C++ outputs different message than Python (Reading from serial port)

Question

Summary

I have two programs; one written in Python3, another in C++. Both perform the same task; read from a serial port, filter out the two header floats, and print the remaining message. The Python script works properly (see the output below for the proper numbers); the C++ one, using a serial library, does not and I can't figure out why. (Running on Raspberry Pi 4, Raspbian Buster).

In order to save anyone from reading this whole thing; if my decision to use this library for serial reading is bad, how can I properly go about reading from a serial port in C++?

What I've tried

I'm very new to C++, so perhaps I'm looking in all the wrong places, but I couldn't find a universally accepted library for reading from a serial port, so I picked the one with the most stars on github (serial). This answer gives a sample for windows and links to a couple libraries, however they either are intended to work with windows, or are in C, not C++. This uses another library. This is in C (my code will be compiled alongside a Simulink-based C++ class, so I think I need to stick with C++ (?))

My Code

Python

Here is the fully functional Python code:

import serial
import struct
import time

PORT_NUMBER = '/dev/ttyACM0'
BAUDRATE = 115200
MESSAGE_LENGTH = 8 
HEADER_NUMBER = float(112)

header_1_received = False       # Has the first header byte been received
header_2_received = False       # Has the second header byte been received
dat = []                        # The actual message

# Open serial port
ser = serial.Serial(
        port=PORT_NUMBER,
        baudrate=BAUDRATE,
        parity=serial.PARITY_NONE,
        stopbits=serial.STOPBITS_ONE,
        timeout=0.001)

ser.isOpen()
readings = 0

print('Receive data from Pixhawk using 2 header floats and {} message floats (32-bit) \nIf you wish to close the program, hit \"Ctrl+C\" on your keyboard and it (should) shut down gracefully.'.format(MESSAGE_LENGTH))
start_time = time.time()        # Start time for the program

try:
    # Main loop
    while (True):
        # Read 4 bytes (32-bits) to get a full float number
        buffer = ser.read(4)
        # Only proceed if the buffer is not empty (an empty buffer is b'')
        if buffer != b'':
            # Since struct.unpack() returns a tuple, we only grab the first element
            try:
                new_dat = struct.unpack("f",buffer)[0]
                if header_1_received==True and header_2_received==True:
                    dat.append(new_dat)
                elif new_dat == HEADER_NUMBER:
                    # We found a header single; treat it
                    if header_1_received == False:
                        header_1_received = True
                    elif header_2_received == False:
                        header_2_received = True
                    else:
                        # Since below, we reset headers once the full message is received, kind of pointless else
                        pass
                else:
                    # If a non-header character is received, but we haven't identified the headers yet, then we're starting in the middle of a message or have lost the rest of our previous message
                    dat = []
                    header_1_received = False
                    header_2_received = False
            except:
                # struct.unpack likely failed; throw away the message and start again
                header_1_received = False
                header_2_received = False
                dat = []
            if(len(dat) == MESSAGE_LENGTH):
                # Reset flags
                #print(dat)
                header_1_received = False
                header_2_received = False
                dat = []
                readings += 1
except KeyboardInterrupt:
    ser.close()
    elapsed_time = time.time() - start_time
    if readings > 0:
        print("Number of readings: {}\nRun time: {}s\nAverage time per reading: {}s ({}ms)".format(readings,elapsed_time,elapsed_time/readings,(elapsed_time/readings)*1000))

C++

Here is the dysfunctional C++ code:

#include <string>
#include <iostream>
#include <cstdio>
#include <unistd.h>
#include "serial/serial.h"

using std::string;
using std::exception;
using std::cout;
using std::cerr;
using std::endl;
using std::vector;

int run(int argc, char **argv)
{
  // Argument 1 is the serial port or enumerate flag
  string port(argv[1]);

  // Argument 2 is the baudrate
  unsigned long baud = 0;
  sscanf(argv[2], "%lu", &baud);

  // port, baudrate, timeout in milliseconds
  serial::Serial my_serial(port, baud, serial::Timeout::simpleTimeout(0.001));

  cout << "Is the serial port open?";
  if(my_serial.isOpen())
    cout << " Yes." << endl;
  else
    cout << " No." << endl;

