如何交换64位整数，而在C ++中从bytearray提取字节？ [英] How to swap 64 bit integer while extracting bytes from bytearray in C++?

问题描述

我正在读取几个字节从 byteData 如下所述在我的C ++代码。 byteData 中的实际值是BIG-ENDIAN字节顺序格式的二进制blob字节数组。因此，我不能简单地将字节数组转换为字符串。

I am tring to read couple of bytes from byteData as mentioned below in my C++ code. The actual value within byteData is a binary blob byte array in BIG-ENDIAN byte order format. So I cannot simply just "cast" the byte array into a String..

byteData 字节数组由这三个东西 -

byteData byte array is composed of these three things -

First is `schemaId` which is of two bytes (short datatype in Java)
Second is `lastModifiedDate` which is of eight bytes (long datatype in Java)
Third is the length of actual `byteArray` within `byteData` which we need from `byteData`.
Fourth is the actual value of that `byteArray` in `byteData`.

现在我试图从 byteData 在C ++ ...不知何故我能够提取 schemaId ，我得到的值也是正确的..现在我不知道如何提取其他的东西...

Now I am trying to extract the above particular information from the byteData in C++... Somehow I am able to extract schemaId and the value which I am getting is also correct.. Now I am not sure how to extract other things from it...

uint16_t schemaId;
uint64_t lastModifiedDate;
uint16_t attributeLength; // or it should be uint32_t?
const char* actual_binary_value;

while (result.next()) {
    for (size_t i = 0; i < result.column_count(); ++i) {
        cql::cql_byte_t* byteData = NULL;
        cql::cql_int_t size = 0;
        result.get_data(i, &byteData, size);

        // I cannot just "cast" the byte array into a String
        // value = reinterpret_cast<char*>(byteData);

        // this works fine
        schemaId = ntohs(*reinterpret_cast<uint16_t*>(byteData));

        // now how to retrieve lastModifiedDate, length of binary value and actual_binary_value from byteData?
        // the below doesn't works..
           lastModifiedDate = be64toh(*reinterpret_cast<uint64_t*>(data));

        // And how to extract other things as well?

    }

    // this prints out `9223090561897746107`  but it should print out `1289811105109`
    cout<< lastModifiedDate <<endl;

    // And print out other things..
}

If somebody needs to see my java code then this is my java code -

    byte[] avroBinaryValue = text.getBytes();

    long lastModifiedDate = 1289811105109L;
    short schemaId = 32767;

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

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

    bbuf.putShort(schemaId);
    bbuf.putLong(lastModifiedDate);
    bbuf.putInt(avroBinaryValue.length);
    bbuf.put(avroBinaryValue);

    // merge everything into one bytearray.
    byte[] bytesToStore = bbuf.array();

    Hex.encodeHexString(bytesToStore)

任何人都可以帮助我什么错我在我的C ++代码，为什么我不能从它和其他字段正确提取 lastModifiedDate ？我理解 lastModifiedDate 是64位整数，所以有什么办法换出64位整数这里？

Can anybody help me what wrong I am doing in my C++ code and why I am not able to extract lastModifiedDate properly from it and other fields as well? As I understand lastModifiedDate is 64 bit integer, so is there any way to swap out 64 bit integer here? Or some other better way to do the conversion?

简而言之，我试图提取 schemaId ，<$ c ++中的字节数组中的c $ c> lastModifiedDate ， avroBinaryValue.length 和 avroBinaryValue ..

In short, I am trying to extract schemaId, lastModifiedDate, avroBinaryValue.length and avroBinaryValue from that byte array in C++..

我可以提取 schemaId 但我现在卡住了其他东西...

I am able to extract schemaId but I am stuck on other things now...

推荐答案

您的方法看起来不错，我看到的只有两个可能的问题。

Your method looks fine, there are just two possible issues I see.

1. 您所显示的代码只是将一个未定义的变量转换为 uint64_t 。请确保您实际上正在通过数据缓冲区并转换正确的数据。

1. Your code as shown is simply casting an undefined variable data as a uint64_t. Make sure that you are actually advancing through the data buffer and converting the correct data.

平台相关性。从我看到的，64位字节交换功能（be64toh，betoh64，ntohll等），并不是所有平台上普遍支持。您可能需要在平台上使用不同的功能，并且/或者如果您希望代码与平台无关，则自动检测哪些功能有效。请参阅类似问题此处和这里。

Platform dependency. From what I've seen, the 64-bit byte swapping functions (be64toh, betoh64, ntohll, etc), are not universally supported on all platforms. You may need to use a different function on your platform, and/or automatically detect what works if you want the code to be platform independent. See similar questions here and here, for example.

至于如何获取数据，这样的工作应该是：

As to how to get the data, something like this should work:

int index=0;
schemaId = ntohs(*reinterpret_cast<uint16_t*>(&byteData[index]));
index += 2;
lastModifiedDate = be64toh(*reinterpret_cast<uint64_t*>(&byteData[index]));
index += 8;
attributeLength = ntohl(*reinterpret_cast<uint32_t*>(&byteData[index]));
index += 4;
actual_binary_data = (const char *)&byteData[index];

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