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#include <stdlib.h>struct _queue {    void *data;    struct _queue *next;};typedef _queue queue;queue queue_allocate(void *value){    queue *node = malloc(sizeof(queue));    if (node)    {        node->data = val;        node->next = NULL;    }    return node;}void queue_free(queue *node){    if (node)    {        free(node);    }}void queue_append(queue **head, void *value) {    if ((*head)->next == NULL)    {        (*head)-> = value;    }    else    {        queue *cur;        for (cur = (*head)->next; cur->next != NULL; cur = cur->next);        cur->next = item;    }}void queue_pop(queue **head){    int value = (*head)->data;    queue *tmp = (*head)->next;    free(*head);    head = tmp;}void queue_display(queue *head){    queue *cur;    for (cur = head->next; cur->next != NULL; cur = cur->next)    {        fprintf(stdout, "%s\n", cur->data);    }}void main(void){    queue *main;    //...}