Read a non-negative number and write it reversed

This is the simplest solution:

#include <stdio.h>

int main(void)

{

        int n;

        printf("Enter a non-negative number\n");

        scanf("%d", &n);

        if(n < 0) {

                printf("*** Error: the number is negative!\n");

                return 1;

        }

        do {

                printf("%d", n % 10);

                n /= 10; // Equivalent to "n = n / 10"

        }

        while(n > 0);

        return 0;

}

Error messages by convention should be written to standard error, so the statement 

printf("*** Error: the number is negative!\n");

should be replaced by

fprintf(stderr, "*** Error: the number is negative!\n");