c - malloc + size_t * 3 的地址是否为任何类型对齐？

Question

我正在构建一种动态数组( vector )，但不是将数据(通常是 void *)嵌入到 struct vector 中，而是保留struct vector + 字节 block 的空间，使用 size_t 数组的示例:

#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:malloc (void *) 的结果地址 + struct 的大小对于任何类型？这段代码的行为是否定义明确？

Answer 1

Is the address of malloc + size_t * 3 aligned for any type?

不，不一定。

通过使前缀数据也基本对齐来确定基本对齐。

一种方法是使用struct vector 和max_align_t 的union。 union uvector 的大小将是 基本对齐 的倍数。

max_align_t which is an object type whose alignment is as great as is supported by the implementation in all contexts;

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;

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答案有一些低效 - 在另一个答案中重新设计..