BUG: No Warning Using Extern "C" in a Namespace

• Microsoft C/C++ Compiler (CL.EXE) included with: - Microsoft Visual C++, 32-bit Edition, versions 4.0, 4.1, 5.0

SYMPTOMS

Unexpected behavior can occur when you use extern "C" in a namespace.

CAUSE

Among other things, extern "C" directs the compiler not to decorate the names of symbols. But a symbol name is not recognized as part of a namespace outside of a translation unit unless the namespace name is part of the decorated name for the symbol.

The compiler should generate a warning when using extern "C" in a namespace, but it does not.

RESOLUTION

Don't use extern "C" linkage in a namespace. If you have to use extern "C" linkage, use it outside of a namespace.

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. We are researching this problem and will post new information here in the Microsoft Knowledge Base as it becomes available.

MORE INFORMATION

The following sample programs (Test1.cpp and Test2.cpp) demonstrate two cases of this.

Sample Code to Reproduce Problem

   //test1.cpp
   /* Compile options needed: -W4
   */
   namespace A{
      extern "C" void printf(int);
   }

   void main(){
      A::printf(1); // actually resolves to CRT printf
                    // which causes an access violation
   }
   //end of test1.cpp

   //test2.cpp

   /* Compile options needed: -W4
   */

   #include <iostream.h>

   namespace A{
      extern "C" void  function(void){cout <<"namespace A" <<endl;};

   }

   namespace B{
      extern "C" void function(void){cout <<"namespace B" <<endl;};

   }

   void main(){
       A::function();
      B::function();

   }
   // end of test2.cpp

   error LNK2005: _function already defined in test2.obj.
   LNK1169: one or more multiply defined symbols found.