[QTBUG-77285] Incorrect initialization of QBitArray from dense byte array - Qt Bug Tracker

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Details

Type: Bug
Status: Closed
Priority: P2: Important
Resolution: Done
Affects Version/s: 5.12.4, 5.13.0
Fix Version/s: 5.12.5, 5.14.0 Alpha
Component/s: Core: Other
Labels:
None

Platform/s:

All
Commits:
6ce9404a6e7ad6ba3ff37f6890fe400c643c3d52 (qt/qtbase/5.12)

Description

When initializing a QBitArray using function:

static QBitArray fromBits(const char *data, qsizetype len);

The resulting bit array isn't always properly initialized.

I wrote an example where I have one byte but want to use only 5 bits.

// Example code
#include <QCoreApplication>
#include <QBitArray>
#include <stdio.h>

int main(int argc, char* argv[])
{
    QCoreApplication a(argc, argv);
    char c = 0xff;
    const int nbits = 5;
    const QBitArray ba = QBitArray::fromBits(&c, nbits);

    for(int b=0 ; b<nbits ; ++b)
        printf("%d ", ba.testBit(b) ? 1 : 0);

    fflush(stdout);

    return a.exec();
}

The above code produces the following result:

1 1 1 0 0

When in fact it should produce:

1 1 1 1 1

This happens always in the last byte of the array, I was able to reproduce this in arrays of several sizes, this example has one byte only for simplicity.