How to display current music position using FMOD and C++?

I want to display the time that has elapsed as music plays in real-time. FMOD's Core API provides Channel::getPosition() function to obtain the current position in milliseconds. I want to update the position every second.

I am a beginner and have no knowledge of multithreaded programming.

I call Channel::getPosition() in a loop and use std::this_thread::sleep_for() to delay the loop for one second before the next iteration.

Here is the code:

unsigned int position = 0;
std::chrono::milliseconds timespan(1000);
while(true) {
    channel -> getPosition(&position, FMOD_TIMEUNIT_MS);
    std::cout << postion / 1000 << "\n"; //Display seconds
    std::this_thread::sleep_for(timespan);
}

However, I get some buggy output :

0
1
...
13
13
14
16
...

13 appears twice and 15 does not even appear. In another case, 5 appears twice.

I am thinking of rounding up or rounding down the number I obtain from Channel::getPosition() to correct the output.

How can I fix this?

Note: Error checking is omitted for simplicity

Solution

Use <chrono> even for trivial timing functions.
Use the C++17 round function for truncating milliseconds to seconds for this example. If you don't have C++17, steal round from here.
Use sleep_until instead of sleep_for in order to keep a more accurate "timespan" for each iteration of the loop.

Putting that all together:

#include <chrono>
#include <iostream>
#include <memory>
#include <thread>

enum unit{FMOD_TIMEUNIT_MS};

struct Channel
{
    void getPosition(unsigned int* position, unit)
    {
        using namespace std::chrono;
        static auto start = steady_clock::now();
        *position = duration_cast<milliseconds>(steady_clock::now()-start).count();
    } 
};

int
main()
{
    using namespace std::chrono;
    auto channel = std::make_unique<Channel>();
    auto constexpr timespan = 1s;
    auto next_start = system_clock::now() + timespan;
    while (true)
    {
        unsigned int position_as_integral;
        channel->getPosition(&position_as_integral, FMOD_TIMEUNIT_MS);
        milliseconds position{position_as_integral};
        std::cout << round<seconds>(position).count() << '\n';
        std::this_thread::sleep_until(next_start);
        next_start += timespan;
    }
}