void main(){
    int n;
scanf("%d",&n);
     head=createcode(n);
    head=createnum(head,n);
}

struct student *createcode(int n)
{
    struct student *head,*p,*q;
    int i=0;
    p=(struct student *)malloc(sizeof(struct student));
    printf("plase input code:");
    scanf("%d",&p->code);
    head=p;
    p->next=NULL;
    p->prior=NULL;
    while(1)
    {
        if(i==n)
            break;
            i++;
        q=(struct student *)malloc(sizeof(struct student));
        if(i<n){
           
            printf("plase input code:");
            scanf("%d",&q->code);
            p->next=q;
            q->prior=p;
            p=q;
            
        }
        
    }
    free(q);
   p->next=head;
   head->prior=p;
    return(head);
}



struct student *createnum(struct student *head,int n)
{
    struct student *p,*q;
    int i=0,j=2;
    p=(struct student *)malloc(sizeof(struct student));
    p->num=1;
    head=p;
    p->next=NULL;
    p->prior=NULL;
    while(1)
    {
        if(i==n)
            break;
            i++;
        q=(struct student *)malloc(sizeof(struct student));
        if(i<n){
           
            q->num=j;
            p->next=q;
            q->prior=p;
            p=q;
            j++;
        }
        
    }
    free(q);
   p->next=head;
   head->prior=p;
    return(head);
}


为什么给num赋值时，code 里面值不见了

在这个struct student *createnum(struct student *head,int n)子函数里，你建立了新的链表，你并没有没有输入code，所以没有值很正常啊。