c# - 奇怪的 SNMP 移位操作

Question

以下摘自 Richard Blum 的《C# Network Programmingm》一书:

public byte[] get(string request, string host, string community, string_ mibstring)
{
    byte[] packet = new byte[1024];
    byte[] mib = new byte[1024];
    int snmplen;
    int comlen = community.Length;
    string[] mibvals = mibstring.Split('.');
    int miblen = mibvals.Length;
    int cnt = 0, temp, i;
    int orgmiblen = miblen;
    int pos = 0;
    // Convert the string MIB into a byte array of integer values
    // Unfortunately, values over 128 require multiple bytes
    // which also increases the MIB length
    for (i = 0; i < orgmiblen; i++)
    {
        temp = Convert.ToInt16(mibvals[i]);
        if (temp > 127)
        {
            mib[cnt] = Convert.ToByte(128 + (temp / 128));
            mib[cnt + 1] = Convert.ToByte(temp - ((temp / 128) * 128));
            cnt += 2;
            miblen++;
        } 
        else
        {
        mib[cnt] = Convert.ToByte(temp);
        cnt++;
        }
    }
    snmplen = 29 + comlen + miblen - 1; //Length of entire SNMP packet
    //The SNMP sequence start
    packet[pos++] = 0x30; //Sequence start
    packet[pos++] = Convert.ToByte(snmplen - 2); //sequence size
    //SNMP version
    packet[pos++] = 0x02; //Integer type
    packet[pos++] = 0x01; //length
    packet[pos++] = 0x00; //SNMP version 1
    //Community name
    packet[pos++] = 0x04; // String type
    packet[pos++] = Convert.ToByte(comlen); //length
    //Convert community name to byte array
    byte[] data = Encoding.ASCII.GetBytes(community);
    for (i = 0; i < data.Length; i++)
    {
        packet[pos++] = data[i];
    }
}

我没看懂下面的代码:

    for (i = 0; i < orgmiblen; i++)
    {
        temp = Convert.ToInt16(mibvals[i]);
        if (temp > 127)
        {
            mib[cnt] = Convert.ToByte(128 + (temp / 128));
             mib[cnt + 1] = Convert.ToByte(temp - ((temp / 128) * 128));
             cnt += 2;
             miblen++;
        } 
        else
        {
            mib[cnt] = Convert.ToByte(temp);
            cnt++;
        }
    }

我知道这是为了在温度大于一个字节的情况下放入两个字节。但是128+(temp/128)然后是第二个字节:temp-(temp/128)*128，这是我不明白的计算是什么。

请帮忙，谢谢。

Answer 1

如果 temp 大于 127，那么它将被分成 2 个字节。让我们看两个例子/

temp = 100;
128 + (temp / 128); //(temp / 128) = 0 + 128 so the mib[cnt] is set to 128
temp - ((temp/128) * 128); // 0*128 = 0. subtracted from temp leaves the original. so you end up with

mib[cnt] = 128;
mib[cnt+1] = 100;

现在如果温度 > 127

temp = 200;
128 + (temp / 128); //(temp / 128) = 1 + 128 so the mib[cnt] is set to 129
temp - ((temp/128) * 128); // 1*128 = 128. 200-128 = 72. so you end up with

mib[cnt] = 129;
mib[cnt+1] = 72;

所以基本上，它采用一个数字，测试它是否 > 7 个字节(-128 => +127 是一个带符号的字节)，如果您提供的数字会溢出该范围，它会将其转换为 2 个字节的值/