Given the following code, how would you make it thread-safe? // basic node structure typedef struct __node_t { int key; struct __node_t *next; } node_t; // basic list structure (one used per list)...

Given the following code, how would you make it thread-safe?

// basic node structure
typedef struct __node_t {
    int key;
    struct __node_t *next;
} node_t;

// basic list structure (one used per list)
typedef struct __list_t {
    node_t *head;

} list_t;

void List_Init(list_t *L) {
    L->head = NULL;

}

int List_Insert(list_t *L, int key) {

    node_t *new = malloc(sizeof(node_t));

    if (new == NULL) {
        perror("malloc");
        return -1; // fail
    }

    new->key = key;
    new->next = L->head;
    L->head = new;

    return 0; // success
  }

int List_Lookup(list_t *L, int key) {
    node_t *curr = L->head;
    while (curr) {
        if (curr->key == key) {
            return 0; // success
        }
    curr = curr->next;
    }

}