Thread: Best way to allow plugins for my program?

I'm in design stages of a program, but I'm a bit stuck because I'm not sure how to best implement a plugin system. I want authors of plugins to be able to add functionality to my program. Should each plugin be contained in a DLL, and if so, how can I provide access to my program? Would COM be a good way?

Thanks for the advice. Say for example, I want the plugin to have three functions, Init(), Draw(HDC) and Quit(). How would the DLL be written, and how would I call those functions from the main program? I'm guessing some of the code would be like this:

Code:

//Dll source
void Init()
{
    MessageBox(NULL,"Plugin loaded",NULL,NULL);
}

void Draw(HDC hdc)
{
    string strTemp="This is the plugin Draw function";
    TextOut(hdc,0,0,strTemp.c_str(),strTemp.length());
}

void Quit()
{
    MessageBox(NULL,"Unloading Plugin",NULL,NULL);
}

Code:

//Main program
void LoadPlugins()
{
	for (int i=0;i<numPlugins;i++)
	{
		Handles[i]=LoadLibrary(Plugins[i]);
		Init(); //Is this right? Somehow I dont think so
	}
}

//Sometime later...
void OnDraw(HDC hdc)
{
	Draw(hdc); //This is surely not right
}

//Sometime later...
void UnloadPlugins()
{
	for (int i=0;i<numPlugins;i++)
	{
		Quit(); //Surely not correct
		FreeLibrary(Handles[i]);
	}
}

Now that I think about it, would I use the GetProcAddress() function?

Thank you.

I have a couple of questions:

1. After I use GetProcAddress(), how do I call the function?
2. Can I call DLL functions that have different return types and parameter lists?

>> . Can I call DLL functions that have different return types and parameter lists? <<
Realistically, you need to know the return type, calling convention and parameter list at compile time.

>> After I use GetProcAddress(), how do I call the function? <<
Here is an example of a function that uses dynamic loading of functions:

Code:

BOOL EnableProcessPrivilege(LPCTSTR lpPrivilege) 
{
	/* This function enables a privilege for the current process */
	/* lpPrivilege can be either a privilege constant eg. SE_SHUTDOWN_NAME
	 * which is DEFINEd in the windows headers to the string 
	 * "SeShutdownPrivilege" or the string can be used directly. */

	typedef BOOL (WINAPI * fnptr_openprocesstoken)(HANDLE,DWORD,PHANDLE);
	typedef BOOL (WINAPI * fnptr_adjusttokenprivileges)(HANDLE,BOOL,PTOKEN_PRIVILEGES,DWORD,PTOKEN_PRIVILEGES,PDWORD);
	typedef BOOL (WINAPI * fnptr_lookupprivilegevalue)(LPCTSTR,LPCTSTR,PLUID);

	TOKEN_PRIVILEGES tp       = { 0 };
	LUID             luid     = { 0 };
	HANDLE           hToken   = NULL;
	HMODULE          hAdvapi  = NULL;

	if (!(hAdvapi = LoadLibrary(TEXT("Advapi32.dll"))))
	{
		return FALSE;
	}

	fnptr_openprocesstoken       openprocesstoken      = (fnptr_openprocesstoken)      GetProcAddress(hAdvapi, "OpenProcessToken");
	fnptr_adjusttokenprivileges  adjusttokenprivileges = (fnptr_adjusttokenprivileges) GetProcAddress(hAdvapi, "AdjustTokenPrivileges");
	fnptr_lookupprivilegevalue   lookupprivilegevalue  = (fnptr_lookupprivilegevalue)  GetProcAddress(hAdvapi, "LookupPrivilegeValueA");

	if (!openprocesstoken || !adjusttokenprivileges || !lookupprivilegevalue)
	{
		FreeLibrary(hAdvapi);
		return FALSE;
	}

	/* Get the local id of our desired privilege */
	if (!lookupprivilegevalue(NULL, lpPrivilege, &luid))
	{
		FreeLibrary(hAdvapi);
		return FALSE; 
	}

	/* Fill in TOKEN_PRIVILEGES structure */
	tp.PrivilegeCount           = 1;
	tp.Privileges[0].Luid       = luid;
	tp.Privileges[0].Attributes = SE_PRIVILEGE_ENABLED;

	/* Get the access token for the current process */
	if (!openprocesstoken(GetCurrentProcess(), TOKEN_ADJUST_PRIVILEGES, &hToken))
	{
		FreeLibrary(hAdvapi);
		return FALSE; 
	}

	/* Enable the privilege */
	if (!adjusttokenprivileges(hToken, FALSE, &tp, sizeof(TOKEN_PRIVILEGES), NULL, NULL))
	{
		FreeLibrary(hAdvapi);
		CloseHandle(hToken);
		return FALSE; 
	}

	FreeLibrary(hAdvapi);
	CloseHandle(hToken);

	return TRUE;
}

Note, the syntax of the typedefs. See http://www.function-pointer.org/ for more details.

Cheers, I understand now.