‘valor’ undeclared (first use in this function) - c

Question

#include<stdio.h>
#include"push.c"
#include"pop.c"
#include"listadopila.c"
#include"structpila.c"
int main ()
{
struct pila valor;
void push();
int pop();
void listado();
int opcion=0;
valor.tope= -1;
int continuar;
printf ("=====ESTRUCTURA DE PILA=====\n\n");
do
{
printf ("Ingresa tu opcion:\n");
printf ("1) Para push\n");
printf ("2) Para pop\n");
printf ("3) Para mostrar tu pila\n");
printf ("4) Para salir.\n");
printf ("\n");
printf ("Ingresa tu opci%cn\n",163);
scanf("%d", &opcion);
switch(opcion)
{
case 1:
push(valor);
break;
case 2:
pop(valor);
break;
case 3:
listado(valor);
break;
default:
printf("Opción no válida.\n");
}
printf("\n¿Desea continuar comprando? Pulse cualquier tecla para si y N para no: ");
getchar();
scanf("%d",&continuar);
continuar=getchar();
}
while(continuar!='n'&&continuar!='N');
}
In other file, I define the struct like this:
struct pila
{
int pila[5];
int tope;
}valor;
And my functions are like this:
int pop()
{
int numero;
if (valor.tope==- 1)
{
printf ("La pila esta vacia.\n");
return (valor.tope);
}
else
{
numero=valor.pila[valor.tope];
printf("El elemento que haz eliminado es= %d", valor.pila[valor.tope]);
valor.tope=valor.tope -1;
}
return(numero);
}
I've tried changing the struct inside and outside the main and to write the functions into the switch case like: pop(valor); but it doesn't work either. Also in the files of each function I've tried to write the function like "int pop(struct pila valor)" but the compiler says I need to add a lenght for valor, the problem is that when I did the code in the same file the compilation finished succesfully, so the problem is when I try to link the struct with the other functions.
Any help?