مشكلة في الكود, ممكن رايكم

تم تعديل الكود لهذا الشكل

#include <stdio.h>
#include <conio.h>
#include <stdlib.h>
#include <ctype.h>
#define null 0
typedef struct students {
  struct students *last;
  int id;
  char name[10];
  float gpa;
  struct students *link;
}student;

typedef struct course {
  struct course *last;
  int id;
  int grade;
  float group;
  struct course *link;
}courses;

student * createstudent(void);
courses * createcourse(void);
void displaystudent(student*);
void displaycourse(courses*);

void main() {
  student *k;
  courses *coa,*cob;
  int ch;
  do {
//    clrscr();
    printf ("\n\n\n\t\t PROGRAM TO PERFORM VARIOUS OPREATION");
    printf("\n\n\n\t\t*******MENU FOR OPERATION**********");
    printf("\n\t\t 1. create student list");
    printf("\n\t\t 2. display student list");
    printf("\n\t\t 3.create student course A");
    printf("\n\t\t 4. display student course A ");
    printf("\n\t\t 5.create student course B");
    printf("\n\t\t6. display student course B ");
    scanf("%d", &ch);
    switch(ch) {
      case '1':
        k = createstudent();
        getch(); break;
      case '2':
        displaystudent(k); break;
      case '3':
        coa = createcourse(); break;
      case '4':
        displaycourse(coa); break;
      case '5':
        cob = createcourse(); break;
      case '6':
        displaycourse(cob); break;
    }
  }while(ch != - 99);
}
student * createstudent() {
  student *head,*temp,*newn;
  int no,i;
up:
//  clrscr();
  printf("\n\t enter number of node:");
  scanf("%d", &no);
  if(no == - 99)
    goto up;
  for(i = 1; i < no; i++) {
    newn = (student*)malloc(sizeof(student));
    newn->link = 0;
    newn->last = 0;
    head = newn;
    printf("\nENTER THE INFORMATION OF STUDENT");
    scanf("%d %s %f", &newn->id, &newn->name, &newn->gpa);
    if(i==1)
      head = temp = newn;
    else
      temp = newn;
    newn = (student*) malloc(sizeof(student));
    newn->link = null;
    newn->last = temp;
    temp->link = newn;
  }
  fflush(stdin);
  free(newn);
  temp->link = null;
  return head;
}

courses * createcourse () {
  courses *head, *temp, * newn;
  int n = sizeof(courses), no, i;
up:
//  clrscr();
  printf("\n\t enter number of node:");
  scanf("%d",&no);
  if( no == - 99)
    goto up;
  for(i = 1; i < no; i++) {
    newn = (courses *) malloc(sizeof(n));
    newn->link = null;
    newn->last = null;
    head = newn;
    printf("\nENTER THE INFORMATION OF STUDENT");
    scanf("%d %d %f", &newn->id, &newn->grade, &newn->group);
    if(i == 1)
      head = temp = newn;
    else
      temp = newn;
    newn = (courses*) malloc(sizeof(n));
    newn->link = null;
    newn->last = temp;
    temp->link = newn;
  }
  fflush(stdin);
  free(newn);
  temp->link = null;
  return head;
}

باقي اضافة الكود الخاص بالدالتين

void displaystudent(student*);
void displaycourse(courses*);