Linux kernel container_of macro example compilation errors

Question

can someone tell me what's wrong with this code:

#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;
}

When i compile i got this errors:

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’

Thanks for your help.

nos · Accepted Answer · 2012-07-28 23:55:24Z

2

There's 2 errors here

That container_of macro expects the last argument to be the member name. So it should be

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

has the wrong operand to sizeof, it should be

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

answered Jul 28, 2012 at 23:55

nos

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