AndrezaWF,
Você precisa verificar se o usuário digitou a tecla "enter" e tomar a atitude necessária.
Exemplo:
https://stackoverflow.com/questions/7144977/reading-enter-key-in-a-loop-in-c?lq=1
char prev = 0;
while(1)
{
char c = getchar();
if(c == '\n' && prev == c)
{
// double return pressed!
break;
}
prev = c;
}
Outro exemplo:
https://stackoverflow.com/questions/19852303/how-to-check-if-user-pressed-enter-key
fgets(input,sizeof(input),stdin);
if(input[0]=='\n') printf("empty string");
[]'s,
Fabio I.