Can I use GetAsyncKeyState() outside of the main() function? - c++

Question

I'm writing, just to get confidence with the programming procedures, a win32 application that responds to keyboard input. To do so I'm using the GetAsyncKeyState() function.
At first I wrote all my code in the main() function, and all seemed well, it worked. So I decided to complicate things, but that requires me to use the GetAsyncKeyState() function in a different function called by the main(). I thought I just needed to declare some variables outside the main() and move the code from the main to the new function like this:
int btnup_down = 0;
int close = 1;
int main(void){
while (1){
Sleep(50);
listentokb();
if (close == 0){
break;
}
}return 0;
}
int listentokb(void){
if ((GetAsyncKeyState(0x4C) & 0x8000) && (ko == 0)){
ko = 1;
printf("Ok you pressed k");
return 0;
} else if (((GetAsyncKeyState(0x4C) == 0) && (ko == 1)) {
ko = 0;
printf("Now you released it");
close = 0;
return 0;
}return 0;
}
When I run this code, the loop keeps going and it doesn't matter if I press the key or not, it keeps looping without printf-ing anything. Any help will be greatly appreaciated.

Answer 1

Your problem has nothing to to with main() or not. You can call winapi function such as GetAsyncKeyState() from wherever you want in your code, as long as you provide the good arrguments.
According to this list of virtual key codes the code 0x4c corresponds to the key L and not to the key K. So after a bracket correction typo in your code, I could run it succesfully interupting the loop with L
Some remarks about your function:
Your function listentokb() always returns 0. On the other hand, you use a global variable close to tell the calling function the result of your keyboard scanning. This is a very bad practice: whenever possible avoid global variables.
Here a slightly updated version of your code that bans global variables, and use return value to communicate the results:
const int KEY_K = 0x4B; // avoid using values directly in the code
int listentokb (void){ // returns 'K' if K is released and 0 otherwise
static int ko; // this is like a global variable: it will keep the value from one call to the other
// but it has teh advantage of being seen only by your function
if((GetAsyncKeyState(KEY_K) & 0x8000) && (ko == 0)){
ko = 1;
printf("Ok you pressed k");
return 0;
}
else if((GetAsyncKeyState(KEY_K) == 0) && (ko == 1)) {
ko = 0;
printf("Now you released it");
return 'K';
}
return 0;
}
int main(void){
bool go_on = true; // The state of the loop shall be local variable not global
while(go_on){
Sleep(50);
go_on= ! listentokb(); // if returns 0 we go on
}
return 0;
}