Why do for(;;) loops behave like infinite loops?

Question

The answers to a recent question about for(;;){} loops (What does a for (;;) loop do) did not seem to answer something for me, so I thought that I would try to refine the question a bit. In particular, beyond knowing that for loops without conditionals are infinite loops, I would like to know why they are infinite loops.

In the statement for (;_;){}, the _ is a conditional expression. My first guess would be that an empty expression might evaluate to 0 or NULL. But if you test:

for (;;){}

is an infinite loop, as everyone has pointed out.

for (;1;){}

is an infinite loop.

But neither of these loop bodies execute at all:

for (;0;){}
for (;NULL;){}

Thus, the empty conditional expression does not seem to evaluate to either 0 or NULL.

So, my question: is the behavior of the for (;;){} loop an artifact of the way that C evaluates expressions, or is it just a special implementation-defined case, because a loop body that never executes is not very useful?

UPDATE: After reading the comments and answers, I realize that my question wasn't as clearly formulated as it might have been. I suppose that the question was two-fold:

1. Is the behavior of for(;;){} loops strictly a result of the way that C evaluates expressions in general, or is this behavior specific to the way that C evaluates for statements?

2. Why was this behavior chosen for for loops lacking conditional expressions?

Answer 1

Both C and C++ guarantee this behaviour.

---

[C99: 6.8.5.3/1]: Both clause-1 and expression-3 can be omitted. An omitted expression-2 is replaced by a nonzero constant.

---

[C++14: 6.5.3/1]: The for statement

for ( for-init-statement conditionopt; expressionopt) statement

is equivalent to

{
   for-init-statement
   while ( condition ) {
      statement
      expression ;
   }
}

[..]

[C++14: 6.5.3/2]: Either or both of the condition and the expression can be omitted. A missing condition makes the implied while clause equivalent to while(true).