C结构的属性的默认值

Question

I'm trying to implement a push function for a linked list, but the following program is causing a segmentation fault. I'm wondering a couple of things: what's the default value for properties of a struct? It seems that I have to manually set the value of head->next to be NULL . What is the default value of head->next then?

I believe the reason the program breaks is because in the push function, head->next != NULL so it then executes the line head = head->next , which leads me to wonder what the value of head->next is if it isn't NULL , and why this causes a segmentation fault?

typedef struct Node {
    struct Node *next;
    int data;
} Node;

void push(Node *head, int data);

int main()
{
    struct Node *head = malloc(sizeof(Node));
    head->data = 1;

    // Works when I uncomment this line
    // head->next = NULL;

    push(head, 2);
    return 0;
}

/* Insert */
void push(Node *head, int data) {
    while (head != NULL) {
        if (head->next == NULL) {
            Node *n = malloc(sizeof(Node));
            n->data = data;
            head->next = n;
            break;
        }
        head = head->next;
    }
}

Answer 1

what's the default value for properties of a struct?

C standard refers to struct fields as members , not properties .

Members of struct s in C have defined values in three situations:

• When you allocate struct s in static/global memory, or
• When you supply member values through an initializer, or
• You use memory allocation routine that zeroes out the memory.

In all other cases struct members must be assigned before the first use; reading them before assignment is undefined behavior.

In your situation, struct is allocated in dynamic memory by malloc . This means that its members must be assigned explicitly.

If you switch to calloc from malloc , struct 's memory would be zeroed out for you:

Node *n = calloc(1, sizeof(Node));