No Memory Alignment with GCC

结构体内存对齐技巧

最新推荐文章于 2020-06-26 19:04:17 发布

原创最新推荐文章于 2020-06-26 19:04:17 发布 · 1k 阅读

0 ·

CC 4.0 BY-SA版权

文章标签：

C++ 专栏收录该内容

19 篇文章

订阅专栏

本文介绍了使用GCC编译器时如何通过属性和预处理器指令控制结构体成员的内存对齐方式，包括使用__attribute__((packed))和#pragmapack来减少结构体的大小，以及如何使用-fpack-struct选项进一步定制对齐策略。

attribute method:

#include <stdio.h>

struct packed
{
    char a;
    int b;
} __attribute__((packed));

struct not_packed
{
    char a;
    int b;
};

int main(void)
{
    printf("Packed:     %zu\n", sizeof(struct packed));
    printf("Not Packed: %zu\n", sizeof(struct not_packed));
    return 0;
}

Output:

$ make example && ./example
cc     example.c   -o example
Packed:     5
Not Packed: 8

pragma pack method:

#include <stdio.h>

#pragma pack(1)
struct packed
{
    char a;
    int b;
};
#pragma pack()

struct not_packed
{
    char a;
    int b;
};

int main(void)
{
    printf("Packed:     %zu\n", sizeof(struct packed));
    printf("Not Packed: %zu\n", sizeof(struct not_packed));
    return 0;
}

Output:

$ make example && ./example
cc     example.c   -o example
Packed:     5
Not Packed: 8

Add -fpack-struct to GCC
-fpack-struct[=n]
Without a value specified, pack all structure members together without holes. When a value is specified (which must be a small power of two), pack structuremembers according to this value, representing the maximum alignment (that is, objects with default alignment requirements larger than this will be outputpotentially unaligned at the next fitting location.
Warning: the -fpack-struct switch causes GCC to generate code that is not binary compatible with code generated without thatswitch. Additionally, it makes the code suboptimal. Use it to conform to a non-default application binary interface.