Q106262: FIX: Direction Flag Is Not Cleared When an Exception Occurs

Article: Q106262
Product(s): Microsoft C Compiler
Version(s): winnt:
Operating System(s): 
Keyword(s): kbCRT kbVCkbbuglist kbfixlist
Last Modified: 26-JUL-2001

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The information in this article applies to:

- The C Run-Time (CRT), included with:
   - *EDITOR Please do not choose this product*Microsoft Visual C++ 32-bit Edition* use 241, 265, 225, version 1.0 
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SYMPTOMS
========


Functions called in an exception handler can cause unexpected behavior such as
access violations.

CAUSE
=====

The direction flag bit may not be cleared when the exception occurs.

RESOLUTION
==========

Clear the direction flag bit within either the exception handler or the
termination handler. The sample code shown below illustrates clearing the flag
in an exception handler.

STATUS
======

Microsoft has confirmed this to be a bug in the products listed at the beginning
of this article. This problem was corrected in Visual C++ version 2.0.

MORE INFORMATION
================

On Intel processors, the direction flag bit in the flags register modifies the
behavior of the string instructions. When the direction flag (DF) is 0 (zero),
the string instructions operate on incrementally higher addresses. When DF is 1,
the string instructions operate on incrementally lower addresses. On Intel
chips, DF can be set to 1 with the STD instruction and can be cleared to 0 with
the CLD instruction.

If a function sets DF to 1, it should clear DF before terminating. This allows
all functions to make the assumption that DF is always 0.

All C run-time functions correctly clear DF upon termination. However, if an
exception occurs before a function has a chance to clear DF, the flag will still
be set when the exception handler is executed. This will cause code in the
handler (which assumes that DF is 0) to fail. The manner in which the code fails
depends on what the code is trying to do. In the example shown below, the
printf() function called in the exception handler causes a memory access
violation.

The problem can be worked around by clearing DF upon entry into an exception or
termination handler. In the sample code shown below, the exception handler
checks the value of the do_cld variable to determine whether or not it will
clear DF. The do_cld variable is set to 0 or 1 depending on whether or not the
CLD command-line argument is specified when the sample is run. To illustrate the
problem with DF being set to 1, run the sample without any command-line
arguments. To allow the sample to run correctly, specify the CLD command-line
argument.

Sample Code
-----------

  /* Compiler options needed: /D_X86_
  */ 

  #include <windows.h>
  #include <memory.h>
  #include <string.h>
  #include <stdio.h>

  int handling = 0;
  int do_cld = 0;

  LONG MyFilter(LPEXCEPTION_POINTERS except_pointers)
  {
      EXCEPTION_RECORD* er = except_pointers->ExceptionRecord;

      if (do_cld)
      {
          __asm cld
      }

      if (er->ExceptionCode==EXCEPTION_ACCESS_VIOLATION && handling==0)
      {
          handling = 1;
          printf("We're in the filter now and printing out a long"
                 "string that's long enough to cause a problem\n");
          handling = 0;
          return EXCEPTION_EXECUTE_HANDLER;
      }
      else return EXCEPTION_CONTINUE_SEARCH;
  }

  int main(int argc,char** argv)
  {
      if ( argc==2 && ((strcmp(argv[1],"cld")==0) ||
                       (strcmp(argv[1],"CLD")==0)))
          do_cld = 1;

      __try
      {
          printf("Starting the test\n");
          memcpy((void*)4,(void*)0,8);
          printf("After exception");
      }
      __except (MyFilter(GetExceptionInformation()))
      {
          printf("In the handler now\n");
      }

      return 0;
  }

Additional query words: 1.00 structured handling protection fault

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Keywords          : kbCRT kbVC kbbuglist kbfixlist
Technology        : kbVCsearch kbAudDeveloper kbCRT
Version           : winnt:
Issue type        : kbbug
Solution Type     : kbfix

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