如何在C ++中使用Big Endian格式而不是Little Endian移位字节？

Question

I am writing Byte Array value into a file using Java with Big Endian Byte Order format.. Now I need to read that file from C++ program...

That Byte Array which I am writing into a file is made up of three Byte Arrays as described below-

short employeeId = 32767;
long lastModifiedDate = "1379811105109L";
byte[] attributeValue = os.toByteArray();

I am writing employeeId , lastModifiedDate and attributeValue together into a single Byte Array and that resulting Byte Array I am writing into a file and then I will be having my C++ program which will retrieve that Byte Array data from file and then deserialize it to extract employeeId , lastModifiedDate and attributeValue from it.

Below is my working Java code, which writes Byte Array value into a file with Big Endian format:

public class ByteBufferTest {

    public static void main(String[] args) {

        String text = "Byte Array Test For Big Endian";
        byte[] attributeValue = text.getBytes();

        long lastModifiedDate = 1289811105109L;
        short employeeId = 32767;

        int size = 2 + 8 + 4 + attributeValue.length; // short is 2 bytes, long 8 and int 4

        ByteBuffer bbuf = ByteBuffer.allocate(size); 
        bbuf.order(ByteOrder.BIG_ENDIAN);

        bbuf.putShort(employeeId);
        bbuf.putLong(lastModifiedDate);
        bbuf.putInt(attributeValue.length);
        bbuf.put(attributeValue);

        bbuf.rewind();

        // best approach is copy the internal buffer
        byte[] bytesToStore = new byte[size];
        bbuf.get(bytesToStore);

        writeFile(bytesToStore);

    }

    /**
     * Write the file in Java
     * @param byteArray
     */
    public static void writeFile(byte[] byteArray) {

        try{
            File file = new File("bytebuffertest");

            FileOutputStream output = new FileOutputStream(file);
            IOUtils.write(byteArray, output);           

        } catch (Exception ex) {
            ex.printStackTrace();
        }
    }
}

Now I need to retrieve Byte Array from that same file using the below C++ program and deserialize it to extract employeeId , lastModifiedDate and attributeValue from it. I am not sure what is the best way on the C++ side. Below is the code I have so far:

int main() {

    string line;

    std::ifstream myfile("bytebuffertest", std::ios::binary);

    if (myfile.is_open()) {

        uint16_t employeeId;
        uint64_t lastModifiedDate;
        uint32_t attributeLength;

        char buffer[8]; // sized for the biggest read we want to do

        // read two bytes (will be in the wrong order)
        myfile.read(buffer, 2);

        // swap the bytes
        std::swap(buffer[0], buffer[1]);

        // only now convert bytes to an integer
        employeeId = *reinterpret_cast<uint16_t*>(buffer);

        cout<< employeeId <<endl;

        // read eight bytes (will be in the wrong order)
        myfile.read(buffer, 8);

        // swap the bytes
        std::swap(buffer[0], buffer[7]);
        std::swap(buffer[1], buffer[6]);
        std::swap(buffer[2], buffer[5]);
        std::swap(buffer[3], buffer[4]);

        // only now convert bytes to an integer
        lastModifiedDate = *reinterpret_cast<uint64_t*>(buffer);

        cout<< lastModifiedDate <<endl;

        // read 4 bytes (will be in the wrong order)
        myfile.read(buffer, 4);

        // swap the bytes
        std::swap(buffer[0], buffer[3]);
        std::swap(buffer[1], buffer[2]);

        // only now convert bytes to an integer
        attributeLength = *reinterpret_cast<uint32_t*>(buffer);

        cout<< attributeLength <<endl;

        myfile.read(buffer, attributeLength);


        // now I am not sure how should I get the actual attribute value here?

        //close the stream:
        myfile.close();
    }

    else
        cout << "Unable to open file";

    return 0;
}

I have specifically set the storage Java-side to big-endian which means I know where each byte belongs I guess. So how do I code it while shifting the bytes into the proper position for each and every value? Right now I am coding it as little-endian I guess which is not what I want...

I was reading somewhere that I can use ntoh in C++ to deserialize the Byte Array.. Not sure whether htons will be much better solution as compared to what I have currently?..

If yes, then I am not sure how to use that in my current C++ code?

Can anybody take a look on C++ code and see what I can do to improve it, as I don't think it is looking much efficient? Any better way to deserialize the Byte Array and extract relevant information on the C++ side?

Answer 1

If the Java and the C++ Code is developed by you then it's perhaps better to use the Google Protocol Buffer ( https://developers.google.com/protocol-buffers/docs/overview ) instead to write your own serializer / deserializer.

If you really want to write your own implementation the best way is to write a buffer class which receives the byte-stream as parameter (eg as Constructor parameter) and make some access methodes readShort / readLong / readInt / readByte ... and only swap the needed bytes.

class ByteBuffer{
  explicit ByteBuffer(uint8_t* byteStream, uint16_t streamLength);
  uint8_t readUInt8(uint16_t readPos)const {return m_byteStream[readPos];} // no conversion needed
  uint16_t readUInt16(uint16_t readPos)const {
    const uint8_t byteCount = 2;
    union{
      uint16_t u16;
      uint8_t u8[byteCount];
    }tmp;
    for(uint8_t i=0; i<byteCount; ++i){
      tmp.u8[i] = readUInt8(readPos+i*8);
    }
    return ntohs(tmp.u16); // do conversion
  }
  ...
}

Checks for read behind the buffer is missing here. If your code should be portable then you have use ntohl / ntohs (see: http://forums.codeguru.com/showthread.php?298741-C-General-What-do-ntohl%28%29-and-htonl%28%29-actually-do ). If you swap the bytes by our own then your code is not portable (runs only on Little-Endian machines). If you use ntoh then it would also run on a such machine.

For convenience I would also write a wrapper class where you can read and write your fields (eg employeeId) directly:

class MyBuffer{
  uint16_t readEmployeeId()const{return m_Buffer.readuint16(EmployeeId_Pos);}
  ....
  static const uint16_t EmployeeId_Pos = 0;
  ....
}