Function kbhit in C is used to determine if a key has been pressed or not. To use it in a program you should include the header file "conio.h". If a key has been pressed, then it returns a non zero value otherwise it returns zero.
Declaration: int kbhit();
C programming code for kbhit
printf("You haven't pressed a key.\n");
As long as in the program a user doesn't press a key kbhit() return zero and (!0), i.e., 1, the condition in while loop is true and "You haven't pressed a key" will be printed again and again. When a key is pressed from the keyboard the condition in while loop becomes false, now kbhit() will return a non-zero value and ( !(non-zero) = 0), so the control will come out of the while loop.