C语言结构体对齐

March 21, 2024

摘自 C99 §6.7.2.1, item 13：https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf

Within a structure object, the non-bit-field members and the units in which bit-fields
reside have addresses that increase in the order in which they are declared. A pointer to a
structure object, suitably converted, points to its initial member (or if that member is a
bit-field, then to the unit in which it resides), and vice versa. There may be unnamed
padding within a structure object, but not at its beginning.

根据规范，结构体的第一个字段保证对齐这段内存的第一个字节。（当然，后面的字段就可能发生字节填充）

因此：可以接受一个指向结构体的指针，并安全地将其转换为指向其第一个字段的指针，反之亦然。

这有点像继承：下面的例子中，结构体 A 和 B 的第一个字段是结构体 Base，
因此保证可以安全地将 A* 或 B* “向下转换”为 Base*

// preview: https://ideone.com/rro44a
#include <stdio.h>
#include <inttypes.h>
#include <stdlib.h>

typedef enum
{
    a = 'a',
    b = 'b',
} BaseType;

typedef struct Base
{
    BaseType type;
} Base;

typedef struct A
{
    struct Base base;
    int a;
} A;

A *newA(int a)
{
    A *p = malloc(sizeof(A));
    p->base.type = 'a';
    p->a = a;
    return p;
}

typedef struct B
{
    struct Base base;
    int64_t b;
} B;

B *newB(int64_t b)
{
    B *p = malloc(sizeof(B));
    p->base.type = 'b';
    p->b = b;
    return p;
}

int main()
{
    A *a = newA(123);
    B *b = newB(456);

    Base *base;
    base = (Base *)a;
    printf("%c\n", base->type); // -> a

    base = (Base *)b;
    printf("%c\n", base->type); // -> b
}

另见：
https://stackoverflow.com/questions/53578631
https://en.wikipedia.org/wiki/Type_punning
https://hackmd.io/@sysprog/c-memory

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C语言 结构体对齐

C语言结构体对齐