Creating Linked List, not passing back to Main

问题

So I am creating a linked list in a separate function, and when I print out the linked list within the function, it seems everything is fine. However; when I go to main and try to access the linked list with printf I get a segmentation fault and am confused exactly why.

void createLL(struct node* head, struct node* curr, char ch, int number){
//lowest digit is the head
    while (((scanf(" %c",&ch)) >= 0)){
        curr = (struct node*)malloc(sizeof(struct node*)); //allocate space
        number = ch - '0' ; //convert char to number
        curr->data = number;
        curr->next = head;
        head = curr;
    }
    curr = head;
    //troubleshoot
    while(curr){
        printf("%d\n",curr->data);
        curr = curr->next;
    }
    curr = head;
    printf("%d\n",curr->data);
}

int main(){
    //initials
    int i, number; 
    char ch;
    //node pointers
    struct node* headOne = NULL;
    struct node* currOne = NULL;
    struct node* headTwo = NULL;
    struct node* currTwo = NULL;
    //create linked list
    createLL(headOne,currOne, ch, number);
    printf("%d\n",currOne->data);
    createLL(headTwo,currTwo, ch, number);
    printf("%d\n",currTwo->data);

回答1:

In C functions pass all parameters by value. So if you want to change a variable in a function, you need to pass the address of that variable and dereference the parameter in the function.

Also, you're not allocating the right amount of space for your node. You want sizeof(struct node), not sizeof(struct node *).

void createLL(struct node **head, struct node **curr, char ch, int number){
//lowest digit is the head
    while (((scanf(" %c",&ch)) >= 0)){
        // don't cast the return value of malloc
        *curr = malloc(sizeof(struct node)); //allocate space
        number = ch - '0' ; //convert char to number
        (*curr)->data = number;
        (*curr)->next = *head;
        *head = *curr;
    }
    *curr = *head;
    //troubleshoot
    while(*curr){
        printf("%d\n",(*curr)->data);
        *curr = (*curr)->next;
    }
    *curr = *head;
    printf("%d\n",(*curr)->data);
}


int main(){
    //initials
    int i, number; 
    char ch;
    //node pointers
    struct node* headOne = NULL;
    struct node* currOne = NULL;
    struct node* headTwo = NULL;
    struct node* currTwo = NULL;
    //create linked list
    createLL(&headOne,&currOne, ch, number);
    printf("%d\n",currOne->data);
    createLL(&headTwo,&currTwo, ch, number);
    printf("%d\n",currTwo->data);
}

来源：https://stackoverflow.com/questions/35350749/creating-linked-list-not-passing-back-to-main

标签

pointers

linked-list