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Problem converting QByteArray to long

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  • VRoninV VRonin

    First of all, you need to understand endianness

    Matching endianness between java and C++

    
    void Receiver::on_receivingByteBuffMessage(const QByteArray& msg){
    Q_ASSERT(msg.size()==sizeof(qint64));
    qint64 value = *reinterpret_cast<const qint64*>(msg.constData());
    }
    

    Mismatching endianness between java and C++

    void Receiver::on_receivingByteBuffMessage(const QByteArray& msg){
    Q_ASSERT(msg.size()==sizeof(qint64));
    QByteArray reversed(sizeof(qint64),0);
    std::copy(msg.crbegin(),msg.crend(),reversed.begin());
    qint64 value = *reinterpret_cast<const qint64*>(reversed.constData());
    }
    
    JonBJ Offline
    JonBJ Offline
    JonB
    wrote on last edited by
    #4

    @VRonin
    Your "reversing" code involves actually copying the bytes in reverse order into a temporary area to "convert" it to a native int64. This is hideously inefficient if you have a file with billions of numbers in it :) Isn't there a solution which swaps bytes directly (e.g. so could be done on a register directly)?

    VRoninV JohanSoloJ 2 Replies Last reply
    0
    • JonBJ JonB

      @VRonin
      Your "reversing" code involves actually copying the bytes in reverse order into a temporary area to "convert" it to a native int64. This is hideously inefficient if you have a file with billions of numbers in it :) Isn't there a solution which swaps bytes directly (e.g. so could be done on a register directly)?

      VRoninV Offline
      VRoninV Offline
      VRonin
      wrote on last edited by
      #5

      @JonB That's a very good question but I'm not smart enough to know the answer

      "La mort n'est rien, mais vivre vaincu et sans gloire, c'est mourir tous les jours"
      ~Napoleon Bonaparte

      On a crusade to banish setIndexWidget() from the holy land of Qt

      JonBJ 1 Reply Last reply
      1
      • VRoninV VRonin

        @JonB That's a very good question but I'm not smart enough to know the answer

        JonBJ Offline
        JonBJ Offline
        JonB
        wrote on last edited by JonB
        #6

        @VRonin
        LOL, of course you're smart enough!

        I was thinking of e.g. https://linux.die.net/man/3/htonl. These functions are "byte-swappers", and they were defined in macros with lots of << & >>s :) But they only go up to int32 size :(

        EDIT I found this sample:

        unsigned long int Endian_DWord_Conversion(unsigned long int dword)
         {
           return ((dword>>24)&0x000000FF) | ((dword>>8)&0x0000FF00) | ((dword<<8)&0x00FF0000) | ((dword<<24)&0xFF000000);
        }
        

        It's a question of expanding that approach from 4 to 8 bytes, and marking it inline!

        1 Reply Last reply
        0
        • M Offline
          M Offline
          mpergand
          wrote on last edited by
          #7

          What about using QDataStream:

          QDataStream data(msg);
          qint64 val;
          data>>val;
          

          QDataStream is smart enough to convert to the host endianess automaticaly.
          By defaut, DataStream is set to big endian, see setByteOrder().

          1 Reply Last reply
          0
          • Christian EhrlicherC Offline
            Christian EhrlicherC Offline
            Christian Ehrlicher
            Lifetime Qt Champion
            wrote on last edited by
            #8

            @mpergand said in Problem converting QByteArray to long:

            What about using QDataStream

            Since QDataStream is a stream format it will not only store the 4 or 8 bytes but also some information to know what is saved - therefore it's totally useless here.

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            1 Reply Last reply
            2
            • SGaistS Offline
              SGaistS Offline
              SGaist
              Lifetime Qt Champion
              wrote on last edited by
              #9

              Hi,

              Another important point, you don't check that you received all your data. The readyRead doesn't signal that the socket got the complete payload, only there's something that arrived. Your payload might be small enough now but it might very well change in the near future.

              Interested in AI ? www.idiap.ch
              Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

              1 Reply Last reply
              2
              • JonBJ JonB

                @nico162
                First let's start by having your actual code. Do you really have:

                    for(int i=0;i<8;i++){
                        qInfo()<<(qint64)hashKeyValue[0];
                    }
                

                or would you like to correct that in either code or pasting here?

                Second, have a look at the docs & example for http://doc.qt.io/qt-5/qbytearray.html#toLong. It's not doing anything like you're expecting it to do: it's for parsing a byte-array-string to a long, not creating a long out of a number already sitting there in binary bytes!

                N Offline
                N Offline
                nico162
                wrote on last edited by
                #10

                @JonB

                my bad, no it's
                for(int i=0;i<8;i++){
                qInfo()<<(qint64)hashKeyValue[i];
                }

                Ah okay.. Thank you

                JonBJ 1 Reply Last reply
                0
                • N nico162

                  @JonB

                  my bad, no it's
                  for(int i=0;i<8;i++){
                  qInfo()<<(qint64)hashKeyValue[i];
                  }

                  Ah okay.. Thank you

                  JonBJ Offline
                  JonBJ Offline
                  JonB
                  wrote on last edited by
                  #11

                  @nico162
                  That's why it's always best to copy & paste code, not type it in, when you want help! :)

                  1 Reply Last reply
                  0
                  • VRoninV VRonin

                    First of all, you need to understand endianness

                    Matching endianness between java and C++

                    
                    void Receiver::on_receivingByteBuffMessage(const QByteArray& msg){
                    Q_ASSERT(msg.size()==sizeof(qint64));
                    qint64 value = *reinterpret_cast<const qint64*>(msg.constData());
                    }
                    

                    Mismatching endianness between java and C++

                    void Receiver::on_receivingByteBuffMessage(const QByteArray& msg){
                    Q_ASSERT(msg.size()==sizeof(qint64));
                    QByteArray reversed(sizeof(qint64),0);
                    std::copy(msg.crbegin(),msg.crend(),reversed.begin());
                    qint64 value = *reinterpret_cast<const qint64*>(reversed.constData());
                    }
                    
                    N Offline
                    N Offline
                    nico162
                    wrote on last edited by
                    #12

                    @VRonin
                    Yes I though it could be that, but the bytes sent from Java to C++, are of same value and in the same order at the sending and receiving (I print the content in java and in C++, so I know the structure of the byte array).
                    So I admit that it wasn't an endianness error (maybe I am wrong ..).

