我在浏览一些route netlink源代码。
我想弄清楚rtnlgrp廑u neigh的价值是什么
来源:http://lxr.free-electrons.com/source/include/linux/rtnetlink.h?v=2.6.35#L550

541 /* RTnetlink multicast groups */
542 enum rtnetlink_groups {
543         RTNLGRP_NONE,
544 #define RTNLGRP_NONE            RTNLGRP_NONE
545         RTNLGRP_LINK,
546 #define RTNLGRP_LINK            RTNLGRP_LINK
547         RTNLGRP_NOTIFY,
548 #define RTNLGRP_NOTIFY          RTNLGRP_NOTIFY
549         RTNLGRP_NEIGH,
550 #define RTNLGRP_NEIGH           RTNLGRP_NEIGH
551         RTNLGRP_TC,
552 #define RTNLGRP_TC              RTNLGRP_TC
553         RTNLGRP_IPV4_IFADDR,
554 #define RTNLGRP_IPV4_IFADDR     RTNLGRP_IPV4_IFADDR
...       ...
...       ...
#define RTNLGRP_PHONET_IFADDR   RTNLGRP_PHONET_IFADDR
585         RTNLGRP_PHONET_ROUTE,
586 #define RTNLGRP_PHONET_ROUTE    RTNLGRP_PHONET_ROUTE
587         __RTNLGRP_MAX
588 };
589 #define RTNLGRP_MAX     (__RTNLGRP_MAX - 1)

define doing的枚举是什么。
RTNLGRP的价值是什么?6 OR 3
谢谢

最佳答案

RTNLGRP_NEIGH的值为3。您可以使用下面的程序轻松地对此进行测试。

#include <stdio.h>

/* RTnetlink multicast groups */
enum rtnetlink_groups {
        RTNLGRP_NONE,
#define RTNLGRP_NONE            RTNLGRP_NONE
        RTNLGRP_LINK,
#define RTNLGRP_LINK            RTNLGRP_LINK
        RTNLGRP_NOTIFY,
#define RTNLGRP_NOTIFY          RTNLGRP_NOTIFY
        RTNLGRP_NEIGH,
#define RTNLGRP_NEIGH           RTNLGRP_NEIGH
        RTNLGRP_TC,
#define RTNLGRP_TC              RTNLGRP_TC
        RTNLGRP_IPV4_IFADDR,
#define RTNLGRP_IPV4_IFADDR     RTNLGRP_IPV4_IFADDR
        /* ... */
#define RTNLGRP_PHONET_IFADDR   RTNLGRP_PHONET_IFADDR
        RTNLGRP_PHONET_ROUTE,
#define RTNLGRP_PHONET_ROUTE    RTNLGRP_PHONET_ROUTE
        __RTNLGRP_MAX
};
#define RTNLGRP_MAX     (__RTNLGRP_MAX - 1)

int
main()
{
  printf("RTNLGRP_NEIGH = %d\n", RTNLGRP_NEIGH);
}

它输出:
RTNLGRP_NEIGH = 3

由于每个宏的名称都是#defined,因此RTNLGRP_NEIGH中的main将替换为RTNLGRP_NEIGH。但由于扩展不是递归的,所以它将在此时停止,程序使用enum常量RTNLGRP_NEIGH,这是第四个常量,因此具有值3。
如果您不确定预处理器做什么,您可以始终使用-E开关编译并查看预处理的输出。用gcc -E编译上面的示例给出(没有显示840行#included标准库头)
# 4 "main.c"
enum rtnetlink_groups {
        RTNLGRP_NONE,

