2026-02-22: Intro: OS And C++ (Live Demo)

Note

This session was entirely live demonstrated by teacher. No need to repeat all the steps.

• Login to Pi at home: ssh -p 2022 jfasch.bounceme.net

$ ls -l /sys/class/hwmon/
$ ls -l /sys/class/hwmon/hwmon2/

• Show how to use that sensor from the commandline (Linux and OneWire (using DS18B20 Temperature Sensor as Slave))

• Write a C program that reads the sensor value in a loop (a “data logger”, so to say). (System Calls)

• Transform the sensor into a dedicated type/class (Data Encapsulation)

code/intro.cpp

#include <iostream>
#include <fcntl.h>
#include <string.h>
#include <unistd.h>
#include <time.h>
#include <sys/types.h>

#include <string>
#include <stdexcept>


class Thermometer
{
public:
    Thermometer(const std::string& filename)
    {
        _fd = open(filename.c_str(), O_RDONLY);
        if (_fd == -1)
            throw std::runtime_error("nix open");
    }

    ~Thermometer()
    {
        close(_fd);
    }

    double get_temperature() const
    {
        char buffer[64];
        ssize_t nbytes = read(_fd, buffer, sizeof(buffer)-1);
        if (nbytes == -1)
            throw std::runtime_error("nix read");

        off_t newpos = lseek(_fd, 0, SEEK_SET);
        if (newpos == -1) {
            throw std::runtime_error("nix lseek64");
        }

        buffer[nbytes] = '\0';
        int milli_celsius = std::stoi(buffer);
        double celsius = milli_celsius / 1000.0;
    
        return celsius;
    }

private:
    int _fd;
};


int main(int argc, char** argv)
{
    if (argc != 2) {
        std::cerr << "nix zwei argumente" << std::endl;
        return 1;
    }
    const char* filename = argv[1];
    Thermometer thermo(filename);

    while (true) {
        std::cout << thermo.get_temperature() << std::endl;

        timespec interval = { 1, 0 };
        int rv = nanosleep(&interval, nullptr);
        if (rv == -1) {
            std::cerr << "nix sleep?" << std::endl;
            return 1;
        }

    }
    
    return 0;
}