Code wont compile - c

Question

I'm trying to use a queue in my program, but it won't compile and I don't know why. The relevant part of the code is as follows.
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <time.h>
#ifndef CUSTOMER
#define CUSTOMER
typedef int bool;
int r;
typedef struct{
int arrival;
int leaving;
} Customer;
static const int MAX_LENGTH = 100;
typedef struct{
int head;
int length;
Customer customer[MAX_LENGTH];
} CustomerLine;
void initializeQueue(CustomerLine* queue)
{
(*queue).head = 0;
(*queue).length = 0;
}
bool hasNext(CustomerLine* queue)
{
return (*queue).length > 0;
}
bool isFull(CustomerLine* queue)
{
return (*queue).length == MAX_LENGTH;
}
bool enqueue(CustomerLine* queue, Customer* customer)
{
if(isFull(queue))
return 0;
int index = ((*queue).head + (*queue).length) % MAX_LENGTH;
(*queue).customer[index] = *customer;
(*queue).length++;
return 1;
}
Customer* dequeue(CustomerLine* queue)
{
if(!hasNext(queue))
return 0;
Customer* result = &(*queue).customer[(*queue).head];
(*queue).length--;
(*queue).head = ((*queue).head + 1) % MAX_LENGTH;
return result;
}
The error says "Variably Modified 'customer' at file scope" I am a beginner at programming and just doing this is starting to get beyond my abilities so any help would be very much appreciated.

Answer 1

The line
static const int MAX_LENGTH = 100
is the problem. Replace it with
#define MAX_LENGTH 100
See why here and more explanations here or here or again here.
Furthermore:
You need an #endif after the #ifndef.
You need a main function somewhere.

Answer 2

In C, const means read-only, not constant and usable just like a macro. You cannot use a variable to specify the dimension of an array as you do here:
static const int MAX_LENGTH = 100;
typedef struct{
int head;
int length;
Customer customer[MAX_LENGTH]; /* Wrong. MAX_LENGTH is not a compile time constant. */
} CustomerLine;