If you read a file and write it to another file you cannot use fopen to read it. When using fopen it adds extra 0x00 marks to the output file.

The example below adds extra 0x00 marks:

QFile fi;
fi.open(fopen("input.txt","r"),QIODevice::ReadOnly);

The example below won't add extra 0x00 marks:

QFile fi("input.txt");
fi.open(QIODevice::ReadOnly);

I attached simple test application (bug.zip) to reproduce the issue. I included input.txt and output.txt to the same package which you can use to see the difference (you have to open those files with a hex editor).

Also, I attahed logs.zip where are logs when running the application while QIODEVICE_DEBUG flag is defined.

This cannot be reproduced on Mac/Linux