访问struct成员时valgrind的读/写错误无效

Question

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)++;
}

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struct list成员变量的访问.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,那么问题出在哪里？

Answer 1

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

哎呦!该list范围内sizeof是你声明的指针,而不是类型.所以你只malloc为一个指针留下足够的内存,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));

Answer 2

问题出在这里:

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

你有一个typedef list和一个名为的变量list.的sizeof操作者走的是变量,而不是类型的大小.

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 ;
}