c - Linux内核container_of宏示例编译错误

Question

有人能告诉我这段代码有什么问题吗：

#include <stdio.h>
#include <stdlib.h>

#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)
#define container_of(ptr, type, member) ({                                     \
                        const typeof( ((type *)0)->member ) *__mptr = (ptr);   \
                        (type *)( (char *)__mptr - offsetof(type,member) );})
typedef struct _elem {
        int     a;
        float   b;
        double  c;
} elem;

int main(int argc, char **argv)
{
        elem    *my_elem = (elem *)malloc(sizeof *elem);

        my_elem->a = 1;
        my_elem->b = 2;
        my_elem->c = 3;
        elem     *new_elem_a = container_of(&(my_elem->a), struct _elem, int);
        fprintf(stdout, "container_of(&(my_elem->a), struct _elem, int) = %p", new_elem_a);

        return 0;
}

当我编译我得到这个错误：

evariste@UnixServer:~$ gcc -Wall container_of_test.c -o container_of_test
container_of_test.c: In function ‘main’:
container_of_test.c:16:51: erreur: expected expression before ‘elem’
container_of_test.c:21:32: erreur: expected identifier before ‘int’
container_of_test.c:21:32: erreur: expected identifier before ‘int’

谢谢你的帮助。

Answer 1

这里有2个错误

1. 该 container_of 宏期望最后一个参数是成员名称。所以应该是

   elem *new_elem_a = container_of(&(my_elem->a), struct _elem, a);

2. elem *my_elem = (elem *)malloc(sizeof *elem)

   sizeof 的操作数错误，应该是

   elem *my_elem = (elem *)malloc(sizeof *my_elem)