Q198477: PRB: Use of /ZI May Cause Access Violation

Article: Q198477
Product(s): Microsoft C Compiler
Version(s): 6.0
Operating System(s): 
Keyword(s): kbCompiler kbVC600
Last Modified: 02-MAY-2002

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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 
- Microsoft Visual C++.NET (2002) 
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NOTE: Microsoft Visual C++ NET (2002) supported both the managed code model that is provided by the .NET Framework and the unmanaged native Windows code model. The information in this article applies to unmanaged Visual C++ code only.

SYMPTOMS
========

Use of the compiler switch /ZI (Program Data Base for Edit and Continue) may
cause an access violation if you try to modify a text string.

RESOLUTION
==========

Use one of the following workarounds:

- In the sample below, change "char * ptr" to "char ptr[]".

- Do not use the /ZI switch.

STATUS
======

As per C++ Standard (2.13.4.2) the effect of attempting to modify a string
literal is undefined.

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

By default the compiler switch /ZI (Program Data Base for Edit and Continue)
uses the compiler switch /GF.

The /GF option enables the compiler to pool strings and place them in read-only
memory. By placing the strings in read-only memory, the operating system does
not need to swap that portion of memory. Instead, it can read the strings back
from the image file. It is a good idea to do this as it saves pages of memory
from being written to and therefore reduces the working set used by the
application. In addition, it allows those pages to be shared between multiple
instances of the process that use that image file (.exe or .dll file), further
reducing total memory usage in the entire system. Strings placed in read-only
memory cannot be modified; if you try to modify them, you will see an
Application Error dialog box.

The following code when executed after compile produces an access violation.

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

     // Test.cpp
     // Compile with: cl /ZI /Od test.cpp

     int main ()
     {
       char* ptr = "Hello World";
       ptr[3] = 'Q';   //Access violation
       return 0;
     }

Since /ZI is used for debugging with Edit and Continue, the above code works fine
in the Release build, where the /ZI switch is not used.

Additional query words: string strings arrays initialize initialise aggregate access violation

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Keywords          : kbCompiler kbVC600 
Technology        : kbVCsearch kbAudDeveloper kbVC600 kbVC32bitSearch kbVCNET
Version           : :6.0
Issue type        : kbprb

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