C语言#pragma pack(n)和__attribute__((aligned(m)))的介绍

一、简单介绍

2、__attribute__((aligned(m)))告诉编译器一个结构体或者类或者联合或者一个类型的变量(对象)分配地址空间时的地址对齐方式。也就是所，如 果将__attribute__((aligned(m)))作用于一个类型，那么该类型的变量在分配地址空间时，其存放的地址一定按照m字节对齐(m必 须是2的幂次方)。并且其占用的空间，即大小,也是m的整数倍，以保证在申请连续存储空间的时候，每一个元素的地址也是按照m字节对齐。

二、实例测试学习

1、例子一，下面的这个例子，如果按自然边界对齐，f5的偏移地址应该是0x10，但设置#pragma pack(4)后f5的偏移地址后变为0xc

测试c代码：

#include<stdio.h>
#pragma pack(4) 
//#pragma pack()//取消1字节对齐，恢复为默认4字节对齐
typedef struct{unsigned int f1;unsigned char f2;unsigned char  f3;unsigned int f4;unsigned long int f5;//}__attribute__((aligned(1024))) ts;
//}__attribute__((packed)) ts;
}ts;
int main(){int n ;ts st1;  printf("\nsizeof(unsigned int)=%d , sizeof(unsigned long int)=%d\n",sizeof(unsigned int),sizeof(unsigned long int));printf("Allocate f1 on address: 0x%x\n",&(st1.f1));printf("Struct size is: %d\n",sizeof(ts));printf("Allocate f1 on address: 0x%x\n",&(((ts*)0)->f1));printf("Allocate f2 on address: 0x%x\n",&(((ts*)0)->f2));printf("Allocate f3 on address: 0x%x\n",&(((ts*)0)->f3));printf("Allocate f4 on address: 0x%x\n",&(((ts*)0)->f4));printf("Allocate f5 on address: 0x%x\n",&(((ts*)0)->f5));return 0;}

2、测试实例二，将一个结构体的对齐方式设置为__attribute__((aligned(1024)))，你会发现，原本大小不到1024个字节的结构体，实际的占用的空间会是1024.其存放的地址一定按照1024字节对齐(即1024的倍数)。

测试c代码

#include<stdio.h>
//#pragma pack(4)
//#pragma pack(4) //取消1字节对齐，恢复为默认4字节对齐
//#pragma pack()
typedef struct{unsigned int f1;unsigned char f2;unsigned char  f3;unsigned int f4;unsigned long int f5;}__attribute__((aligned(1024))) ts;
//}__attribute__((packed)) ts;
//}ts;
int main(){int n ;ts st1;  printf("\nsizeof(unsigned int)=%d , sizeof(unsigned long int)=%d\n",sizeof(unsigned int),sizeof(unsigned long int));printf("Allocate f1 on address: 0x%x\n",&(st1.f1));printf("Struct size is: %d\n",sizeof(ts));printf("Allocate f1 on address: 0x%x\n",&(((ts*)0)->f1));printf("Allocate f2 on address: 0x%x\n",&(((ts*)0)->f2));printf("Allocate f3 on address: 0x%x\n",&(((ts*)0)->f3));printf("Allocate f4 on address: 0x%x\n",&(((ts*)0)->f4));printf("Allocate f5 on address: 0x%x\n",&(((ts*)0)->f5));return 0;}

 编译&执行