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From: Iain Sandoe <>
To: Jonathan Wakely <>
Subject: Re: [RFA/C] Reimplementation of std::call_once.
Date: Wed, 9 Aug 2023 13:51:48 +0100	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

Hi Jonathan,

> On 9 Aug 2023, at 13:38, Jonathan Wakely <> wrote:
> On Wed, 9 Aug 2023 at 13:22, Iain Sandoe <> wrote:
>> Hello.
>> With the current [posix-y] implementation of std::call_once, we are seeing significant issues for Darwin. The issues vary in strength from “hangs or segvs sometimes depending on optimisation and the day of the week” (newer OS, faster h/w) to “segvs or std::terminates every time” older OS slower h/w.
>> Actually, I think that in the presence of exceptions it is not working on Linux either.
>> As a use-case: this problem prevents us from building working LLVM/clang tools on Darwin using GCC as the bootstrap compiler.
>> the test code I am using is: (derived from a cppreference example, IIRC).

>> This is a prototype, so I’m looking for advice/comment on:
>> (a) if this can be generalised to be part of libstdc++
>> (b) what that would take ...
>> (c) ..or if I should figure out how to make this a darwin-specific impl.
> It's an ABI break. I already tried to replace std::call_once once, see
> r11-4691-g93e79ed391b9c6 and r11-7688-g6ee24638ed0ad5 and PR 99341.


>> ——  here is the prototype implementation as a non-patch .. (patch attached).

>> =====
>> the implementation in can sit togethe with the old version so that the symbols for that remain available (or, for versioned libraries, the old code can be deleted).
> If you have old and new code sharing a std::once_flag object you're dead.

indeed - I was only considering the case where existing binaries needed to run with the new lib, not the case where code compiled with two different impls was combined.

> Something like the abi_tag transition for std::string in GCC 5 would be needed.

That sounds quite complicated and likely to produce similar pain?


Presumably an alternative is that I just have to accept that Darwin needs to use a versioned library (which is a direction I am close to heading in because of co-existence of mulitple c++ runtimes anyway).


  reply	other threads:[~2023-08-09 12:51 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-08-09 12:21 Iain Sandoe
2023-08-09 12:38 ` Jonathan Wakely
2023-08-09 12:51   ` Iain Sandoe [this message]
2023-08-09 12:53     ` Jonathan Wakely
2023-08-09 13:12       ` Iain Sandoe
2023-08-09 14:10         ` Jonathan Wakely

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