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Q304497: BUG: _wsetlocale Is Not Threadsafe

Article: Q304497
Product(s): Microsoft C Compiler
Version(s): 6.0
Operating System(s): 
Keyword(s): 
Last Modified: 13-MAY-2002

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The information in this article applies to:

- Microsoft Visual C++, 32-bit Editions, version 6.0 
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SYMPTOMS
========

A heap corruption can occur if _wsetlocale is called simultaneously from
multiple threads.

CAUSE
=====

_wsetlocale does not synchronize access to a local static variable, outwlocale.

RESOLUTION
==========

To work around this, you must synchronize calls to _wsetlocale.

STATUS
======

Microsoft has confirmed this to be a bug in the Microsoft products listed at the
beginning of this article.

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();
  }

Additional query words:

======================================================================
Keywords          :  
Technology        : kbVCsearch kbAudDeveloper kbVC600 kbVC32bitSearch
Version           : :6.0
Issue type        : kbbug
Solution Type     : kbfix

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