struct的字节对齐问题（通过pragma pack(n)改变字节对齐的例子） 佟强 2008.10.9
通过#pragma pack(n)改变C编译器的字节对齐方式，在C语言中，结构是一种复合数据类型，其构成元素既可以是基本数据类型（如int、long、float等）的变量，也可以是一些复合数据类型（如数组、结构、联合等）的数据单元。在结构中，编译器为结构的每个成员按其自然对界（alignment）条件分配空间。看下面例子。
其输出是：
sizeof(char)=1 sizeof(int)=4 sizeof(short)=2 sizeof(long)=4
struct1(char,int,short,long) offset: 0 4 8 12 sizeof(mystruct1)=16
struct2(char,int,short,long) offset: 0 2 6 8 sizeof(mystruct2)=12
struct3(char,int,short,long) offset: 0 1 5 7 sizeof(mystruct3)=11
struct3(char,int,short,long) offset: 0 4 8 12 sizeof(mystruct3)=16
#include <stdio.h>
/*默认字节对齐*/
struct mystruct1{
char a;
int b;
short c;
long d;
};
/*2字节对齐*/
#pragma pack(2)
struct mystruct2{
char a;
int b;
short c;
long d;
};
/*1字节对齐*/
#pragma pack(1)
struct mystruct3{
char a;
int b;
short c;
long d;
};
/*恢复默认字节对齐*/
#pragma pack()
struct mystruct4{
char a;
int b;
short c;
long d;
};
int main(int argc,char* argv[]){
int a_off;
int b_off;
int c_off;
int d_off;
struct mystruct1 s1;
struct mystruct2 s2;
struct mystruct3 s3;
struct mystruct4 s4;
printf("sizeof(char)=%d sizeof(int)=%d sizeof(short)=%d sizeof(long)=%d/n",
sizeof(char), sizeof(int), sizeof(short), sizeof(long));
/*mystruct1*/
a_off = (char*)&(s1.a) - (char*)&s1;
b_off = (char*)&(s1.b) - (char*)&s1;
c_off = (char*)&(s1.c) - (char*)&s1;
d_off = (char*)&(s1.d) - (char*)&s1;
printf("struct1(char,int,short,long) offset: %d %d %d %d",a_off,b_off,c_off,d_off);
printf(" sizeof(mystruct1)=%d/n",sizeof(mystruct1));
/*mystruct2*/
a_off = (char*)&(s2.a) - (char*)&s2;
b_off = (char*)&(s2.b) - (char*)&s2;
c_off = (char*)&(s2.c) - (char*)&s2;
d_off = (char*)&(s2.d) - (char*)&s2;
printf("struct2(char,int,short,long) offset: %d %d %d %d",a_off,b_off,c_off,d_off);
printf(" sizeof(mystruct2)=%d/n",sizeof(mystruct2));
/*mystruct3*/
a_off = (char*)&(s3.a) - (char*)&s3;
b_off = (char*)&(s3.b) - (char*)&s3;
c_off = (char*)&(s3.c) - (char*)&s3;
d_off = (char*)&(s3.d) - (char*)&s3;
printf("struct3(char,int,short,long) offset: %d %d %d %d",a_off,b_off,c_off,d_off);
printf(" sizeof(mystruct3)=%d/n",sizeof(mystruct3));
/*mystruct4*/
a_off = (char*)&(s4.a) - (char*)&s4;
b_off = (char*)&(s4.b) - (char*)&s4;
c_off = (char*)&(s4.c) - (char*)&s4;
d_off = (char*)&(s4.d) - (char*)&s4;
printf("struct3(char,int,short,long) offset: %d %d %d %d",a_off,b_off,c_off,d_off);
printf(" sizeof(mystruct3)=%d/n",sizeof(mystruct4));
return 0;
}