c - 使用 max_align_t 存储一大块字节

Question

在this thread我被建议使用 max_align_t 来为任何类型正确对齐地址，我最终创建了动态数组的这个实现:

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

struct vector {
    size_t capacity;
    size_t typesize;
    size_t size;
    max_align_t data[];
};

#define VECTOR(v) ((struct vector *)((unsigned char *)v - offsetof(struct vector, data)))

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

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

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((unsigned char *)vector->data + vector->typesize * iter);
        }
    }
    free(vector);
}

static void *vadd(void *data)
{
    struct vector *vector = VECTOR(data);
    struct vector *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;
        }
        new->capacity = capacity;
        new->size++;
        return new->data;
    }
    vector->size++;
    return vector->data;
}

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

static void vsort(void *data, int (*comp)(const void *, const void *))
{
    struct vector *vector = VECTOR(data);

    if (vector->size > 1) {
        qsort(vector->data, vector->size, vector->typesize, comp);
    }
}

static char *vgetline(FILE *file)
{
    char *data = valloc(sizeof(char), 32);
    size_t i = 0;
    int c;

    while (((c = fgetc(file)) != '\n') && (c != EOF)) {
        data = vadd(data);
        data[i++] = (char)c;
    }
    data = vadd(data);
    data[i] = '\0';
    return data;
}

struct data {
    int key;
    char *value;
};

static int comp_data(const void *pa, const void *pb)
{
    const struct data *a = pa;
    const struct data *b = pb;

    return strcmp(a->value, b->value);
}

static void free_data(void *ptr)
{
    struct data *data = ptr;

    vfree(data->value, NULL);
}

int main(void)
{
    struct data *data;

    data = valloc(sizeof(struct data), 1);
    if (data == NULL) {
        perror("valloc");
        exit(EXIT_FAILURE);
    }
    for (size_t i = 0; i < 5; i++) {
        data = vadd(data);
        if (data == NULL) {
            perror("vadd");
            exit(EXIT_FAILURE);
        }
        data[i].value = vgetline(stdin);
        data[i].key = (int)vsize(data[i].value);
    }
    vsort(data, comp_data);
    for (size_t i = 0; i < vsize(data); i++) {
        printf("%d %s\n", data[i].key, data[i].value);
    }
    vfree(data, free_data);
    return 0;
}

但我不确定我是否可以使用 max_align_t 来存储一大块字节:

struct vector {
    size_t capacity;
    size_t typesize;
    size_t size;
    max_align_t data[]; // Used to store any array,
                        // for example an array of 127 chars
}; 

它是否打破了数组规则的最后一个元素？

Answer 1

Does it break the one past the last element of an array rule?

没有。

Using max_align_t to store a chunk of bytes

OP 的问题并不特殊，因为它使用了一个灵活的数组成员。

As a special case, the last element of a structure ... have an incomplete array type; this is called a flexible array member. ... However, when a . (or ->) operator has a left operand that is (a pointer to) a structure with a flexible array member and the right operand names that member, it behaves as if that member were replaced with the longest array (with the same element type) ...

这与访问任何分配的内存或一种类型的数组是同一个问题，就好像它是另一种类型一样。

从 max_align_t * 到 char * 到 void * 的转换在正确完成对齐时定义明确.

A pointer to an object type may be converted to a pointer to a different object type. If the resulting pointer is not correctly aligned for the referenced type, the behavior is undefined. C11dr §6.3.2.3 7

所有经过审查的代码访问都不会尝试访问“as if”数组之外的内容。