C / C++ / MFC

23-Mar-05 21:15

How do you export a member function / class from and exe so that is can be called from a dll?

David Crow24-Mar-05 5:43

David Crow

24-Mar-05 5:43

I've never had the need to export from an EXE, only from a DLL. Are you sure this is something you need/want?

"Opinions are neither right nor wrong. I cannot change your opinion. I can, however, change what influences your opinion." - David Crow

Steve Messer24-Mar-05 7:01

24-Mar-05 7:01

David,

I don't see away around it for what I am trying to accomplish.

Let me explain what I am trying to do and maybe it will make for sense.

My main application is just a framework that most useful functionality will be added thru plugins.
Each screen is described in XML and all communication is done by messages.

So, here is a simple example. The main application has a screen file that says it has two buttons on it. First a theme selector button which will call the theme.dll and exit button which sends a command to close the applcation. The Theme selector button will load the Theme selector dll which loads its own screen file which has the following information.

1) Theme name
2) Theme version
3) Theme author
4) List of screenshots
5) Next Button
6) Prev Button
7) Apply Button

Since the main application has no prior knowledge of this information the plugin must be responsible to maintaining and updating this data to the main application.

That is why I need my message function to be able to send data both directions.

bool OnMessage( CSystem *pSys, CMessage * pMsg );

The main application knows that the plugin registered the following controls:

1) 2 text controls
2) screenshot control
3) 3 buttons

In the main application the user presses the Theme Selector button. The theme plugin is loaded.

On the plugins screen the next button is pushed which sends a simple message to the plugin "theme.next"

OnMessage( CSystem *pSys, CMessage *pMsg )
{
   if( pMsg->GetSubject() == "theme.next")
   {
      // Send message back to main application
      // containing the information of the next theme in the list
      // My CMessage can contain lists of data which can't be send in
      // a normal windows message.
      // this is where I would like to call
      mainapplication->OnMessage( ... )
      
      // therefore this is why I want to export OnMessage from the main application
      // so the dll can repsond to messages from main.

   }

}

Hope that makes sense.

cmk24-Mar-05 8:34

24-Mar-05 8:34

In your exe source:

#ifdef EXE_EXPORTS
#define EXE_API __declspec(dllexport)
#else
#define EXE_API __declspec(dllimport)
#endif

EXE_API bool OnMessage( CSystem *pSys, CMessage *pMsg )
{
...
return(true);
}

Define EXE_EXPORTS in the project settings when building the .exe.
This will export the function in the exe and create a .lib for the .exe when you build it.

For each plugin dll you need to include a header that defines the exported function(s) e.g. OnMessage(), and link the dll with the .exe's .lib file.

...cmk

Save the whales - collect the whole set

Steve Messer24-Mar-05 8:41

24-Mar-05 8:41

Okay,

But, can you get the name of function dynamically without including the .lib like you would get the address of a function in a dll.

Oh, does it matter if this function is a member function? Would you export it the same way?

Thanks,

Steve

cmk24-Mar-05 10:17

24-Mar-05 10:17

smesser wrote:
But, can you get the name of function dynamically without including the .lib like you would get the address of a function in a dll.

You can use LoadLibrary() on an exe with exported functions; however first read all the MSDN remarks for LoadLibrary().

If you exported a c++ function or a class method you have to use the mangled name in GetProcAddress().

I am guessing that it would work. LoadLibrary() would see that the specified module (the exe) is already loaded into the process and not try and call DllMain() in it. Just remember to call FreeLibrary() as well to decrease the reference count for the module (exe).

smesser wrote:
Oh, does it matter if this function is a member function? Would you export it the same way?

You would export it as you would a member function from a dll.

That is, either export the entire class:

class EXE_API MyClass {
...
);

or just the method:

class MyClass {
...
EXE_API bool MyMethod( ... );
...
);

...cmk

Save the whales - collect the whole set

cmk24-Mar-05 10:24

24-Mar-05 10:24

As David mentioned the more usual way is that the exe just calls known interfaces in the dll.

For you i would suggest a similar approach.
When the exe loads the dll have it call an Init() function that passes a pointer (or reference) for the App object to the dll, then the dll can just use that pointer (or reference) to call back to the exe. You don't need to use LoadLibrary() on the exe from the dll.

