extern "C" and Symbian DEF files

Rule for using extern "C"

When porting or writing libraries that export C APIs to the user, care should be taken to ensure that extern "C" is used properly if those APIs are implemented in a C++ source file. Otherwise these libraries may either not be built for the target platform (ARMV5) or the user of such library will get a linker error when using exported APIs of such library.

If extern "C" is declared in the header and not defined in the source, then after building the library there will be a similar entry in DEF files. The compiler will not give external linkage to these APIs. This can be observed when trying to use these APIs from some other C source file. To overcome this, extern "C" must be present in both declarations and definitions.

Example code

       /*File: SampleDLL.h*/
/*extern "C" is not used for declaration!*/ 
#ifndef SAMPLEDLL_H
#define SAMPLEDLL_H

typedef int MyInt;
typedef int (*MyFunPtr) ();

IMPORT_C int MyLibLibFun1(MyInt aParam);
IMPORT_C int MyLibLibFun3(MyFunPtr);
IMPORT_C int MyLibLibFun2(int (*MyFun1)());

#endif /*SAMPLEDLL_H*/



//File: SampleDLL.cpp
#include "SampleDLL.h"
// extern "C" is used for definition
extern "C"  {

EXPORT_C int MyLibLibFun1(MyInt aParam)
   {
   return aParam + 10;
   }

EXPORT_C int MyLibLibFun2(int (*MyFun1)())
   {
   return MyFun1();
   }

EXPORT_C int MyLibLibFun3(MyFunPtr aPtr)
   {
   return aPtr();
   }
}
      

Building for WINSCW

This code will get compiled for WINSCW . The DEF file for WINSCW will have the following entries:

       EXPORTS
     ?MyLibLibFun1@@YAHH@Z @ 1 NONAME ; int MyLibLibFun1(int)
     ?MyLibLibFun2@@YAHP6AHXZ@Z @ 2 NONAME ; int MyLibLibFun2(int (*)(void))
     ?MyLibLibFun3@@YAHP6AHXZ@Z @ 3 NONAME ; int MyLibLibFun3(int (*)(void))
      

If the user comments extern "C" from the above SampleDLL.cpp file, the DEF file entries will be the same.

Building for ARMv5

If the user try's the above code without extern "C" in SampleDLL.cpp , the code will get built and the DEF file entry for ARMv5 will be:

       EXPORTS
     _Z12MyLibLibFun1i @ 1 NONAME
     _Z12MyLibLibFun2PFivE @ 2 NONAME
     _Z12MyLibLibFun3PFivE @ 3 NONAME
      

If the user keeps the extern "C" in SampleDLL.cpp , this code will not get compiled for ARMv5 and will result in the error given below:

       "..\src\Sampledll.cpp", line 7: Error:  #337: linkage specification is incompatible with previous "MyLibLibFun1" (declared at line 9 of "..\inc\SampleDLL.h")
   EXPORT_C int MyLibLibFun1(MyInt aParam)
                ^ 
"..\src\Sampledll.cpp", line 17: Error:  #337: linkage specification is incompatible with previous "MyLibLibFun3" (declared at line 11 of "..\inc\SampleDLL.h")
   EXPORT_C int MyLibLibFun3(MyFunPtr aPtr)
               ^
      

The reason for the errors is that MyLibLibFun1 and MyLibLibFun3 use typedef datatypes. There is no issue with MyLibLibFun2() since it does not use typedef datatypes. To overcome these errors, the user must keep both the declaration and the definition within extern "C" linkage.

       /*File: SampleDLL.h*/    
#ifndef SAMPLEDLL_H  
#define SAMPLEDLL_H  
/*extern "C" is used for declaration also*/

typedef int MyInt;  
typedef int (*MyFunPtr) ();    

#ifdef __cplusplus  
extern "C" {  
#endif //__cplusplus    

IMPORT_C int MyLibLibFun1(MyInt aParam);  
IMPORT_C int MyLibLibFun3(MyFunPtr);  
IMPORT_C int MyLibLibFun2(int (*MyFun1)());    

#ifdef __cplusplus  
}  
#endif //__cplusplus    

#endif /*SAMPLEDLL_H*/
      

When everything is within extern "C" (extern "C" in header and implementation), the DEF file for ARMv5 and WINSCW looks as follows:

       EXPORTS
    MyLibLibFun1 @ 1 NONAME
    MyLibLibFun2 @ 2 NONAME
    MyLibLibFun3 @ 3 NONAME
      

So, as one can observe in the above section where different DEF file entries are created with and without extern "C", the user must make sure that the rule is followed. extern "C" has to be used in both declaration and definition: otherwise the user will either, not be able to build it for all the platforms or, it will not be possible to use those exported C APIs in other applications.