Tag - Length - Value parse in qt

Damian7546

Hi,

I try understand below class:
https://codereview.stackexchange.com/questions/115395/tlv-implementation-in-c
to deserialize (from bytes array to TLV (tag - length - value) object).

I run this class in my qt project in this way:

QByteArray buff = "045002124354";
std::vector<uint8_t> output(buff.constData(), buff.constData() + buff.size());

TLVObject obj1;
obj1.Deserialize(output); // Reconstruct TLV from byte array

And doesn't work.... anyway.. How this class know which one is Tag and wchich one is value ?
How can I access this data from the obj1 object?

JonB

@Damian7546
How do you/we know that 045002124354 is a valid serialization for TLV to deserialize?? For example, are you sure these bytes are supposed to be decimal character digits, not actual binary values...?? Glancing --- and I do mean only glancing --- at that code I am unconvinced they are what they are supposed to be.

jsulm

@Damian7546 said in Tag - Length - Value parse in qt:

How this class know which one is Tag and wchich one is value ?

The code is there, why don't you read it?

void  TLVObject::Deserialize(vector<uint8_t> value)
{
    // Get type
    TLV_TYPE tlvType =  static_cast<TLV_TYPE>(value.at(0));

    // Get tag name size.
    vector<uint8_t> tmp;
    tmp.push_back(value.at(1));
    tmp.push_back(value.at(2));
    uint16_t tagLength = LEToUINT16(tmp);

    if(tagLength > MAXTAGLENGTH)
        throw exception("Tag name length exceeded");

    // Now get tag name
    std::string tagName;
    for(int i = 0; i<tagLength; i++)
        tagName += value.at(3 + i); // Use an offset.
...

Damian7546

@jsulm You are right.
But I do not still know why my app doesn't work.
When I try run my app

QByteArray buff = "045002124354";
    std::vector<uint8_t> output(buff.constData(), buff.constData() + buff.size());

    TLVObject obj1;
    obj1.Deserialize(output); // Reconstruct TLV from byte array

I get error:
terminate called after throwing an instance of 'std::out_of_range' what(): vector::_M_range_check: __n (which is 12) >= this->size() (which is 12)

Damian7546

@jsulm for the help of this class is possible Parse data to object TLV form QbyteArray?
Do you know any library to can do this ?

JonB

@Damian7546
How do you/we know that 045002124354 is a valid serialization for TLV to deserialize?? For example, are you sure these bytes are supposed to be decimal character digits, not actual binary values...?? Glancing --- and I do mean only glancing --- at that code I am unconvinced they are what they are supposed to be.

stryga42

JonB is completely right. You initialize the QByteArray with "numerical garbage" and so the de-serialization fails. Details: If you initialize QByteArray with a const char * you will get the ASCII codes as values. A "0" will thus yield a value of 48, which is definitely not a valid enum TLV_TYPE. And even if it would be a numerical zero, it would decode to TLV_TYPE::UNDEFINED, which doesn't sound reasonable. Please use data which was generated from a serialization.