#pragma pack()
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转自:http://gaona021973.blog.163.com/blog/static/3735227200810103131555/
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;
}
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