Doubly linked circular list - c

Question

This code tries to implement a doubly linked circular list
The code compiles, does not throw error, but when entering a number is hung where is the fault? Pretend that the signs in Spanish are in English
#include <stdio.h>
#include <conio.h>
#include <windows.h>
#include <stdlib.h>
struct lista
{
int info;
struct lista *sig,*ant;
}*CAB=NULL,*AUX=NULL,*Q=NULL,*P=NULL,*F=NULL,*QD=NULL,*FD=NULL;
main menu
int main() {
void insertard(void);
void extraerd(void);
void visualizard(void);
void visualizari(void);
void eliminardespues();
void eliminarantes();
char opc;
do{
system("cls");
printf("___________________________________________________\n");
printf("_¡¡¡MENU DE LISTA CIRCULAR DOBLEMENTE ENLAZADA!!!__\n");
printf("___________________________________________________\n");
printf("____________SELECCIONE UNA OPCION__________________\n");
printf("___________________________________________________\n");
printf("___________________________________________________\n");
printf("__________1) INSERTAR______________________________\n");
printf("__________2) VISUALIZAR ASCENDIENTE________________\n");
printf("__________3) VISUALIZAR DESCENDIENTE_______________\n");
printf("__________4) INGRESAR Y ELIMINAR SIGUIENTE_________\n");
printf("__________5) INGRESAR Y ELIMINAR ANTERIOR__________\n");
printf("__________6) SALIR_________________________________\n");
printf("___________________________________________________\n");
opc=getch();
switch(opc)
{
case '1':
insertard();
break;
case '2':
visualizard();
break;
case '3':
visualizari();
break;
case '4':
eliminarantes();
break;
}
} while(opc!='6');
getch();
return 0;
}
Process insertard
void insertard(void)
{
P=CAB;
AUX=(struct lista *)malloc(sizeof(struct lista));
system("cls");
printf("INGRESE UN NUMERO ENTERO:");
scanf("%d",&AUX->info);
AUX->sig=CAB;
AUX->ant=CAB;
F=AUX;
if(CAB==NULL)
CAB=AUX;
}else{ //ESTO SI
while (P->sig!=CAB){
P=P->sig;
}
P->sig=AUX;
AUX->ant=P;
AUX->sig=CAB;
}
eliminardespues Process (Deletes an item after the list)
void eliminardespues(){
int x;
system("cls");
printf("INGRESE UN NUMERO PARA ELIMINAR EL SIGUIENTE:");
scanf("%d",&x);
FD=CAB;
QD=CAB;
while(FD->info!=x&&FD->sig!=CAB){
FD=FD->sig;
}QD=FD->sig;
if(FD->sig==CAB&&FD->info!=x){
printf("\nEL NUMERO INGRESADO NO SE ENCUENTA EN LA LISTA");
}else{
if(FD->info==x){
FD->sig=QD->sig;
(QD->sig)->ant=FD;
printf("\nELIMINADO %d",QD->info);
free(Q);
}
}
getch();
}
Process eliminarantes
void eliminarantes()
{
int x;
system("cls");
printf("INGRESE UN NUMERO PARA ELIMINAR EL ANTERIOR");
scanf("%d",&x);
FD=CAB;
QD=CAB;
while (FD->info!=x&&FD->sig!=CAB){
FD=FD->sig;
}QD=FD->ant;
if(FD->sig==CAB&&FD->info!=x){
printf("\nEL NUMERO INGRESADO NO SE ENCUENTA EN LA LISTA");
}else{
if(FD->info==x){
FD->ant=QD->ant;
(QD->ant)->sig=FD;
printf("\nELIMINADO %d",QD->info);
free(Q);
}
}
getch();
}
void visualizard(void)
{
system("cls");
if(CAB==NULL){
printf("LISTA VACIA");
getchar();
return;
}
AUX=CAB;
printf("LISTA:\n\n");
while(AUX->sig!=CAB){
printf("-> %d\n",AUX->info);
AUX=AUX->sig;
}
if(AUX->sig==CAB){
printf("-> %d\n",AUX->info);
}
getch();
}
void visualizari(void){
system("cls");
if(F==NULL){
printf("LISTA VACIA");
getchar();
return;
}
AUX=F;
printf("LISTA:\n\n");
do{
printf("-> %d\n",AUX->info);
AUX=AUX->ant;
}while(AUX->sig!=CAB);
getch();
}

Answer 1

In your first execution of method 'insertard' you are actually assigning P=CAB; i.e. P=NULL and in the blow line of code you are referring a null pointer to assign value.
P->sig=AUX;
I have modified your method which might fix it.
void insertard(void)
{
P=CAB;/* very first execution of this method P=NULL */
AUX=(struct lista *)malloc(sizeof(struct lista));
system("cls");
printf("INGRESE UN NUMERO ENTERO:");
scanf("%d",&AUX->info);
AUX->sig=CAB;
AUX->ant=CAB;
F=AUX;
if(CAB==NULL){
CAB=AUX;
P = AUX;/*first execution of this method P is no longer NULL but P is pointing to AUX */
}else{ //ESTO SI
while (P->sig!=CAB){
P=P->sig;
}
P->sig=AUX;
AUX->ant=P;
AUX->sig=CAB;
}