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在Linux下查询RTC和NTP时间?

posix time linux c c++

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Vincent · 技术社区 · 8 年前

Linux内核处理时间的方式有些复杂,因为它使用不同的时间源,并定期重新同步所有内容。有一些命令行工具(如 hwclock )明确查询RTC时间。但是,我想在C或C++程序内部编程。如果Linux提供命令行工具,我假设有一种方法可以通过编程实现,但是到目前为止,我还没有找到正确的函数以及如何使用它。

所以我的问题是:

如何明确地向系统请求 RTC C或C++程序中的时间?(多么 get_rtc() 功能看起来像?)
如何明确地向系统请求 NTP C或C++程序中的时间?(多么 get_ntp() 功能看起来像?)

1 回复 | 直到 8 年前

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dvhh 8 年前

使用 time(time_t*) 或 clock_gettime(clockid_t,struct timespec *) 如果你希望更精确,从 time.h header 对于RTC时间。

有一个 article describing how to get query NTP time (这是一个 google search 离开)。

描述NTP包的结构

typedef struct
{

  uint8_t li_vn_mode;      // Eight bits. li, vn, and mode.
                           // li.   Two bits.   Leap indicator.
                           // vn.   Three bits. Version number of the protocol.
                           // mode. Three bits. Client will pick mode 3 for client.

  uint8_t stratum;         // Eight bits. Stratum level of the local clock.
  uint8_t poll;            // Eight bits. Maximum interval between successive messages.
  uint8_t precision;       // Eight bits. Precision of the local clock.

  uint32_t rootDelay;      // 32 bits. Total round trip delay time.
  uint32_t rootDispersion; // 32 bits. Max error aloud from primary clock source.
  uint32_t refId;          // 32 bits. Reference clock identifier.

  uint32_t refTm_s;        // 32 bits. Reference time-stamp seconds.
  uint32_t refTm_f;        // 32 bits. Reference time-stamp fraction of a second.

  uint32_t origTm_s;       // 32 bits. Originate time-stamp seconds.
  uint32_t origTm_f;       // 32 bits. Originate time-stamp fraction of a second.

  uint32_t rxTm_s;         // 32 bits. Received time-stamp seconds.
  uint32_t rxTm_f;         // 32 bits. Received time-stamp fraction of a second.

  uint32_t txTm_s;         // 32 bits and the most important field the client cares about. Transmit time-stamp seconds.
  uint32_t txTm_f;         // 32 bits. Transmit time-stamp fraction of a second.

} ntp_packet;              // Total: 384 bits or 48 bytes.

发送查询

// Call up the server using its IP address and port number.

if ( connect( sockfd, ( struct sockaddr * ) &serv_addr, sizeof( serv_addr) ) < 0 )
  error( "ERROR connecting" );

// Send it the NTP packet it wants. If n == -1, it failed.

n = write( sockfd, ( char* ) &packet, sizeof( ntp_packet ) );

if ( n < 0 )
  error( "ERROR writing to socket" );

读取响应

// Wait and receive the packet back from the server. If n == -1, it failed.

n = read( sockfd, ( char* ) &packet, sizeof( ntp_packet ) );

if ( n < 0 )
  error( "ERROR reading from socket" );

分析它

// These two fields contain the time-stamp seconds as the packet left the NTP server.
// The number of seconds correspond to the seconds passed since 1900.
// ntohl() converts the bit/byte order from the network's to host's "endianness".

packet.txTm_s = ntohl( packet.txTm_s ); // Time-stamp seconds.
packet.txTm_f = ntohl( packet.txTm_f ); // Time-stamp fraction of a second.

// Extract the 32 bits that represent the time-stamp seconds (since NTP epoch) from when the packet left the server.
// Subtract 70 years worth of seconds from the seconds since 1900.
// This leaves the seconds since the UNIX epoch of 1970.
// (1900)------------------(1970)**************************************(Time Packet Left the Server)

time_t txTm = ( time_t ) ( packet.txTm_s - NTP_TIMESTAMP_DELTA );