wujinq](https://discourse.julialang.org/u/wujinq)
#### Post date: [December 2, 2023, 8:20pm UTC](https://discourse.julialang.org/t/fortran-calling-convention-when-passing-an-array-with-unknown-size/107057/1 "2023-12-02T20:20:23Z")
In modern Fortran when an array is passed to a subroutine, we don’t really need to specify its size: the size can be read from `size(arr)`. So the follows
```fortran
function my_length(arr)
real(8), intent(in), dimension(:) :: arr
integer :: my_length
my_length = size(arr)
end function my_length
```
works.
Now suppose I’m going to pass a Julia array to some Fortran code, and definitely the latter needs to know the size of the array. If I naively compile the Fortran `my_length` into a `so` file, then then call the function in that `so` file from Julia, then that’s what I get:
```julia
arr = [1.0, 2.0, 3.0]
res = @ccall "./fortran-lib.so".my_length_(arr::Ptr{Float64})::Int32
println(res)
```
```julia
0
```
So since the Julia vector is passed to `my_length_` as a pointer, the length information expectedly is not received by `my_length_`.
What makes me curious then is how the size of the array is passed to `my_length` when the latter is called from _Fortran_, or in other words, what exactly is the calling convention of a Fortran function accepting an array with unknown size: is the size being passed to the function as a hidden argument, or something else? If so is it possible to not explicitly declare a size argument in the Fortran part of the code, and do
```julia
@ccall "./fortran-lib.so".my_length_(arr::Ptr{Float64},
arr_size::Ref{Int32} # Argument not explicitly declared in the Fortran code but still there
)::Int32
```
at the Julia side?
I don’t know if there is a standard concering this or it’s compiler dependent.
---
### Author:  [@Ralph\_Smith](https://discourse.julialang.org/u/Ralph_Smith)
#### Post date: [December 2, 2023, 10:17pm UTC](https://discourse.julialang.org/t/fortran-calling-convention-when-passing-an-array-with-unknown-size/107057/2 "2023-12-02T22:17:11Z")
Such a Fortran routine requires that you provide the array argument in the form of an “array descriptor” which is a compiler-dependent data structure. If you’re curious about these, search for information about `ISO_Fortran_binding` (which allows C code to call modern Fortran).
---
### Author:  [@wujinq](https://discourse.julialang.org/u/wujinq)
#### Post date: [December 2, 2023, 10:26pm UTC](https://discourse.julialang.org/t/fortran-calling-convention-when-passing-an-array-with-unknown-size/107057/3 "2023-12-02T22:26:59Z")
Yeah I realized the problem is not really about Julia, but about how to call Fortran from C (and `@ccall` follows the C convention). There is also a stack exchange question about that: [Fortran90 and size of arrays created in C++ - Stack Overflow](https://stackoverflow.com/questions/9844326/fortran90-and-size-of-arrays-created-in-c).
It will be very helpful if someone can make a `@fcall` macro to make calling Fortran subroutine slightly easier…