New tricks learned in C

learnt.c

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

typedef struct vec3D vec3D;
struct vec3D {
  float x, y, z;
};

_Static_assert(sizeof(vec3D) == 12, "May not work");

vec3D *add_vec3D(vec3D *a, vec3D *b, int size) {
  vec3D *ret = calloc(sizeof(vec3D), size);
  assert(ret != NULL);
  for (int i = 0; i < size; i++) {
    ret[i].x += a[i].x + b[i].x;
    ret[i].y += a[i].y + b[i].y;
    ret[i].z += a[i].z + b[i].z;
  }
  return ret;
}
int main() {
  const int len = 3;
  vec3D *matrix3x3 = add_vec3D(
      (vec3D[len]){
          [0].x = 1,
          [0].y = 2,
          [0].z = 0,
          [1].x = 0,
          [1].y = 0,
          [1].z = 0,
          [2].x = 0,
          [2].y = 0,
          [2].z = 5,
      },
      (vec3D[len]){
          [0].x = 1,
          [0].y = 3,
          [0].z = 0,
          [1].x = 0,
          [1].y = 0,
          [1].z = 0,
          [2].x = 0,
          [2].y = 0,
          [2].z = 2,
      },
      len);
  for (int i = 0; i < len; i++) {
    printf("%c %f %f %f\n", "xyz"[i], (&matrix3x3[0].x)[i],
           (&matrix3x3[1].x)[i], (&matrix3x3[2].x)[i]);
  }
  free(matrix3x3);
  return 0;
}