CODEWITHSUNDEEP
c
Krzysztofik
Krzysztofik

Reputation: 391

C - dynamically sized array of struct pointers in another struct, segmentation fault

I have segfault while trying to run such code.

struct list;
struct node;

typedef struct list {
    struct node ** links;
    int size;
    int content;
} list;

typedef struct node {

    wchar_t value;
    struct list* children;
    int exists;

} node;


node* newNode(wchar_t value, int exists) {
    node *q = (struct node*)malloc(sizeof(struct node));
    q->value = value;
    q->children = newList();
    q->exists = exists;
    return q;
}

list* newList(){

    list *result = (list*)malloc(sizeof(list));
    result->size = 2;
    result->content = 0;
    result->links = (struct node**) malloc(result->size * sizeof(struct node*));

    return result;

}

void resizeList(list* list_pointer){

    if(list_pointer->size <= list_pointer->content){

        list_pointer->size *= 2;
        list_pointer->links = (struct node**) realloc(list_pointer->links, (list_pointer->size) * sizeof(struct node*));

    }
}

void pushList(list* list_pointer, node* node_pointer){
    if(node_pointer == NULL)
        return;

    resizeList(list_pointer);
    list_pointer->content++;

    int i;
    node* temp_pointer;

    for(i = 0; i < list_pointer->content; i++){

        if(list_pointer->links[i] == NULL){
            list_pointer->links[i] = node_pointer;
            break;
        }

        if(list_pointer->links[i]->value > node_pointer->value){
            temp_pointer = list_pointer->links[i];
            list_pointer->links[i] = node_pointer;
            node_pointer = temp_pointer;
        }
    }

}

Calling.

struct list* l = newList();

struct node* n1 = newNode(L'a', 1);
struct node* n2 = newNode(L'b', 1);
struct node* n3 = newNode(L'c', 1);
struct node* n4 = newNode(L'd', 1);
struct node* n5 = newNode(L'e', 1);
struct node* n6 = newNode(L'f', 1);
struct node* n7 = newNode(L'g', 1);
struct node* n8 = newNode(L'h', 1);

pushList(l, n1);
pushList(l, n2);
pushList(l, n3);
pushList(l, n4);
pushList(l, n5);
pushList(l, n6);
pushList(l, n7);
pushList(l, n8);

After first two pushes it fails.

It's supposed to create list based on values stored in nodes. But... it doesn't. It throws segfault. When I changed allocation of memory from "sizeod(node*)" to "sizeof(node)", it works, but presumably cause of allocation of bigger memory. I want to store POINTERS, not STRUCTS in this array.

I'm figthing that 6 hours with no idea what to do.

Upvotes: 1

Views: 62

Answers (1)

Ewan Mellor
Ewan Mellor

Reputation: 6847

If you are getting a segfault, then you should include the backtrace in your question. That will make it a lot easier to figure out what's going on.

Looking at the code, I see that you're not clearing result->links when you malloc or realloc it, but you are relying on the pointers being NULL inside pushList. You increase list_pointer->content and then check if (list_pointer->links[i] == NULL). That's going to result in undefined behavior for sure.

Memory is not filled with zeros when you use malloc or realloc. If you need that to be the case, you need to do it yourself. (You could use calloc as a replacement for malloc, but that doesn't help you with the realloc.)

This code is fine if you are learning, though I agree with the comment above that it's a bit of a convoluted way of doing it. If this is for production code then you should use an open-source list library because that will already be debugged and tuned for you.

Upvotes: 2

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