I get the impression that you want this as generic as possible, but at some point you have to tell each side how to talk to the other. Smile | :)

...cmk

Save the whales - collect the whole set

Steve Messer24-Mar-05 13:21

24-Mar-05 13:21

cmk wrote:
When the exe loads the dll have it call an Init() function that passes a pointer (or reference) for the App object to the dll, then the dll can just use that pointer (or reference) to call back to the exe. You don't need to use LoadLibrary() on the exe from the dll.

I tried something like that and I couldn't get it to compile. I tried the following:

// Set Pointer to CCOREDlg to g_System-----------------------------------------
HWND hCore;		
hCore = AfxGetApp()->m_pMainWnd->GetSafeHwnd();

CDialog* pDlg = DYNAMIC_DOWNCAST(CDialog, CWnd::FromHandlePermanent(hCore));
if (pDlg != NULL)
{
   CCOREDlg *pCore1;
   pCore1 = (CCOREDlg*)pDlg->GetRuntimeClass();

   CSystem * pSys = NULL;
   IMessage *pMsg = NULL;
   g_pSystem->pCore = pCore1;
   g_pSystem->pCore->OnMessage( pSys, pMsg );
}
//-------------------------------------------------------------------------

This code works fine from the exe but externally it complains that CCOREDlg is unknown.

Maybe I was doing something wrong??

cmk wrote:
I get the impression that you want this as generic as possible, but at some point you have to tell each side how to talk to the other.

Well, that is the crux of my dilemma. Communication to the dll is simple but the other direction is some what of a buggard.

Best case scenario for me would be to export the OnMessage function from the exe and then import it in the dll. But, I am a little fuzzy on the details for doing this.

Thanks for all the input.

cmk24-Mar-05 20:49

24-Mar-05 20:49

smesser wrote:
This code works fine from the exe but externally it complains that CCOREDlg is unknown.
Maybe I was doing something wrong??

Yeah, maybe. Smile | :)

Here are two chunks that i whipped up to test things - it works.
- build the exe as a console application
- build the dll as a non-mfc dll
- exe project defines MYEXE_EXPORTS
- dll project defines MYDLL_EXPORTS
- build exe first, copy .lib file to dll project directory
- build dll, copy .dll file to exe output directory (Debug/Release, where .exe is)
- debug exe and step through code
- or copy exe and dll to empty dir and run exe dll, watch the magic happen.

First the exe code:

