I can alter a struct member from one location but not from the other

Question

I am trying to implement a linked list in C - starting simple, with one list containing one node. However, I stumble upon some issues when trying to add data to the node. Here's my implementation thus far:

struct mylist_node {
  int data;
};

struct mylist {
  struct mylist_node *head_pt;
};

void mylist_init(struct mylist* l){
    struct mylist_node head_node;
    head_node.data = 5; //First try
    l->head_pt = &head_node;
    l->head_pt->data = 5; //Second try
};

And my main method:

int main()
{
    struct mylist ml, *ml_pointer;
    ml_pointer = &ml;
    mylist_init(ml_pointer);

    printf("%d\n", ml_pointer->head_pt->data);
    ml_pointer->head_pt->data = 4;
    printf("%d\n", ml_pointer->head_pt->data);

    return 0;
}

This should print out

5
4

If my knowledge of pointers is correct. However, it prints out

0
4

As you can see I try to set the node data twice within the mylist_init method. Neither appears to be working - meanwhile, writing to and reading from it from my main method works just fine. What am I doing wrong?

Answer 1

In mylist_init, you're storing the address of a local variable in the struct pointed to by l. That variable goes out of scope when the function returns, so the memory it occupied is no longer valid, and thus the pointer that previously pointed to it now points to an invalid location. Returning the address of a local variable a dereferencing that address invokes undefined behavior.

Your function needs to allocate memory dynamically using malloc so the memory will still be valid when the function returns.

void mylist_init(struct mylist* l){
    struct mylist_node *head_node = malloc(sizeof(*head_node));
    l->head_pt = head_node;
    l->head_pt->data = 5; 
};

Also, don't forget to free the memory when you're done using it.