Qdatastreams and binary files.

Christian Ehrlicher

@Styx said in Qdatastreams and binary files.:

Qfile read or qbytearray readall then cover the data from there?

Yes

@jsulm: the data is not created with QDataStream, so reading with QDataStream will not (really) work.

jsulm

@Christian-Ehrlicher Ah, OK. I was thinking other way around - it is early morning here :-)

JKSH

@Styx said in Qdatastreams and binary files.:

How should i go about reading the data then?

First, you need to clarify:

What is the binary layout of your file?
Are the numbers in your file stored in little-endian or big-endian format?
How are your text strings encoded? ASCII? UTF-8? Something else?

@Styx said in Qdatastreams and binary files.:

So the binary file looks like this.

0x01 0x00 0x00 0x00 0x00 0x02 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0xcb 0x3b 0x8d
0x12 ....

its suppose to be

m_binaryVersion = 0x01
bookCount = 0x02
bookhash = 0xcb 0x3b 0x8d 0x12

OK, so your file layout is something like this...

Byte Offset	Content
0	`m_binaryVersion`
1-4	???
5	`bookCount`
6-12	????
13-16	`bookHash`

There are 4 null bytes between 0x01 and 0x02, and there are 7 null bytes between 0x02 and 0xcb 0x3b 0x8d 0x12. What are these null bytes for?

Is bookCount supposed to occupy Bytes 5-12 as a qint64?

Styx

how do i read chunks or sections with qbytearray? Should i store it in one buffer then read the section with qfile read?

1-4 bytes are just padding i believe 5 is the ammount of books that is written to the struct

0xcb 0x3b 0x8d is the bookhash

0x12 is the bookhashId

jsulm

@Styx said in Qdatastreams and binary files.:

how do i read chunks or sections with qbytearray?

https://doc.qt.io/qt-5/qbytearray.html#mid

JKSH

@Styx said in Qdatastreams and binary files.:

1-4 bytes are just padding

OK

i believe 5 is the ammount of books that is written to the struct

So the maximum number of books is 255?

What are bytes 6-12?

Styx

The null bytes are just null padding. max number of books are 5 each book has its own bookhash and bookhashId booktype bookdir and bookfilename

Would use qbytearray mid grab the max number of books which is 5 then loop thru the rest of the qbytearray to grab each bookinformation.

JKSH

@Styx said in Qdatastreams and binary files.:

The null bytes are just null padding. max number of books are 5 each book has its own bookhash and bookhashId booktype bookdir and bookfilename

OK

Would use qbytearray mid grab the max number of books which is 5 then loop thru the rest of the qbytearray to grab each bookinformation.

Sounds good

Styx

is there a way to loop thru the bytearray without using mid?

was using...

Books bookinfo;
 bookinfo.bookCount =  (bytearray.at(5) & 0xFFFF);

jsulm

@Styx said in Qdatastreams and binary files.:

is there a way to loop thru the bytearray without using mid?

Sure (https://doc.qt.io/qt-5/qbytearray.html):
QByteRef operator[](int i)
char operator[](int i) const
char operator[](uint i) const
QByteRef operator[](uint i)

So

for (int i = 0; i < 3; ++i)
    bytearray[i];

Styx

So once i readall the file into the qbytearray i would use mid to break up the byte offset and copy them to another bytearray?

Is there away to get around not having to use so many qbytearray to copy data?

How would seek and read work from a Qfile?

02 - book count 

00 00 00 00 00 00 00 - (padding)

fb 2b 7d 13 - bookhash

09 - bookhashid

00 00 00 00 00 00 00 - (padding)

44 6e 49 4f 43 44 61 62 64 - booktype

42 - booktypeid

00 00 00 00 00 00 00 - (padding)

00 00 00 00 00 00 00 00 00 00 00 00 - bookDir and bookFileName (Qstring)

00 00 00 00 00 00 00 - (padding)

1a 10 a2 ae - bookhash

08 - bookhashid

00 00 00 00 00 00 00 

41 6S 64 4f 47 61 49 44 - booktype

3c - booktypeid

00 00 00 00 00 00 00 

00 00 00 00 00 00 00 00 00 00 00 00 - bookDir and bookFileName (Qstring)

This is a example of a file i am reading i was trying to read it into a struct but not sure if that is the correct method. Qdatastreams cant be used because the file wasn't written by qdatastreams.

as you seen in the code the book count is looped base on the same information provided.

mrjj

Hi
What app produces the file ?
Its not open source so you could get the actual record definitions ?

Christian Ehrlicher

@Styx said in Qdatastreams and binary files.:

Is there away to get around not having to use so many qbytearray to copy data?

