Question

这是一个小测试，以帮助演示和审查valgrind的memcheck的输出。有人可以帮我弄清楚如何从列表中间删除AND释放节点？如果我从删除节点部分注释掉free（cur）和free（cur-＆gt; lock），那么memcheck告诉我我有内存泄漏，但是如果我把它们保留在那里那么我在顶部做了一个无效的读取循环。有没有办法解决这个难题？

TEST(UtilityGeneralUnittest, valgrindTests)
{
    //declare a node type
    typedef struct node{
        int size;
        int value;
        struct node *next;
        pthread_mutex_t *lock;
    }node_t;

    //make the head
    node_t *head;
    head = (node_t*)malloc(1 * sizeof(node_t));
    head->size = 0;
    head->next = NULL;
    head->lock = (pthread_mutex_t*)malloc(1 * sizeof(pthread_mutex_t));
    pthread_mutex_init(head->lock, NULL);

    //create array for storing values
    int array[10];


    //build a list with random numbers
    for (int i = 0; i < 10; i++)
    {
        node_t *newNode;
        newNode = (node_t*)malloc(1 * sizeof(node_t));
        newNode->value = rand() % 100 + 1;
        newNode->next = NULL;
        newNode->lock = (pthread_mutex_t*) malloc(1 * sizeof(pthread_mutex_t));
        pthread_mutex_init(newNode->lock, NULL);
        array[i] = newNode->value;
        if (head->next == NULL)
        {
            head->next = newNode;
            head->size++;
        }
        else
        {
            node_t *tmp = head->next;
            head->next = newNode;
            newNode->next = tmp;
            head->size++;
        }
    }
    // assert the list added nodes
    ASSERT_EQ(10, head->size);

    //sanity check; print the list
    node_t *printer = head;
    while(printer->next != NULL)
    {
        printer = printer->next;
        std::cout << "value: ";
        std::cout <<  printer->value << ", ";
    }
    std::cout << "\n";
    // the meat and potatoes: deleting with locks.
    int removeMe = array[rand() % 10];
    bool verifyDel = true;
    int checkVal = removeMe;
    node_t *prev;
    node_t *cur;

    prev = head;
    pthread_mutex_lock(prev->lock);
    while((cur = prev->next) != NULL) //******** this is the problem
    {
        pthread_mutex_lock(cur->lock);
        if(cur->value == removeMe)
        {
            prev->next = cur->next;
            pthread_mutex_unlock(cur->lock);
            pthread_mutex_unlock(prev->lock);
            cur->next = NULL;
            head->size--;
            free(cur->lock); ///******** this is the problem
            free(cur); ///****** this is the problem
        }
        pthread_mutex_unlock(prev->lock);
        prev = cur;
    }
    //pthread_mutex_unlock(prev->lock);


    //verify node has been removed
    printer = head;
    while(printer->next != NULL)
    {
        printer = printer->next;
        if(printer->value == checkVal)
        {
            verifyDel = false;
        }
        std::cout << "value: ";
        std::cout <<  printer->value << ", ";
    }
    std::cout << "\n";
    ASSERT_TRUE(verifyDel);

    //clean up: delete the list
    while((printer = head) != NULL)
    {
        head = head->next;
        free(printer->lock);
        free(printer);
        std::cout << "free!!!" << std::endl;
    }


}

Answer 1

查看你的循环（简化）：

while ((cur = prev->next) != NULL) //* problem
{
    if (cur->value == removeMe)
    {
        prev->next = cur->next;
        cur->next = NULL;
        free(cur);              //* problem
    }
    prev = cur;
}

问题在于赋值prev = cur，但仅在评估if块时。这已释放cur，因此下次循环时，cur=prev->next会引用已删除的存储空间。

只有在未删除else的情况下，您才可以通过插入prev来分配cur来解决此问题：

while ((cur = prev->next)) {
    if (cur->value == removeMe) {
        prev->next = cur->next;
        cur->next = NULL;
        free(cur);
    } else {
        prev = cur;
    }
}

如何在没有valgrind错误的列表中间释放已删除的节点？

1 个答案: