Modifying Tuples with Julia ccall

generated.jl

using CEnum

const mytype = NTuple{3, Cdouble}

struct mystruct
    x::mytype
    y::Ptr{mytype}
    n::Cint
end

function get()
    ccall((:get, libtest), Ptr{mystruct}, ())
end

function f(a)
    # ccall((:f, libtest), Cvoid, (mytype,), a)  # as generated by default (does not work)
    ccall((:f, libtest), Cvoid, (Ptr{mytype},), a)  # manually changed (works)
end

const N = 3

test.c

#include <stdio.h>
#include <stdlib.h>

#include "test.h"

// arbitrary code
mystruct * get(void){
    mystruct * r = malloc(sizeof(mystruct));
    for(int j = 0; j < N; j++){
        r->x[j] = 1;
    }
    int n = 5;
    r->y = malloc(n * sizeof(mystruct));
    for(int i = 0; i < n; i++){
        for(int j = 0; j < N; j++){
            r->y[i][j] = i + 1;
        }
    }
    r->n = n;
    return r;
}

// arbitrary code to modify a `mytype`
void f(mytype a){
    for(int j = 0; j < N; j++){
        a[j] = -a[j];
    }
}

test.h

#define N 3

typedef double mytype[N];  // no control over this typedef

typedef struct {    // no control over this typedef
   mytype x;
   mytype * y;
   int n;
} mystruct;  

mystruct * get(void); // retrieve mystruct object

void f(mytype a);   // modify mytype object

test.jl

include("LibTest.jl")

const libtest = "./libtest.so"

qp = get()
q = unsafe_load(qp)

function myprint()
    println("x = ", q.x[1])
    for i = 1:q.n
        println("y = ", unsafe_load(q.y, i))
    end
end

myprint()

f(Ptr{mytype}(qp))  # making a change to `x` member
for i = 1:q.n
    f(q.y + (i - 1) * sizeof(mytype))
end

q = unsafe_load(qp) # have to reload for changes in x member
myprint()