use Visual C++ dll in c++ builder

[Jorge López]

Hi all, I have a similar problem:

I have a dll compiled with Visual C++ but I don´t have the .lib or .def associated files. This dll has C style functions and methods inside classes, there is no problem when I call a C style function, but I can´t call the methods of the different classes implemented on the dll library (The linker says: unresolved external...).

I think the problem is something with the names used, because when I generate de .def file, the C style functions names are of this type:

pcre_get_substring_list @2841; pcre_get_substring_list
pcre_malloc @2842; pcre_malloc
pcre_stack_free @2843; pcre_stack_free
pcre_stack_malloc @2844; pcre_stack_malloc
pcre_study @2845; pcre_study
yychar @2846; yychar
yyin @2847; yyin
yyleng @2848; yyleng

But the names of the methods implemented in the different classes are really strange:

_Z12PrintDepTreeN4treeI7depnodeE17preorder_iterato rEi @1 ; _Z12PrintDepTreeN4treeI7depnodeE17preorder_iterato rEi
_Z13yy_scan_bytesPKci @2 ; _Z13yy_scan_bytesPKci
_Z14yy_init_bufferP15yy_buffer_stateP6_iobuf @3 ; _Z14yy_init_bufferP15yy_buffer_stateP6_iobuf
_Z14yy_scan_bufferPcj @4 ; _Z14yy_scan_bufferPcj
_Z14yy_scan_stringPKc @5 ; _Z14yy_scan_stringPKc
_Z15yy_flush_bufferP15yy_buffer_state @6 ; _Z15yy_flush_bufferP15yy_buffer_state
_Z16yy_create_bufferP6_iobufi @7 ; _Z16yy_create_bufferP6_iobufi
_Z16yy_delete_bufferP15yy_buffer_state @8 ; _Z16yy_delete_bufferP15yy_buffer_state
_Z19yy_switch_to_bufferP15yy_buffer_state @9 ; _Z19yy_switch_to_bufferP15yy_buffer_state
_Z20yy_load_buffer_statev @10 ; _Z20yy_load_buffer_statev

Is it possible to call a method of a class? what should I do?

Thank you very much.

[weaknessforcats]

What happens is that a DLL is an old C device. In C function names are unique so you can call the function by using the function name.

However, to support function overloading in C++, the compiler hashes (camoonly called mangling) the function anme plus the arguments into a unique name. It this unique, or mangled, name that is actually called in the compiled code.

Now you can't call the C++ function by name.

What you have to do is add and exports section to your .def file an export the mangled name as something else. Of course, you lose function overloading but you never had that in C anyway.

.def formats vary but it will be something like:

;BEGIN ADLL.DEF FILE
;This DEF file is required becuase the argument to GetProcAddress()
;for the function is a C-string and it will never be equal to the
;C++ mangled name for the function
;This DEF file maps the mangled name to a name that can be used with GetProcAddress()
;Note also: Change project settings in Visual Studio to send the LINK this def file.
;Visual Studio.NET: Project Properties/Linker/Input/Module Definition File/...Path to the def file\Adll.def
LIBRARY ADll 
EXPORTS 
;Exported Name    C++ Mangled Name
AreaOfSquare   =  ?AreaOfSquare@@YGHHH@Z
AreaOfSquare1 =  ?AreaOfSquare@@YAXH2W3
;END DEF FILE 
 

[Jorge López]

Thank you very much for your help.

I tried what you told me and I have errors. I have a file called 'morfo.dll', with the 'impdef' tool of C++ Builder I get the 'morfo.def' file, which has teh format I told you:

LIBRARY MORFO.DLL

EXPORTS

_ZN9tokenizer8tokenizeERKSs @1105; _ZN9tokenizer8tokenizeERKSs
_ZN9tokenizer8tokenizeERKSsRm @1106; _ZN9tokenizer8tokenizeERKSsRm
_ZN9tokenizerC1ERKSs @1107; _ZN9tokenizerC1ERKSs
_ZN9tokenizerC2ERKSs @1108; _ZN9tokenizerC2ERKSs

...

(This functions correspond to a class 'tokenizer' with 2 constructors and 2 functions named 'tokenize'). As you told me, I tried modifiying morfo.def:

LIBRARY MORFO.DLL

EXPORTS

tokenize = _ZN9tokenizer8tokenizeERKSs @1105; _ZN9tokenizer8tokenizeERKSs
tokenize1 = _ZN9tokenizer8tokenizeERKSsRm @1106; _ZN9tokenizer8tokenizeERKSsRm
tokenizer = _ZN9tokenizerC1ERKSs @1107; _ZN9tokenizerC1ERKSs
tokenizer1 = _ZN9tokenizerC2ERKSs @1108; _ZN9tokenizerC2ERKSs

...

I included 'morfo.def' in my project but I had two problems:

[Linker warning]: Attemp to export non-public symbol '_ZN9tokenizer8tokenizeERKSs'

If I get 'morfo.def' automatically from morfo.dll I suppose that morfo.dll really export this symbols but... It could be something about the format??

And:

[Linker error]: Unresolved external: tokenizer::tokenizer(const STL::basic_string)

So the problem persists...

Another thing I tried it's getting a static library 'morfo.lib' with 'implib' tool of C++ Builder, and I tried adding it to my project, but the problem continues...

What should I do?

Thank you very much!

[Jorge López]

Just one more thing, I tried too with 'morfo.def' modified and added to my project and dynamic loading. To call the constructor I define:

typedef tokenizer * __stdcall (* FPTR)(const string &);

and I use this code:

tokenizer *tok;

HMODULE dllHandle = LoadLibrary("morfo.dll");
FPTR cons_tok = (FPTR) GetProcAddress(dllHandle, "tokenizer1");

tok = cons_tok("hola");

(tokenizer1 is the name I assigned to the constructor that recives a string, see it in my earlier post) But I have the same linker Warning I told you before and a run-time error, How can I call the constructor?

Thank you!!

[weaknessforcats]

(tokenizer1 is the name I assigned to the constructor that recives a string, see it in my earlier post) But I have the same linker Warning I told you before and a run-time error, How can I call the constructor?

Remember, this is C.

There are no constructors, methods, etc. Member functions of classes require that the address of the object be passed as the first argument. The compiler does this secretly so the arguments in the class do not have one for a pointer to a class object. If you try to put one in, things don't compile.

Bottom line: You cannot call member functions (that incudes constructors) from C.

You can only call C functions.

So to create an object, you write a C function in the dll that you can call and have that function create the object:

void* Create(char* str)
{
     return (void*)new MyClass(str);
}
 

Then in C you can:

void* ptr;
void* (*Create)(char*);   //the Create function pointer
HMODULE dllHandle = LoadLibrary("morfo.dll"); 
Create = void*(*)(char*) GetProcAddress(dllHandle, "Create");
ptr  = Create("hola");
 

Now ptr has the address of a MyClass object.

To call a member function MyClass::MethodA(void* obj,int arg), you write another C function in the dll:

void* MyClassMethodA(void* obj, int arg)
{
     MyClass* ptr = (MyClass*)obj;
     obj->MethodA(arg)
     return (void*) obj;
 
}
 

Then back in C:

void* (*MyClassMethodA)(void*, int);  //the function pointer
MyClassMethodA = void*(*)(void*, int) GetProcAddress(dllHandle, "MyClassMethodA");
ptr  = MyClassMethodA(ptr, 25);
 

Functions like MyClassMethodA are called wrappers and you will need to write one of these inthe dll for each of your class member functions that you want to call from C.

[Jorge López]

Thank you so much for your help!!!!!
It´s been really useful!!