Simple C++ example to send serial AT commands to and receive data from a modem

I tried using many C/C++ libraries trying to coax a 5G modem to respond to my input AT commands for a long time but I was not successful. The command I was trying to send was the Quectel modem command to query for PDN channels:

AT+CGDCONT?

After a while, I figured out I had to simulate a keyboard Enter press in code to actually tell the modem the command is complete. So all I had to do was append the carriage return (\r) and new line (\n) characters to the AT command string, e.g:

string cmd = "AT+CGDCONT?\r\n";

A working C++ code example is shown below:

#include <string>
#include <iostream>
#include <cstdio>
#include <unistd.h>

// Using header only library from 
// https://github.com/karthickai/serial
#include "Serial.h"


using namespace std;


int main ( int argc, char** argv) {

        string commPort = "/dev/ttyUSB2";
        unsigned int baud = 115200;
        serial::Serial serial;

        serial.open ( commPort, baud);

        if ( !serial.isOpen()) {
                cout << "comm port is not open" << endl;
                return 1;
        }

        // A modem AT query command
        string cmd = "AT+CGDCONT?\r\n";
        vector<uint8_t> cmdVec (cmd.begin(), cmd.end());

        // send command to the modem
        size_t bytesSent = serial.transmitAsync(cmdVec);
        cout << "Bytes sent " << bytesSent << endl;

        int received_bytes = -1;

        while (received_bytes != 0 ) {
                // read one byte from the modem and timeout if the
                // there is no response in more than 1 sec.
                future<vector<uint8_t>> future = serial.receiveAsync(1, 1000);
                vector<uint8_t> const received_data = future.get();
                received_bytes = received_data.size();

                string str(received_data.begin(), received_data.end());
                cout << "[" << received_data[0] << "] " << endl;
        }
        // Close the serial port
        serial.close();

        cout << "End of process" << endl;

        return 0;

}

The example prints out the data sent by the modem to the calling program, as shown below:

Note: this example is using the modern serial header only C++ library from this site: https://github.com/karthickai/serial