This article was previously published under Q304497
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A heap corruption can occur if _wsetlocale is called simultaneously from multiple threads.
_wsetlocale does not synchronize access to a local static variable, outwlocale.
To work around this, you must synchronize calls to _wsetlocale.
Microsoft has confirmed that this is a bug in the Microsoft products that are listed at the beginning of this article.
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.