QFile - read bytes, x at a time, x +offset?

Dariusz

Hey

I'm trying to read part of the file to decide if I wish to read all of the file... can any1 help me out fill in the blank?

    QByteArray someData;
    QByteArray da;
    QDataStream s(&da, QIODevice::WriteOnly);
    s << (int) 1412 << (quint16) 123143 << (bool) true << (int) someData.size() << someData;
    QFile f(path);
    if (f.open(QIODevice::ReadOnly)) {
        int initial = 0;
        quint16 qinitial = 0;
        bool good = false;
        int dataSize = 0;
        QByteArray inData = f.read(sizeof(int) + sizeof(quint16) + sizeof(bool) + sizeof(int));/// Is this correct?

        QDataStream in(&inData,QIODevice::ReadOnly);
        in>>initial>>qinitial>>good>>dataSize;
        if(good){
            /// Read f.read(dataSize) ??? How to read rest?
        }
    }

TIA

KroMignon

@Dariusz Why you not simply use a QDataStream from file?

    QByteArray someData;
    QByteArray da;
    QDataStream s(&da, QIODevice::WriteOnly);
    s << (int) 1412 << (quint16) 123143 << (bool) true << (int) someData.size() << someData;
    QFile f(path);
    if (f.open(QIODevice::ReadOnly)) {
        QDataStream inData(&f);
        int initial;
        quint16 qinitial;
        bool good;
        int dataSize;

        inData>>initial>>qinitial>>good>>dataSize;
        if(good){
            QByteArray buffer;
            buffer.resize(dataSize);
            inData.readRawData(buffer.data(), dataSize);
        }
    }

Dariusz

@KroMignon looks interesting, but I want to read from xx byte to xx byte. Not all of it.

Also ohh hmmmm that looks interesting, did not know I can do it. So if I do dataStream, hell only read specific bytes automaticly? Damn thats neat o.o / still what about reading just xx to xx ?

Hmm I suppose I could just use skiPrawData(int) < I guess its skipRawData(sizeof(int)) or just the size I stored early.

jsulm

@Dariusz Simply seek to the position where you want to read and then read the amount of bytes you need.
https://doc.qt.io/qt-5/qfiledevice.html#seek
https://doc.qt.io/qt-5/qiodevice.html#read-1

You can seek to the beginning of the file if you decide to read whole file.