I don’t think this means it’s not standardizing existing practices. The
existing practice is #pragma once with the intention of preventing multiple
inclusion. This paper proposes what I think are reasonable, and maybe the
only robust, semantics to achieve this.

Jeremy

On Tue, Aug 27, 2024 at 11:37 Gašper Ažman <gasper.azman_at_[hidden]> wrote:

> Yes, but then you're not standardizing existing practice.
>
> On Tue, Aug 27, 2024 at 5:35 PM Jeremy Rifkin <rifkin.jer_at_[hidden]>
> wrote:
>
>> This paper proposes #pragma once mean don’t #include the same contents
>> again. Unlike GCC which will perform multiple inclusion if mtime differs.
>> This paper leaves open the possibility of implementations doing their own
>> fast path checks of inode, stat, etc., but ultimately the file contents are
>> what matter.
>>
>> Jeremy
>>
>> On Tue, Aug 27, 2024 at 11:32 Gašper Ažman <gasper.azman_at_[hidden]>
>> wrote:
>>
>>> We have issues even with GCC - it doesn't work reliably, because mtime
>>> precision differs between filesystems, and so you end up with differing
>>> mtimes for the same file.
>>>
>>> On Tue, Aug 27, 2024 at 5:25 PM Jeremy Rifkin <rifkin.jer_at_[hidden]>
>>> wrote:
>>>
>>>> Hi,
>>>> Thank you for your comments.
>>>>
>>>> > #pragma once is unimplementable.
>>>>
>>>> Every notable C/C++ compiler has implemented it.
>>>>
>>>> > That's because it doesn't mean "don't include a file with the same
>>>> bytes"
>>>>
>>>> On GCC it more or less does.
>>>>
>>>> > And the second you have the same file accessible through two
>>>> separate fuse mountpoints, no amount of stat() will help you identify it's
>>>> the same file.
>>>>
>>>> This proposal does not propose any use of stat() for determining file
>>>> identity. I understand concerns regarding paths, links, and mounts and have
>>>> taken care in this proposal to propose a robust solution. This solution
>>>> proposed here should not cause problems in cases you refer to with your
>>>> build farms. If you can think of a way it would, I would be interested.
>>>>
>>>> Cheers
>>>> Jeremy
>>>>
>>>> On Tue, Aug 27, 2024 at 10:34 Gašper Ažman via Std-Proposals <
>>>> std-proposals_at_[hidden]> wrote:
>>>>
>>>>> #pragma once is unimplementable.
>>>>>
>>>>> That's because it doesn't mean "don't include a file with the same
>>>>> bytes", it means "don't include the same file". And the second you have the
>>>>> same file accessible through two separate fuse mountpoints, no amount of
>>>>> stat() will help you identify it's the same file.
>>>>>
>>>>> We hit this in our build-farms all the time, I had to write tooling to
>>>>> kill it.
>>>>>
>>>>> Don't make me explain this in-committee, again.
>>>>>
>>>>> On Tue, Aug 27, 2024 at 4:32 PM Tiago Freire via Std-Proposals <
>>>>> std-proposals_at_[hidden]> wrote:
>>>>>
>>>>>> I like pragma once, should be standardized, but the problem is in the
>>>>>> details of the wording.
>>>>>>
>>>>>> I know that the paper acknowledges that some compiler issues a
>>>>>> warning if they appear in the source file, and that the paper doesn't
>>>>>> advocate for the warning either way. But the note seems to suggest
>>>>>> otherwise.
>>>>>> It would be best if the standard had no notion of what a source file
>>>>>> is, and you should remove the note that a compiler could issue a warning,
>>>>>> let that just be a vendor detail.
>>>>>>
>>>>>> The description of how the directive should behave isn't the best,
>>>>>> describing in in terms of an ifdef block is not good.
>>>>>>
>>>>>>
>>>>>> ------------------------------
>>>>>> *From:* Std-Proposals <std-proposals-bounces_at_[hidden]> on
>>>>>> behalf of Jeremy Rifkin via Std-Proposals <
>>>>>> std-proposals_at_[hidden]>
>>>>>> *Sent:* Tuesday, August 27, 2024 4:01:08 PM
>>>>>> *To:* Jeremy Rifkin via Std-Proposals <std-proposals_at_[hidden]
>>>>>> >
>>>>>> *Cc:* Jeremy Rifkin <rifkin.jer_at_[hidden]>
>>>>>> *Subject:* [std-proposals] Revising #pragma once
>>>>>>
>>>>>> Hi,
>>>>>> I have drafted a proposal for standardizing #pragma once. This has
>>>>>> been previously proposed a few years ago and I recognize that on top of
>>>>>> being difficult to standardize, existing opinions on this topic may render
>>>>>> the paper dead on arrival. However, due to its widespread nature and
>>>>>> concerns about portability contributing to it not being used more I think
>>>>>> it's worth revisiting. I have uploaded the first draft at:
>>>>>> https://jeremy-rifkin.github.io/cpp-proposals/drafts/pragma-once-draft-1.html
>>>>>> .
>>>>>>
>>>>>> Cheers,
>>>>>> Jeremy
>>>>>> --
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>>>>>> Std-Proposals_at_[hidden]
>>>>>> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
>>>>>>
>>>>> --
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>>>>> Std-Proposals_at_[hidden]
>>>>> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
>>>>>
>>>>