'How to unpack 32bit integer packed in a QByteArray?

I'm working with serial communication, and I receive 32bit integers in a QByteArray, packed in 4 separate bytes (little-endian). I attempt to unpack the value from the 4 bytes using QByteArray::toLong() but it fails the conversion and returns the wrong number:

quint8 packed_bytes[] { 0x12, 0x34, 0x56, 0x78 };
QByteArray packed_array { QByteArray(reinterpret_cast<char*>(packed_bytes),
                                     sizeof(packed_bytes)) };
bool isConversionOK;
qint64 unpacked_value { packed_array.toLong(&isConversionOK) };
// At this point:
// unpacked_value == 0
// isConversionOK == false

The expected unpacked_value is 0x78563412 (little-endian unpacking). Why is the conversion failing?

qttype-conversionqt5unpackqbytearray


Solution 1:[1]

toLong() converts a char * digits string to long. Not bytes. And your values likely don't make the up the string "0x78563412" or its decimal equivalent. Hence the 0 result.

If you need the byte values interpreted as long you can do something like:

long value;
value == *((long*)packed_bytes.data());

Or to access an array of bytes as long array:

long * values;
values == (long*)packed_bytes.data();

values[0]; // contains first long
values[1]; // contains second long
...

Don't know whether my examples work out of the box but it should make clear the principle.

Check out this example:

char bytes[] = {255, 0};

QByteArray b(bytes, 2);

QByteArray c("255");

qDebug() << b.toShort() << c.toShort();

qDebug() << *((short*)b.data()) << *((short*)c.data());

the output is:

0 255 
255 13618

You may need to change the byte order depending on the endianess. But it does what you need.

Solution 2:[2]

you can build your qint64 with bit manipulators:

#include <QtGlobal>
#include <QByteArray>
#include <QDebug>

int main()
{
    quint8 packed_bytes[] { 0x12, 0x34, 0x56, 0x78 };
    QByteArray packed_array { QByteArray(reinterpret_cast<char*>(packed_bytes),
                                         sizeof(packed_bytes)) };

    qint64 unpacked_value = 0;

    unpacked_value |=   packed_array.at(0)       |
                        packed_array.at(1) << 8  |
                        packed_array.at(2) << 16 |
                        packed_array.at(3) << 24;

    qDebug() << QString("0x%1").arg(unpacked_value, 0, 16);
}

Solution 3:[3]

Here's a generic solution for converting a QByteArray to "some other type" (such as what is specifically asked in the question) by running it through a QDataStream (as done by the accepted answer).

DISCLAIMER: I am only advocating for using this in a private implementation. I am aware there are many ways one could abuse the macro!

Using this macro, you can easily produce many conversion functions such as the examples I've provided. Defining a series of such functions in this way may be useful if you need to pull a variety of types out of a stream. Obviously, you could tweak the macro for your use case, the point is the pattern can remain basically same and be put in a macro like this.

#define byteArrayToType( data, order, type ) \
    QDataStream stream( data ); \
    stream.setByteOrder( order ); \
    type t; \
    stream >> t; \
    return t;

Example functions, which simply wrap the macro:

16 bit, signed

qint16 toQInt16( const QByteArray &data,
                 const QDataStream::ByteOrder order=QDataStream::BigEndian )
{ byteArrayToType( data, order, qint16 ) }

32 bit, signed

qint32 toQInt32( const QByteArray &data,
                 const QDataStream::ByteOrder order=QDataStream::BigEndian )
{ byteArrayToType( data, order, qint32 ) }

64 bit, signed

qint64 toQInt64( const QByteArray &data,
                 const QDataStream::ByteOrder order=QDataStream::BigEndian )
{ byteArrayToType( data, order, qint64 ) }

Solution 4:[4]

Cast the Byte array to the required format and use the built-in function qFromBigEndian or qFromLittleEndian to set the Byte order. Example code is shown below,

QByteArray byteArray("\x45\x09\x03\x00");
quint32 myValue = qFromBigEndian<quint32>(byteArray);
qDebug() << "Hex value: " << QString("0x%1").arg(myValue, 8, 16, QLatin1Char( '0' ));

myValue holds the converted value.

Don't forget to include the header file <QtEndian>

Sources

This article follows the attribution requirements of Stack Overflow and is licensed under CC BY-SA 3.0.

Source: Stack Overflow

Solution Source
Solution 1
Solution 2 Paraboloid87
Solution 3
Solution 4 Vinu Raja Kumar C

Related Questions