Is there a way to wait for a file to be written to in pure c++?

Question

I am looking to write some code to monitor a file. When it gets written to I would like to read the new lines and act upon them.

So I found this thread: how-to-read-a-growing-text-file-in-c and it shows me how to do this.

However, its a bit of a "polling" approach. Here is the code snippet for convenience. Note: this is not my work (its the answer from the link):

#include <iostream>
#include <string>
#include <fstream>

int main()
{
    std::ifstream ifs("test.log");

    if (ifs.is_open())
    {
        std::string line;
        while (true)
        {
            while (std::getline(ifs, line)) std::cout << line << "\n";
            if (!ifs.eof()) break; // Ensure end of read was EOF.
            ifs.clear();

            // You may want a sleep in here to avoid
            // being a CPU hog.
        }
    }

    return 0;
}

You can see there is the comment: You may want a sleep in here to avoid being a CPU hog.

Is there a way (and there might not be) to wait for the file to be written to, such that some event/condition triggers our thread to wake up? I am thinking along the lines of select() like function... But I would really like it to be pure c++.

Failing that - is there a non-pure c++ way (for me I require it to work for Linux OS and possibly windows as well)?

I have not written any code yet because I am not even sure where the best place to start is.

Answer 1

you just need to add sleep function that works on both Win and Linux, thus you can use it std::this_thread::sleep_for(std::chrono::milliseconds(500)); in your code. it's from std library so you can use it on Linux or Windows.

#include <chrono>
#include <thread>
#include <iostream>
#include <string>
#include <fstream>

int main()
{
    std::ifstream ifs("test.log");

    if (ifs.is_open())
    {
        std::string line;
        while (true)
        {
            while (std::getline(ifs, line)) std::cout << line << "\n";
            if (!ifs.eof()) break; // Ensure end of read was EOF.
            ifs.clear();

            // You may want a sleep in here to avoid
            // being a CPU hog.
            std::this_thread::sleep_for(std::chrono::milliseconds(10));
        }
    }

    return 0;
}