        RTNLGRP_LINK,

        RTNLGRP_NOTIFY,

        RTNLGRP_NEIGH,

        RTNLGRP_TC,

        RTNLGRP_IPV4_IFADDR,



        RTNLGRP_PHONET_ROUTE,

        __RTNLGRP_MAX
};


int
main()
{
  printf("RTNLGRP_NEIGH = %d\n", RTNLGRP_NEIGH);
}

希望这不会那么令人困惑。
#define定义中的enums对enum定义没有影响。#defines位于何处并不重要。它们可以(也可能应该)放在定义之前或之后。
/* RTnetlink multicast groups */
enum rtnetlink_groups {
        RTNLGRP_NONE,
        RTNLGRP_LINK,
        RTNLGRP_NOTIFY,
        RTNLGRP_NEIGH,
        RTNLGRP_TC,
        RTNLGRP_IPV4_IFADDR,
        /* ... */
        RTNLGRP_PHONET_ROUTE,
        __RTNLGRP_MAX
};

#define RTNLGRP_NONE            RTNLGRP_NONE
#define RTNLGRP_LINK            RTNLGRP_LINK
#define RTNLGRP_NOTIFY          RTNLGRP_NOTIFY
#define RTNLGRP_NEIGH           RTNLGRP_NEIGH
#define RTNLGRP_TC              RTNLGRP_TC
#define RTNLGRP_IPV4_IFADDR     RTNLGRP_IPV4_IFADDR
#define RTNLGRP_PHONET_IFADDR   RTNLGRP_PHONET_IFADDR
/* ... */
#define RTNLGRP_PHONET_ROUTE    RTNLGRP_PHONET_ROUTE
#define RTNLGRP_MAX     (__RTNLGRP_MAX - 1)

他们写这些怪代码的原因可能是他们想用
#define RTNLGRP_NONE          0
#define RTNLGRP_LINK          1
#define RTNLGRP_NOTIFY        2
#define RTNLGRP_NEIGH         3
#define RTNLGRP_TC            4
#define RTNLGRP_IPV4_IFADDR   5
/* ... */

改为使用enum。但是,因为现有代码可能依赖于标识符是宏(例如测试#ifdef RTNLGRP_NEIGH)的事实,所以它们希望提供具有相同值的宏。注意,这种方法是有缺陷的,但是,因为预处理器不知道常量的值,所以您不能做像#if RTNLGRP_NEIGH >= 3这样的事情,如果RTNLGRP_NEIGH已经#defined到3字面上的话。因此,本质上,他们的方法结合了使用宏(名称空间污染)和使用enums(在预处理时不可用)的缺点。
我以前见过的一个更有用的模式是将常量转换成实际整数。
enum rtnetlink_groups {
        RTNLGRP_NONE
#define RTNLGRP_NONE            0
        = RTNLGRP_NONE,
        RTNLGRP_LINK
#define RTNLGRP_LINK            1
        = RTNLGRP_LINK,
        RTNLGRP_NOTIFY
#define RTNLGRP_NOTIFY          2
        = RTNLGRP_NOTIFY,
        RTNLGRP_NEIGH
#define RTNLGRP_NEIGH           3
        = RTNLGRP_NEIGH,
        RTNLGRP_TC
#define RTNLGRP_TC              4
        = RTNLGRP_TC,
        RTNLGRP_IPV4_IFADDR
#define RTNLGRP_IPV4_IFADDR     5
        = RTNLGRP_IPV4_IFADDR,
        /* ... */
};

它将被预处理为以下内容。
enum rtnetlink_groups {
        RTNLGRP_NONE

        = 0,
        RTNLGRP_LINK

        = 1,
        RTNLGRP_NOTIFY

        = 2,
        RTNLGRP_NEIGH

        = 3,
        RTNLGRP_TC

        = 4,
        RTNLGRP_IPV4_IFADDR

        = 5,

};

注意,在这里,#defines必须混合到#define定义中,否则我们将得到诸如enum之类的无效代码,而不是所需的3 = 3,
哦,请不要使用RTNLGRP_NEIGH = 3作为标识符。包含两个相邻下划线或以下划线开头,后跟大写字母的名称由C标准保留。在自己的代码中使用它们会导致未定义的行为。

关于c - 令人困惑的MACRO和枚举定义,我们在Stack Overflow上找到一个类似的问题:https://stackoverflow.com/questions/45778074/

10-11 23:20
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