  /* MY CUSTOM VARIABLES */
  const float header = 112;
  const int msg_size = 8;
  int msg_index = 0;                
  float f;                  // the read float
  float msg [msg_size] = { };           // the collected floats will be placed here, auto-filled with 0s
  bool header_1_received = false;
  bool header_2_received = false;
  uint8_t *buffer = new uint8_t[sizeof(f)]; // buffer that will be converted to 32-bit float
  int count = 0;

  while (count < 1000) {

    size_t number_of_bytes_read = my_serial.read(buffer, sizeof(f));
    memcpy(&f, buffer, sizeof(f));

    // Logic for adding new element to array
    if (header_1_received and header_2_received){
      msg[msg_index] = f;
      msg_index += 1;
    } else if (f == header) {
      if (header_1_received == false){
        header_1_received = true;
      } else if (header_2_received == false){
        header_2_received = true;
      } else {
        // Do nothing
      }
    } else {
      // A non-header character was received, but headers are also not identified;
      // Throw partial message away and restart
      std::fill_n(msg, msg_size, 0);       // Fill with zeroes
      msg_index = 0;
      header_1_received = false;
      header_2_received = false;
    }

    // Check to see if message is full
    if(msg_index == msg_size){
      cout << "Msg: [";
      for (int i = 0; i < msg_size; i += 1){
        cout << msg[i] << ",";
      }
      cout << "]" << endl;
      // Reset flags
      header_1_received = false;
      header_2_received = false;
      std::fill_n(msg, msg_size, 0);
      msg_index = 0;
    }
    count += 1;
  }
  return 0;
}

int main(int argc, char **argv) {
  try {
    return run(argc, argv);
  } catch (exception &e) {
    cerr << "Unhandled Exception: " << e.what() << endl;
  }
}

The C++ library can be found here, and the documentation on the read method here. As far as I understand it, his read method writes the requested number of bytes (if available) to the buffer; there is a constant stream of incoming bytes from the other device, so I don't see this being the issue.

Expected Result

The Python3 script functions properly and outputs the following:

[0.08539174497127533, 0.17273111641407013, -9.816835403442383, 0.0, 0.0, 0.0, 0.0, 0.0]
[0.08539174497127533, 0.17273111641407013, -9.816835403442383, 0.0, 0.0, 0.0, 0.0, 0.0]
[0.08539174497127533, 0.17273111641407013, -9.816835403442383, 0.0, 0.0, 0.0, 0.0, 0.0]
[0.08539174497127533, 0.17273111641407013, -9.816835403442383, 0.0, 0.0, 0.0, 0.0, 0.0]

(it should be two very smaller numbers, followed by approximately -9.81, then 5 zeroes).

Actual Result

The C++ program can be built and executed by running the following command:

g++ serial_example.cc -lserial -L ../build/devel/lib -I../include -o test_serial
LD_LIBRARY_PATH=`pwd`/../build/devel/lib ./test_serial

and outputs the following:

[112,112,112,112,112,112,112,112,]
[112,-9.82691,-9.82691,-9.82691,-9.82691,-9.82691,-9.82691,0,]
[112,112,112,112,-9.82691,-9.82691,-9.82691,-9.82691,]
[112,112,112,112,112,112,112,112,]

If I add the following line

cout << "Float extracted from buffer: " << f << endl;

then it outputs every float it reconstructs from the read operation, resulting in a mish-mash of 9.81, 112 and 0s.

Question

What has gone wrong in my C++ program that causes it to read bytes/float differently than the Python program, and if the library is at fault, what is an alternative method or library for reading serial messages in C++?

Edit

After some troubleshooting with @Barmar and @Gaspa79, it seems that the number of bytes read by the library's read() method is inconsistent. I'll try re-writing my program and leave the new version as an answer.

Answer 1

After verifying that the conversions were indeed correct, we realized that OP never actually checked the number_of_bytes_read variable, and the underlying library was reading different numbers of bytes for some reason.