Passing structures and arrays back and forth in Julia with ccall

test_ccall.jl

immutable Cdvec
    val::Ptr{Cdouble}
    len::Clong
end

a = [1.0, 2.0, 3.0]
b = [3.1, 4.2, 7.3]

Ca = Cdvec(pointer(a), length(a))
Cb = Cdvec(pointer(b), length(b))
Cc = Ptr{Cdvec}[C_NULL]

push!(DL_LOAD_PATH, ".")

x = unsafe_load(ccall((:get_dvec, "test_ccall_lib"), Ptr{Cdvec}, ()))

l = ccall((:get_dvec_ptr, "test_ccall_lib"), Cint, (Ptr{Cdvec},), Cc)

err = ccall((:dvec_sum, "test_ccall_lib"), Int32, (Ptr{Cdvec}, Ptr{Cdvec}, Ptr{Cdvec}), &Ca, &Cb, Cc)

c = unsafe_load(Cc[1])

if err != 0
    print("Error: vectors have different length")
else
    for i = 1:(c.len)
        println(unsafe_load(c.val, i))
    end
end

test_ccall_lib.c

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

typedef struct dvec {
    double* val;
    long len;
} dvec;

typedef struct lvec {
    long* val;
    long len;
} lvec;

int dvec_sum(dvec* a, dvec* b, dvec** c) {
    printf("Original c address: %p\n", c);
    if (a->len != b->len) {
	return -1; // different lengths, can't sum
    } else {
	int l = a->len;
	dvec *cc = (dvec*)malloc(sizeof(dvec));
	cc->val = (double*) malloc(sizeof(double)*l);
	cc->len = l;
	for(int i = 0; i < l; i++) {
	    cc->val[i] = a->val[i] + b->val[i];
	    printf("%f\n", cc->val[i]);
	}
	*c = cc;
	printf("Return val address: %p\n", c);
	return 0;
    }
}

dvec* get_dvec() {
    dvec* c = (dvec*)malloc(sizeof(dvec));
    c->val = (double*) malloc(sizeof(double)*4);
    c->val[0] = 1.0;
    c->val[1] = 2.0;
    c->val[2] = 3.0;
    c->val[3] = 4.0;
    c->len = 4;
    return c;
}

int get_dvec_ptr(dvec** cc) {
    dvec *c = (dvec*)malloc(sizeof(dvec));
    c->val = (double*) malloc(sizeof(double)*4);
    c->val[0] = 1.0;
    c->val[1] = 2.0;
    c->val[2] = 3.0;
    c->val[3] = 4.0;
    c->len = 4;
    *cc = c;
    return c->len;
}

This is the output:

julia> require("test_ccall.jl")
4.100000
6.200000
10.300000
ERROR: type Array has no field len
 in anonymous at no file:20
 in reload_path at loading.jl:144
 in _require at loading.jl:59
 in require at loading.jl:43
while loading /home/jfsantos/Projects/test_ccall/test_ccall.jl, in expression starting on line 17

julia> Cc
1-element Array{Ptr{T},1}:
 Ptr{Void} @0x0000000000000000