How to do it...

We will create an application that reads the current time from the Real-Time Clock (RTC) connected to the Raspberry Pi board:

1. In your ~/test working directory, create a subdirectory called rtc.
2. Use your favorite text editor to create a rtc.cpp file in the rtc subdirectory.
3. Let's put the required include functions into the rtc.cpp file:

#include <iostream>
#include <system_error>

#include <time.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/ioctl.h>
#include <linux/rtc.h>

4. Now, we define a class called Rtc that encapsulates the communication to the real-clock device:

class Rtc {
  int fd;
  public:
    Rtc() {
      fd = open("/dev/rtc", O_RDWR);
      if (fd < 0) {
        throw std::system_error(errno,
            std::system_category(),
            "Failed to open RTC device");
      }
    }

    ~Rtc() {
      close(fd);
    }

    time_t GetTime(void) {
      union {
        struct rtc_time rtc;
        struct tm tm;
      } tm;
      int ret = ioctl(fd, RTC_RD_TIME, &tm.rtc);
      if (ret < 0) {
        throw std::system_error(errno,
            std::system_category(),
            "ioctl failed");
      }
      return mktime(&tm.tm);
    }
};

5. Once the class is defined, we put a simple usage example into the main function: 

int main (void)
{
  Rtc rtc;
  time_t t = rtc.GetTime();
  std::cout << "Current time is " << ctime(&t)
            << std::endl;

  return 0 ;
}

6. Create a CMakeLists.txt file containing the build rules for our program:

cmake_minimum_required(VERSION 3.5.1)
project(rtc)
add_executable(rtc rtc.cpp)

set(CMAKE_SYSTEM_NAME Linux)
set(CMAKE_SYSTEM_PROCESSOR arm)

set(CMAKE_CXX_COMPILER /usr/bin/arm-linux-gnueabi-g++)

7. Build your application and copy the resulting rtc binary to our Raspberry Pi emulator.