Question

I need to create a linked list in my program. In my program the list is allocated on the heap with malloc(), than i try to visit it but i get a segmetation fault;

EDIT: i get the SIGSEGV at this line “while(!(node->nodeType == TYPE_END_LIST)){“

    struct dagNode *createList(int k);
    struct dagNode *newNodeXInterval(int type, int val);


    struct dagNode *createList(int k){
    struct dagNode *head, *node;
    printf("\nList %d = ", k);
    head = newNodeXInterval(TYPE_EDGE_OR_GAP, getVal(k,1));
    node = head;
    int i;
    for (i=1; i<LENGTH_OF(k); i++){
            node->next = newNodeXInterval(TYPE_XTEST, getRightPointOf(k,i));
            node = node->next;
            node->next = newNodeXInterval(TYPE_EDGE_OR_GAP, getVal(k,i+1));
            node = node->next;
            }     
    node = newNodeXInterval(TYPE_END_LIST, 0);
    node = head;     // i think that here there is the error
    printf("%d", node->val); i=0;
                    while(!(node->nodeType == TYPE_END_LIST)){
                        printf("%d ", i);
                        node = node->next;}
    return head;}


    struct dagNode *newNodeXInterval(int type, int val){
        struct dagNode *node = (struct dagNode *) malloc(sizeof(struct dagNode));
        if (type == TYPE_EDGE_OR_GAP){
        *node = (struct dagNode) {(val<0)? TYPE_GAP:TYPE_EDGE, val, NULL, NULL, NULL};
        }
        else{
        *node = (struct dagNode) {type, val, NULL, NULL, NULL};
        }
        return node;    }

The caller function would get the head of the list.

Answer 1

As far as I can tell, the problem is with the line

node = newNodeXInterval(TYPE_END_LIST, 0);

Before the assignement, node points to the last node in your linked list, and the previous node has its next pointer equal to node. After the assignement, node points to the newly created node with type TYPE_END_LIST, but the previous node has its next pointer unchanged (i.e. it still holds the original value of node). In other words, your newly created node is not a part of the list, and thus the condition node->nodeType == TYPE_END_LIST in the following while() loop will never evaluate to true, and you’ll end up dereferencing a null pointer when you go past the end of the list. Changing the line to

node->next = newNodeXInterval(TYPE_END_LIST,0);     

should fix the problem.