Q:

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?

A:

You can use a QDataStream to read binary data. 
quint8 packed_bytes[] { 0x12, 0x34, 0x56, 0x78 };
QByteArray packed_array { QByteArray(reinterpret_cast<char*>(packed_bytes), sizeof(packed_bytes)) };
QDataStream stream(packed_array);
stream.setByteOrder(QDataStream::LittleEndian);
int result;
stream >> result;
qDebug() << QString::number(result,16);

A:

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.

