Работа со списками - C (СИ)

#ifndef LINKED_LIST_H_
#define LINKED_LIST_H_
 
struct node {
    char* data;
    int key;
    struct node *next;
};
 
void print_list(struct node* head);
 
void insert_first(struct node* head, int key, char* data);
 
_Bool is_empty(struct node* head);
 
int length(struct node* head);
 
struct node* find(struct node* head, int key);
 
struct node* delete_first(struct node* head);
struct node* delete(struct node* head, int key);
 
void sort(struct node* head);
 
void reverse(struct node** head_ref);
 
#endif /* LINKED_LIST_H_ */

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <stdbool.h>
 
#include "linked_list.h"
 
struct node* current = NULL;
 
void print_list(struct node* head) {
    struct node *current = head;
    printf("\n[ ");
 
    while (current != NULL) {
        printf("(%d,%s) ", current->key, current->data);
        current = current->next;
    }
 
    printf(" ]");
}
 
void insert_first(struct node* head, int key, char* data) {
    struct node *link = (struct node*) malloc(sizeof(struct node));
 
    link->key = key;
    link->data = data;
 
    /* point it to old first node */
    link->next = head;
 
    /* point first to new first node */
    head = link;
}
 
struct node* delete_first(struct node* head) {
    struct node *temp_link = head;
 
    /* mark next to first link as first */
    head = head->next;
 
    return temp_link;
}
 
_Bool is_empty(struct node* head) {
    return head == NULL;
}
 
int length(struct node* head) {
    int length = 0;
    struct node *current;
 
    for (current = head; current != NULL; current = current->next) {
        length++;
    }
 
    return length;
}
 
struct node* find(struct node* head, int key) {
    struct node* current = head;
 
    if (head == NULL) {
        return NULL;
    }
 
    while (current->key != key) {
 
        if (current->next == NULL) {
            return NULL;
        } else {
            current = current->next;
        }
    }
 
    return current;
}
 
struct node* delete(struct node* head, int key) {
    struct node* current = head;
    struct node* previous = NULL;
 
    if (head == NULL) {
        return NULL;
    }
 
    while (current->key != key) {
        if (current->next == NULL) {
            return NULL;
        } else {
            previous = current;
            current = current->next;
        }
 
    }
 
    if (current == head) {
        head = head->next;
    } else {
        previous->next = current->next;
    }
 
    current->next = NULL;
    free(current);
    current = NULL;
 
    return current;
}

struct node* first_list = NULL;
struct node* second_list = NULL;
 
insert_first(first_list, 1, "node_1");
insert_first(first_list, 2, "node_2");