Print a linked list

#include <stdio.h>
#include <stdlib.h>

struct digit {
    int num;
    struct digit *next;
};

struct digit * createDigit(int);
struct digit * append(struct digit * end, struct digit * newDigptr);
void printNumber(struct digit *start);

int main(void) {
   
    struct digit *start, *newDigptr, *end, *tmp;
    int first = 5;
    int second = 3;
    int third = 7;
    start = createDigit(first);
    end = start;
    newDigptr = createDigit(second);
    end = append(end, newDigptr);
    newDigptr = createDigit(third);
    end = append(end, newDigptr);
    printNumber(start);
    tmp = start->next;
    free(start);
    start = tmp;
    tmp = start->next;
    free(start);
    free(tmp);
    return 0;
}

struct digit *createDigit(int dig) {
    struct digit *ptr;
    ptr = (struct digit *) malloc(sizeof(struct digit));
    ptr->num = dig;
    ptr->next = NULL;
    return ptr;
}

struct digit * append(struct digit * end, struct digit * newDigptr) {
    end->next = newDigptr;
    return(end->next);
}

void printNumber(struct digit *start){
   
    struct digit * ptr = start;
    while (ptr!=NULL) {
        printf("%d", ptr->num);
        ptr = ptr->next;
    }
    printf("\n");
}