关于C的struct结构的几个常见疑问。

Code：

 

#include<stdio.h>

struct bit{
        unsigned int a0:8;
        unsigned int a1:16;
        unsigned int a2:8;

        unsigned int b;
};

struct bit2
{
        unsigned char  a0;
        unsigned short a1;
        unsigned char  a2;

        unsigned int b;
};

struct nobit
{
        unsigned int a;
        unsigned int b;
};

 

int main()
{
        struct bit  t1;
        struct bit2 t3;
        struct nobit t2;

        unsigned char data[8]={0xA5, 0xCE, 0x12, 0xA5, 0xB0, 0xB1, 0xB2,0xB3};
        memcpy( &t1, data, 8);
        memcpy( &t2, data, 8);
        memcpy( &t3, data, 8);

        printf("++ sizeof(t1)=%d,  sizeof(t3)=%d, sizeof(t2)=%d \n",
                   sizeof(t1), sizeof(t3), sizeof(t2) );

        printf("++ t1.a0=%2x  t1.a1=%4x t2.a2=%2x t1.b=%8x \n", t1.a0, t1.a1, t1
.a2, t1.b);
        printf("++ t3.a0=%2x  t3.a1=%4x t3.a2=%2x t3.b=%8x \n", t3.a0, t3.a1, t3
.a2, t3.b);
        printf("++ t2.a=0x%8x  t2.b=%8x \n", t2.a, t2.b);

        return 0;
}

 

//运行结果：<Linux 2.6.18  X86>

++ sizeof(t1)=8,  sizeof(t3)=12, sizeof(t2)=8
++ t1.a0=a5  t1.a1=12ce t2.a2=a5 t1.b=b3b2b1b0
++ t3.a0=a5  t3.a1=a512 t3.a2=b0 t3.b=bfb59d7c
++ t2.a=0xa512cea5  t2.b=b3b2b1b0

//结果说明：

1. struct对齐。

2. little_endian.

3.__LITTLE_ENDIAN_BITFIELD (这个测试还看不出来。)

 

        0xA5, 0xCE, 0x12, 0xA5, 0xB0, 0xB1, 0xB2,0xB3

bit： | a0  |      a1         |  a2   |           b                       |

bit2  | a0  |        |         a1       |  a2  |                           | b         |

nobit|         a                          |            b                      |

 

end.