QFile/QDataStream writing on existing data

Question

Hello,

I have got a file which is let's say 8 bytes length. For example it looks like that:

22222222

Now, I read first let's say 5 bytes and changing them. For ex. to 11111

Finally, I want to write them ONTO EXCISTING DATA to the file, so I expect the file to look like that:

11111222

But I get only 11111, because file is erased. How can I disable erasing? (Maybe this question exists, but couldn`t find question like this one)

Answer 1

void fileopen()
{
QDataStream Input(&file);
Input>>"11111";
Input>>"22222";
file.close();
}

this function write the data.

QDataStream &operator<<(QDataStream &ds,const QString &data)
{

ds<<data.toLatin1().data();
ds<<data.toLatin1().data();
return ds;
}

Answer 2

Try to open the QFile with | QIODevice::Append, then QFile::seek(), then create the QDataStream on the QFile object. But note that QDataStream adds control information to the output, so that probably doesn't result in exactly what you wants.

Answer 3

Depending on what you are exactly doing with the file, you might want to memory map it:

QFile f("The file");
f.open(QIODevice::ReadWrite);
uchar *buffer = f.map(0, 5);

// The following line will edit (both read from and write to)
// the file without clearing it first:
for (int i=0; i<5; ++i) buffer[i] -= 1;

f.unmap(buffer);
f.close();