How is GdkPixbuf pixel data stored?

[Christopher Andrew Chenery <cheneryc cs curtin edu au>]

Hi

I'm looking to manipulate a GdkPixbuf at the pixel level and have found 
that the actual pixel data is not stored in the way I expected.

In order to get the pixel data I have used the following code (where 
image is a static GtkWidget declared in the header file):

void pixel_data(GtkWidget *widget, GdkEvent *event, gpointer data)
{
	int i, array_size;
	GdkPixbuf *image_data;

	i = 0;
	image_data = gtk_image_get_pixbuf(GTK_IMAGE(image));
	array_size = 3 * (gdk_pixbuf_get_width(image_data) * gdk_pixbuf_get_height(image_data));
	
	for(i=0; i < array_size; i++)
		g_print("%d, ", (int) gdk_pixbuf_get_pixels(image_data)[i]);
}


My assumption was that the guchar pointer returned by 
gdk_pixbuf_get_pixels() would store the pixel data as an 8 bit int for 
the R, G and B channels for each pixel. ie. a 2*2 pixel image with 24bit 
colour would print out the following (assuming the image was white):

255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,

However the following was output:

255, 255, 255, 255, 255, 255, 80, 64, 255, 255, 255, 255,

I then output the rowstride for the image which was 8 (bytes per row) 
which prompted me to hack the code to loop 16 times and got:

255, 255, 255, 255, 255, 255, 80, 64, 255, 255, 255, 255, 255, 255, 0, 0, 
rowstride: 8

My Question is simply why are there two extra bytes per image row, what do 
they represent and what is a rowstride? - the documentation assumes this 
knowledge!

I'm using GTK2.0 and redhat 7.3

Cheers