Read binary data from file

[HB76]

QDataStream s(&file);
s.setByteOrder(QDataStream::LittleEndian);

s.skipRawData(80);
qint32 value;
s >> value;
qDebug() << value;

give me the same result, 0...

[Pablo J. Rogina]

@HB76 just in case, could you please post:

1. the values of the bytes at positions 80, 81, 82, 83 for the file you're working with?
2. the size of the data (or the whole file) for such file?

[HB76]

I've found a way to do it :

QByteArray data = file.read(4);
qint32 facet_count;
memcpy(&facet_count, data.constData(), 4);
qDebug() << facet_count;

Just for knowledge, why this simple code doesn'y work ?

file.seek(80);
ulong length =  file.read(4).toULong();
qDebug() << length;

it would be much more simplier..

[Christian Ehrlicher]

@HB76 said in Read binary data from file:

ulong length = file.read(4).toULong();

Because you did not read the documentation: https://doc.qt.io/qt-5/qbytearray.html#toLong

[HB76]

@Pablo-J-Rogina the values are :

80 : "\x8C"
81 : "\x13"
82 : "\x01"
83 : "\x00"

the file length is 3527084 using

int length = int(file.size());

[HB76]

@Christian-Ehrlicher I was just having a look at it but I supposed I didn't understand the whole explaination the first time as I'm not native english ^^'

[Christian Ehrlicher]

The example there should be obvious:

QByteArray str("FF");
bool ok;
int hex = str.toInt(&ok, 16);     // hex == 255, ok == true
int dec = str.toInt(&ok, 10);     // dec == 0, ok == false

As you can see it converts a string value into an integer

[HB76]

ok I got it now, but why every time the conversion failed with this method whereas the conversion with memcpy is working ?

[Christian Ehrlicher]

@HB76 said in Read binary data from file:

ut why every time the conversion failed with this method whereas the conversion with memcpy is working ?

Again: toUint() interprets your string as ascii text and tries to convert it to an integer, whereas memcpy simply copies the plain data - C basics on how a value is interpreted.

[HB76]

Ok thank you very much !
I don't know if it is the most efficient way to read data but it is working !