Chequea los cambios.quizas sea lo q tu querias.
#include <iostream.h>
#include <process.h>
int i=0;
char *dectobin(int);
void main(){
int dec;
cout << "\n\nIntroducir Decimal: ";
cin >> dec;
if (dec < 0){
cout << "\nERROR; Entrada no valida";
exit(0);
}
cout << "El numero en Binario es: " << dectobin(dec);
}
char *dectobin(int d){
char *chain;
int aux;
if (d > 0){
aux = d % 2;
d /= 2;
chain=dectobin(d);
chain[i++]=aux+'0';
chain[i]='\0';
}
return chain;
}