Questions

ptr pointer is constant. In ptr = yourbuf the program is trying to modify it, hence an error.

If you read newspaper daily, then you may be aware of the four coloured dots on the corner of the newspaper. 

The four dots CMYK — blue (Cyan), pink (Magenta), Yellow and black (Kalio) are registration marks used during printing to help ensure the print is aligned properly. Any colour can be obtained by adding the correct proportion of these four colours.

CMYK is used as a base for the colours used in printing newspapers, and these four colours are often printed on the paper in either a heart, dot, or square shape.

To print an image, the plates of all these colours are laid out on a page separately and lined up in the same spot while printing. If those four dots are blur or you find the colours overlapping on the image it means the printing is not good. To print an accurate and perfect image, the colors and shape must be arranged precisely and must be claear sharp in order to get the best output.

That's why CMYK is also called registration mark. As physically checking all the pages of the paper is not possible for a printer who prints number of news papers daily, he just checks these dots.