Swapping bytes in a QByteArray - c++

Question

I want to write a simple function to swap bytes in a QByteArray. This is what I have come up with:
int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
QString t = "abcde";
QByteArray test;
test.append(t.at(0));
test.append(t.at(1));
test.append(t.at(2));
test.append(t.at(3));
test.append(t.at(4));
qDebug() << t;
qDebug() << QString(swapBytes(test,0,3));
return a.exec();
}
QByteArray swapBytes(QByteArray in, int swapOffset, int quantity) {
if (swapOffset < 0) {
return in;
}
if(quantity>(in.length()/2)) {quantity=in.length()/2;}
if(quantity < 1) {quantity=1;}
int k;
char buf[quantity];
char buf2[quantity];
qDebug() << quantity;
for (int i = 0; i + quantity*2 + swapOffset <= in.length(); i=i+2*quantity) {
k=i;
for(int b = 0;b<i+quantity;b++){
buf[b]=in.at(k);
buf2[b]=in.at(k+swapOffset+quantity);
k++;
}
qDebug() << buf;
qDebug() << buf2;
qDebug() << in;
in.replace(0,quantity,buf2);
qDebug() << in;
in.replace(quantity+swapOffset,quantity,buf);
}
return in;
}
For some reason when I run this code I get the following output :
abcde
ab(
cd
abcde
cdcde
cdab(e
Where does the parentheses come from? As far as I know there is only one char per byte, so what is wrong?

Answer 1

You need to allocate one byte more than quantity to leave room for the null terminator. Try this:
char* buf = new char[quantity+1];
char* buf2 = new char[quantity+1];
memset(buf, 0, quantity+1);
memset(buf2, 0, quantity+2);
Then before your function returns, remember to deallocate:
delete [] buf;
delete [] buf2;
return in;
Alternately, you can just use a QByteArray in place of a char array:
QByteArray buf(quantity, 0x00);
QByteArray buf2(quantity, 0x00);
This allows you to skip calls to memset and to avoid worrying about deallocation.
Unrelated to all of this, note that you can initialize a QByteArray from a QString like this:
QByteArray test = t.toAscii();