Two semicolons inside a for-loop parentheses

Each clause of a for loop is optional. So when they are excluded, it still loops. for loops compile into while loops.

The end result becomes a check to initialize any variables, which concludes after nothing happening since it is empty, a check to the boolean condition in the second clause, which is not present so the loop starts, and once the loop hits the end bracket, a check to see if there is any code to run before checking the boolean condition again.

In code it looks like:

while(true){

}

The for loop has 3 components, separated by semi-colons. The first component runs before the looping starts and is commonly used to initialize a variable. The second is a condition. The condition is checked at the beginning of each iteration, and if it evaluates to true, then the code in the loop runs. The third components is executed at the end of the loop, before another iteration (starting with condition check) begins, and is often used to increment a variable.

In your case for(;;) means that it will loop forever since the condition is not present. The loop ends when the code returns or breaks.

for(;;) {
}

functionally means

 while (true) {
 }

It will probably break the loop/ return from loop based on some condition inside the loop body.

The reason that for(;;) loops forever is because for has three parts, each of which is optional. The first part initializes the loop; the second decides whether or not to continue the loop, and the third does something at the end of each iteration. It is full form, you would typically see something like this:

for(i = 0; i < 10; i++)

If the first (initialization) or last (end-of-iteration) parts are missing, nothing is done in their place. If the middle (test) part is missing, then it acts as though true were there in its place. So for(;;) is the same as for(;true;)', which (as shown above) is the same as while (true).