大端、小端與結構體位域

文字說明待後續補充

#include <stdio.h>

struct st1
{
    unsigned short a:4;
    unsigned short b:4;
    unsigned short c:4;
    unsigned short d:4;
};


struct st2
{
    unsigned short e:6;
    unsigned short f:6;
    unsigned short g:4;
};

int main()
{

    unsigned short num = 0x1234;
    unsigned char *cp = (unsigned char *) &num;
    // 小端：低位存放在低地址中
    if ((*cp) == 0x34){
        printf("Little Endian\n");
    }
    else{
        printf("Big Endian\n");
    }

    printf("\n");

    printf("num = 0x%x\n",num);

    printf("\n");

    printf("*cp = 0x%x\n",*cp);
    printf("*cp & 0x0F = 0x%x\n", *cp & 0x0F);
    printf("*cp >> 4 = 0x%x\n",*cp >> 4);

    printf("\n");

    struct st1 *p1 = (struct st1 *) &num;

    printf("*p1 : 0x%x\n",*( (char *) p1 ) );
    printf("*p1 & 0x0F : 0x%x\n",(*( (char *) p1 )) & 0x0F );
    printf("p1->a = 0x%x\np1->b = 0x%x\np1->c = 0x%x\np1->d = 0x%x\n",p1->a,p1->b,p1->c,p1->d);

    printf("\n");

    struct st2 *p2 = (struct st2 *) &num;

    printf("*p2 : 0x%x\n",*( (char *) p2 ) );
    printf("*p2 & 0x0F : 0x%x\n",(*( (char *) p2 )) & 0x0F );
    printf("p2->e = 0x%x\np2->f = 0x%x\np2->g = 0x%x\n",p2->e,p2->f,p2->g);

}

運行結果

Little Endian

num = 0x1234

*cp = 0x34
*cp & 0x0F = 0x4
*cp >> 4 = 0x3

*p1 : 0x34
*p1 & 0x0F : 0x4
p1->a = 0x4
p1->b = 0x3
p1->c = 0x2
p1->d = 0x1

*p2 : 0x34
*p2 & 0x0F : 0x4
p2->e = 0x34
p2->f = 0x8
p2->g = 0x1

其他：

1. 結構體位域不能被尋址，例如 &(p1->a) 無法編譯。