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I added this to main


printf("Number of digits in 153: %d\n", digitn(153));
printf("Sum of digits in 153: %d\n", sum(153,digitn(153)));
printf("Number of digits in 10: %d\n", digitn(10));...

Learn what "\n" means; add it to all of your printf, Or use flush stdout. This might not be problem, but not sure it is not a problem.

Learn to indent correctly.
Learn to comment your code....