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Date: Wed, 19 Jul 2023 18:10:52 +0100
From: Alastair Houghton <>
Subject: Re: setlocale() behaviour

On 19 Jul 2023, at 17:51, Markus Wichmann <> wrote:
> Well, you must not depend on implementation internals. According to
> POSIX, the form of the locale environment variables and the strings to
> be plugged into setlocale() (except for "POSIX", "C", "", and the null
> pointer) are implementation-defined, and musl defines that absolutely
> any name is supported and is a copy of C.UTF-8 (again, except for
> "POSIX" and "C"). The name handed in must be returned back out again for
> gettext to work.

As was pointed out in the previous thread, POSIX does say, in the Rationale section:

> If the string does not correspond to a valid locale, setlocale() shall return a null pointer and the international environment is not changed.

And while I think musl probably does get out of it on the technicality that the “contents of this string are implementation-defined” so it can claim that any old string “correspond[s] to a valid locale” that just happens to be C.UTF-8 unless there’s a data file installed, I think the current behaviour is very much not in the spirit of what was intended here.

But I don’t actually care about the standards lawyering here.  As Rich Felker noted previously about this behaviour:

> Unfortunately this turns out to have been something of a tradeoff,
> since there's no way for applications (and, as it turns out,
> especially tests/test suites) to query whether a particular locale is
> "really" available. I've been asked to change the behavior to fail on
> unknown locale names, but of course that's not a working option in
> light of the above.

He then went on in a subsequent message to the list to suggest

> 1. setlocale(cat, explicit_locale_name) - succeeds if the locale
>    actually has a definition file, fails and returns a null pointer
>    otherwise.
> 2. setlocale(cat, "") - always succeeds, honoring the environment
>    variable for the category if a locale definition file by that name
>    exists, but otherwise (the unspecified behavior) treating it as if
>    it were C.UTF-8.

Which would work just fine for libc++.  What it’s trying to do is to check whether e.g. fr_FR is supported, then it can enable additional tests that rely on the French localisation being present.  I appreciate that, per the POSIX standard, you don’t *technically* know what fr_FR means, but in practice that isn’t really true --- an implementation that had a locale installed called fr_FR that *wasn’t* French would be pretty silly.  Unfortunately, because of musl’s current behaviour, it *looks like* such an implementation, even though it actually isn’t (and it totally could have a genuine fr_FR locale if you had the right data in the right place).

I can, of course, check whether setlocale(LC_ALL, “something ridiculous”) succeeds and returns “something ridiculous”, then disable all locales except for POSIX, C and C.UTF-8.  That will work around the current musl behaviour without causing trouble with other C libraries, but libc++’s maintainer isn’t terribly keen on it and would rather we explore the possibility of musl changing its implementation.

TL/DR: What I’m really enquiring about here is the fact that there was discussion about changing it to work in a more useful manner, but nothing changed (and I don’t see anything in that email thread to explain that it was decided to not make the proposed change; but maybe I missed it?)

Kind regards,


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