访问struct member

时间:2016-07-07 18:06:58

标签: c memory struct valgrind

我试图在C中实现一个纯粹作为练习的链表。我有这样的结构:

typedef struct node {
    int data;
    struct node* next;
} 
node;

typedef struct list {
    size_t size;
    node* head;
}
list;

现在,valgrind抱怨的功能是:

创建()

list* create() {
    // alocate memory for a new list
    list* list = malloc(sizeof(list));

    if (list != NULL) {
        list->head = NULL; // this is line 65
        list->size = 0;
    }

    // return pointer to the allocated memory
    return list;
}

插入()

void insert(int data, list* list) {
    if (list == NULL)
        return;

    // allocate memory for new node
    node* newNode = malloc(sizeof(node));

    // check if allocation was successful
    if (newNode == NULL)
        return;

    // initialize new node's data
    newNode->data = data;

    // make newNode the head of the list
    newNode->next = list->head; // this is line 88
    list->head = newNode;

    // increment size
    (list->size)++;
}

破坏()

void destroy(list* list) {
    if (list == NULL)
        return;

    node* current = list->head; // this is line 154
    while (current != NULL) {
        node* temp = current;
        current = current->next;
        free(temp);
    }

    free(list);
}

main()如下:

int main(void) {
    list* list = create();
    insert(1, list);
    destroy(list);
    return 0;
}

这就是valgrind的输出:

==10601== 1 errors in context 1 of 4:
==10601== Invalid read of size 8
==10601==    at 0x400A33: destroy (slist.c:154)
==10601==    by 0x400AAE: main (slist.c:167)
==10601==  Address 0x51fc048 is 0 bytes after a block of size 8 alloc'd
==10601==    at 0x4C2AB80: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==10601==    by 0x4007C3: create (slist.c:62)
==10601==    by 0x400A93: main (slist.c:165)
==10601== 
==10601== 
==10601== 1 errors in context 2 of 4:
==10601== Invalid write of size 8
==10601==    at 0x400866: insert (slist.c:89)
==10601==    by 0x400AA5: main (slist.c:166)
==10601==  Address 0x51fc048 is 0 bytes after a block of size 8 alloc'd
==10601==    at 0x4C2AB80: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==10601==    by 0x4007C3: create (slist.c:62)
==10601==    by 0x400A93: main (slist.c:165)
==10601== 
==10601== 
==10601== 1 errors in context 3 of 4:
==10601== Invalid read of size 8
==10601==    at 0x400852: insert (slist.c:88)
==10601==    by 0x400AA5: main (slist.c:166)
==10601==  Address 0x51fc048 is 0 bytes after a block of size 8 alloc'd
==10601==    at 0x4C2AB80: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==10601==    by 0x4007C3: create (slist.c:62)
==10601==    by 0x400A93: main (slist.c:165)
==10601== 
==10601== 
==10601== 1 errors in context 4 of 4:
==10601== Invalid write of size 8
==10601==    at 0x4007DA: create (slist.c:65)
==10601==    by 0x400A93: main (slist.c:165)
==10601==  Address 0x51fc048 is 0 bytes after a block of size 8 alloc'd
==10601==    at 0x4C2AB80: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==10601==    by 0x4007C3: create (slist.c:62)
==10601==    by 0x400A93: main (slist.c:165)
==10601== 
==10601== ERROR SUMMARY: 4 errors from 4 contexts (suppressed: 0 from 0)

如果我正确理解输出,问题似乎是访问struct list成员变量。但是,我不明白为什么访问这些变量是一个问题。 malloc(sizeof(list))应该为两个成员分配足够的内存,那么问题在哪里呢?

3 个答案:

答案 0 :(得分:5)

list* list = malloc(sizeof(list));

糟糕! list中的sizeof是您要声明的指针,而不是类型。所以你只需要malloc - 为一个指针编写足够的内存,而不是你想要的结构。

避免使用变量名称遮蔽类型名称。或者,如果必须,请使用

list* list = malloc(sizeof(struct list));

答案 1 :(得分:5)

问题在于:

list* list = malloc(sizeof(list));

您有list的typedef和名为list的变量。 sizeof运算符取的是变量的大小,而不是类型。

通常,不要将变量命名为与类型相同的名称:

list* create() {
    // alocate memory for a new list
    list* mylist = malloc(sizeof(list));

    if (mylist != NULL) {
        mylist ->head = NULL;
        mylist ->size = 0;
    }

    // return pointer to the allocated memory
    return mylist ;
}

答案 2 :(得分:2)

@aschepler@dbush

可以很好地识别问题

提供不同的解决方案:使用sizeof()时,请使用取消引用变量的大小而不是类型的大小。

// some_type *ptr = malloc(sizeof (some_type));
some_type *ptr = malloc(sizeof *ptr);

即使使用了阴影类型list和变量list,也可以使用

list* create() {

  list* list;

  printf("%zu\n", sizeof(list)); // compiles but the needed code
  printf("%zu\n", sizeof list);  // compiles but the needed code
  printf("%zu\n", sizeof *list); // good

  // allocate memory for a new list
  list = malloc(sizeof *list);
  ...

打印

4
4
8