Q172396: PRB: Access Violation When Accessing STL Object in DLL
Article: Q172396
Product(s): Microsoft C Compiler
Version(s): WINDOWS NT:5.0
Operating System(s):
Keyword(s): kbCRT kbVC500 kbVC600
Last Modified: 17-JUL-2001
-------------------------------------------------------------------------------
The information in this article applies to:
- Microsoft Visual C++, 32-bit Enterprise Edition, versions 5.0, 6.0
- Microsoft Visual C++, 32-bit Professional Edition, versions 5.0, 6.0
- Microsoft Visual C++, 32-bit Learning Edition, version 6.0
-------------------------------------------------------------------------------
SYMPTOMS
========
When accessing an STL object created in one DLL or EXE through a pointer or
reference in a different DLL or EXE, you may experience an access violation or
other serious program errors including the appearance of data corruption or data
loss.
CAUSE
=====
Most classes in the Standard C++ Libraries use static data members directly or
indirectly. Since these classes are generated through template instantiation,
each executable image (usually with DLL or EXE file name extensions) will
contain its own copy of the static data member for a given class. When a method
of the class that requires the static data member is executed, it uses the
static data member in the executable image in which the method code resides.
Since the static data members in the executable images are not in sync, this
action could result in an access violation or data may appear to be lost or
corrupted.
RESOLUTION
==========
1. Export accessor methods from the executable image that created the STL
object. These methods wrap the required functionality of the STL object. In
this way, the STL object will only be directly accessed inside a single
executable image. For example, suppose MyProgram.EXE needs to get the next
element in deque<MyClass> that resides in MyLibrary.DLL. MyLibrary.DLL
could export an accessor method, MyClass* DequeNextItem (/*...*/). Then
MyProgram.EXE could execute this method to get the next item in the deque.
See the code sample below for a more complete example.
This option works for STL objects that are either global, static, or static
data members of a class that are not exported from a DLL. This option will
not work for non-static data members of a class that are exported from a DLL
or for automatic data.
2. Export the template class instantiation from one executable image and import
it into the other executable images. For example, if MyLibrary.DLL passes a
pointer to vector<MyClass> back to a function in MyProgram.EXE, then
export the classes MyClass and vector<MyClass> from MyLibrary.DLL. Then
import these classes into MyProgram.EXE. By doing this, you will have one
copy of the static class members residing in MyLibrary.DLL. For more
information about exporting and importing STL classes, please see the
following article in the Microsoft Knowledge Base:
Q168958 HOWTO: Exporting STL Components Inside & Outside of a Class
STATUS
======
This behavior is by design.
MORE INFORMATION
================
Steps to Reproduce Behavior
---------------------------
//---------------------------------------------------------
// AVEXE.CPP
// Compile options needed: /GX
#pragma warning (disable : 4786)
#include <map>
#include <string>
#include <stdio.h>
__declspec(dllimport)
std::map<int,std::string>* GiveMeAMap(int n);
__declspec(dllimport)
void ShowMeTheMap(std::map<int,std::string> *amap);
__declspec(dllexport)
const char* MapItemX (std::map<int,std::string> *m, int x);
int main () {
// Create the map in the DLL
int x = 6;
std::map<int,std::string> *p = GiveMeAMap(x);
// Display the contents of the map from the DLL
printf("Showing contents from the DLL\n");
ShowMeTheMap(p);
// Display the contents of the map from the EXE
// using the accessor function from the DLL so we
// aren't directly accessing the map
printf("Showing contents from the EXE using accessor\n");
int i = x;
while (i--) {
printf("%d = %s\n",i,MapItemX(p,i));
}
// Access Violation when accessing the map that
// was created in the DLL from the EXE
printf("Showing contents from the EXE directly\n");
while (x--) {
printf("%d = %s\n",x,(*p)[x].c_str());
}
return 0;
}
//---------------------------------------------------------
// AVDLL.CPP
// Compile options needed /GX
#pragma warning (disable : 4786)
#include <map>
#include <string>
#include <stdlib.h>
// Create the map here in the DLL
__declspec(dllexport)
std::map<int,std::string>* GiveMeAMap(int n) {
std::map<int,std::string> *m = new std::map<int,std::string>;
while(n--) {
char b[33];
itoa(n,b,2);
(*m)[n] = std::string(b);
}
return m;
}
// We can access the map without error from the executable
// image where the map was created
__declspec(dllexport)
void ShowMeTheMap(std::map<int,std::string> *p) {
int x = p->size();
while (x--) {
printf("%d = %s\n",x,(*p)[x].c_str());
}
}
// An accessor method to return the associated C string
// for key x
__declspec(dllexport)
const char* MapItemX (std::map<int,std::string> *m, int x) {
return (*m)[x].c_str();
}
======================================================================
Keywords : kbCRT kbVC500 kbVC600
Technology : kbVCsearch kbAudDeveloper kbVC500 kbVC600 kbVC32bitSearch kbVC500Search
Version : WINDOWS NT:5.0
Issue type : kbprb
=============================================================================
THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY.
Copyright Microsoft Corporation 1986-2002.