大家看看我写的这个双链表有什么问题吗？

[Dan1980]

#include <stdio.h>

typedef int DataType;
typedef struct node
{
DataType v;
struct node *previous;
struct node *next;
} Node;

Node *head;

add(DataType v)
{
Node *newNode = (Node*) malloc(sizeof (*head));
newNode->v = v;
newNode->next = head;
newNode->previous = head->previous;
newNode->next->previous = newNode;
newNode->previous->next = newNode;
}

main()
{
Node *n;
DataType i;
head->next = head;
head->previous = head;

printf("please enter some integers,\nenter a non-integer to finish:\n");
while(scanf("%d", &i) == 1)
add(i);

printf("the integers you entered are:\n");
for(n = head->next; n != head; n = n->next) {
if(n != head->next) printf(", ");
printf("%d", (*n).v);
}
}

编译没问题，但运行的时候，总是在输出所有元素以后，出现一句“null pointer asignment”，空指针赋值。我输出完以后就没有语句了呀，空指针赋值倒底在哪里啊？请大家指教。

[duduhaha]

head->next = head;
head->previous = head;
你只是定义了一个全局指针，都没有 struct node型的变量。怎么可以指来指去呢？
head->next这样操作是违法的。违反了先定义后使用的原则。。。

[Dan1980]

哦，谢谢。
我在
head->next = head;
head->previous = head;
这两句前面加一句：head = (Node*) malloc(sizeof (*head)); 就没问题了是吧？

[duduhaha]

应该先malloc
#include <stdio.h>
#include <stdlib.h>

typedef int DataType;
typedef struct node
{
DataType v;
struct node *previous;
struct node *next;
} Node;

Node *head;

void add(DataType v)
{
Node *newNode = (Node*) malloc(sizeof (*head));
newNode->v = v;
newNode->next = head;
newNode->previous = head->previous;
newNode->next->previous = newNode;
newNode->previous->next = newNode;
}

int main(void)
{
Node *n;
DataType i;
printf("please enter some integers,\nenter a non-integer to finish:\n");
head = (Node*) malloc(sizeof (*head));
head->next = head;
head->previous = head;
scanf("%d", &i);
add(i);


while(scanf("%d", &i) == 1)
add(i);

printf("the integers you entered are:\n");
for(n = head->next; n != head; n = n->next) {
if(n != head->next) printf(", ");
printf("%d", (*n).v);
}
return 0;
}