Error copying data into iplimage - c++

Question

Hi I have the following piece of code:
void ViewPlane::colourPixel(const int u, const int v, const Colour c){
int step = img->widthStep;
cout << "U: " << u << ", V: " << v << ", step: " << step << "\n";
cout << "Colour: " << c.r << ", " << c.g << ", " << c.b << "\n";
cout << "Data: " << v*step+u*3 << ", " << v*step+u*3+1 << ", " << v*step+u*3+2 << "\n";
cout << "Data: " << data[v*step+u*3] << ", " << data[v*step+u*3+1] << ", " << data[v*step+u*3+2] << "\n";
//swap colours round as opencv default image is bgr, not rgb
data[v*step+u*3] = c.b;
cout << "done it\n";
data[v*step+u*3+1] = c.g;
cout << "done it\n";
data[v*step+u*3+2] = c.r;
cout <<"done it\n";
}
img is an IplImage pointer, and data is
data = (uchar *)img->imageData;
I am looping through lots of values of u and v (image coordinates) and assigning a colour to each pixel. However the code just stops half way through this pixel when it reaches u = 148.
Printing out the values reveals the green pixel at this point originally contains '*', and is the only one up to that point which does so. Here is a copy of the output:
....
U: 146, V: 0, step: -1219934912
Colour: 0, 0, 0
Data: 438, 439, 440
Data:
done it
done it
done it
U: 147, V: 0, step: -1219934912
Colour: 0, 0, 0
Data: 441, 442, 443
Data:
done it
done it
done it
U: 148, V: 0, step: -1219934912
Colour: 0, 0, 0
Data: 444, 445, 446
Data: �, *, �
done it
So my question is, what does this star mean? And why is it stopping me from assigning data there?
EDIT: The image size is 400x400px
PS: I just tried commenting out the green channel assignment and this error happens again for the red channel further on. This time it held '#' before assignment, so this makes me think it has no bearing on what's going wrong...In that case, what on earth? I'm not out of bounds of the array, the value I'm inserting is an integer 0...

Answer 1

Your program's output shows step being "-1219934912". It makes it look like you have a stray img pointer. It's rather strange that each line in image would take ~1GB