2字节表示C语言中的3个整数

Question

我有两个字节，8位八位字节，应该读作:3个bits3位。

示例：

unsigned char octet1 = 0b11111111;  // binary values
unsigned char octet2 = 0b00000011;

作为整数: 77。

谁能给我一个提示，从哪里开始？

Answer 1

在提取我们想要的位之前，不需要将两个字节放在一起。

#include <stdio.h>

main()
{
    unsigned char octet1 = 0b11111111;
    unsigned char octet2 = 0b00000011;
    unsigned char n1 = octet1 & 0b111;
    unsigned char n2 = (octet1 >> 3) & 0b1111;
    unsigned char n3 = (octet1 >> 7) | (octet2 + octet2);

    printf("octet1=%u octet2=%u n1=%u n2=%u n3=%u\n",
              octet1, octet2, n1, n2, n3);

}

Answer 2

在一种pseudocode中

octet1 = 0b11111111
octet2 = 0b00000011
word = octet1 | octet2<<8
n1 = word & 0b111
n2 = word>>3 & 0b1111
n3 = word>>7 & 0b111

Answer 3

您好，这里有一个使用VC++9测试和编译的方法

#pragma pack( 1 )

union 
{
    struct 
    {
        unsigned short val1:3;
        unsigned short val2:4;
        unsigned short val3:3; 
        unsigned short val4:6; 
    } vals;
    struct 
    {
        unsigned char octet1:8;
        unsigned char octet2:8;
    } octets;
    short oneVal;
} u = {0xFFFF};

unsigned char octet1 = 0xFF;  //1 1111 111
unsigned char octet2 = 0x03;  //000000 11
//000000 111 1111 111 0 7 15 7
u.octets.octet1 = octet1;
u.octets.octet2 = octet2;
cout << "size of u.vals:" << sizeof(u.vals)<< endl;
cout << "size of u.octets:" << sizeof(u.octets)<< endl;
cout << "size of u.oneVal:" << sizeof(u.oneVal)<< endl;
cout << "size of u:" << sizeof(u)<< endl;
cout << endl;

cout << "Your values:" << endl;
cout << "oneVal in Hex: 0x";
cout.fill( '0' );
cout.width( 4 );
cout<< hex << uppercase << u.oneVal << endl;
cout << "val1: " << (int)u.vals.val1 << endl;
cout << "val2: " << (int)u.vals.val2 << endl;
cout << "val3: " << (int)u.vals.val3 << endl;
cout << "val4: " << (int)u.vals.val4 << endl;
cout << endl;

octet1 = 0xCC;  //1 1001 100
octet2 = 0xFA;  //111110 10
//111110 101 1001 100 62 5 9 4
u.octets.octet1 = octet1;
u.octets.octet2 = octet2;

cout << "Some other values:" << endl;
cout << "oneVal in Hex: 0x";
cout.fill( '0' );
cout.width( 4 );
cout<< hex << uppercase << u.oneVal << endl;
cout << dec;
cout << "val1: " << (int)u.vals.val1 << endl;
cout << "val2: " << (int)u.vals.val2 << endl;
cout << "val3: " << (int)u.vals.val3 << endl;
cout << "val4: " << (int)u.vals.val4 << endl;
cout << endl;

octet1 = 0xCC;  //1 1001 100
octet2 = 0xFA;  //111110 10
//111110 101 1001 100 62 5 9 4
u.oneVal = ( (( unsigned short )octet2 ) << 8 ) | ( unsigned short )octet1;
cout << "Some thing diffrent asignment:" << endl;
cout << "oneVal in Hex: 0x";
cout.fill( '0' );
cout.width( 4 );
cout<< hex << uppercase << u.oneVal << endl;
cout << dec;
cout << "val1: " << (int)u.vals.val1 << endl;
cout << "val2: " << (int)u.vals.val2 << endl;
cout << "val3: " << (int)u.vals.val3 << endl;
cout << "val4: " << (int)u.vals.val4 << endl;
cout << endl;

还要注意的是，我使用#杂注包(1 )将结构打包设置为1字节。

我还包含了一种将两个二进制八位数赋值为一个短值的方法。使用按位移位"<<“和按位或"|”

您可以通过删除命名结构来简化对u的访问。但我想展示的是用于结构的大小。

如下所示：

union 
{
    struct 
    {
        unsigned short val1:3;
        unsigned short val2:4;
        unsigned short val3:3; 
        unsigned short val4:6; 
    };
    struct 
    {
        unsigned char octet1:8;
        unsigned char octet2:8;
    };
    short oneVal;
} u = {0xFFFF};

现在访问就像下面这样简单

u.oneVal = 0xFACC;

或

u.octet1 = 0xCC;
u.octet2 = 0xFA;

您还可以根据您喜欢的访问方法删除oneVal或octet1和octet2。