Passing a BigInt (mpz_t for GMP) array to C

General Usage
1

Lately, I’ve managed to pass a BigInt to C and back (Passing GMP variables from C(++) to Julia - #10 by Questioner). Now suppose I want to generate Fibonacci numbers in C and pass them to Julia. I thought I just take the analogous code for arrays

#include<gmp.h>

void test(mpz_t* var[]) {
	mpz_init_set_ui(*var[1], 2);
}

but upon calling it

@ccall "./art.so".test(x::Ptr{Ref{BigInt}})::Cvoid

where x was

 x
2-element Vector{Ref{BigInt}}:
 Base.RefValue{BigInt}(3)
 Base.RefValue{BigInt}(3)

it ran, but crashed upon typing “x” and hitting enter, producing a lengthy error message.

I’d be grateful for any input on how to solve this problem.

2

I’ve finally found a workaround: I’m first creating an array in Julia

grr = Array{BigInt}(undef, 2)

then I save pointers to the elements of that array into a different array

arr = Array{Ptr{Nothing}}(undef, 2)
for j in 1:2
   arr[j] = pointer_from_objref(grr[j])
end

This last array is what I pass to C:

@ccall "./gmptest.so".modify2(arr::Ref{Ptr{Nothing}})::Cvoid

C then operates on arr, like in the following code:

void modify2(mpz_t* arr[]) {
	mpz_set_ui(*arr[1], 10);
}
3

Can’t you just pass grr to modify2 directly? i.e.

@ccall "./gmptest.so".modify2(grr::Ptr{BigInt})::Cvoid

since a Vector{BigInt} is stored under the hood as an array of pointers to BigInt objects (which aren’t stored inline since they are mutable structs), and BigInt objects are laid out identically to mpz_t (so that they can be passed by reference to GMP functions).