Question

我正在学习如何在 C 中构建链接列表。我的程序编译但由于某种原因我无法弄清楚，我得到分段错误。我一直试图找出问题一段时间，但我没有运气。这是错误的代码：

int len()
{
    struct list * current = head;
    int length = 0; 

    while (current != NULL)
    {
        length++;
        current = current -> next; //move to next node
    }
    return length; 
}


struct list * search ( int key)
{
    struct list * current = head;

    while (current != NULL && current->data != key)
        current = current -> next;

    if (current != NULL && current -> data == key)
        return current;
    return NULL;
}



/* Insert a new data element with key d into the end of the list. */
void insert(int d )  //  at the end
{
    struct list * current = head; 
    struct list * new;
    while (current -> next != NULL)
        current = current -> next;
    new = (struct list *)malloc(sizeof(struct list));
    new -> data = d; 
    current -> next = new;
    new -> next = NULL;     
}


void insertAfter(int d, int where )  //  insert at the middle
{
    struct list * marker = head;
    struct list * new;

    while(marker -> data != where)
        marker = marker -> next;
    new = (struct list*)malloc(sizeof(struct list));

    new -> next = marker -> next; 
    marker -> next = new;
    new -> data = d; 
}


/* Remove the node with value d from the list */
/* assume no duplicated keys in the list */
/* If the list is empty, call prtError() to display an error message and return -1. */

void delete(int d)
{
    struct list * current1 = head; 
    struct list * current2;

    if (len() == 0)
    { //prtError("empty");
        exit(0);
    }
    if (head -> data == d)
    { 
        head = head -> next;
    }

    //Check if last node contains element
    while (current1->next->next != NULL)
        current1 = current1->next;

    if(current1->next->data == d)
            current1->next == NULL; 


    current1 = head; //move current1 back to front */

    while(current1 -> next -> data != d)
        current1 = current1 -> next; 

    current2 = current1 -> next;
    current1 -> next = current2 -> next; 

}

我在第一行的删除方法中遇到了分段错误：

while(current1 -> next -> data != d)

为什么这是错的？

Answer 1

您发布的代码存在许多问题，但您在评论中提到您关注insert()。它崩溃的原因是，如果调用head时insert()为NULL，您将取消引用NULL指针。

当它为NULL时，您需要在head处插入一个特殊情况：

if (head) {
    while (current -> next != NULL)
        current = current -> next;
}
new = (struct list *)malloc(sizeof(struct list));
new -> data = d;
if (current) {
    current -> next = new;
} else {
    head = new;
}
new -> next = NULL;

您应该检查其他功能中的类似问题。使用search()函数作为避免在循环中取消引用NULL指针的示例。

Answer 2

您在insert中有几个问题：

while (current -> next != NULL)

您不会检查current是否为NULL。 delete中存在类似问题：

if (head -> data == d)

您需要在此处查看head并：

while (current1->next->next != NULL)

也是一个问题。

Answer 3

插入后出现错误，您将超出列表末尾未找到元素。

Answer 4

在insert，

while(current->next != NULL) current = current->next;

这将保证current == NULL。

current->next = new;

每次都会崩溃。

Answer 5

也许它在insertAfter：

while(marker -> data != where)
    marker = marker -> next;

如果找不到data == where的节点，marker将成为NULL指针。
稍后在您的代码中取消引用NULL指针：

new = marker -> next;

它会产生段错误。检查marker->next != NULL是否应避免：

while(marker->next != NULL && marker -> data != where)
    marker = marker -> next;

但是你应该尝试使用调试符号（-g选项）编译你的程序，并在像GDB这样的调试器中逐行运行。

分段故障 - 链接列表

5 个答案: