Question

I wrote a server using c++ and the code looks like this

char *sendBuffer = new char[4+4+1+8+48];
m_magicNumberBuffer[0] = 0xA9;
m_magicNumberBuffer[1] = 0xA9;
m_magicNumberBuffer[3] = 0xA9;
m_magicNumberBuffer[4] = 0xA9;
int m_dataLength = 4+4+1+8+48;
unsigned char m_type = 1;
long long m_deviceId = 12123122123;
char m_msg = new char[48];
m_msg = "NO ERROR";

/*
 * Magic number;
 */
memcpy(sendBuffer, m_magicNumberBuffer + offset, 4);
offset += 4;

/*
 * Data length
 */
memcpy(sendBuffer, &m_dataLength + offset, 4);
offset += 4;

/*
 * Packet type
 */
memcpy(sendBuffer, &m_type + offset, 1);
offset += 1;

/*
 * Device ID
 */

memcpy(sendBuffer, &m_deviceId + offset, 8);
offset += 8;

memcpy(sendBuffer, m_msg + offset, 48);
offset += 48;

I am writing the sendBuffer

 write(client->getFd(), sendBuffer, MAX_LENGTH);

Client is written in Java

I am doing something like this

        InputStream in = mSocket.getInputStream();
        DataInputStream dis = new DataInputStream(in);

        int magicNumber = dis.readInt();
        int length = dis.readInt();
        byte[] data = new byte[length];
        if(length > 0){
            dis.readFully(data);
        }

And it reads something wrong. I am getting unreasonable number for the magic number and always reads 0 for the data length.

How do I fix this ? Thanks in advance..

Answer 1

Pretty simple, really: you want to add your offset to the destination sendBuffer, not your sources. E.g. instead of

memcpy(sendBuffer, &m_deviceId + offset, 8);

you want

memcpy(sendBuffer + offset, &m_deviceId, 8);

As it is, you are storing various random data to the first few bytes of sendBuffer, and nothing to the rest of it.