reverse a linked list -- C

/******************************************************************************************
 * main.c
 * Created on:      24-Jan-2018 20:25:28
 * Original author: linx
 * Computer Science & Technology, Linx
 * Copyright (c) 2018-2020. All rights reserved.
 ******************************************************************************************/

#include <stdio.h>
#include <stdlib.h>

typedef int datatype;

typedef struct node_t {
    datatype value;
    struct node_t *next;
} node, *link;

static const int kLIST_CNT = 5;

int add_node(link *head, datatype data);
void reverse_link(link *head);
/*void reverse_link_1(link *head);*/
link reverse_link_ret(link head);
link reverse_recursive(link head);
void traverse_link(link head);
void release_link(link *head);

int runtest();

int main() {
    runtest();
    return 0;
}

int runtest() {
    link head = NULL;
    int loop = kLIST_CNT;
    while (loop > 0) 
        add_node(&head, loop--);
    
    traverse_link(head);

    reverse_link(&head);
    traverse_link(head);

    link relnk = reverse_link_ret(head);
    traverse_link(relnk);

    link recursive = reverse_recursive(relnk);
    traverse_link(recursive);

    release_link(&recursive);
    return 0;
}

int add_node(link *head, datatype data) {
    link newnode = (link)malloc(sizeof(*newnode));
    if (NULL == newnode) {
        return -1;
    }
    newnode->value = data;
    newnode->next = *head;
    *head = newnode;
    return 0;
}

/*
void reverse_link_1(link *head) {
    printf("%s\n", __FUNCTION__);
    if (NULL == *head)
        return;
    link head_node = *head;
    link current_node = (*head)->next;
    head_node->next = NULL;
    while (current_node) {
        link next_node = current_node->next;
        current_node->next = head_node;
        head_node = current_node;
        *head = current_node;
        current_node = next_node;
    }
}
*/

void reverse_link(link *head) {
    printf("%s\n", __FUNCTION__);
    link prev = NULL;
    link current = *head;
    link next = NULL;
    while (NULL != current) {
        next = current->next;
        current->next = prev;
        prev = current;
        current = next;
    }
    *head = prev;
}

link reverse_link_ret(link current) {
    printf("%s\n", __FUNCTION__);
    link prev = NULL;
    link next = NULL;
    while (NULL != current) {
        next = current->next;
        current->next = prev;
        prev = current;
        current = next;
    }
    return prev;
}

link reverse_recursive(link head) {
    printf("%s\n", __FUNCTION__);
    if (NULL == head || NULL == head->next)
        return head;
    link tmp = reverse_recursive(head->next);
    head->next->next = head;
    head->next = NULL;
    return tmp;
}

void traverse_link(link head) {
    printf("%s\n", __FUNCTION__);
    while (NULL != head) {
        printf("%d->%s", head->value, head->next ? "": "NULL\n");
        head = head->next;
    }
    printf("\n");
}

void release_link(link *head) {
    printf("%s\n", __FUNCTION__);
    while (NULL != *head) {
        link tmp = *head;
        printf("%d->%s", tmp->value, tmp->next ? "": "NULL\n");
        *head = (*head)->next;
        free(tmp);
        tmp = NULL;
    }
}

• original geeksforgeeks