Question

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

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();