What does sprintf do? (was: FPS Calculation in OpenGL) - c

Question

For FPS calculation, I use some code I found on the web and it's working well. However, I don't really understand it. Here's the function I use:
void computeFPS()
{
numberOfFramesSinceLastComputation++;
currentTime = glutGet(GLUT_ELAPSED_TIME);
if(currentTime - timeSinceLastFPSComputation > 1000)
{
char fps[256];
sprintf(fps, "FPS: %.2f", numberOfFramesSinceLastFPSComputation * 1000.0 / (currentTime . timeSinceLastFPSComputation));
glutSetWindowTitle(fps);
timeSinceLastFPSComputation = currentTime;
numberOfFramesSinceLastComputation = 0;
}
}
My question is, how is the value that is calculated in the sprint call stored in the fps array, since I don't really assign it.

Answer 1

This is not a question about OpenGL, but the C standard library. Reading the reference documentation of s(n)printf helps:
man s(n)printf: http://linux.die.net/man/3/sprintf
In short snprintf takes a pointer to a user supplied buffer and a format string and fills the buffer according to the format string and the values given in the additional parameters.
Here's my suggestion: If you have to ask about things like that, don't tackle OpenGL yet. You need to be fluent in the use of pointers and buffers when it comes to supplying buffer object data and shader sources. If you plan on using C for this, get a book on C and thoroughly learn that first. And unlike C++ you can actually learn C to some good degree over the course of a few months.

Answer 2

This function is supposedly called at every redraw of your main loop (for every frame). So what it's doing is increasing a counter of frames and getting the current time this frame is being displayed. And once per second (1000ms), it's checking that counter and reseting it to 0. So when getting the counter value at each second, it's getting its value and displaying it as the title of the window.
/**
* This function has to be called at every frame redraw.
* It will update the window title once per second (or more) with the fps value.
*/
void computeFPS()
{
//increase the number of frames
numberOfFramesSinceLastComputation++;
//get the current time in order to check if it has been one second
currentTime = glutGet(GLUT_ELAPSED_TIME);
//the code in this if will be executed just once per second (1000ms)
if(currentTime - timeSinceLastFPSComputation > 1000)
{
//create a char string with the integer value of numberOfFramesSinceLastComputation and assign it to fps
char fps[256];
sprintf(fps, "FPS: %.2f", numberOfFramesSinceLastFPSComputation * 1000.0 / (currentTime . timeSinceLastFPSComputation));
//use fps to set the window title
glutSetWindowTitle(fps);
//saves the current time in order to know when the next second will occur
timeSinceLastFPSComputation = currentTime;
//resets the number of frames per second.
numberOfFramesSinceLastComputation = 0;
}
}