C++ Logo


Advanced search

Re: [wg14/wg21 liaison] [isocpp-parallel] Proposal: Trivial infinite loops are not Undefined Behavior

From: Aaron Peter Bachmann <aaron_ng_at_[hidden]>
Date: Fri, 17 Feb 2023 10:51:47 +0100
Hello Robert!
I understand the while(1)-loop can be assumed to terminate.
But who calls unreachable()? It is not in the body of main().
I would expect main() to return with 0 for C and an unspecified value
for C++
or alternatively remain in the endless loop for both languages.

Regards, Aaron Peter Bachmann

On 2/16/23 05:42, Robert Seacord via Liaison wrote:
> I'm not sure if someone has posted this example yet, but here is a fun
> example that compiles in both C and C++ with different results:
> #include <stdio.h>
> int main() {
> while (1)
> ;
> }
> void unreachable() {
> puts("Hello world!");
> }
> If you want to try it: https://godbolt.org/z/1d3hzseh4
> In C, this code produces an infinite loop while in C++ it does the
> following:
> image.png
> On Wed, Feb 15, 2023 at 11:31 PM Robert Seacord <rcseacord_at_[hidden]>
> wrote:
> Hans,
> I initiated this latest discussion on the WG14 reflector and I've
> recently become a bit more active in WG21 (and my boss is JF who
> wrote this paper).
> Here is a recap of some of the history of this issue:
> I can't find any forward progress guarantee in the C Standard.
> The word "progress" does not appear anywhere in the standard.
> Subclause 6.8.5, "Iteration statements" paragraph 5 says:
> An iteration statement may be assumed by the implementation to
> terminate if its controlling
> expression is not a constant expression197), and none of the
> following operations are performed in its
> body, controlling expression or (in the case of a for statement)
> its expression-3198):
> — input/output operations
> — accessing a volatile object
> — synchronization or atomic operations.
> 197) An omitted controlling expression is replaced by a nonzero
> constant, which is a constant expression.
> 198) This is intended to allow compiler transformations such as
> removal of empty loops even when termination cannot be
> proven.
> N1509 proposes to eliminate 6.8.5p6 in the current WP as the
> author believes it to be incompatible with C99. See also N1528,
> Item 6.30, Response to N1509.
> N1528 covers why the rules were added. N1509 was written to show
> a compiler can determine the opposite as well.
> In https://open-std.org/jtc1/sc22/wg14/www/docs/n1509.pdf [Walls]
> contends that many C90/C99 programs rely on entrance to an
> unending loop to proceed no further (within that thread). Those
> programs are relying on the semantics of C99 6.8.5p4: "An
> iteration statement causes a statement called the loop body to be
> executed repeatedly until the controlling expression compares
> equal to 0."
> N1528 breaks existing conforming programs. Adding an exception may
> solve the intent of both papers. That approach should be run by
> Hans. We can defer a decision and not affect getting a CD out,
> then respond to an NB comment. We do want to be compatible with WG21.
> ACTION Douglas to work with Larry to come up with the proposed
> words are:
> Change 6.8.5p6 as follows:
> An iteration statement whose controlling expression is not a
> constant expression (or omitted), that performs no input/output
> operations, does not access volatile objects, and performs no
> synchronization or atomic operations in its body, controlling
> expression, or (in the case of a for
> statement) its expression, may be assumed by the implementation to
> terminate.
> Decision: Adopt N1509, as modified above, to the C1X WP. result:
> 14, 0, 1
> On Wed, Feb 15, 2023 at 8:29 PM Hans Boehm via Parallel
> <parallel_at_[hidden]> wrote:
> I read the current C wording, and the one proposed here, as
> saying that
> while(true) {
> if (!cond) break;
> ...
> }
> may not be assumed to terminate, and is thus very different from
> while(cond) {
> ...
> }
> This is a very confusing shorthand as "..." seems to imply any
> syntactically correct statements and thus the first pattern is a
> subset of the second pattern.
> I don't remember considering that before. This seems to be
> completely at odds with Olivier's goals. I also don't think
> this is a good idea, for purely loop optimization and
> understandability reasons. OTOH, some people on the WG14 list
> seem to think it is intentional and desired.
> Unlike what I said to JF at the meeting, it no longer sounds
> easy to me to remove this C vs. C++ divergence. There seem to
> be some substantive issues here.
> There is a strong desire in C to remain compatible with C++ and
> it's less clear what the C requirement is. There is a requirement
> that "while (1);" cannot be assumed to terminate as it is
> ubiquitous in environments where you don't have the luxury of
> calling exit() or yielding to the scheduler. I certainly wouldn't
> give up on compatibility at this point, and there is still a small
> window to change the C23 language as we are going to a second CD.
> Thanks,
> rCs
> _______________________________________________
> Liaison mailing list
> Liaison_at_[hidden]
> Subscription: https://lists.isocpp.org/mailman/listinfo.cgi/liaison
> Link to this post: http://lists.isocpp.org/liaison/2023/02/1175.php

Received on 2023-02-17 09:52:05