<br />
//////////////////////////////////////////////////////////////////////////<br />
//////////////////////////////////////////////////////////////////////////<br />
<br />
#include "stdafx.h"<br />
<br />
//////////////////////////////////////////////////////////////////////////<br />
//////////////////////////////////////////////////////////////////////////<br />
<br />
#ifdef  MYEXE_EXPORTS<br />
#define MYEXE_API __declspec(dllexport)<br />
#else<br />
#define MYEXE_API __declspec(dllimport)<br />
#endif<br />
<br />
//////////////////////////////////////////////////////////////////////////<br />
//////////////////////////////////////////////////////////////////////////<br />
<br />
// forward declare<br />
class  MyApp;<br />
<br />
// function prototypes for plugin exports<br />
typedef  bool  (*PLUGININIT)  ( MyApp *APP );<br />
typedef  void  (*PLUGINTERM)  ( void );<br />
typedef  bool  (*APPTOPLUGIN) ( long MSGID, char *MSGSTR );<br />
<br />
// function pointers to be used by app<br />
PLUGININIT   PluginInit  = NULL;<br />
PLUGINTERM   PluginTerm  = NULL;<br />
APPTOPLUGIN  AppToPlugin = NULL;<br />
<br />
//////////////////////////////////////////////////////////////////////////<br />
//////////////////////////////////////////////////////////////////////////<br />
<br />
class  MYEXE_API  MyApp {<br />
public:<br />
	int   Run         ( void );<br />
	bool  PluginToApp ( long MSGID, char *MSGSTR );<br />
};<br />
<br />
//...............................................<br />
<br />
int  MyApp::Run( void )<br />
{<br />
	// simple test, app sends plugin msg, plugin echos back<br />
	AppToPlugin(1, "First  Msg");<br />
	AppToPlugin(2, "Second Msg");<br />
	return(0);<br />
}<br />
<br />
//...............................................<br />
<br />
// plugin calls this to send msg to app<br />
bool  MyApp::PluginToApp( long MSGID, char *MSGSTR )<br />
{<br />
	if( MSGSTR )  fprintf(stdout, "App got from Plugin: %ld %s \n", MSGID, MSGSTR);<br />
	return(true);<br />
}<br />
<br />
//////////////////////////////////////////////////////////////////////////<br />
//////////////////////////////////////////////////////////////////////////<br />
<br />
int main( int argc, char* argv[] )<br />
{<br />
	MyApp  app;<br />
<br />
	// load the dll - the first arg to the program<br />
	// e.g. run as: > MyExe.exe  MyDll<br />
	HMODULE  lib = ::LoadLibrary(argv[1]);<br />
	if( !lib )  return(-1);<br />
<br />
	// get plugin exported functions<br />
	// note: the ones exported as C++ functions use the mangled names<br />
	PluginInit  = (PLUGININIT)  ::GetProcAddress(lib, "?PluginInit@@YA_NPAVMyApp@@@Z");<br />
	PluginTerm  = (PLUGINTERM)  ::GetProcAddress(lib, "?PluginTerm@@YAXXZ");<br />
	AppToPlugin = (APPTOPLUGIN) ::GetProcAddress(lib, "AppToPlugin");<br />
<br />
	if( !PluginInit )   return(-2);<br />
	if( !PluginTerm )   return(-3);<br />
	if( !AppToPlugin )  return(-4);<br />
<br />
	// tell plugin what app object it should talk to <br />
	if( !PluginInit(&app) )  return(-5);<br />
<br />
	// start app processing<br />
	int  rc = app.Run();<br />
<br />
	// tell plugin we are shutting down<br />
	PluginTerm();<br />
<br />
	::FreeLibrary(lib);<br />
<br />
	return(rc);<br />
}<br />
<br />
//////////////////////////////////////////////////////////////////////////<br />
//////////////////////////////////////////////////////////////////////////<br />
<br />
<br />
Now the dll code:<br />
<br />
//////////////////////////////////////////////////////////////////////////<br />
//////////////////////////////////////////////////////////////////////////<br />
<br />
#include "stdafx.h"<br />
#include "MyDll.h"<br />
#include <stdio.h><br />
<br />
// need to link to MyExe.lib to get MyApp methods<br />
#pragma  comment(lib, "MyExe")<br />
<br />
//////////////////////////////////////////////////////////////////////////<br />
//////////////////////////////////////////////////////////////////////////<br />
<br />
BOOL APIENTRY DllMain( HANDLE MOD, DWORD REASON, LPVOID RES )<br />
{<br />
    switch( REASON ) {<br />
		case DLL_PROCESS_ATTACH :<br />
		case DLL_THREAD_ATTACH  :<br />
		case DLL_THREAD_DETACH  :<br />
		case DLL_PROCESS_DETACH :  break;<br />
    }<br />
    return TRUE;<br />
}<br />
<br />
//////////////////////////////////////////////////////////////////////////<br />
//////////////////////////////////////////////////////////////////////////<br />
// Header File:<br />
//////////////////////////////////////////////////////////////////////////<br />
//////////////////////////////////////////////////////////////////////////<br />
<br />
#ifdef  MYDLL_EXPORTS<br />
#define MYDLL_API __declspec(dllexport)<br />
#else<br />
#define MYDLL_API __declspec(dllimport)<br />
#endif<br />
<br />
//===============================================<br />
<br />
// included from app header<br />
class  MyApp {<br />
public:<br />
	MyApp( void );<br />
<br />
	long  MsgId;<br />
	int   Run         ( void );<br />
	bool  PluginToApp ( long MSGID, char *MSGSTR );<br />
};<br />
<br />
//===============================================<br />
<br />
// these we will export as C++ functions<br />
MYDLL_API  bool  PluginInit  ( MyApp *APP );<br />
MYDLL_API  void  PluginTerm  ( void );<br />
<br />
#ifdef __cplusplus<br />
extern "C" {<br />
#endif<br />
<br />
// this will be exported as a C function<br />
MYDLL_API  bool  AppToPlugin ( long MSGID, char *MSGSTR );<br />
<br />
#ifdef __cplusplus<br />
}<br />
#endif<br />
<br />
//////////////////////////////////////////////////////////////////////////<br />
//////////////////////////////////////////////////////////////////////////<br />
// Source File:<br />
//////////////////////////////////////////////////////////////////////////<br />
//////////////////////////////////////////////////////////////////////////<br />
<br />
MyApp  *App = NULL;<br />
<br />
//===============================================<br />
<br />
bool  PluginInit( MyApp *APP )<br />
{<br />
	App = APP;<br />
	return(App?true:false );<br />
}<br />
<br />
//...............................................<br />
<br />
void  PluginTerm( void )<br />
{<br />
	App = NULL;<br />
}<br />
<br />
//...............................................<br />
<br />
#ifdef __cplusplus<br />
extern "C" {<br />
#endif<br />
<br />
bool  AppToPlugin( long MSGID, char *MSGSTR )<br />
{<br />
	if( MSGSTR )  fprintf(stdout, "Plugin got from App: %ld %s \n", MSGID, MSGSTR);<br />
<br />
	// echo back<br />
	if( App )  App->PluginToApp(MSGID, MSGSTR);<br />
<br />
	return(true);<br />
}<br />
<br />
#ifdef __cplusplus<br />
}<br />
#endif<br />
<br />
//////////////////////////////////////////////////////////////////////////<br />
//////////////////////////////////////////////////////////////////////////<br />

