Question

typedef struct _DListNode
{
    struct _DListNode* prev;
    struct _DListNode* next;
    void*  data;
}DListNode;

in gcc :

DListNode *p = NULL;

p = (DListNode*)malloc(sizeof(DListNode));

p->data = (int*)malloc(sizeof(int));   

scanf("%d", (int*)p->data);

compile correctly.

in gcc :

DListNode *p = NULL;

p = (DListNode*)malloc(sizeof(DListNode));

(int*)p->data = (int*)malloc(sizeof(int)); 

scanf("%d", (int*)p->data);

There is a problem on :

(int*)p->data = (int*)malloc(sizeof(int)); -------error: lvalue required as left operand of assignment.

the difference:

scanf("%d", (int*)p->data) ,

(int*)p->data = (int*)malloc(sizeof(int)),

p->data = (int*)malloc(sizeof(int));

Answer 1

How about (int*&)p->data = (int*)malloc(sizeof(int));“?

(int *) gives the pointer value itself, i.e. it’s akin to saying 0x12345678 = malloc(sizeof(int));.

Btw, mallocing for something as small as an int is extremely wasteful.

EDIT:

1. This might only work in C++, not C.

2. Why do you cast in the first place? You’re assigning a void pointer to a void pointer. Isn’t that convenient? Why cast?

3. If you really want to amuse yourself, you could do something silly like *(int **) &(p->data) = (int *) malloc(sizeof(int));. Nonsense, right? Right. Just assign the value. Better yet, replace your data with a union.