C 指向结构的指针在每次函数调用时都变为NULL

问题是，每次调用函数addNodePos时，头指针都是NULL，这在调试器中看到，它只创建一个节点列表，该列表指向自身，因为它是一个循环双链接列表。并显示列表为空。因为list在传递给printList函数时也是NULL。一直在试图理解为什么，但仍然没有结果

下面是根据删除的代码

---

head的地址是通过值传递的，因此您的更改只反映在函数本身中。您必须传递一个指向head地址的指针，以便更改该值

int main() { 
   ...
   addNodePos(&list, value, position);
   ...
}

void addNodePos(struct DoubleList** headPtr, int value, int position)
{
    struct DoubleList *head = *headPtr;
    int i;
    struct DoubleList *node;

    if ( (node = malloc (sizeof(struct DoubleList))) != NULL ){
        node->id=value;
        if (head==NULL) {
            // points to itself as it is the only node in a list
            node->next=node;
            node->prev=node;
            head=node;
        } else {
            struct DoubleList *current=head;
            for (i = position; i > 1; i--)
                current=current->next;
            // reassign pointers -- relink nodes
            current->prev->next=node;
            node->prev=current->prev;
            node->next=current;
            current->prev=node;
        }
    }

    printf("Element has been added.\n\n");
}

---

在分享了一些非常有用的链接的用户的大力帮助下，我成功地解决了这个问题

解决方案：要修改调用方内存，请传递指向该内存的指针

经过一点更新后，我在这里留下了正确的工作代码：

#include <stdio.h>
#include <stdlib.h>

struct DoubleList
{
    int id;
    struct DoubleList *next;
    struct DoubleList *prev;
};

void addNodePos(struct DoubleList** headRef, int value, int position);
void printList (struct DoubleList** headRef);
//void clearList (struct DoubleList** headRef);


int main()
{
    int value, position;

    struct DoubleList *list = NULL;

    printf("\nvalue: ");
    scanf("%x", &value);
    printf("position: ");
    scanf("%d", &position);
    addNodePos(&list, value, position);

    printf("\nvalue: ");
    scanf("%x", &value);
    printf("position: ");
    scanf("%d", &position);
    addNodePos(&list, value, position);


    printList(&list);
    //clearList(head);
    return 0;
}




void addNodePos(struct DoubleList** headRef, int value, int position)
{
    int i;
    struct DoubleList *node;

    if ( (node = malloc (sizeof(struct DoubleList))) != NULL ){
        node->id=value;
        if ( (*headRef)==NULL) {
            // points to itself
            node->next=node;
            node->prev=node;
            (*headRef)=node;
        } else {
            struct DoubleList *current=(*headRef);
            for (i = position; i > 1; i--)
                current=current->next;
            // reassign pointers -- relink nodes
            current->prev->next=node;
            node->prev=current->prev;
            node->next=current;
            current->prev=node;
        }
    }
    printf("Element has been added.\n\n");
}



void printList(struct DoubleList** headRef)
{
    if ( (*headRef)==NULL)
        printf("\nList is empty.\n\n");
    else {
        struct DoubleList *current=(*headRef);
        printf("\nThe list: ");
        do {
            printf("%x", current->id);
            current=current->next;
            if(current != (*headRef))
                printf("<->");
        } while(current!=(*headRef));
        printf("\n\n");
    }
}

---

看起来你也犯了同样的错误。一旦你修改了head，你就再也不会更新*headPtr了。你可以写&list而不是head。@MattMcNabb，这样代码看起来会更清晰一些。更新！

#include <stdio.h>
#include <stdlib.h>

struct DoubleList
{
    int id;
    struct DoubleList *next;
    struct DoubleList *prev;
};

void addNodePos(struct DoubleList** headRef, int value, int position);
void printList (struct DoubleList** headRef);
//void clearList (struct DoubleList** headRef);


int main()
{
    int value, position;

    struct DoubleList *list = NULL;

    printf("\nvalue: ");
    scanf("%x", &value);
    printf("position: ");
    scanf("%d", &position);
    addNodePos(&list, value, position);

    printf("\nvalue: ");
    scanf("%x", &value);
    printf("position: ");
    scanf("%d", &position);
    addNodePos(&list, value, position);


    printList(&list);
    //clearList(head);
    return 0;
}




void addNodePos(struct DoubleList** headRef, int value, int position)
{
    int i;
    struct DoubleList *node;

    if ( (node = malloc (sizeof(struct DoubleList))) != NULL ){
        node->id=value;
        if ( (*headRef)==NULL) {
            // points to itself
            node->next=node;
            node->prev=node;
            (*headRef)=node;
        } else {
            struct DoubleList *current=(*headRef);
            for (i = position; i > 1; i--)
                current=current->next;
            // reassign pointers -- relink nodes
            current->prev->next=node;
            node->prev=current->prev;
            node->next=current;
            current->prev=node;
        }
    }
    printf("Element has been added.\n\n");
}



void printList(struct DoubleList** headRef)
{
    if ( (*headRef)==NULL)
        printf("\nList is empty.\n\n");
    else {
        struct DoubleList *current=(*headRef);
        printf("\nThe list: ");
        do {
            printf("%x", current->id);
            current=current->next;
            if(current != (*headRef))
                printf("<->");
        } while(current!=(*headRef));
        printf("\n\n");
    }
}