Issue when scheduling tasks using clock() function

If I was to implement some timer mechanism in C++, I would probably be using the std::chrono namespace together with std::priority_queue.

#include <functional>
#include <queue>
#include <chrono>
#include <sys/time.h>  // for `time_t` and `struct timeval`

namespace events
{
    struct event
    {
        typedef std::function<void()> callback_type;
        typedef std::chrono::time_point<std::chrono::system_clock> time_type;

        event(const callback_type &cb, const time_type &when)
            : callback_(cb), when_(when)
            { }

        void operator()() const
            { callback_(); }

        callback_type callback_;
        time_type     when_;
    };

    struct event_less : public std::less<event>
    {
            bool operator()(const event &e1, const event &e2) const
                {
                    return (e2.when_ < e1.when_);
                }
    };

    std::priority_queue<event, std::vector<event>, event_less> event_queue;

    void add(const event::callback_type &cb, const time_t &when)
    {
        auto real_when = std::chrono::system_clock::from_time_t(when);

        event_queue.emplace(cb, real_when);
    }

    void add(const event::callback_type &cb, const timeval &when)
    {
        auto real_when = std::chrono::system_clock::from_time_t(when.tv_sec) +
                         std::chrono::microseconds(when.tv_usec);

        event_queue.emplace(cb, real_when);
    }

    void add(const event::callback_type &cb,
             const std::chrono::time_point<std::chrono::system_clock> &when)
    {
        event_queue.emplace(cb, when);
    }

    void timer()
    {
        event::time_type now = std::chrono::system_clock::now();

        while (!event_queue.empty() &&
               (event_queue.top().when_ < now))
        {
            event_queue.top()();
            event_queue.pop();
        }
    }
}

To use, simply add events using events::add, and call events::timer a few times every second.

Example:

void foo()
{
    std::cout << "hello from foo\n";
}

void done()
{
    std::cout << "Done!\n";
}

struct bar
{
    void hello()
        { std::cout << "Hello from bar::hello\n"; }
};

auto now = std::chrono::system_clock::now();
bar b;

events::add(foo, now + std::chrono::seconds(2));

events::add(std::bind(&bar::hello, b), now + std::chrono::seconds(4));

events::add(done, now + std::chrono::seconds(6));

while (true)
{
    usleep(10000);
    events::timer();
}

The above example will print:

hello from foo
hello from bar::hello
Done!

One line will be printed every two second. After "Done!" the program will just loop forever, doing nothing.

Note that this program contains lots of C++11 functionality, but has been tested with GCC 4.4.5 and 4.7.1. VC++2010 unfortunately does not have the <chrono> header, but the VC++2012RC apparently have it.

CLOCKS_PER_SEC is integer in your system. in other systems, it could be float too. put (float) near it too

The problem could be because of your floating point comparison. This can provide unexpected results. Please avoid this.

Refer this link