C++技巧：通過pragmapack(n)改變字節(jié)對齊

通過#pragma pack(n)改變C編譯器的字節(jié)對齊方式，在C語言中，結(jié)構(gòu)是一種復(fù)合數(shù)據(jù)類型，其構(gòu)成元素既可以是基本數(shù)據(jù)類型（如int、long、float等）的變量，也可以是一些復(fù)合數(shù)據(jù)類型（如數(shù)組、結(jié)構(gòu)、聯(lián)合等）的數(shù)據(jù)單元。在結(jié)構(gòu)中，編譯器為結(jié)構(gòu)的每個成員按其自然對界（alignment）條件分配空間?？聪旅胬印?BR>    其輸出是：
    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
    /*默認(rèn)字節(jié)對齊*/
    struct mystruct1{
    char a;
    int b;
    short c;
    long d;
    };
    /*2字節(jié)對齊*/
    #pragma pack(2)
    struct mystruct2{
    char a;
    int b;
    short c;
    long d;
    };
    /*1字節(jié)對齊*/
    #pragma pack(1)
    struct mystruct3{
    char a;
    int b;
    short c;
    long d;
    };
    /*恢復(fù)默認(rèn)字節(jié)對齊*/
    #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;
    }