D problems with static arrays and C interoperability - interop

Question

I have problems passing D struct that contains static array to C.
D code:
extern (C){
struct vec3f{
float[3] array;
}
void testVec3f(vec3f v);
}
...
void test(){
float[3] ar = [1f,2f,3f];
vec3f v = vec3f(ar);
testVec3f(v);
}
C code:
extern "C" struct vec3f{
float array[3];
};
extern "C" void testVec3f(vec3f a){
printf("x=%f, y=%f, z=%f\n", a.array[0], a.array[1], a.array[2]);
}
Result: x=0.000000, y=0.000000, z=0.000000.
I also checked that both structs in D and C have the same size - 12 bytes.

Answer 1

You are going to run into a whole host of trouble trying to pass structs by value between C and D; especially if your D compiler uses gnu compiler on the back end whereas you may be using clang++ for some parts of your project.
The way gnu c++ and clang++ treat object passing is different, extern "C" adds another layer of complexity. You are best off avoiding all of that and pass your values by reference:
Here your example (working correctly) using clang-c++ and dmd:
test.d:
import std.stdio;
struct vec3f {
float [3] array;
}
extern (C) {
void print_vec3f(vec3f *v);
}
void main()
{
vec3f v = vec3f([1f, 2f, 3f]);
print_vec3f(&v);
}
vect3.cc:
#include <stdio.h>
#ifdef __cplusplus
extern "C" {
#endif
struct vec3f {
float array[3];
};
void print_vec3f(struct vec3f *v)
{
printf("x: %f, y: %f, z: %f\n", v->array[0], v->array[1], v->array[2]);
}
#ifdef __cplusplus
};
#endif
Compile using:
clang++ -o vect3.o -c vect3.cc
dmd test.d vect3.o
./test
x: 1.000000, y: 2.000000, z: 3.000000