A loop that stops at five
`for (size_t i = 0; i < 5; ++i) { printf("%zu\n", i); }` prints 0 through 4. The loop condition `i < 5` is checked before each execution of the body. When it is false, the loop stops; `++i` changes the value tested on the next check. section 2.4.1
A stopping criterion is a condition used to decide when repeated work ends. In C, the expression making that decision is a controlling expression. Its value determines a control path: numerical zero is false, and any nonzero value is true. Read the surrounding construct carefully—a true `for` condition continues iteration, whereas a true `if` condition executes its dependent statement once. section 3.0 section 3.1 section 2.4.1
8 size_t gcd2(size_t a, size_t b) {
9 assert(a <= b);
10 if (!a) return b;
11 size_t rem = b % a;
12 return gcd2(rem, a);
13 }
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In `gcd2`, `if (!a) return b;` checks for termination before the recursive call. Otherwise, the function computes a remainder and calls itself again. The precondition check with `assert` serves a different purpose: it checks whether the arguments are valid, not whether recursion should stop. section 7.3
TAKEAWAY: Check a recursive function’s termination condition first. Without that check, repeated calls can exhaust system resources. In the `for` example, the changing counter eventually makes the continuation condition false; in `gcd2`, reaching zero makes the return branch run instead of another recursive call. section 7.3 section 2.4.1
A wait loop adds another distinction. A `cnd_t` condition variable identifies what a thread waits for; it is not the condition expression that decides whether waiting should continue. Because a wait can return without that expression changing, check the expression again in a loop. A `finished` test can also let that loop end when the task is over. section 18.5