Work with a plain const char * pointer

JKSH

@Christian-Ehrlicher said in Qdatastreams and binary files.:

Work with a plain const char * pointer

To add to @Christian-Ehrlicher's point: CallQByteArray:data() or QByteArray::constData() to get a raw pointer to your data. Then, you can use pointer arithmetic to extract your data.

QByteArray ba = file.readAll();
const char* data = ba.constData();

// Assuming that your file is little-endian...
memcpy(&m_binaryVersion, data +  0, sizeof(quint8 ));
memcpy(&bookCount,       data +  5, sizeof(quint8 ));
memcpy(&bookHash,        data + 13, sizeof(quint32));

EDIT: Code above changed from reinterpret_cast<> to memcpy() for cross-platform safety

Styx

@JKSH Since .data() is null terminated. Think it would be better to use shift left.

// Assuming that your file is little-endian...
m_binaryVersion = *reinterpret_cast<const quint8* >(data +  0 ) >> 8;
bookCount       = *reinterpret_cast<const quint8* >(data +  5) >> 12;
bookHash        = *reinterpret_cast<const quint32*>(data + 13) >> 16;

// example
bookCount=256 the first byte is '\0' then all the rest will be undetermined.

Shouldn't have issues calling the index and then looping thru the qbytearray to print out the data as well.

Styx

I have to read some where around 2000 binary files non of them the same but some contain same data.

How would i use seek and read dynamically to read each file. (Qfile api).

JKSH

@Styx said in Qdatastreams and binary files.:

@JKSH Since .data() is null terminated. Think it would be better to use shift left.

m_binaryVersion = *reinterpret_cast<const quint8* >(data +  0 ) >> 8;
bookCount       = *reinterpret_cast<const quint8* >(data +  5) >> 12;
bookHash        = *reinterpret_cast<const quint32*>(data + 13) >> 16;

// example
bookCount=256 the first byte is '\0' then all the rest will be undetermined.

I don't get it. Could you please explain how this works?

@Styx said in Qdatastreams and binary files.:

How would i use seek and read dynamically to read each file. (Qfile api).

Take the code that reads one file and put it in a loop. Pass a different filename each loop iteration.

JonB

@Styx said in Qdatastreams and binary files.:

@JKSH Since .data() is null terminated. Think it would be better to use shift left.

// Assuming that your file is little-endian...
m_binaryVersion = *reinterpret_cast<const quint8* >(data +  0 ) >> 8;
bookCount       = *reinterpret_cast<const quint8* >(data +  5) >> 12;
bookHash        = *reinterpret_cast<const quint32*>(data + 13) >> 16;

// example
bookCount=256 the first byte is '\0' then all the rest will be undetermined.

I don't know what you're trying to achieve here (as @JKSH said), but:

You are using shift right, not left.
*reinterpret_cast<const quint8* >(data + 0 ) returns a quint8. Since that is (unsigned) 8-bits in size, >> 8 always returns 0 regardless of content.
Similarly for *reinterpret_cast<const quint8* >(data + 5) >> 12, except that >> 12 makes even less sense for an 8-bit value.
QByteArray:data() is indeed (extra) \0 terminated, but that has no relevance to any of the lines of code you wrote.

The code without any shifts written by @JKSH makes sense. I'm afraid yours does not!

JonB

@JKSH said in Qdatastreams and binary files.:

bookHash = *reinterpret_cast<const quint32*>(data + 13);

Have you actually tried this line? Because I would assume it will "segment fault" (or whatever, probably something else). You are trying to dereference a 32-bit int from data + 13, which will be an odd numbered address. Whoops! :) [I have a feeling static_cast<> would warn/prohibit this at compile-time?]

You must be very careful recommending to treat a binary block like this as though you can index into it directly for the types you know were serialized there, for this kind of reason. Here you need to pull the 4 bytes out of the buffer (e.g. memcpy() directly into an &quint32 if you know endian-ness is same on host as in file), or some other safe approach.

JKSH

@JonB said in Qdatastreams and binary files.:

@JKSH said in Qdatastreams and binary files.:

bookHash = *reinterpret_cast<const quint32*>(data + 13);

Have you actually tried this line? Because I would assume it will "segment fault" (or whatever, probably something else). You are trying to dereference a 32-bit int from data + 13, which will be an odd numbered address. Whoops! :)

Thanks for the heads-up. I tried compiling it using MinGW 7.3.0 32-bit, MSVC 2017 32-bit, and MSVC2017 64-bit (all with Qt 5.14.0, release mode) and got the expected results every time. However, your comment prompted me to do some digging which led me to this question: Should I worry about the alignment during pointer casting?

I'll update my sample code.

[I have a feeling static_cast<> would warn/prohibit this at compile-time?]

Static casting cannot be used to convert a byte array into an integer at all, no matter where the bytes sit in memory.