Read binary data from file

[HB76]

Hi everyone !

I am trying to read data from a file that can be either in ascii or binary format. For the ascii one, I figured out a code to read it and get the data. But I'm stuck with the binary format.

My binary file contain a 80 bytes header wich is a string, and then a 4 bytes unigned int that correspond to the data length, followed by the data. I've tried differents approach :

QFile file(file_name);

if (!file.open(QIODevice::ReadOnly))
        return;

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

s.skipRawData(80);    // I've already read the header before to determine the file format
s.readRawData(buffer,4);
ulong length = ulong(buffer);
qDebug() << length;

But the value returned is totaly inconsistent and change each time I run my code, wich makes me seriously doubt its validity.
Other way tested :

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

But it return 0 every time.
And the last one :

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

ulong length;
s >> length;
qDebug() << length;

But it also return 0.

Do you have any idea of what I am doing wrong ? If differents ways exist, wich one is the fastest ?
Thank you for your help !

HB76

[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:

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 !