                    1 Reply Last reply
                    0
                    • VRoninV VRonin

                      First of all, you need to understand endianness

                      Matching endianness between java and C++

                      
                      void Receiver::on_receivingByteBuffMessage(const QByteArray& msg){
                      Q_ASSERT(msg.size()==sizeof(qint64));
                      qint64 value = *reinterpret_cast<const qint64*>(msg.constData());
                      }
                      

                      Mismatching endianness between java and C++

                      void Receiver::on_receivingByteBuffMessage(const QByteArray& msg){
                      Q_ASSERT(msg.size()==sizeof(qint64));
                      QByteArray reversed(sizeof(qint64),0);
                      std::copy(msg.crbegin(),msg.crend(),reversed.begin());
                      qint64 value = *reinterpret_cast<const qint64*>(reversed.constData());
                      }
                      
                      N Offline
                      N Offline
                      nico162
                      wrote on last edited by
                      #13

                      @VRonin
                      Ah no okay I misunderstood... Now I see what you mean :D! And it works fine

                      N 1 Reply Last reply
                      0
                      • N nico162

                        @VRonin
                        Ah no okay I misunderstood... Now I see what you mean :D! And it works fine

                        N Offline
                        N Offline
                        nico162
                        wrote on last edited by nico162
                        #14

                        @nico162 said in Problem converting QByteArray to long:

                        @VRonin
                        Ah no okay I misunderstood... Now I see what you mean :D! And it works fine

                        So as @JonB explained the QByteArray::toLong() is not adapted to my need (to convert a byte array to long).
                        @VRonin gave a solution to interpret the byteArray in the correct endianness mode (from Java to C++):

                        void Receiver::on_receivingByteBuffMessage(const QByteArray& msg){
                        Q_ASSERT(msg.size()==sizeof(qint64));
                        QByteArray reversed(sizeof(qint64),0);
                        std::copy(msg.crbegin(),msg.crend(),reversed.begin());
                        qint64 value = *reinterpret_cast<const qint64*>(reversed.constData());
                        }
                        

                        Maybe better solution could exist to avoid create a temp array to copy the byte in the reverse order as mentioned @JonB .
                        I personally have little bytebuffer received, so the solution that @VRonin gave it's enough efficient in my use case, that's why I close this topic.
                        @SGaist I take in account your remark thank you :).
                        I thank you all for your help :)! First time I posted a topic, and I am happy to see that there is an helpful and comprehensive community.

                        See ya !

                        1 Reply Last reply
                        1
                        • JonBJ JonB

                          @VRonin
                          Your "reversing" code involves actually copying the bytes in reverse order into a temporary area to "convert" it to a native int64. This is hideously inefficient if you have a file with billions of numbers in it :) Isn't there a solution which swaps bytes directly (e.g. so could be done on a register directly)?

                          JohanSoloJ Offline
                          JohanSoloJ Offline
                          JohanSolo
                          wrote on last edited by
                          #15

                          @JonB said in Problem converting QByteArray to long:

                          Isn't there a solution which swaps bytes directly (e.g. so could be done on a register directly)?

                          Would by chance _byteswap_uint64 or __builtin_bswap64 do the trick?

                          `They did not know it was impossible, so they did it.'
                          -- Mark Twain

                          JonBJ 1 Reply Last reply
                          0
                          • JohanSoloJ JohanSolo

                            @JonB said in Problem converting QByteArray to long:

                            Isn't there a solution which swaps bytes directly (e.g. so could be done on a register directly)?

                            Would by chance _byteswap_uint64 or __builtin_bswap64 do the trick?

                            JonBJ Offline
                            JonBJ Offline
                            JonB
                            wrote on last edited by
                            #16

                            @JohanSolo
                            They would indeed, but each of those is MSVC/GCC specific ... :(

                            1 Reply Last reply
                            0
                            • JohanSoloJ Offline
                              JohanSoloJ Offline
                              JohanSolo
                              wrote on last edited by
                              #17

                              @JonB
                              I know, that's why I gave the two of them... Not the first time a #ifdef WIN32 would help.

                              `They did not know it was impossible, so they did it.'
                              -- Mark Twain

                              JonBJ 1 Reply Last reply
                              0
                              • JohanSoloJ JohanSolo

                                @JonB
                                I know, that's why I gave the two of them... Not the first time a #ifdef WIN32 would help.

                                JonBJ Offline
                                JonBJ Offline
                                JonB
                                wrote on last edited by JonB
                                #18

                                @JohanSolo
                                Yeah, but I think (for Qt) we still have to support MinGW and whatever on MacOS and ...

                                Unlike the functions for swapping 16 or 32 bytes, I couldn't find standard ones for 64 bytes anywhere when I searched. Shame!

                                1 Reply Last reply
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