C / C++ / MFC

24-Feb-03 3:36

Hello all. I have a little issue that I'm unsure of how to do. I have a bunch of dlls that are used by various applications. I have 1 application though which needs to call some functions located in ALL the dlls. My problem is how do I dynamically Load and Unload the Dlls from this application so that my app will still run if there are NO dlls present?

What do I have to do to allow this? Any help on the matter is grealy appreciated.
Thank you.

Dan Willis

AlexO24-Feb-03 3:40

AlexO

24-Feb-03 3:40

LoadLibrary

João Paulo Figueira24-Feb-03 3:41

24-Feb-03 3:41

Check out the LoadLibrary API. To retrieve the address of the function you need to call, use the GetProcAddress API with the handle returned by LoadLibrary.
Have fun.

will138324-Feb-03 3:52

24-Feb-03 3:52

Doh!

I knew it was simple one. 1 more question. I thought there was some linking option that I needed to turn on or off to force a dynamic load of the dlls. Is this true, or does the adding of the Load Library take care of that issue?

Once again, Thanks alot!

Dan Willis

João Paulo Figueira24-Feb-03 4:02

24-Feb-03 4:02

You should prototype your export functions with extern "C" in order to avoid name mangling. Anyway, take a look at the DLL's link map or export list to make sure the functions do get exported with human-readable names (the ones you will have to use in GetProcAddress).
Have more fun!

will138324-Feb-03 4:08

24-Feb-03 4:08

Even Better! Woo Hoo!

Another question to have even more fun! Smile | :)

How do I look at the dll link map and/or export list?

Dan Willis

João Paulo Figueira24-Feb-03 4:29

24-Feb-03 4:29

Try this: go to the Project Settings dialog (Alt+F7), select the Link tab, in the Category combo choose "Debug". A checkbox named "Generate mapfile" should be there. Use it when compiling your DLL, and look for any generated .MAP files on your output directory. These are text files.

will138324-Feb-03 4:47

24-Feb-03 4:47

Ok,

Great! Thank you soo much! I really appreciate this! It helps me alot in there becuase sometimes the MSDN just gives a little bit too much information and/or it can be difficult to find exactly what you are looking for.

I greatly appreciate your time Joao (I can't make the squiggly).

Thanks!

Dan Willis

João Paulo Figueira24-Feb-03 6:32

24-Feb-03 6:32

Anytime! This is the spirit of the CodeProject. You cannot imagine how much I learn here everyday, so I feel as my obligation to return some back.

João Paulo

Big Art24-Feb-03 5:32

Big Art

24-Feb-03 5:32

Use the Dependency Walker tool (Depends.exe). It should be part of your VC++ studio installation.

Art

will138324-Feb-03 9:16

24-Feb-03 9:16

Well, I'm having fun, more fun, wahoo! now I'm stuck again. Here's my exported function declaration:

EXPORT char *get_compile_date();

And here's what I'm doing to get it:

typedef LPSTR (CALLBACK* LPFNDLLFUNC1)();

LPSTR strResult;
HINSTANCE hToDLL;
LPFNDLLFUNC1 lpfnDllFunc1; // Function pointer

hToDLL = LoadLibrary(app);
// If the app doesn't exist, then forgettaboutit
if (app == NULL) return NULL;
sprintf(dllVersionString,"%s compiled on ",app);
lpfnDllFunc1 = (LPFNDLLFUNC1)GetProcAddress(hToDLL,"get_compile_date");
if (lpfnDllFunc1 != NULL)
{
// call the function
strResult = lpfnDllFunc1();
if (strResult == NULL) return NULL;
}

I get the dll handle back fine, but the GetProcAddress always returns NULL. Here's the line in the mapping file from the dll containing that function:

0001:000110a0 _get_compile_time 100120a0 f timestamp.obj

What am I doing wrong here that is causing the GetProcAddress to fail?

Dan Willis