...cmk

Save the whales - collect the whole set

Steve Messer25-Mar-05 5:25

25-Mar-05 5:25

cmk,

Thanks alot, your DA MAN.

I will try this out very shortly.

Steve

Steve Messer25-Mar-05 9:51

25-Mar-05 9:51

cmk,

It there any way you can email the files you made. I tried to reconstruct the projects from your code snipits and I am getting linking errors. I am putting something in the wrong place.

thanks,

Steve

smesser at xmission dot com

Steve Messer24-Mar-05 19:11

24-Mar-05 19:11

I tried this method but I am having linking problems

Here is what I have in my exe

#define EXE_API __declspec(dllexport)
EXE_API BOOL Message(CSystem * pSystem, IMessage * pMessage ); // Member function

Here is what I have in my plugin:

// import.h
#pragma comment( lib, "core.lib"  )   // lib of the exe
#define EXE_API __declspec(dllimport)
EXE_API BOOL Message(CSystem * pSystem, IMessage * pMessage );

Here is the function in the plugin trying to return a message back to the .exe

BOOL PluginA::OnMessage( CSystem* pSystem, IMessage* pMessage, BOOL* Result )
{
   CSystem * pSys = NULL;
   IMessage * pMsg = NULL;

   Message( pSys, pMsg ); // back to exe
   return TRUE;
}

Here is the linker error

HTPCPLUGIN error LNK2019: unresolved external symbol 
"int __cdecl Message(class CSystem *,class IMessage *)" 
(?Message@@YAHPAVCSystem@@PAVIMessage@@@Z) referenced 
in function "public: virtual int __thiscall 
PluginA::OnMessage(class CSystem *,class IMessage *,int *)" (?OnMessage@PluginA@@UAEHPAVCSystem@@PAVIMessage@@PAH@Z)

Did I miss something?

David Crow24-Mar-05 8:38

David Crow

24-Mar-05 8:38

I am working on a plug-in application similar to this. The actual EXE belongs to IBM and my part is just a DLL. The two communicate via COM interfaces. We add a few entries to the registry so that the EXE knows that our plug-in exists, and when certain events happen our plug-in is "notified" through a few standard interfaces. I know this is unlike what you are doing, but I just wanted to mention that this is at least one other way for "generic" EXEs to communicate with plug-in DLLs.

"Opinions are neither right nor wrong. I cannot change your opinion. I can, however, change what influences your opinion." - David Crow

Steve Messer24-Mar-05 13:27