c - Linux内核链表和 'typeof'错误

Question

我正在尝试像 Linux 内核一样实现链表。

我认为我的头文件很好，但是当我尝试编译主文件来测试我所做的事情时，我真的不明白出了什么问题......

我的文件list.h:

#ifndef _LIST_H
#define _LIST_H

struct list_head {
    struct list_head *next, *prev;
};

static inline void
INIT_LIST_HEAD (struct list_head *head)
{
    head->next = head->prev = head;
}

static inline void
list_add (struct list_head *node,struct list_head *head)
{
    head->next->prev = node;
    node->next = head->next;
    node->prev = head;
    head->next = node;
}
static inline void
list_del (struct list_head *node)
{
    node->next->prev = node->prev;
    node->prev->next = node->next;
}

#define container_of(ptr, type, member) ({\
        const typeof( ((type *)0)->member ) *__mptr = (ptr); \
        (type *)( (char *)__mptr - offsetof(type, member) );})

#define list_for_each_entry(cur, head, member) \
        for (cur = container_of((head)->next, typeof(*cur),member);&cur->member != (head); \
        cur = container_of(cur->member.next,typeof(*cur), member))

#endif /* _LIST_H */

我的主文件:

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

struct kool_list{
    int to;
    struct list_head list;
    int from;
};

int
main(){
    struct kool_list *tmp;
    struct list_head *pos, *q;
    unsigned int i;

    struct kool_list mylist;
    INIT_LIST_HEAD(&mylist.list);

    /* adding elements to mylist */
    for(i=5; i!=0; --i){
        tmp= (struct kool_list *)malloc(sizeof(struct kool_list));
        printf("enter to and from:");
        scanf("%d %d", &tmp->to, &tmp->from);
        list_add(&(tmp->list), &(mylist.list));
    }
    printf("\n");

    printf("traversing the list using list_for_each_entry()\n");
    list_for_each_entry(tmp, &mylist.list, list)
         printf("to= %d from= %d\n", tmp->to, tmp->from);
    printf("\n");

    return 0;
}

当我用 gcc *.c -o main 编译它时，出现以下错误:

In file included from main.c:3:0:
main.c: In function ‘main’:
list.h:35:41: error: expected expression before ‘typeof’
   for (cur = container_of((head)->next, typeof(*cur),member);&cur->member != (head); \
                                         ^
list.h:32:37: note: in definition of macro ‘container_of’
 (type *)( (char *)__mptr - offsetof(type, member) );})
                                     ^
main.c:77:2: note: in expansion of macro ‘list_for_each_entry’
  list_for_each_entry(tmp, &mylist.list, list)
  ^
list.h:36:39: error: expected expression before ‘typeof’
   cur = container_of(cur->member.next,typeof(*cur), member))
                                       ^
list.h:32:37: note: in definition of macro ‘container_of’
 (type *)( (char *)__mptr - offsetof(type, member) );})
                                     ^
main.c:77:2: note: in expansion of macro ‘list_for_each_entry’
  list_for_each_entry(tmp, &mylist.list, list)
  ^

看来我没有以正确的方式操作“typeof”，但我不明白为什么......如果有人可以解释什么是错误的，谢谢你:)

Answer 1

您的错误是由于缺少 include 引起的 - offsetof 不是内置运算符(如 sizeof)，but rather a true macro ，在 stddef.h 中定义。

如果不包含此文件，编译器会假定它必须引用一个函数，这会导致编译器完全误解语法并给出不清楚的错误消息(因为函数不能接受类型作为其参数)。

您可以通过包含 stddef.h 来修复即时语法错误。通过在编译时启用尽可能多的严格开关和警告(至少使用 -Wall)，您可以更深入地了解这些情况下出现的问题。 GCC 默认情况下以相当宽容的模式进行编译，除其他外，它不认为隐式函数声明是错误，因此这个问题的真正根本原因是编译器完全忽略了。