# [Avoid cfunction closure] GSL root finding problem with function passed as argument

I have a usage scenario in which the functions that represent the equations for the multidimentional root-finding problem are generated in run time. I use GSL.jl to solve the root-finding problem, which requires wrapping the functions for the C library.

What I used to do is to make use of the `@cfunction` runtime enclosure. But, now I realize that this is not supported on ARM platforms. The way how I do that is illustrated below. So, I need to somehow circumvent this issue with `@cfunction`. How would that be done?

What I used to do is something like below (what matters the most is just the beginning part). The use of `@cfunction` is giving me error on Apple Silicon.

``````ERROR: cfunction: closures are not supported on this platform
``````
``````using GSL
# For Mac
using LinearAlgebra, OpenBLAS32_jll
LinearAlgebra.BLAS.lbt_forward(OpenBLAS32_jll.libopenblas_path)

function solve!(f!, x0::AbstractArray, maxiter)
# Do what GSL.@gsl_multiroot_function does to wrap the function
function _f(x_vec, p, y_vec)
x = GSL.wrap_gsl_vector(x_vec)
y = GSL.wrap_gsl_vector(y_vec)
f!(y, x)
return Cint(GSL.GSL_SUCCESS)
end
# Does not work on ARM !
cfunc = @cfunction(\$_f, Cint, (Ptr{GSL.gsl_vector}, Ptr{Cvoid}, Ptr{GSL.gsl_vector}))
N = length(x0)
gsl_func = GSL.gsl_multiroot_function(Base.unsafe_convert(Ptr{Cvoid}, cfunc), N, 0)

# The remaining code solves the root-finding problem
vinit = GSL.vector_alloc(N)
solver = GSL.multiroot_fsolver_alloc(GSL.gsl_multiroot_fsolver_hybrids, N)

for (i, v) in enumerate(x0)
v = convert(Cdouble, v)
GSL.vector_set(vinit, i-1, v)
end

GSL.multiroot_fsolver_set(solver, gsl_func, vinit)

iter = 0
while iter < maxiter
iter += 1
status = GSL.multiroot_fsolver_iterate(solver)
end
x = GSL.multiroot_fsolver_root(solver)
# Copy the solution
sol = Vector{Float64}(unsafe_wrap(Array{Cdouble}, GSL.vector_ptr(x, 0), N))
GSL.multiroot_fsolver_free(solver)
GSL.vector_free(vinit)
return sol
end

# The function for the actual scenario is only known in run time
function f!(x::Array{Float64}, out::Array{Float64})
out[1:5] =  (x .- collect(1.0:5.0)).^2
end

x0 = [1.0, 5.0, 2.0, 1.5, -1.0]

solve!(f!, x0, 5)
``````