Submitter: UK C Panel
Submission Date: 2001-09-07
Source: Clive D.W. Feather <clive@demon.net>
Version: 1.4
Date: 2006-03-29
Subject: jumps into iteration statements
Consider the code:
int x = 0;
goto centre;
while (++x < 10)
{
// Some code centre:
// More code }
"Everyone knows" that, when the end of the block is reached,
the loop test is evaluated in the normal way. Nevertheless, I
can find nothing in the Standard that says so (it is implied by
the example in 6.8.6.1#3, but that is all). Note that in:
int x;
// ... if (condition) { x = -1; goto true_case; }
// ... if (x > 0)
true_case:
do_something ();
else
do_something_else ();
the else case is not executed after a jump to
true_case, even though the condition x
> 0 is false. Therefore it is not possible to argue
from analogy; note also that this latter case is spelled out in
the Standard. Since this technique is well-known, it ought to
be well-defined.
Suggested Technical Corrigendum
Add a new paragraph after 6.8.5#4:
Possibly also add an example either as 6.8.5#6 or 6.8.6.1#5 (with appropriate editorial changes):[#4a] If the loop body is reached by a jump from outside the iteration statement, the behavior is as if the body were entered in the normal way. That is, when the end of the body is reached the controlling expression is evaluated (and, in the case of a for statement, expr-3 is evaluated first) and the body re-executed if it is not 0. Similarly, a break or continue statement has the appropriate effect. However, the code jumped over - including the controlling expressions in the case of a while or for statement - is not evaluated when the jump happens.
[#6] EXAMPLE: A jump into a for statement does not execute clause-1 at all or expr-2 during the jump:int i = 5; if (condition) goto body; for (i = 0; i < 10; i++) { if (i > 2) i++; body: printf (" %d", i); } printf ("\n");If condition is true, this prints:5 7 9while if it is false it prints:0 1 2 4 6 8 10
Committee Discussion
While we agree that this may be a defect, we are not happy with the proposed words, and processing this defect is postponed pending improved wording. Specifically, "as if the body were entered in the normal way" raises a few new questions.
Technical Corrigendum
Append to 6.8.5#4:
The repetition occurs regardless of whether the loop body is entered from the iteration statement or by a jump.** Code jumped over is not executed. In particular, the controlling expression of a for or while statement is not evaluated before entering the loop body, nor is clause-1 of a for statement.