main function and extern "C" programming considerations

If a C shared object (CSO) contains an entry point that is in a C++ program, the C++ compiler mangles the function name when a CSO does not contain the main function.

Specify the extern "C" linkage specification to produce an entry point other than main. For more information about coding a program with a main function, see main function support.

The extern "C" linkage specification allows a C++ application entry point to be called through z/TPF enter/back services. This linkage specification forces the linkage to the entry point of the shared object to be C linkage instead of C++ linkage.

The entry point function must have the extern "C" linkage specification. Other functions in a C++ application do not need this linkage specification. A function in a C++ application that is called by another function in the same shared object can have C++ linkage. See z/TPF header files and C++ for more information about extern "C" linkage.

If you do not code the extern "C" linkage specification, the z/TPF offline loader provides an error message that the entry point is not found in the program.

In the following example, the extern "C" linkage specification produces an entry point with the same name as the QZZ0 shared object name. The call to ReadIt in EmpClass does not require this linkage specification. 
     class EmpClass                                                     
     {                                                                  
     …                                                                  
     }                                                                  
                                                                        
     extern "C" void QZZ0 ();                                           
     {                                                                  
       double raise;                                                    
       EmpClass *EmpPtr = new EmpClass[total_employees];                
     …                                                                  
       raise = EmpPtr[i].ReadIt;                                        
     …                                                                  
     }                                                                  
                                                                        
     double & EmpClass::ReadIt                                          
     {                                                                  
                                                                        
     return EmpClass.rate;                                              
     …                                                                  
     }