How to check in C++ if the system is active?

Question

I\'m writing code that need to run only when there is no human activity on the PC, like when the screensaver is running. Any suggestions on how to do this in c++ under windows?<

Answer 1

This is just a minor update to Niklas B. answer which is tested on Windows 8.1 and Windows 10.

Description of changes is as follows:

• LASTINPUTINFO has cbSize initialized

• instead of checking SPI_GETSCREENSAVERACTIVE, SPI_GETSCREENSAVERRUNNING is checked

• idle_time is calculated and compared with idle_milliseconds

• user is notified only on change, check is executed every ~500ms

• since windows can trigger event id 4803 immediately after 4802 one can determine that screensaver is still on with user idle_time, see the problem I had

Additionally this only works if screensaver timer is smaller then screen power off timer!

#define start

#include 
#include 

// do something after 10 minutes of user inactivity
static const unsigned int idle_milliseconds = 60*10*1000;

int main() {
    LASTINPUTINFO last_input;
    // without setting cbSize GetLastError() returns the parameter is incorrect
    last_input.cbSize = sizeof(last_input);  
    BOOL screensaver_running;

    DWORD idle_time;
    bool screensaverOn = false;

    // main loop to check if user has been idle long enough
    for (;;) {
        if ( !GetLastInputInfo( &last_input )
          || !SystemParametersInfo( SPI_GETSCREENSAVERRUNNING, 0,  
                               &screensaver_running, 0 ) )
        {
            std::cerr << "WinAPI failed!" << std::endl;
            return ERROR_FAILURE;
        }

        // calculate idle time
        idle_time = GetTickCount() - last_input.dwTime;

        if ( idle_time > this->user_idle_time && TRUE == screensaver_running )
        {
            if ( !screensaverOn )
            {
                // screensaver is running
                screensaverOn = true;
                notify( true );
            }
        }
        else if ( idle_time < this->user_idle_time && screensaverOn )
        {
            // screensaver is not running
            screensaverOn = false;
            notify( false );
        }
        // wait 500ms before next loop
        Sleep( 500 );
    }
}