与释放 C 中的内存混淆？

Question

我对我在这里做错了什么感到非常困惑。我有一个节点链接列表（结构见下文），我对如何在修剪列表时释放内存感到困惑。我认为将它们添加到数组 (eventArr) 会使它们以后更容易释放，但我仍然遇到问题！谁能帮我指出正确的方向？

static void trim_list(int room, int keep, char timestamp[]) {

        struct room_t *the_room;
        struct room_t *the_room_copy;

        struct evnode_t *eventArr[20];
        int index = 0;

        the_room_copy = malloc(sizeof(struct room_t));
        the_room = find_room(room);

        the_room_copy->room = the_room->room;
        the_room_copy->ecount = the_room->ecount;
        the_room_copy->evl_head = the_room->evl_head;

        struct evnode_t *node;

        int counter;
        int removed = 0;

        for(counter = 0; counter != keep; counter++){
          the_room_copy->evl_head = the_room_copy->evl_head->next;                 
        }

        while(the_room_copy->evl_head){
          eventArr[index] = the_room_copy->evl_head;
          the_room_copy->evl_head = the_room_copy->evl_head->next;
          index++;
          removed++;
        }

        for(index = 0; index < removed; index++){
          free(eventArr[index]);
        }

        printf("Trim log @ %s: room %i log trimmed to length %i (%i entries removed)\n",timestamp, room, keep, removed);

        return ;
}

结构：

struct evnode_t {
        struct event_t event ;
        struct evnode_t *next ;
} ;

struct room_t {
        int room ;
        int ecount ;
        struct evnode_t *evl_head ;
        struct room_t *next_room ;
} ;

struct event_t {
        int sensor ;
        char time_stamp[MAX_TIMESTRING+1];
        int event_id ;
        int event_info ;
} ;

我应该注意，我运行通过 gdb 进行此操作是为了更好地理解它，但由于某种原因，这些值变得一团糟。这是我的输出：

之前运行 trim_list...

(gdb) p room_list->room
 = 1
(gdb) p room_list->evl_head->event.sensor
 = 9
(gdb) p room_list->evl_head->next->event.sensor
 = 4
(gdb) p room_list->evl_head->next->next->event.sensor
 = 2
(gdb) p room_list->evl_head->next->next->next->event.sensor
 = 2
(gdb) p room_list->evl_head->next->next->next->next->event.sensor
 = 3
(gdb) p room_list->evl_head->next->next->next->next->next->event.sensor
 = 2

在运行之后trim_list...

(gdb) p room_list->evl_head->next->event.sensor
 = 4
(gdb) p room_list->evl_head->next->next->event.sensor
 = 2
(gdb) p room_list->evl_head->next->next->next->event.sensor
 = 2
(gdb) p room_list->evl_head->next->next->next->next->event.sensor
 = 0
(gdb) p room_list->evl_head->next->next->next->next->next->event.sensor
 = 6396880

任何关于我为什么会遇到这些问题的见解都将非常有帮助！我不确定我是否正确理解了如何释放值...但是我知道被释放的值都已 malloced。澄清一下，room_list 是我试图从中释放值的列表，the_room_copy 是 room_list.

的新 malloced 版本

Answer 1

看起来您正在正确释放节点。您没有做的是在新位置终止列表。例如，如果旧列表是：

1 -> 9 -> 4 -> 2 -> 2 -> 3 -> 2 -> NULL

而你想在第 5 个元素之后 trim，你现在所做的结果是这样的：

1 -> 9 -> 4 -> 2 -> 2 -> (freed memory) -> (unknown pointer) -> ...

你需要做的是在你想保留的链表之后终止链表：

1 -> 9 -> 4 -> 2 -> 2 -> NULL

下面是一些示例代码，显示了一种终止列表的方法：

    struct evnode_t *end_of_list = NULL;

    // This is your existing loop to skip past "keep" elements.
    for(counter = 0; counter != keep; counter++){
      end_of_list = the_room_copy->evl_head;
      the_room_copy->evl_head = the_room_copy->evl_head->next;                 
    }

    // Free stuff as you did before...

    // At the very end:
    if (end_of_list == NULL) {
        // Keep nothing.
        the_room->evl_head = NULL;
    } else {
        // Terminate the list at the proper place.
        end_of_list->next = NULL;
    }

    // Oh, you need to free the copy also, it's a memory leak:
    free(the_room_copy);

Answer 2

添加到 JS1 先生的答案中，要终止链表，您需要添加类似这样的代码：

struct evnode_t *node_to_terminate = the_room->evl_head;
for (int i = 1; i < keep; ++i)
{
    node_to_terminate = node_to_terminate->evl_head;
}

node_terminate->next = NULL;

与释放 C 中的内存混淆？

Confusion with freeing memory in C?

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