DOCUMENT:Q262515  20-AUG-2001  [visualc]
TITLE   :FIX: /GT Compiler Switch May Cause AV During Thread Switch
PRODUCT :Microsoft C Compiler
PROD/VER:winnt:6.0
OPER/SYS:
KEYWORDS:kbCompiler kbVC600bug kbGrpDSVCCompiler kbVS600sp4fix kbVS600sp5fix

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The information in this article applies to:

 - Microsoft Visual C++, 32-bit Enterprise Edition, version 6.0 
 - Microsoft Visual C++, 32-bit Professional Edition, version 6.0 
 - Microsoft Visual C++, 32-bit Learning Edition, version 6.0 
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SYMPTOMS
========

During the switching of threads, an access violation may occur, with the
following message at the top of the call stack:

   KERNEL32! IsBadReadPtr@8 + 82 bytes

In general, this problem can occur whenever a fiber is switched between two
threads; for instance:

 - when switching between preemptive mode and nonpreemptive mode in User Mode
   Schedulers (UMS)

 - when accessing a variable that is stored in thread-local storage (TLS) before
   and after a switch.

CAUSE
=====

A fiber is a lightweight object that consists of a stack and a register context,
and which can be scheduled on various threads. A fiber can run on any thread.
Because a fiber may be swapped out and then restarted on a different thread, it
must not be cached or optimized as a common subexpression across a function
call.

The /GT compiler switch prevents such optimizations. The /GT option supports
fiber safety for data that is allocated by using static thread-local storage; in
other words, data that is allocated with the __declspec( thread ) keyword.

The /GT compiler switch leaves the TLSArray references as constant. When
accessing a TLS variable before and after a switch, the address is computed
incorrectly. The pointer to the TLS variable is stored in the esi register, the
offset is stored in the eax register, and the address is calculated by adding
these two values. After a thread switch, eax is recalculated but esi is not. The
address is still evaluated by adding esi and eax, and when the fiber switch
changes the value in esi, an access violation occurs.

RESOLUTION
==========

Obtain the latest service pack for Visual Studio 6.0.

STATUS
======

Microsoft has confirmed this to be a bug in the Microsoft products listed at the
beginning of this article. This bug was corrected in the next service pack for
Visual Studio 6.0.

For additional information about Visual Studio service packs, click the article
numbers below to view the articles in the Microsoft Knowledge Base:

   Q194022 INFO: Visual Studio 6.0 Service Packs, What, Where, Why

   Q194295 HOWTO: Tell That a Visual Studio Service Pack Is Installed

You can download the latest Visual Studio service pack from the following
Microsoft Web site:

   Visual Studio Product Updates
   (http://msdn.microsoft.com/vstudio/downloads/updates.asp)

Additional query words: sp4

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Keywords          : kbCompiler kbVC600bug kbGrpDSVCCompiler kbVS600sp4fix kbVS600sp5fix 
Technology        : kbVCsearch kbAudDeveloper kbVC600 kbVC32bitSearch
Version           : winnt:6.0
Issue type        : kbbug
Solution Type     : kbfix

=============================================================================

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