Why am I getting "error: invalid conversion from 'void' to 'nlattr'" when I use "nla_for_each_nested" on a nested array?

Question

我正在使用 libnl3 和 nl80211.h 获取已连接无线接口的可用频段。

当我尝试编译时得到 error: invalid conversion from ‘void*’ to ‘nlattr*’。 IDE (CLion) 也在抱怨 nla_for_each_nested(nl_band, tb_msg[NL80211_ATTR_WIPHY_BANDS], rem_band) 行并说 incompatible pointer type 'nlattr *' and 'void *'.

什么被视为 void *，/usr/include/libnl3/netlink/attr.h 中的 pos = nla_data(nla) 是否被视为 void *？

编译错误：

/home/username/Documents/Development/appname/fs_nl80211Class.cpp: In function ‘int callback(nl_msg*, void*)’:
/usr/include/libnl3/netlink/attr.h:328:21: error: invalid conversion from ‘void*’ to ‘nlattr*’ [-fpermissive]
  for (pos = nla_data(nla), rem = nla_len(nla);
                     ^

来自 libnl3/netlink/attr.h 文件的宏：

#define nla_for_each_nested(pos, nla, rem) \
    for (pos = nla_data(nla), rem = nla_len(nla); \
         nla_ok(pos, rem); \
         pos = nla_next(pos, &(rem)))

我的代码：

static int callback(struct nl_msg *msg, void *arg) {

    genlmsghdr *gnlh = (genlmsghdr *) nlmsg_data(nlmsg_hdr(msg));

    struct nlattr *tb_msg[NL80211_ATTR_MAX + 1];
    struct nlattr *tb_band[NL80211_BAND_ATTR_MAX + 1];

    struct nlattr *nl_band;

    int rem_band;
    static int last_band = -1;
    static bool band_had_freq = false;

    nla_parse(tb_msg, NL80211_ATTR_MAX, genlmsg_attrdata(gnlh, 0), genlmsg_attrlen(gnlh, 0), NULL);

    if (tb_msg[NL80211_ATTR_WIPHY_BANDS]) {

        // macro from libnl3/netlink/attr.h
        nla_for_each_nested(nl_band, tb_msg[NL80211_ATTR_WIPHY_BANDS], rem_band) {
            if (last_band != nl_band->nla_type) {
                cout << ("Band %d:", nl_band->nla_type) << endl;
                band_had_freq = false;
            }
            last_band = nl_band->nla_type;

            nla_parse(tb_band,
                      NL80211_BAND_ATTR_MAX,
                      nla_data(nl_band),
                      nla_len(nl_band), NULL);

            if (tb_band[NL80211_BAND_ATTR_FREQS]) {
                if (!band_had_freq) {
                    cout << "Frequencies:" << endl;
                    band_had_freq = true;
                }
            }
        }
    }

    //nl_msg_dump(msg, stdout);

    return NL_SKIP;
}

Answer 1

问题是由于从 void* 到 nlattr* 的隐式类型转换，g++ 无法使用 nla_for_each_nested 宏编译代码。

从 libnl 列表中查看 link：

http://lists.infradead.org/pipermail/libnl/2016-February/002096.html

nla_for_each_nested 宏中的修复是为 nla_for_each_nested 宏添加类型转换。

更改自：

for (pos = nla_data(nla), rem = nla_len(nla); \

收件人：

for (pos = (struct nlattr *) nla_data(nla), rem = nla_len(nla); \

Why am I getting "error: invalid conversion from 'void*' to 'nlattr*'" when I use "nla_for_each_nested" on a nested array?