截获数据包并打印时间和数据包长度。
/* Callback function invoked by libpcap for every incoming packet */
// 定义回调函数，一定要符合接口，这个是麻烦的地方啊。。。
void packet_handler(u_char *param, const struct pcap_pkthdr *header, const u_char *pkt_data)
{
       struct tm *ltime;
       char timestr[16];
       time_t local_tv_sec;
 
       /* convert the timestamp to readable format */
       local_tv_sec = header->ts.tv_sec;
       ltime=localtime(&local_tv_sec);
       // strftime是win32的API?或者Linux、Windows下都有这个函数？
       // 把ltime里的时间，格式化后，输到字符数组中？
       strftime( timestr, sizeof timestr, "%H:%M:%S", ltime);
 
       // 前面是标准时间，后面是毫秒
       printf("%s,%.6d len:%d\n", timestr, header->ts.tv_usec, header->len);
}

以下是windows定义的时间结构：
struct tm {
        int tm_sec;     /* seconds after the minute - [0,59] */
        int tm_min;     /* minutes after the hour - [0,59] */
        int tm_hour;    /* hours since midnight - [0,23] */
        int tm_mday;    /* day of the month - [1,31] */
        int tm_mon;     /* months since January - [0,11] */
        int tm_year;    /* years since 1900 */
        int tm_wday;    /* days since Sunday - [0,6] */
        int tm_yday;    /* days since January 1 - [0,365] */
        int tm_isdst;   /* daylight savings time flag */
        };