Simple C trie program goes wrong (segmentation fault)

Question

In order to understand tries I am creating this very simple C program that takes from user 10 nums from 0 to 9 as children of the trie. The final step is to print this nums with the function print, but I am getting a segmentation fault:

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

typedef struct list
{
    int data;
    struct list *ar[10];
} list;

void insert(list *head);
void print(list *head);

int main(void)
{
    printf("hello\n");
    list *root = malloc(sizeof(list));

    insert(root);
    print(root);    
}

void insert(list *head)
{
    int a, i;

    if (head == NULL) {
        return;
    }

    for (i = 0; i < 10; i++) {
        printf("Give Num 0-9\n");
        scanf("%i", &a);
        head->ar[a] = malloc(sizeof(head));
        head = head->ar[a];
        head->data = a;
    }
}

void print(list *head)
{
    if (head == NULL) {
        return;
    }

    while (head != NULL) {
        for (int i = 1; i < 10; i++) {
            if (head->ar[i] != NULL) {
                printf("%i", i);
                head = head->ar[i];
                break;
            }
        }
    }
    printf("\n");
}

Answer 1

There are several issues with your code:

1. The first mention of malloc doesn't actually initialize the memory (the ar field). You should initialize it properly

   list *root = malloc(sizeof(list));
   

   Is missing initialization, e.g.

   root->data = 0;
   for (size_t ii = 0; ii < 10; ii++) {
       root->ar[ii] = NULL;
   }
   

2. When you are actually gathering input, you allocate only enough memory for the pointer, not for the actual list itself.

   head->ar[a] = malloc(sizeof(head));
   

   should be initialized as above (head = malloc(sizeof(list)); for (size_t ...

3. There seems to be an infinite loop when actually running your program (after correcting all these issues).

EDIT: Remove calloc...