dynamic vector implementation

vec.c

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

typedef int i32;
typedef unsigned int u32;
typedef long long i64;
typedef unsigned long long u64;

struct _DynamicVectorHead {
  u32 cap;
  u32 size;
  char content[1];
};

#define vec(T) T*
#define vec_head(v) ((struct _DynamicVectorHead*)((u64)v - offsetof(struct _DynamicVectorHead, content)))
#define vec_content(vh) ((void*)vh->content)
#define vec_capacity(v) ((vec_head(v))->cap)
#define vec_size(v) ((vec_head(v))->size)
#define vec_new(T, size) ((T*)(_vec_new(sizeof(T), size)->content))
#define vec_free(v) free(vec_head(v))
#define vec_push(v, p) \
{ \
  struct _DynamicVectorHead* vh = vec_head(v); \
  if(vh->cap == vh->size) \
  { \
    vh = _vec_resize(vh, sizeof(*v)); \
    v = vec_content(vh); \
  }\
  v[vh->size] = p; \
  vh->size++; \
}
#define vec_erase(v, i, n) _vec_erase(vec_head(v), i, n, sizeof(*v))

struct _DynamicVectorHead* _vec_new(u32 dsize, u32 size)
{
  struct _DynamicVectorHead* v = malloc(sizeof(struct _DynamicVectorHead) + dsize * size - 1);
  v->cap = size;
  v->size = 0;
  return v;
}

struct _DynamicVectorHead* _vec_resize(struct _DynamicVectorHead* v, u32 dsize)
{
  struct _DynamicVectorHead* new = _vec_new(dsize, v->cap * 1.5);
  new->size = v->size;
  memmove(new->content, v->content, dsize * v->cap);
  free(v);
  return new;
}

void _vec_erase(struct _DynamicVectorHead* v, u32 idx, u32 n, u32 dsize)
{
  memmove((void*)((u64)v->content + (idx * dsize)), (void*)((u64)(v->content) + (idx * dsize) + (n * dsize)), (v->size * dsize) - (idx * dsize) - (n * dsize));
  v->size -= n;
}

int main()
{
  struct Vector3 { i32 x, y, z; };
  vec(struct Vector3) v = vec_new(struct Vector3, 4);

  // create some dummy data
  for(u32 i = 0; i < 20 * 3; i += 3)
    vec_push(v, ((struct Vector3){ i, i + 1, i + 2}));

  // erase the first two items
  vec_erase(v, 0, 2);

  // print the vec
  for(u32 i = 0; i < vec_size(v); i++)
    printf("[%2d] <%2d, %2d, %2d>\n", i, v[i].x, v[i].y, v[i].z);

  // free the memory allocated for the vec
  vec_free(v);

  return 0;
}