- 为什么要地址对齐？
- * 我们在访问内存时，如果地址是按4字节对齐，则访问效率会高很多。
- * 这个问题的原因在于访问内存的硬件电路。一般情况下，地址总线总是按照对齐后的地址来访问。
- * 例如你想得到0x00000001开始的4字节内容，系统首先需要以0x00000000读4字节，
- * 然后从中取得3字节，然后在用0x00000004作为开始地址，获得下一个四字节，
- * 在从中得到第一个字节，两次组合出你想得到的内容。但是如果地址一开始就是对齐到
- * 0x00000000，则系统只要一次读写即可。
- 所以，为了性能考虑，编译器会做地址对齐。
- 地址对齐的原则：
- 每个基本类型有自己默认的对齐值。 然后可以通过#pragma pack(2) 设置编译器的默认对齐值。取两者最小的一个作为最终对齐值。
- 若没有#pragma pack(2) 设置，那么只看基本类型自己的对齐值。
- 总之： 在允许的情况下，尽量依次填充，但是需要保证（32位系统）
- 1 char,bool只从X地址处开始存储；
- 2 short只从2X地址出开始存储；
- 3 int，float，long只从4X地址处开始存储；
- 4 double只从8X地址处开始存储；
下面是列子：
#include <iostream>
using namespace std;
//sizeof(union1) = 16
union union1 {
long i; //4
int x[4]; //16
char ch; //1
};
//sizeof(union2) = 16
union union2{
char a[13];
int i; //由于int的存在，使得union按照四个字节来对其了
};
//sizeof(union3) = 13
union union3{
char a[13];
char i;
};
/////////////// 对比node1和node2 ///////////
struct node1{ //24
int j; //4
double f; //8
int i; //4
};
struct node2{ //16
int i; //4
int j; //4
double f; //8
};
/////////////// 对比node3和node4 ///////////
struct node3{ //32(注意)
char i; //1
int d[5]; //20
double f; //8
};
struct node4{ //40
int d[5]; //20
double f; //8
int i; //4
};
///////////////////////////////////////////
struct node5{ //32(注意)
char d[10]; //10
double f; //8
int i; //4
};
struct node6{ //20
char d; //1
int f; //4
char i[10]; //10
};
//}__attribute__((aligned (1)));
///////////////////////////////////////
struct node7{ //12
char x1; //1
short x2; //2
float x3; //4
char x4; //4
};
#pragma pack(1) //指定按照1字节对齐
struct node8{ //8
char x1; //1
short x2; //2
float x3; //4
char x4; //1
};
#pragma pack() //恢复缺省的字节对齐设置
////////////////////////////////////////
struct node9{ //20
char ch,*p; //1,4
union{ //4
short a,b;
unsigned int c:2,d:1;
}u;
bool f; //1
struct node9 *next; //4
};
//测试字节对齐
void test2(){
int size;
struct node1 n1;
struct node2 n2;
struct node3 n3;
struct node4 n4;
struct node5 n5;
struct node6 n6;
struct node7 n7;
struct node8 n8;
struct node9 n9;
//栈生长的方向与地址增长方向相反，所以a的地址比b的地址大四字节
int a = 1,b = 2,c=3;
int *p = &b;
printf("%d\n",*(p+1)); //1
//测试union和struct的大小(后者会地址对齐，前者仍然输出16)
printf("union1:%d\n",sizeof(union union1)); //16
printf("union2:%d\n",sizeof(union union2)); //16
printf("union3:%d\n",sizeof(union union3)); //13
//测试结构体的地址对齐
printf("************************\n");
printf("n1:%d\n",sizeof(n1));
printf("%ld\n",(unsigned long)&n1.j);
printf("%ld\n",(unsigned long)&n1.f);
printf("%ld\n",(unsigned long)&n1.i);
printf("************************\n");
printf("n2:%d\n",sizeof(n2));
printf("%ld\n",(unsigned long)&n2.i);
printf("%ld\n",(unsigned long)&n2.j);
printf("%ld\n",(unsigned long)&n2.f);
printf("************************\n");
printf("n3:%d\n",sizeof(n3));
printf("%ld\n",(unsigned long)&n3.i);
printf("%ld\n",(unsigned long)&n3.d);
printf("%ld\n",(unsigned long)&n3.f);
printf("************************\n");
printf("n4:%d\n",sizeof(n4));
printf("%ld\n",(unsigned long)&n4.d);
printf("%ld\n",(unsigned long)&n4.f);
printf("%ld\n",(unsigned long)&n4.i);
printf("************************\n");
printf("n5:%d\n",sizeof(n5));
printf("%ld\n",(unsigned long)&n5.d);
printf("%ld\n",(unsigned long)&n5.f);
printf("%ld\n",(unsigned long)&n5.i);
printf("************************\n");
printf("n6:%d\n",sizeof(n6));
printf("%ld\n",(unsigned long)&n6.d);
printf("%ld\n",(unsigned long)&n6.f);
printf("%ld\n",(unsigned long)&n6.i);
printf("************************\n");
printf("n7:%d\n",sizeof(n7));
printf("%ld\n",(unsigned long)&n7.x1);
printf("%ld\n",(unsigned long)&n7.x2);
printf("%ld\n",(unsigned long)&n7.x3);
printf("%ld\n",(unsigned long)&n7.x4);
printf("************************\n");
printf("n8:%d\n",sizeof(n8));
printf("%ld\n",(unsigned long)&n8.x1);
printf("%ld\n",(unsigned long)&n8.x2);
printf("%ld\n",(unsigned long)&n8.x3);
printf("%ld\n",(unsigned long)&n8.x4);
printf("************************\n");
printf("n9:%d\n",sizeof(n9));
}
int main(){
int *ptr;
test2();
printf("************************\n");
printf("char:%d\n",sizeof(char)); //1
printf("short:%d\n",sizeof(short)); //2
printf("int:%d\n",sizeof(int)); //4
printf("long:%d\n",sizeof(long)); //4
printf("float:%d\n",sizeof(float)); //4
printf("double:%d\n",sizeof(double)); //8
printf("pointer:%d\n",sizeof(ptr)); //4
return 0;
}