BUG: _wsetlocale Is Not Threadsafe (304497)

MORE INFORMATION

The C Runtime locale mechanism in Microsoft Visual C++ is distinct from the locale mechanism in Microsoft Windows. Though Windows supports per-thread locales, C Runtime supports only a single global locale for any program.

If multiple threads call _wsetlocale, then no thread's locale can be trusted. For example, if one thread calls _wsetlocale for French and tries to use that locale for _wcslwr, and another thread calls _wsetlocale for English before the first thread executes the _wcslwr function call, then the first thread uses the English locale instead of the French locale.

Steps to Reproduce the Behavior

#define _UNICODE
#include <stdio.h>
#include <locale.h>
#include <process.h>

void __cdecl test(void *i) {
	_wsetlocale(LC_ALL, L"English");
	_wsetlocale(LC_ALL, L"French");
	_wsetlocale(LC_ALL, L"German");
	_wsetlocale(LC_ALL, L"English");
	printf("%d exit\n",i);
}

void wmain () {
	for (int i=0; i<30; ++i) _beginthread(test,0,(void*)i);
	getchar();
}