c++ - Visual C++ Windows 中的基本 NTP 客户端

Question

你好，

我正在尝试实现一个非常基本的 NTP 客户端，这样我就可以查询远程 NTP 服务器(即:pool.ntp.org)的互联网时间以自动设置我的 Windows 上的时间每个引导上的开发板。

我在这里重构了另一个用户的代码:

Get time/date from server with sntp(windows c++)

但是程序在recv()操作处挂了。 DNS 解析和 send() 操作似乎可以毫无问题地执行。

有谁知道我在哪里可以找到 Windows 中的简单 NTP 客户端示例(首选 GPL，但此时任何东西都可以)，或者他们可以评论为什么以下代码块会从示例中挂起(我不应该不要说“挂起”，它似乎永远不会收到回复)。

NTP_CLIENT.CPP

/******************************************************************************
 * Project Headers
 *****************************************************************************/
#include "stdafx.h"
#include "ntp_client.h"

/******************************************************************************
 * System Headers
 *****************************************************************************/
#include <winsock2.h>
#include <winsock.h>
#include <ws2tcpip.h>


/******************************************************************************
 * Preprocessor Directives and Macros
 *****************************************************************************/


/******************************************************************************
 * Class Member Function Definitions
 *****************************************************************************/
void Timestamp::ReverseEndian(void) {
    ReverseEndianInt(seconds);
    ReverseEndianInt(fraction);
}

time_t Timestamp::to_time_t(void) {
    return (seconds - ((70 * 365 + 17) * 86400))&0x7fffffff;
}

void NTPMessage::ReverseEndian(void) {
    ref.ReverseEndian();
    orig.ReverseEndian();
    rx.ReverseEndian();
    tx.ReverseEndian();
}

int NTPMessage::recv(int sock) {
    int ret = ::recv(sock, (char*)this, sizeof(*this), 0);
    ReverseEndian();
    return ret;
}

int NTPMessage::sendto(int sock, struct sockaddr_in* srv_addr) {
    ReverseEndian();
    int ret = ::sendto(sock, (const char*)this, sizeof(*this), 0, (sockaddr*)srv_addr, sizeof(*srv_addr));
    ReverseEndian();
    return ret;
}

void NTPMessage::clear() {
    memset(this, 0, sizeof(*this));
}

NTP_CLIENT.H

#ifndef __NTP_CLIENT_H__
#define __NTP_CLIENT_H__

#include <ctime>

#define ReverseEndianInt(x) ((x) = \
    ((x)&0xff000000) >> 24 |       \
    ((x)&0x00ff0000) >> 8  |       \
    ((x)&0x0000ff00) << 8  |       \
    ((x)&0x000000ff) << 24)

/**
 * NTP Fixed-Point Timestamp Format.
 * From [RFC 5905](http://tools.ietf.org/html/rfc5905).
 */
struct Timestamp {
    unsigned int seconds;   /**< Seconds since Jan 1, 1900. */
    unsigned int fraction;  /**< Fractional part of seconds. Integer number of 2^-32 seconds. */

    /**
     * Reverses the Endianness of the timestamp.
     * Network byte order is big endian, so it needs to be switched before
     * sending or reading.
     */
    void ReverseEndian(void);

    /**
     * Convert to time_t.
     * Returns the integer part of the timestamp in unix time_t format,
     * which is seconds since Jan 1, 1970.
     */
    time_t to_time_t(void);
};

/**
 * A Network Time Protocol Message.
 * From [RFC 5905](http://tools.ietf.org/html/rfc5905).
 */
struct NTPMessage {
    unsigned int mode :3;           /**< Mode of the message sender. 3 = Client, 4 = Server */
    unsigned int version :2;        /**< Protocol version. Should be set to 3. */
    unsigned int leap :2;           /**< Leap seconds warning. See the [RFC](http://tools.ietf.org/html/rfc5905#section-7.3) */
    unsigned char stratum;          /**< Servers between client and physical timekeeper. 1 = Server is Connected to Physical Source. 0 = Unknown. */
    unsigned char poll;             /**< Max Poll Rate. In log2 seconds. */
    unsigned char precision;        /**< Precision of the clock. In log2 seconds. */
    unsigned int sync_distance;     /**< Round-trip to reference clock. NTP Short Format. */
    unsigned int drift_rate;        /**< Dispersion to reference clock. NTP Short Format. */
    unsigned char ref_clock_id[4];  /**< Reference ID. For Stratum 1 devices, a 4-byte string. For other devices, 4-byte IP address. */
    Timestamp ref;                  /**< Reference Timestamp. The time when the system clock was last updated. */
    Timestamp orig;                 /**< Origin Timestamp. Send time of the request. Copied from the request. */
    Timestamp rx;                   /**< Recieve Timestamp. Reciept time of the request. */
    Timestamp tx;                   /**< Transmit Timestamp. Send time of the response. If only a single time is needed, use this one. */


    /**
     * Reverses the Endianness of all the timestamps.
     * Network byte order is big endian, so they need to be switched before
     * sending and after reading.
     *
     * Maintaining them in little endian makes them easier to work with
     * locally, though.
     */
    void ReverseEndian(void);

    /**
     * Recieve an NTPMessage.
     * Overwrites this object with values from the received packet.
     */
    int recv(int sock);

    /**
     * Send an NTPMessage.
     */
    int sendto(int sock, struct sockaddr_in* srv_addr);

    /**
     * Zero all the values.
     */
    void clear();
};

#endif  /* __NTP_CLIENT_H__ */

被测设备已禁用防火墙和防病毒软件。

谢谢。

编辑

---

根据要求，这是 main 的主体:

WSADATA wsaData;
DWORD ret = WSAStartup(MAKEWORD(2,0), &wsaData);

char *host = "pool.ntp.org"; /* Don't distribute stuff pointing here, it's not polite. */
//char *host = "time.nist.gov"; /* This one's probably ok, but can get grumpy about request rates during debugging. */

NTPMessage msg;
/* Important, if you don't set the version/mode, the server will ignore you. */
msg.clear();
msg.version = 3;
msg.mode = 3 /* client */;

NTPMessage response;
response.clear();

int sock = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP);
sockaddr_in srv_addr;
memset(&srv_addr, 0, sizeof(srv_addr));
dns_lookup(host, &srv_addr); /* Helper function defined below. */

msg.sendto(sock, &srv_addr);
response.recv(sock);

time_t t = response.tx.to_time_t();
char *s = ctime(&t);
printf("The time is %s.", s);

WSACleanup();

---

引用资料

1. Get time/date from server with sntp(windows c++)

Answer 1

来自 RFC 5905 的图 8 (需要向下滚动到第 18 页):

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|LI | VN  |Mode |    Stratum     |     Poll      |  Precision   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

我在图 8 中看到的一件事是模式、版本和 leap 应该构成第一个字节。

将模式、版本和跳跃类型从 unsigned int 更改为 unsigned char，这样您的按位变量仅跨越 1 个字节而不是 4 个字节。这将使您的 NTPMessage 大小预计 48 字节。

喜欢:

struct NTPMessage {
    unsigned char mode :3;
    unsigned char version :2;
    unsigned char leap :2;
    ...