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Subject: [PHP-DEV] Change `setlocale()` to use the `uselocale()` C function if available on POSIX too
Thread-Topic: Change `setlocale()` to use the `uselocale()` C function if
 available on POSIX too
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Date: Tue, 28 Oct 2025 15:52:26 +0000
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From: m@pyc.ac (Marc Henderkes)

--_000_ZR1P278MB112080300D6FB5871FFD7E9AD1FDAZR1P278MB1120CHEP_
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Hello everyone,

We recently noticed that the setlocale function currently uses `setlocale` =
in its C implementation as well [https://github.com/php/frankenphp/pull/194=
1], which is a) not multithreading safe to use and b) when it doesn't crash=
, leads to overriding the current locale for the entire process.
For NTS builds with php-fpm this is not an issue, but for FrankenPHP and mo=
d_php, this leads to unexpected behaviour described here: https://github.co=
m/php/frankenphp/pull/1941#issuecomment-3457137937

On Windows builds, since PHP 7, it is already per-thread, but on POSIX syst=
ems it is not.

I would therefore like to get your reactions on whether it would make sense=
 to unify the behaviour between POSIX systems and Windows and make the `set=
locale()` function thread-safe and per-thread, rather than per-process.
In case my proposal sees positive reception, I'd like to go ahead and creat=
e an RFC. I will also implement the changes in php-src, I've already done s=
o as a proof of concept.

Thank you for your time,
Marc Henderkes


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<p class=3D"MsoNormal">Hello everyone,<o:p></o:p></p>
<p class=3D"MsoNormal"><o:p>&nbsp;</o:p></p>
<p class=3D"MsoNormal">We recently noticed that the setlocale function curr=
ently uses `setlocale` in its C implementation as well [<a href=3D"https://=
github.com/php/frankenphp/pull/1941">https://github.com/php/frankenphp/pull=
/1941</a>], which is a) not multithreading
 safe to use and b) when it doesn&#8217;t crash, leads to overriding the cu=
rrent locale for the entire process.<o:p></o:p></p>
<p class=3D"MsoNormal">For NTS builds with php-fpm this is not an issue, bu=
t for FrankenPHP and mod_php, this leads to unexpected behaviour described =
here:
<a href=3D"https://github.com/php/frankenphp/pull/1941#issuecomment-3457137=
937">https://github.com/php/frankenphp/pull/1941#issuecomment-3457137937</a=
><br>
<br>
On Windows builds, since PHP 7, it is already per-thread, but on POSIX syst=
ems it is not.<br>
<br>
I would therefore like to get your reactions on whether it would make sense=
 to unify the behaviour between POSIX systems and Windows and make the `set=
locale()` function thread-safe and per-thread, rather than per-process.<o:p=
></o:p></p>
<p class=3D"MsoNormal">In case my proposal sees positive reception, I&#8217=
;d like to go ahead and create an RFC. I will also implement the changes in=
 php-src, I&#8217;ve already done so as a proof of concept.<br>
<br>
Thank you for your time,<o:p></o:p></p>
<p class=3D"MsoNormal">Marc Henderkes<o:p></o:p></p>
<p class=3D"MsoNormal"><o:p>&nbsp;</o:p></p>
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