What is a busy loop?

Question

Can it be a loop with or without statements?

while (1)
{
   //Empty
}

OR

int i = 0;

while (1)
{
   i++;
}

Answer 1

I'm pretty sure you're talking about busy wait where you have a loop polling for some condition every iteration.

Answer 2

To me, a busy loop is a loop that never blocks.

Blocking is a behavior provided by the operating system that allows a thread to consume NO cpu cycles until some condition is met (a condition variable is signaled, or perhaps just data arriving on a socket (as recv() will block)).

In a traditional Win32 main loop, you potentially block every time your thread calls GetMessage(). All event driven windowing system are similar in this way.

Answer 3

A "busy loop" or more commonly "busy wait" is an active polling where the application is waiting on some event to occur and continuously checks for it. Typically this includes a timed sleep or other task which gives up CPU time so that another process can provide the expected input.

Contrast this with a callback. While waiting on a callback, the program consumes no CPU cycles. Typically the program will "register" a callback routine which some monitoring application invokes based on some event.

The distinction is that the program in a busy-loop consumes CPU and time slices while waiting while the callback mechanism allows a program to consume no (or almost no) CPU while waiting.