c-分段故障-链表

Question

我正在学习如何在C中构建链表。我的程序编译了，但由于某种原因我无法理解，我遇到了分段错误。我想弄清楚这个问题已经有一段时间了，但运气不好。这是故障代码：

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; 
}

我在以下行的删除方法中遇到分段错误：

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

为什么这是错误的？

Answer 1

在insert:中，您有几个问题

while (current -> next != NULL)

您没有检查current是否为NULL。delete:中也存在类似问题

if (head -> data == d)

你需要在这里检查head和：

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

也是一个问题。

Answer 2

插入后出现错误，因为您将溢出未找到元素的列表末尾。

Answer 3

您发布的代码有很多问题，但您在评论中提到了insert()。它崩溃的原因是，如果调用insert()时head为NULL，则将取消引用NULL指针。

当head为NULL时，您将需要一个特殊的案例来插入：

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;

您应该在其他功能中检查类似的问题。使用search()函数作为避免在循环中取消引用NULL指针的示例。

Answer 4

在insert中，

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

这将保证current == NULL。

current->next = new;

每次都会崩溃。

Answer 5

可能在insertAfter:中

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

如果没有找到具有data == where的节点，则marker将是NULL指针
NULL指针稍后在您的代码中被取消引用：

new = marker -> next;

这是一个错误。检查marker->next != NULL是否应避免：

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

但您应该尝试使用调试符号（-g选项）编译程序，并在类似GDB的调试器中逐行运行。