Add two linked lists representing a non-negative number

linked_addition.c

struct ListNode *linked_addition(struct ListNode *l1, struct ListNode *l2)
{
    struct ListNode *l1Start = l1;
    struct ListNode *l2Start = l2;
    struct ListNode *lSum = (struct ListNode *)malloc(sizeof(struct ListNode));
    struct ListNode *lSumStart = lSum;
    bool flag = true;
    while (flag)
    {
        lSum->val = l1->val + l2->val;
        if (l1->next && l2->next)
        {
            l1 = l1->next;
            l2 = l2->next;
            lSum->next = (struct ListNode *)malloc(sizeof(struct ListNode));
            lSum = lSum->next;
            lSum->next = NULL;
            flag = true;
        }
        else
        {
            lSum->next = NULL;
            flag = false;
        }
    }
    while (l1->next || l2->next)
    {
        if (l1->next)
        {
            l1 = l1->next;
            lSum->next = (struct ListNode *)malloc(sizeof(struct ListNode));
            lSum = lSum->next;
            lSum->next = NULL;
            lSum->val = l1->val;
        }
        if (l2->next)
        {
            l2 = l2->next;
            lSum->next = (struct ListNode *)malloc(sizeof(struct ListNode));
            lSum = lSum->next;
            lSum->next = NULL;
            lSum->val = l2->val;
        }
    }
    l1 = l1Start;
    l2 = l2Start;
    lSum = lSumStart;
    flag = true;
    int carry = 0;
    while (flag)
    {
        lSum-> val += carry;
        if(lSum->val >= 10)
        {
            carry = 1;
            lSum->val %= 10;
        }
        else carry = 0;
        if (lSum->next) lSum = lSum->next;
        else flag = false;
    }
    if (carry)
    {
        lSum->next = (struct ListNode *)malloc(sizeof(struct ListNode));
        lSum = lSum->next;
        lSum->next = NULL;
        lSum->val = carry;
        carry = 0;
    }
    lSum = lSumStart;
    return lSum;
}