example C++17 gpsd program using libgpsmm

gpsd-example.cpp

//  Example C++17 gpsd program using libgpsmm

//  Has no actual purpose other than output some of libgpsmm struct variables of your choosing.
//  reference declarations: https://fossies.org/dox/gpsd-3.22/index.html
//
//  compiles without warning as follows:
//  g++ -std=c++17 -Wall -pedantic -pthread $(pkg-config --cflags --libs libgps) gpsd-example.cpp -o gpsd-example
//  clang++ -std=c++17 -Wall -pedantic -pthread $(pkg-config --cflags --libs libgps) gpsd-example.cpp -o gpsd-example
//  Free for the taking.
//  Version 1.84

#include <libgpsmm.h>

#include <ctime>
#include <iomanip>
#include <iostream>
#include <sstream>
#include <thread>

enum TimeFormat { LOCALTIME, UTC, UNIX, ISO_8601 };

auto TimespecToTimeStr(const timespec& gpsd_time, TimeFormat time_format = LOCALTIME) {
  // example showing different ways of formating GPSD's timespec, depending on requirement
  std::ostringstream oss;
  switch (time_format) {
    case LOCALTIME: {
      // convert timespec_t into localtime (dd-mm-YY HH:MM:SS)
      const auto tm = *std::localtime(&gpsd_time.tv_sec);
      oss << std::put_time(&tm, "%d-%m-%Y %H:%M:%S");
      break;
    }
    case UTC: {
      // convert timespec_t into UTC (dd-mm-YY HH:MM:SS)
      const auto tm = *std::gmtime(&gpsd_time.tv_sec);
      oss << std::put_time(&tm, "%d-%m-%Y %H:%M:%S");
      break;
    }
    case UNIX:
      // returns seconds since the Epoch
      oss << gpsd_time.tv_sec;
      break;
    case ISO_8601: {
      // convert timespec_t into ISO8601 UTC time (yyyy-MM-dd'T'HH:mm:ss'Z')
      constexpr size_t kScrSize{128};
      std::array<char, kScrSize> scr{};
      timespec_to_iso8601(gpsd_time, scr.data(), kScrSize);
      oss << scr.data();
      break;
    }
  }
  return oss.str();
}

auto main() -> int {
  gpsmm gps_rec("localhost", DEFAULT_GPSD_PORT);

  if (gps_rec.stream(WATCH_ENABLE | WATCH_JSON) == nullptr) {
    std::cerr << "No GPSD running.\n";
    return 1;
  }

  constexpr auto kWaitingTime{1000000};  // 1000000 is 1 second

  for (;;) {
    if (!gps_rec.waiting(kWaitingTime)) {
      continue;
    }

    struct gps_data_t* gpsd_data;

    if ((gps_rec.read()) == nullptr) {
      std::cerr << "GPSD read error.\n";
      return 1;
    }

    while (((gpsd_data = gps_rec.read()) == nullptr) || (gpsd_data->fix.mode < MODE_2D)) {
      // Do nothing until fix, block execution for 1 second (busy wait mitigation)
      std::this_thread::sleep_for(std::chrono::seconds(1));
    }

    const auto latitude{gpsd_data->fix.latitude};
    const auto longitude{gpsd_data->fix.longitude};
    const auto hdop{gpsd_data->dop.hdop};
    const auto vdop{gpsd_data->dop.vdop};
    const auto pdop{gpsd_data->dop.pdop};
    const auto s_vis{gpsd_data->satellites_visible};
    const auto s_used{gpsd_data->satellites_used};
    const auto time_str{TimespecToTimeStr(gpsd_data->fix.time, ISO_8601)};  // you can change the 2nd argument to LOCALTIME, UTC, UNIX or ISO8601

    std::cout << std::setprecision(8) << std::fixed;  // set output to fixed floating point, 8 decimal precision
    std::cout << time_str << "," << latitude << "," << longitude << "," << hdop << "," << vdop << "," << pdop << "," << s_vis << "," << s_used << '\n';
  }
}

@garyv100, I've had a USB unit (u-blox driver) running for about 40 minutes and a raw UART (nmea driver) running for about 20 minutes and both aren't showing the drift (other than the normal cold boot drift which is quickly corrected as satellites are received). I suggest you post an issue here: https://gitlab.com/gpsd/gpsd/issues and the main gpsd dev (also named Gary) can maybe help if it's a driver issue with your hardware or within the gpsd code itself. I suggest you link this gist so he can get some context as well. Wish I could be of more help.

edit: are you seeing the same behavior with gpsd's cgps tool?

@ncoder-1, I am using VK-162 GPS Receiver Dongle Navigation Module USB Interface G-mouse Antenna DEN. I think this is the problem. Not the Linux drivers or software. Can you recommend a different receiver for Linux?
Thank you for your help.
Gary

@garyv100, I use a GR-8013 USB GPS (one of the rare USB ones that also have PPS) and also the ND-100S which both work well with gpsd.

@ncoder-1, I tried to lookup GR-8013 USB GPS, but I am coming up w/ all kind of stuff. If you could send me the link - it would be great.
Thanks,
Gary

@garyv100, they are quite rare now, but this is the one I have: https://www.globalsources.com/gsol/I/PC-PDA/p/sm/1147069913.htm#1147069913

@ncoder-1, Thank you!