Deque in C

deque.c

#include <stdlib.h>
#include <assert.h>
#include "deque.h"

struct node_struct {
	struct node_struct *next;
	struct node_struct *prev;
	deque_val_type val;
};

struct deque_struct {
	struct node_struct *head;
	struct node_struct *tail;
};

deque_type * deque_alloc() {
	deque_type *d = malloc(sizeof(deque_type));
	if (d != NULL)
		d->head = d->tail = NULL;
	return d;
}

void deque_free(deque_type *d) {
	free(d);
}

bool deque_is_empty(deque_type *d) {
	return d->head == NULL;
}

void deque_push_front(deque_type *d, deque_val_type v) {
	struct node_struct *n = malloc(sizeof(struct node_struct));
	assert(n != NULL);
	n->val = v;
	n->next = d->head;
	n->prev = NULL;
	if (d->tail == NULL) {
		d->head = d->tail = n;
	} else {
		d->head->prev = n;
		d->head = n;
	}
}

void deque_push_back(deque_type *d, deque_val_type v) {
	struct node_struct *n = malloc(sizeof(struct node_struct));
	assert(n != NULL);
	n->val = v;
	n->prev = d->tail;
	n->next = NULL;
	if (d->head == NULL) {
		d->head = d->tail = n;
	} else {
		d->tail->next = n;
		d->tail = n;
	}
}

deque_val_type deque_pop_front(deque_type *d) {
	deque_val_type v = d->head->val;
	struct node_struct *n = d->head;
	if (d->head == d->tail)
		d->head = d->tail = NULL;
	else
		d->head = n->next;
	free(n);
	return v;
}

deque_val_type deque_pop_back(deque_type *d) {
	deque_val_type v = d->tail->val;
	struct node_struct *n = d->tail;
	if (d->head == d->tail)
		d->head = d->tail = NULL;
	else
		d->tail = n->prev;
	free(n);
	return v;
}

deque_val_type deque_peek_front(deque_type *d) {
	return d->head->val;
}

deque_val_type deque_peek_back(deque_type *d) {
	return d->tail->val;
}

deque.h

/* An implementation of a Double Ended Queue.
 * Pushes, peeks, and pops from both ends can be done and
 * an allocate and free function is provided to instantiate a 
 * deque.  As the implementation of the deque struct is in the
 * source file "deque.c" it is required that the deque_alloc function
 * be used to create a new deque providing a form of encapsulation.
 */
#ifndef DEQUE_H
#define DEQUE_H
#include <stdbool.h>

/* deque_type is a deque object that the deque_foo functions
 * act upon.  New deque_type objects can be created with deque_alloc.
 */
typedef struct deque_struct deque_type;

/* This deque_val_type is the type of all values stored in the deque.
 * Change this to the type you want to store in your deque.
 */
typedef int deque_val_type;

/* Instantiates a new deque_type.
 * Returns a pointer to a new deque_type with memory allocated by malloc,
 * returns NULL if it fails.
 */
deque_type * deque_alloc();

/* Frees a deque_type pointed to by *d */
void deque_free(deque_type *d);

/* Returns true if there are no values in the deque, false otherwise. */
bool deque_is_empty(deque_type *d);

/* Adds a new value to the front/back of the deque_type *d */
void deque_push_front(deque_type *d, deque_val_type v);
void deque_push_back(deque_type *d, deque_val_type v);

/* Removes and returns the front/back value of the deque *d */
deque_val_type deque_pop_front(deque_type *d);
deque_val_type deque_pop_back(deque_type *d);

/* Returns the front/back value of the deque *d */
deque_val_type deque_peek_front(deque_type *d);
deque_val_type deque_peek_back(deque_type *d);

#endif

main.c

#include <stdio.h>
#include <assert.h>
#include "deque.h"

int main() {
	deque_type *d = deque_alloc();
	assert(d != NULL);

	for (int i = 1; i <= 256; i+=i) {
		deque_push_back(d, i);
	}

	while (deque_is_empty(d) != true) {
		printf("%d\n", deque_pop_front(d));
	}

	deque_free(d);

	return 0;
}

If any of these functions is called when the deque is empty it will result in error:
deque_val_type deque_peek_front(deque_type *d) {
return d->head->val;
}
deque_val_type deque_peek_back(deque_type *d) {
return d->tail->val;
}

Check first if head or tail is not NULL to avoid it.

deque_val_type deque_peek_front(deque_type *d) {
return d->head ? d->head->val : NULL;
}
deque_val_type deque_peek_back(deque_type *d) {
return d->tail ? d->tail->val : NULL;
}

Can I use this code?
How is this licensed?