Giving instruction to an executing C code

I've tried to think of a solution, but I can't do it without some help. Lets say I have a c code that is executing and splitting out data using printf() statements constantly. Is there a way I can change a variable within the program on-the-fly? Or at least instruct it to fetch another constant from a file that I modify? Thanks!

for example, in the following code, I want to change the variable "variable_of_interest" while it is executing.

Code:

float variable_of_interest
main(){
    extern float variable_of_interest;
    while(1){
        printf( "variable = %f\n", variable_of_interest);
    }
}

Depending on the OS, yes. In Linux, for example, you could have a signal handler.

ah. Is it like catching Ctrl+C? I've done that before.