C issue 0268: jumps into iteration statements

Issue 0268: jumps into iteration statements

This issue has been automatically converted from the original issue lists and some formatting may not have been preserved.

Authors: Clive D.W. Feather <clive@demon.net>, UK C Panel
Date: 2001-09-07
Submitted against: C99
Status: Fixed
Fixed in: C99 TC3
Converted from: summary-c99.htm, dr_268.htm

Problem

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:

[#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.

Possibly also add an example either as 6.8.5#6 or 6.8.6.1#5 (with appropriate editorial changes):

[#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 9

while if it is false it prints:

  0 1 2 4 6 8 10

Comment from WG14 on 2006-03-29:

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.