while循环终止后的代码不运行

Question

我正在编写一个对链表进行排序的算法。但是，我遇到了我的 while 循环似乎已终止的问题，但以下代码都不会运行。我的 while 循环终止很可能是错误的，但根据我的打印输出，while 循环的条件（cur！=NULL）已经满足。

void LL_sort (struct node **headpp){
//if list is null or size 1
if((*headpp)==NULL || (*headpp)->next ==NULL){
    return;
}
struct node* sortedHead = NULL;
struct node* sortedCur = NULL;
struct node* cur = *headpp;
struct node* fwd = (*headpp)->next;
int emptyflag = 0;

while(cur!=NULL){
    fwd=cur->next;
    printf("start of loop cur is %p wtih value %i\n", cur, cur->data);
    printf("start of loop fwd is %p\n", fwd);
    //if sorted list is empty
    if(sortedHead == NULL){
        cur->next = NULL;
        sortedHead = cur;
        emptyflag = 1;
        printf("[sorted list NULL]sorted head is %p with value %i\n", sortedHead, sortedHead->data );
        printf("[sorted list NULL]reg head is %p with value %i\n", *headpp, (*headpp)->data );
    }
    //if cur is smaller than head
    if(sortedHead->data >= cur->data && emptyflag==0){
        cur->next = sortedHead;
        sortedHead = cur;
        printf("[sortedHead change]sorted head is %p with value %i\n", sortedHead, sortedHead->data );
        printf("[sortedHead change]reg head is %p with value %i\n", *headpp, (*headpp)->data );
    }

    sortedCur=sortedHead;
    printf("sortedCur is %p with value %i\n",sortedCur, sortedCur->data );
    printf("sortedCur->next is %p \n",sortedCur->next );
    printf("sortedhead is %p with value %i\n",sortedHead, sortedHead->data );
    printf("sortedHead->next is %p \n",sortedHead->next );
    while((sortedCur->next!=NULL) && (sortedCur->data < cur->data)){
        sortedCur=sortedCur->next;
    }
    if(sortedCur->next == NULL && emptyflag ==0){
        sortedCur->next = cur;
        sortedCur->next = NULL;
    }

    if(sortedCur->data < cur->data && emptyflag ==0){
        sortedCur->next = cur;
        cur->next = sortedCur->next->next;
    }
    emptyflag=0;

    cur = fwd;

    printf("end of loop cur is %p\n", cur);
    printf("end of loop cur has value %i\n", cur->data);
    printf("end of loop sortedHead is %p\n", sortedHead);
    printf("end of loop sortedHead has value %i\n", sortedHead->data);
    printf("end of loop sortedHead->next is %p \n", sortedHead->next);
    printf(" -------------------------------------------------- \n");
  }

  printf("this won't print\n");
  *headpp=sortedHead;
  }

我希望循环结束并将链表的头部更改为已排序的链表，以便调用该列表的任何后续函数都会生成一个已排序的列表。

Answer 1

如果cur 是NULL 那么这一行

printf("end of loop cur has value %i\n", cur->data);

通过取消引用NULL 来调用未定义的行为。那时任何事情都可能发生，很可能程序刚刚崩溃。

Answer 2

正如@alk 已经提到的，这条线可能会导致分段错误。

printf("end of loop cur has value %i\n", cur->data);

将其更改为：

if (cur != NULL)
    printf("end of loop cur has value %i\n", cur->data);

您的功能运行良好。链表的头部变为已排序的。 我用这个程序来测试它：

int main(){
    // Create the head node
    struct node *head = malloc(sizeof(struct node));
    head->next = NULL;
    head->data = 10;

    // Create 9 more nodes, 10 in total
    struct node *ptr = head;
    int i;
    for (i = 9; i > 0; i--){
        ptr->next = malloc(sizeof(struct node));
        ptr->next->next = NULL;
        ptr->next->data = i;
        ptr = ptr->next;
    }

    // Print all the nodes
    ptr = head;
    while (ptr != NULL){
        printf("Node: %p, Next: %p, Data: %d\n", ptr, ptr->next, ptr->data);
        ptr = ptr->next;
    }

    LL_sort(&head);

    // Print the sorted nodes
    ptr = head;
    while (ptr != NULL){
        printf("Node: %p, Next: %p, Data: %d\n", ptr, ptr->next, ptr->data);
        ptr = ptr->next;
    }

    return 0;
}