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C++技巧：通过pragmapack(n)改变字节对齐
作者：佚名  文章来源：不详  点击数  更新时间：2008/10/22 21:33:32  文章录入：杜斌  责任编辑：杜斌
　　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
　　/*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;
　　}
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