Code

 Header...
#define MAXLEN 16
typedef struct food {
  char name[MAXLEN];
  float calories;
  struct food *next;
}Food;

void new_food (void);
void calculate_calories (void);

Food *firstfood;

 int main(void){
   ... 

»  int choice;

»  do{
»    printf("\nChoice?\n\n"
     "0. Quit\n"
     "1. Add new food\n"
     "2. Calculate calories\n");
»    scanf("%d", &choice);

»    switch (choice) {
»    case 0:
»      break;

»    case 1:
»      new_food();
»      break;

»    case 2:
»      calculate_calories();
»      break;

»    default :
»      fprintf(stderr, "Input out of range\n");
}
  }while (choice) ;

»  return 0;
}

 void new_food(void){
   ... 

»  Food *new=NULL;

»  new=xmalloc(sizeof *new);
»  new->next=firstfood;
»  firstfood=new;

»  printf("Name of food?\n");
»  scanf("%15s", new->name);
»  printf("Calories per gramme?\n");
»  scanf("%f", &new->calories);
}

 void calculate_calories(void){
   ... 

»  Food *tmp;
»  float calories=0, grammes;

»  for (tmp=firstfood; tmp!=NULL; tmp=tmp->next) {
»    printf("How many grammes of %s?\n", tmp->name);
»    scanf("%f", &grammes);
»    calories+=grammes*tmp->calories;
  }
»  printf("Your food contains %.2f calories\n", calories);

}