访问结构成员时 valgrind 出现无效 read/write 错误

Question

我试图在 C 中实现链表纯粹是为了练习。我有这样定义的结构：

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

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

现在，valgrind 抱怨的功能是这些：

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)++;
}

销毁()

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)

如果我对输出的理解正确，问题似乎出在 struct list 成员变量的访问上。但是，我不明白为什么访问这些变量是个问题。 malloc(sizeof(list))应该为两个成员分配足够的内存，那么问题出在哪里？

Answer 1

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

糟糕！ sizeof 中的 list 是您声明的指针，而不是类型。所以你只 malloc-ed 足够的内存用于一个指针，而不是你想要的结构。

避免用变量名遮盖类型名。或者，如果必须，请使用

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

Answer 2

问题出在这里：

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

您有一个 list 的类型定义和一个名为 list 的变量。 sizeof 运算符取的是变量的大小，而不是类型。

作为规则，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 ;
}

Answer 3

and

明确指出了问题

提供不同的解决方案：使用 sizeof() 时，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);

即使使用阴影类型 list 和变量 list，这也有效

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

访问结构成员时 valgrind 出现无效 read/write 错误

Invalid read/write errors by valgrind when accessing struct members

c

memory

struct

valgrind