malloc + size_t * 3 的地址对于任何类型都是对齐的吗？

我正在构建一种动态数组（向量），但不是嵌入数据（通常是void *) 变成struct vector，我正在预留空间struct vector+ 一大块字节，使用数组的示例size_t's:

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

struct vector {
    size_t capacity;
    size_t typesize;
    size_t size;
};

#define VECTOR(v) ((struct vector *)((unsigned char *)v - sizeof(struct vector)))

static void *valloc(size_t typesize, size_t size)
{
    struct vector *vector;
    unsigned char *data;

    data = calloc(1, sizeof(*vector) + typesize * size);
    if (data == NULL) {
        return NULL;
    }
    vector = (struct vector *)data;
    vector->typesize = typesize;
    vector->capacity = size;
    vector->size = 0;
    return data + sizeof(*vector);
}

static void *vadd(void *data)
{
    struct vector *vector = VECTOR(data);
    unsigned char *new;
    size_t capacity;

    if (vector->size >= vector->capacity) {
        capacity = vector->capacity * 2;
        new = realloc(vector, sizeof(*vector) + vector->typesize * capacity);
        if (new == NULL) {
            return NULL;
        }
        vector = (struct vector *)new;
        vector->capacity = capacity;
        vector->size++;
        return new + sizeof(*vector);
    }
    vector->size++;
    return data;
}

static size_t vsize(void *data)
{
    return VECTOR(data)->size;
}

static void vfree(void *data, void (*func)(void *))
{
    struct vector *vector = VECTOR(data);

    if (func != NULL) {
        for (size_t iter = 0; iter < vector->size; iter++) {
            func(*(void **)((unsigned char *)data + iter * vector->typesize));
        }
    }
    free(vector);
}

int main(void)
{
    size_t *data;

    data = valloc(sizeof(size_t), 1);
    if (data == NULL) {
        perror("valloc");
        exit(EXIT_FAILURE);
    }
    for (size_t i = 0; i < 10; i++) {
        data = vadd(data);
        if (data == NULL) {
            perror("vadd");
            exit(EXIT_FAILURE);
        }
        data[i] = i;
    }
    for (size_t i = 0; i < vsize(data); i++) {
        printf("%zu\n", data[i]);
    }
    vfree(data, NULL);
    return 0;
}

问题是：结果的地址是malloc (void *) + 的大小struct适合任何类型？这段代码的行为定义是否明确？

malloc + size_t * 3 的地址对于任何类型都是对齐的吗？

不，不一定。

确定基本对齐通过使前缀数据也为基本对齐.

一种方法是使用union of struct vector and max_align_t。的大小union uvector将是的倍数基本对齐.

max_align_t这是一种对象类型，其对齐方式与实现所支持的一样大 在所有情况下；

union uvector {
  max_align_t a;
  struct vector {
    size_t capacity;
    size_t typesize;
    size_t size;
  } v;
};

union uvector *uvector;
unsigned char *data;

data = calloc(1, sizeof *uvector ...
...
return data + sizeof *uvector;

答案有一些低效 - 在另一个答案中重新设计..