Read binary data from file

SGaist

Hi,

@HB76 said in Read binary data from file:

ulong length = ulong(buffer);

You are casting a pointer to a char array to ulong not its content. That's why you get a different value every time. You are seeing the address of the first item of that array.

Rondog

You can use pointers to do this sort of thing. I have an example that takes some part of a QByteArray and extracts an integer from it:

int i;
char *c;
		
c = (char*)(&i); // set address of char to point to address of integer.  The two are now tied together
	
// indirectly copy bytes to integer.  Var 'index' is first byte of integer value in the byte_array
for(unsigned int cntr = 0;cntr < sizeof(int);++cntr)
    *(c + cntr) = array[index + cntr];
	
// var 'i' now contains integer value

HB76

Thank you for your response,

indeed I noticed the pointer problem in my code, but even when I use the normal variable, the value returned is not correct :

QDataStream s(&file);
s.setByteOrder(QDataStream::LittleEndian);
char buffer[4];

s.skipRawData(80);
s.readRawData(buffer,4);
ulong length = ulong(buffer);
qDebug() << length;

Should return 70540 in this case but return 2686300.
Same for :

QDataStream s(&file);
s.setByteOrder(QDataStream::LittleEndian);
char *buffer = new char[4];

s.skipRawData(80);
s.readRawData(buffer,4);
ulong length = ulong(*buffer);
qDebug() << length;

wich return 0...

I would just like to understand what is incorrect in this code to know what I am doing wrong

JonB

@HB76 said in Read binary data from file:
ulong length = ulong(*buffer);

only looks at the first char/byte in buffer, i.e. buffer[0], and that's probably a 0, so that's not much good!

As for ulong length = ulong(buffer); not returning what you expect, the first thing I'd do is print it out in hex rather than decimal/look at the 4 bytes and see if they are the wrong way round/not what you expect in the 4 bytes. [I'm tired now, but doesn't ulong(buffer) just do the same thing as (ulong)buffer, and hence all you're printing is the address of the variable and nothing about its content?]

Apart from that, and I haven't looked up the mechanics, but I don't see how QDataStream::setByteOrder() can cooperate with your choice of using QDataStream::readRawData(). If it reads raw bytes it's not going to be dealing with numbers and observing any byte ordering. Presumably you have to use QDataStream &QDataStream::operator>>(quint32 &i), https://doc.qt.io/qt-5/qdatastream.html#operator-gt-gt-5, if you want to read what you know to be numbers and have the byte order taken into account.

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:

the values of the bytes at positions 80, 81, 82, 83 for the file you're working with?
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 !