c - 结构体中的 malloc 和指针

Question

我有以下 C 代码:

typedef struct DListNode_ {
    void    *data;
    struct DListNode_ *prev;
    struct DListNode_ *next;
} DListNode;


typedef struct DList_ {
    int size;
    DListNode  *tail;
    DListNode  *head;
} DList;

void insert(DList * list, DListNode * element, int data) {
    DListNode * new_element = (DListNode *)malloc(sizeof(DListNode));
    new_element->data = &data;
    if (list->head==NULL) {
        list->head=list->tail=new_element;
        list->size++;
        return;
    }
    if(element == NULL) {
        // handle size==0?
        new_element->next=list->head;
        list->head->prev=new_element;
        list->head=new_element;
        list->size++;
    } else {
        printf("Not yet implemented!\n");
    }
}

void printNodes(DList *list) {
    DListNode * pointer = list->head;
    if (pointer!=NULL) {
        int v= *((int*)pointer->data);
        printf("Node has value: %d\n", v);
        while (pointer->next != NULL) {
            v = *((int*)pointer->data);
            printf("Node has value: %d\n", v);
            pointer=pointer->next;
        }
    }
}

int main(int argc, const char * argv[])
{

    int e0 = 23;
    int e1 = 7;
    int e2 = 11;
    DList *list = (DList *)malloc(sizeof(DList));
    initList(list);
    assert(count(list)==0);
    insert(list, NULL, e0);
    assert(count(list)==1);

    insert(list,NULL, e1);
    assert(count(list)==2);

    insert(list,NULL, e2);
    assert(count(list)==3);
    printNodes(list);

    return 0;
}

我有一些问题:

1. 是否还为 data、prev、next 指针分配空间，或者我是否需要手动在每个指针上调用 malloc那些指针？
2. 当我打印每个节点中数据指针的内容时，即使我插入 23、7 和 11 并将数据指针设置为 int 的地址，它们的值仍然是 3:* * new_element->data = &data;**。

(C入门教材已订)

编辑:

插入现在采用指向数据的空指针:

// Insert data as the new head
void insert(DList *list, DListNode *element, void *data) {
    DListNode *new_element = malloc(sizeof(DListNode));
    new_element->data = data;
    if (list->head==NULL) {
        list->head=list->tail=new_element;
        list->size++;
        return;
    }
    if(element == NULL) {
        new_element->next=list->head;
        list->head->prev=new_element;
        list->head=new_element;
        list->size++;
    } else {
        printf("Not yet implemented!\n");
    }
}

我主要做的是:

int main(int argc, const char * argv[])
{
    int i0=7;
    int *ip0 = malloc(sizeof(int));
    ip0 = &i0;

    int i1=8;
    int *ip1 = malloc(sizeof(int));
    ip1 = &i1;

    int *ip2 = malloc(sizeof(int));
    int i2=44;
    ip2 = &i2;

    DList *list = malloc(sizeof(DList));
    initList(list);
    // create some nodes
    assert(count(list)==0);
    insert(list, NULL, ip0);
    assert(count(list)==1);

    insert(list,NULL, ip1);
    assert(count(list)==2);

    insert(list,NULL, ip2);
    assert(count(list)==3);
    printNodes(list);

    return 0;
}

输出:

Node has value: 44
Node has value: 44
Node has value: 8

但应该是:

Node has value: 44
Node has value: 8
Node has value: 7

Answer 1

1. malloc(sizeof(DListNode)) 为一个 DListNode 分配空间，根据定义，它由一个 void* 和两个 DListNode 指针。但它不会初始化这些指针。

2. 您正在将 data 参数的地址分配给 insert。这是一个指向临时对象的指针，一旦 insert 返回就会失效。程序的行为是未定义的。简单的解决方案是将 void *data 替换为 int data。