[HELP] C Programming

Hi!

I was wondering why this program is not working. When it gets to function uniao() it enters in an infinite loop. Can somebody please help me????

Here is the code:

#include <stdio.h>
#include <stdlib.h>
#define NUM 4

typedef struct NODO{

int id;
struct NODO *nseg;

}Nodo;

Nodo *makeNode(int n);
Nodo *push(Nodo *L, Nodo *nv);
Nodo *makeList();
Nodo *insertLast(Nodo *L, Nodo *nv);
Nodo *pop(Nodo **L);
Nodo *uniao(Nodo **s1, Nodo **s2);
int totalElementos(Nodo *L);
void printList(Nodo *L);


int main(int argc, char *argv){

Nodo *Pilha1 = NULL;
Nodo *Pilha2 = NULL;
Nodo *Pilha3 = NULL;
Nodo *nN = NULL;
Pilha1 = makeList();
Pilha2 = makeList();
Pilha3 = uniao(&Pilha1, &Pilha2);
printList(Pilha3);

}

Nodo *makeNode(int n){

Nodo *L;
L = (Nodo *)malloc(sizeof(Nodo));
L->id = n;
L->nseg = NULL;
return L;
}

Nodo *push(Nodo *L, Nodo *nv){
if(L == NULL)
return nv;
nv->nseg = L;
return nv;
}

Nodo *makeList(){

int i,n;

Nodo *L = NULL;
Nodo *aux;


for(i=0;i<NUM;i++){
printf("Introduza o id: \n");
scanf("%d",&n);
aux = makeNode(n);
L = push(L,aux);
}
return L;

}

Nodo *insertLast(Nodo *L, Nodo *nv){

Nodo *aux = L;

if(L == NULL)
return nv;
while(L->nseg!=NULL)
L = L->nseg;
L->nseg = nv;
nv->nseg = NULL;
return aux;
}

Nodo *pop(Nodo **L){

Nodo *aux = *L;

if(*L == NULL)
return *L;
*L = (*L)->nseg;
aux->nseg = NULL;

return (*L);
}

Nodo *removeFirst(Nodo *L){

Nodo *aux = L;

if(L == NULL)
return NULL;
L = L->nseg;
free(aux);
return L;
}

Nodo *uniao(Nodo **s1, Nodo **s2){

Nodo *aux;
Nodo *s3 = NULL;

while(*s1 != NULL || *s2 != NULL){

printf("Estou no ciclo\n");

*s1 = pop(s1);
s3 = push(s3, *s1);
*s2 = pop(s2);
s3 = push(s3, *s2);


}
return (s3);
}

void printList(Nodo *L){

if(L == NULL)
printf("NULL");

while(L!=NULL){
printf("%d ----> ", L->id);
L = L->nseg;
}
printf("NULL.\n");
}

Find the significant changes to pop and to uniao.
main returns an integer regarding its status. Fulfill your promise to the compiler, and honor your operating system.

Ok thank you!