list.c

#include <stdlib.h>
#include <stdbool.h>
#include "list.h"

Node *list_append(Node *list, void *data) {
	if (list) list_last(&list);
	Node *node = malloc(sizeof(Node));
	if (node) {
		node->data = data;
		node->next = NULL;
		node->prev = list;
		if (list) {
			list->next = node;
		}
	}
	return node;
}

void list_traverse(Node *list, void (*traverser)(void *data)) {
	list_head(&list);
	for (; list; traverser(list->data), list = list->next);
}

void list_free(Node *list) {
	list_head(&list);
	for (Node *next; list; list = next) {
		next = list->next;
		free(list->data);
		free(list);
	}
}

int list_length(Node *list) {
	int len = 1;
	Node *temp;
	if (!list) return 0;
	for (temp = list; temp->prev; ++len, temp = temp->prev);
	for (temp = list; temp->next; ++len, temp = temp->next);
	return len;
}

Node *list_search(Node *list, bool (*comparer)(void const *data)) {
	if (!list) return NULL;
	list_head(&list);
	for (; list; list = list->next) {
		if (comparer(list->data)) {
			return list;
		}
	}
	return NULL;
}

Node *list_remove(Node *node) {
	if (!node->prev) {
		Node *temp = node->next;
		temp->prev = NULL;
		free(node->data);
		free(node);
		return temp;
	} else {
		node->prev->next = node->next;
		if (node->next) node->next->prev = node->prev;
		free(node->data);
		free(node);
		return NULL;
	}
}

list.h

#ifndef LIST_H
#define LIST_H

#include <stdbool.h>

typedef struct _Node {
	void *data;
	struct _Node *next;
	struct _Node *prev;
} Node;


// Appends a new node to the tail of the list
Node *list_append(Node *list, void *data);

// Frees the memory used by the list
void list_free(Node *list);

// Returns length of the list
int list_length(Node *list);

// Finds the first node where comparer returns true.
// Returns first matching node or NULL
Node *list_search(Node *list, bool (*comparer)(void const *data));

// Applies function to each of the nodes
void list_traverse(Node *list, void (*traverser)(void *data));

// Removes node from the list and returns NULL.
// If removed node was head of the list (prev = NULL) return pointer to the new list head
Node *list_remove(Node *node);

// Traverses pointer to the first node of the list
static inline void list_head(Node **list) {
	for (; (*list)->prev; *list = (*list)->prev);
}

// Traverses pointer to the last node of the list
static inline void list_last(Node **list) {
	for (; (*list)->next; *list = (*list)->next);
}

#endif