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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>>>>
>>>