Question

I am writing a program to control a flashbulb. The flash fires in response to a key press by the user. I am trying to limit the occurence regularity of the flash to prevent the bulb burning out. I have already received some help from this forum, but am unable to implement the code with my own. A user suggested using a class, as follows:

class bulb
{
    __int64 clocks;
    __int64 frequency;
    public:
    bulb()
    {
        LARGE_INTEGER li;
        QueryPerformanceFrequency(&li);
        frequency = li.QuadPart;
        clocks = 0;
    }
    void WINAPI flash (HINSTANCE hThisInstance,
           HINSTANCE hPrevInstance,
           LPSTR lpszArgument,
           int nFunsterStil)
    {
        LARGE_INTEGER li;
        QueryPerformanceCounter(&li);

        // If this is the first occurence, set the 'clocks' to system time (+10000 to allow flash to occur)
        if (clocks == 0) clocks = li.QuadPart + 10000;

        __int64 timepassed = clocks - li.QuadPart;
        if (timepassed >= (((double)frequency) / 10000))
        {
            //Set the clock
            clocks = li.QuadPart;
            //Define the serial port procedure
            HANDLE hSerial;
            //Open the serial port (fire the flash)
            hSerial = CreateFile("COM1", GENERIC_WRITE, 0, 0, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, 0);
            //Close the serial port
            CloseHandle(hSerial);
        }
    }
};

I receive a few syntax errors that I can’t seem to shift, all of which are at either the first or last bracket of the class – “syntax error : identifier ‘bulb'”, “syntax error : ‘;'”, “syntax error : ‘}'” and “syntax error : ‘}'”. I have never worked with classes before though, so expect this is something to do with that. Where am I going wrong?

Please note ‘10000’ is the minimum delay between flashes.

Answer 1

There are a few major issues with your code:

• Missing a ‘;’ at the end of the class definition.
• Missing a definition HANDLE hSerial before it is used.

• You are comparing the time and frequency incorrectly on the line if (timepassed >= (((double)frequency) / 10000)). If you wish to convert the counter from QueryPerformanceCounter into a real time use something like:

  double RealTime = (double) clocks / (double) frequency;

If you are getting other error messages they are related to the code before or after the snippet you posted. A few more minor issues and comments:

• Both QueryPerformanceFrequency and QueryPerformanceCounter can fail. Unless getting invalid values doesn’t matter you should be checking the return values from these.
• You open a COM port but don’t write anything to it or confirm that the open succeeded or not.
• To avoid future issues the if statement if (clocks == 0) should all be on one line or include brackets, i.e., one of:

:

if (clocks == 0) clocks = li.QuadPart + 10000;

if (clocks == 0) {
    clocks = li.QuadPart + 10000;
}

Edit: Example for converting QueryPerformanceCounter into real times (error checking not included):

LARGE_INTEGER Frequency;
LARGE_INTEGER Counter;

QueryPerformanceFrequency(&Frequency);
QueryPerformanceCounter(&Counter);

   //Time in seconds
double RealTime = (double) Counter.QuadPart / (double)Frequency.QuadPart;

LARGE_INTEGER Counter1;
QueryPerformanceCounter(&Counter1);

   //Elapsed time in seconds
double DeltaTime = (double) (Counter1.QuadPart - Counter.QuadPart) / (double)Frequency.QuadPart;

See Also: How to use QueryPerformanceCounter?