Skip to content
  • Categories
  • Recent
  • Tags
  • Popular
  • Users
  • Groups
  • Search
  • Get Qt Extensions
  • Unsolved
Collapse
Brand Logo
  1. Home
  2. Qt Development
  3. General and Desktop
  4. Read binary data from file
Forum Updated to NodeBB v4.3 + New Features

Read binary data from file

Scheduled Pinned Locked Moved Solved General and Desktop
15 Posts 6 Posters 3.1k Views 2 Watching
  • Oldest to Newest
  • Newest to Oldest
  • Most Votes
Reply
  • Reply as topic
Log in to reply
This topic has been deleted. Only users with topic management privileges can see it.
  • R Offline
    R Offline
    Rondog
    wrote on last edited by Rondog
    #3

    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
    
    1 Reply Last reply
    0
    • H Offline
      H Offline
      HB76
      wrote on last edited by HB76
      #4

      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

      JonBJ 1 Reply Last reply
      0
      • H 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

        JonBJ Offline
        JonBJ Offline
        JonB
        wrote on last edited by JonB
        #5

        @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.

        1 Reply Last reply
        0
        • H Offline
          H Offline
          HB76
          wrote on last edited by
          #6
          QDataStream s(&file);
          s.setByteOrder(QDataStream::LittleEndian);
          
          s.skipRawData(80);
          qint32 value;
          s >> value;
          qDebug() << value;
          

          give me the same result, 0...

          Pablo J. RoginaP 1 Reply Last reply
          0
          • H 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. RoginaP Offline
            Pablo J. RoginaP Offline
            Pablo J. Rogina
            wrote on last edited by
            #7

            @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?

            Upvote the answer(s) that helped you solve the issue
            Use "Topic Tools" button to mark your post as Solved
            Add screenshots via postimage.org
            Don't ask support requests via chat/PM. Please use the forum so others can benefit from the solution in the future

            1 Reply Last reply
            0
            • H Offline
              H Offline
              HB76
              wrote on last edited by
              #8

              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..

              1 Reply Last reply
              0
              • Christian EhrlicherC Offline
                Christian EhrlicherC Offline
                Christian Ehrlicher
                Lifetime Qt Champion
                wrote on last edited by
                #9

                @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

                Qt Online Installer direct download: https://download.qt.io/official_releases/online_installers/
                Visit the Qt Academy at https://academy.qt.io/catalog

                H 1 Reply Last reply
                2
                • H Offline
                  H Offline
                  HB76
                  wrote on last edited by
                  #10

                  @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());
                  
                  1 Reply Last reply
                  0
                  • Christian EhrlicherC 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

                    H Offline
                    H Offline
                    HB76
                    wrote on last edited by
                    #11

                    @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 ^^'

                    1 Reply Last reply
                    0
                    • Christian EhrlicherC Offline
                      Christian EhrlicherC Offline
                      Christian Ehrlicher
                      Lifetime Qt Champion
                      wrote on last edited by Christian Ehrlicher
                      #12

                      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

                      Qt Online Installer direct download: https://download.qt.io/official_releases/online_installers/
                      Visit the Qt Academy at https://academy.qt.io/catalog

                      1 Reply Last reply
                      0
                      • H Offline
                        H Offline
                        HB76
                        wrote on last edited by
                        #13

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

                        1 Reply Last reply
                        0
                        • Christian EhrlicherC Offline
                          Christian EhrlicherC Offline
                          Christian Ehrlicher
                          Lifetime Qt Champion
                          wrote on last edited by
                          #14

                          @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.

                          Qt Online Installer direct download: https://download.qt.io/official_releases/online_installers/
                          Visit the Qt Academy at https://academy.qt.io/catalog

                          1 Reply Last reply
                          2
                          • H Offline
                            H Offline
                            HB76
                            wrote on last edited by
                            #15

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

                            1 Reply Last reply
                            0

                            • Login

                            • Login or register to search.
                            • First post
                              Last post
                            0
                            • Categories
                            • Recent
                            • Tags
                            • Popular
                            • Users
                            • Groups
                            • Search
                            • Get Qt Extensions
                            • Unsolved