hostapd wpa_supplicant madwifi详细分析（十五）——supplicant扫描结果排序规则

int (*compar)(const void *, const void *) = wpa_scan_result_compar;

qsort(scan_res->res, scan_res->num, sizeof(struct wpa_scan_res *),compar);  // qsort函数介绍

static int wpa_scan_result_compar(const void *a, const void *b)  //这个函数用于scan result结果降序排序
{
#define MIN(a,b) a < b ? a : b
    struct wpa_scan_res **_wa = (void *) a;  //scan result的二级指针
    struct wpa_scan_res **_wb = (void *) b;
    struct wpa_scan_res *wa = *_wa; //scan result的一级指针
    struct wpa_scan_res *wb = *_wb;
    int wpa_a, wpa_b;
    int snr_a, snr_b, snr_a_full, snr_b_full;

    /* WPA/WPA2 support preferred */
    wpa_a = wpa_scan_get_vendor_ie(wa, WPA_IE_VENDOR_TYPE) != NULL ||
        wpa_scan_get_ie(wa, WLAN_EID_RSN) != NULL;
    wpa_b = wpa_scan_get_vendor_ie(wb, WPA_IE_VENDOR_TYPE) != NULL ||
        wpa_scan_get_ie(wb, WLAN_EID_RSN) != NULL;

    if (wpa_b && !wpa_a) //这里先比较是否有WPA_IE_VENDOR_TYPE和WLAN_EID_RSN这两个IE，谁有就谁排在前面，如果都没有，就不将他们看作排序因子
        return 1;  //返回正数表示升序，反会负表示逆序，返回0表示相等，所以正数时表示a比b小，负数时表示a比b大
    if (!wpa_b && wpa_a)
        return -1; //这里返回负数，表示将a放在b的后面

    /* privacy support preferred */
    if ((wa->caps & IEEE80211_CAP_PRIVACY) == 0 &&  //如果有标志位IEEE80211_CAP_PRIVACY的，就排在前面
        (wb->caps & IEEE80211_CAP_PRIVACY))
        return 1;
    if ((wa->caps & IEEE80211_CAP_PRIVACY) &&
        (wb->caps & IEEE80211_CAP_PRIVACY) == 0)
        return -1;

    if (wa->flags & wb->flags & WPA_SCAN_LEVEL_DBM) {
        snr_a_full = wa->snr;
        snr_a = MIN(wa->snr, GREAT_SNR); //如果信噪比大于30，那么信噪比snr_a的值就赋值为30，所以当wa->snr大于30的时候，snr_a_full是真实值，snr_a是阈值
        snr_b_full = wb->snr;
        snr_b = MIN(wb->snr, GREAT_SNR); //通过log可知，其实排序的时候没有用到信噪比这个因子
    } else {
        /* Level is not in dBm, so we can't calculate
         * SNR. Just use raw level (units unknown). */
        snr_a = snr_a_full = wa->level;   //level表示信号强度，一般用百分比表示，和信噪比的算法不一样
        snr_b = snr_b_full = wb->level;
    }

    /* if SNR is close, decide by max rate or frequency band */
    if ((snr_a && snr_b && abs(snr_b - snr_a) < 5) ||     //如果两者的level相差在5以内，就认为两者是相等的
        (wa->qual && wb->qual && abs(wb->qual - wa->qual) < 10)) {  //如果两者的信号质量相差在10以内，就认为两者是相等的
        if (wa->est_throughput != wb->est_throughput)  //Enrollment over Secure Transport, 我们没有用这种协议，所以两者都是0, 非排序因子
            return wb->est_throughput - wa->est_throughput;
        if (IS_5GHZ(wa->freq) ^ IS_5GHZ(wb->freq))   //如果两者都是5G，那就不用比较了，如果其中一个是5G，另外一个不是，那么5G的排在前面
            return IS_5GHZ(wa->freq) ? -1 : 1;
    }

    /* all things being equal, use SNR; if SNRs are
     * identical, use quality values since some drivers may only report
     * that value and leave the signal level zero */
    if (snr_b_full == snr_a_full)   //如果运行到这里还没有return，就表示前面的那些参数都是相等的，所以这里看看两者的真是信噪比是否相等
        return wb->qual - wa->qual; //如果相等，那就比信号质量，信号质量高的排在前面
    return snr_b_full - snr_a_full; //如果不相等，那就比真实信噪比，信噪比高的排在前面
#undef MIN
}