dynamic queue in C

/* a simple dynamic queue in C

  functions:
  - insert and remove element (no)
  - show the queue
  - free queue

  writer: Alexandre (azk4n)
*/

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

typedef struct NO{
     int data;
     struct NO *nxt;
 }no;

int size;
no *tmp;

int empty(no *queue){
    if(queue->nxt == NULL) return 1;
    else return 0;
}


void showQueue(no *queue){
    if(empty(queue)){
        printf("\nempty queue\n\n");
    }else{
        tmp = queue->nxt;
        printf("\n");
        while(tmp != NULL){
            printf("%d ", tmp->data);
            tmp = tmp->nxt;
        }printf("\nsize of queue: %d\n\n", size);
    }
}

void enqueue(no *queue){
    no *new = (no*) malloc(sizeof(no));
    new->nxt = NULL;
    printf("\ntake the new node: \n");
    scanf("%d", &new->data);
    printf("\n");
    if(empty(queue)){
        queue->nxt = new;
        size++;
    }else{
        tmp = queue->nxt;
        while(tmp->nxt != NULL){
            tmp = tmp->nxt;
        }
        tmp->nxt = new;
        size++;
    }
}

no *dequeue(no *queue){
    if(empty(queue)){
        printf("\nempty queue\n\n");
        return NULL;
    }else{
        tmp = queue->nxt;
        queue->nxt = tmp->nxt;
        size--;
        return tmp;
    }
}

void freeQueue(no *queue){
    if(empty(queue)){
        printf("\nempty queue\n\n");
    }else{
        no *nxtNode,
           *actualNode;
        actualNode = queue->nxt;
        while(actualNode != NULL){
            nxtNode = actualNode->nxt;
            free(actualNode);
            actualNode = nxtNode;
        }
        printf("\nqueue clear\n\n");
    }
}

int main(){
    no *queue = (no*) malloc(sizeof(no));
    if(!queue){
        printf("no memory");
        exit(1);
    }else{
        queue->nxt = NULL;
        size = 0;
        int opt;

        do{
            printf("0 -> quit\n");
            printf("1 -> show queue\n");
            printf("2 -> add node in queue\n");
            printf("3 -> remove node in queue\n");
            printf("4 -> free queue\n");
            printf("opt: "); scanf("%d", &opt);

            switch(opt){
                case 1:
                    showQueue(queue);
                    break;
                case 2:
                    enqueue(queue);
                    break;
                case 3:
                    tmp = dequeue(queue);
                    if(tmp != NULL)
                        printf("\nelement removed: %d\n\n", tmp->data);
                        break;
                case 4:
                    freeQueue(queue);
                    queue->nxt = NULL;
                    size = 0;
                    break;
                default:
                    if(opt != 0) printf("\ninsert valid option!\n\n");
             }
        }while(opt != 0);
    }
    return 1;
}