Use getlocalename_l() if available

[khwilliamson]

This POSIX 2024 function solves long-standing thread unsafe behavior on platforms that properly implement it.

• I'm unsure if this set of changes requires a perldelta entry

[tonycoz]

The change looks reasonable, but I assume you see the cygwin failure.

As far as I can see cygwin is the only platform CI tests that has getlocalename_l().

[khwilliamson]

As with many of my PRs, if they go smoothly fine; if not, it'll have to wait until I get time to dig further. In this case, I will need to get a current cygwin set up for debugging.

Given that basically every OS we test on has some bug or another with locale handling, I'm more inclined to think this is their problem, and it just hasn't been tested sufficiently until we came along. But it's premature to make that assertion.

I've converted this to draft

[khwilliamson]

I can't see how it could be anything other than a bug(s) in cygwin. It's dying in getlocalename_l() when called with category LC_ALL, and the object parameter has been set to the C locale. If instead I loop through the individual categories, it doesn't die and each properly returns C. That indicates it can't handle LC_ALL on this simple case.

I'm not convinced that is the only bug.

I presume they will want a non-Perl simple reproducer.

[tonycoz]

https://cygwin.com/pipermail/cygwin/2026-August/259965.html

It's easy to make it crash:

tony@GANYMEDE ~/play
$ uname -a
CYGWIN_NT-10.0-19045 GANYMEDE 3.6.10-1.x86_64 2026-07-13 20:20 UTC x86_64 Cygwin

tony@GANYMEDE ~/play
$ cat getlcn_l.c
#include <locale.h>
#include <stdio.h>

int main(int argc, char **argv) {
  if (argc < 2) {
    fprintf(stderr, "Usage: %s locale-name\n");
    return 1;
  }
    
  locale_t lc = newlocale(LC_ALL_MASK, argv[1], 0);
  puts("newlocale");
  if (!lc) {
    perror("newlocale");
    return 1;
  }
  const char *name = getlocalename_l(LC_ALL, lc);
  printf("name '%s'\n", name);
}

tony@GANYMEDE ~/play
$ cc -ogetlcn_l.exe -g getlcn_l.c 

tony@GANYMEDE ~/play
$ ./getlcn_l C
newlocale

tony@GANYMEDE ~/play
$ ./getlcn_l en_AU.UTF-8
newlocale
name 'en_AU.UTF-8'

tony@GANYMEDE ~/play
$ gdb --args ./getlcn_l C
GNU gdb (GDB) (Cygwin 15.2-1) 15.2
Copyright (C) 2024 Free Software Foundation, Inc.
...
Reading symbols from ./getlcn_l...
(gdb) r
Starting program: /home/tony/play/getlcn_l C
[New Thread 7356.0x2d20]
[New Thread 7356.0x4f88]
[New Thread 7356.0x6684]
newlocale

Thread 1 "getlcn_l" received signal SIGTRAP, Trace/breakpoint trap.
0x00007ff88aba9823 in KERNELBASE!DebugBreak ()
   from /cygdrive/c/WINDOWS/System32/KERNELBASE.dll
(gdb) bt
#0  0x00007ff88aba9823 in KERNELBASE!DebugBreak ()
   from /cygdrive/c/WINDOWS/System32/KERNELBASE.dll
#1  0x00007ff8561264a7 in break_here ()
    at /usr/src/debug/cygwin-3.6.10-1/winsup/cygwin/dcrt0.cc:473
#2  0x00007ff8561403e2 in try_to_debug ()
    at /usr/src/debug/cygwin-3.6.10-1/winsup/cygwin/exceptions.cc:599
#3  exception::handle (e=0x7ffffc970, frame=<optimized out>, in=0x7ffffc480, 
    dispatch=<optimized out>)
    at /usr/src/debug/cygwin-3.6.10-1/winsup/cygwin/exceptions.cc:812
#4  0x00007ff88d2729df in ntdll!.chkstk ()
   from /cygdrive/c/WINDOWS/SYSTEM32/ntdll.dll
#5  0x00007ff88d222554 in ntdll!RtlRaiseException ()
   from /cygdrive/c/WINDOWS/SYSTEM32/ntdll.dll
#6  0x00007ff88d2714ee in ntdll!KiUserExceptionDispatcher ()
   from /cygdrive/c/WINDOWS/SYSTEM32/ntdll.dll
#7  0x00007ff8562c70b4 in strcpy (dst0=0x0, src0=<optimized out>)
    at /usr/src/debug/cygwin-3.6.10-1/newlib/libc/string/strcpy.c:70
#8  0x00007ff8562d043f in __currentlocale (locobj=0x7ff8563e10c0 <__C_locale>, 
    locale_string=0x7ff8563e11a0 <__C_locale+224> "C")
    at /usr/src/debug/cygwin-3.6.10-1/newlib/libc/locale/locale.c:460
#9  0x00007ff856276cf4 in _sigfe () at sigfe.s:35
#10 0x0000000100401118 in main (argc=2, argv=0xa00002900) at getlcn_l.c:16

[tonycoz]

https://sourceware.org/pipermail/newlib/2026/022610.html