[DLL Call] Cleaning up memory

ID:155880
 
Feb 22 2011, 1:50 am
Ok, i will be honest, i am realy a novice wehen it comes to concepts of C/C++.

I played around a bit with calling a dll and I am a bit confused about how and when to clean up the memory used by the dll.

The dll i call (just for testing) is nothing more than this:
extern "C" __declspec(dllexport) char *myFunction(int argc, char *argv[])
{
    return "blabla";
}


Actually calling this function creates a small memory leak and i am not even sure what causes that leak here in this case, since that function does almost nothing.

Maybe someone can give me a hint, or maybe show me a example of a library that does proper cleaning up.

Feb 22 2011, 2:09 am
How are you determining that there is a memory leak? The code you've got there should be fine...
Feb 22 2011, 3:20 am
This does actually bring up an interesting point with the DLL interface, that I have no answer for. Should we be cleaning up the strings provided in argv?

I would assume they are the DLL's responsibility to clean up, not BYOND's.
Feb 22 2011, 3:43 am (Edited on Feb 22 2011, 3:51 am)
In response to Stephen001
Actually, it seems to be the returned value causing the problem:
extern "C" __declspec(dllexport) char* myFunction(int argc, char *argv[])
{

    return NULL; //or return ""; or return "abcdefg"; gives same results
}

Creates Overhead

extern "C" __declspec(dllexport) char* myFunction(int argc, char *argv[])
{

    return *argv;
        //return argv[0]; works too
}

seems to work

Seems like i just failed in understanding, that i need to return a pointer.
Feb 22 2011, 3:47 am
In response to Amsel
Well I suppose your initial example was providing a new string each time. Can you show us your DM code for the first example, that you used to determine a memory leak? It may be that BYOND isn't releasing the string when you'd expect it to.
Feb 22 2011, 3:49 am
In response to Stephen001
"asfasfas" is a const char* to an entry in the strings section in the DLL. He's not returning a new string each time - he's returning the same one every time. There should be no leak (unless your speculations re: having to clean up argv are correct. Although that would be bizarre.)
Feb 22 2011, 3:52 am
In response to Jp
Now the magic of a black box comes into play. As part of incorporating that string into the VM, does BYOND clone it and shove it in a data structure?
Feb 22 2011, 3:56 am
In response to Stephen001
And if it does, how can the DLL clean it up? How can DM clean it up?

If there is a memory leak here, it is a bug in DM.
Feb 22 2011, 4:09 am
In response to Jp

Diagramm for the dreamseeker process.

mob
    proc
        doCalls(var/num)
            for(var/i = 10000, i > 0, i--)
                set background = 0
                call("mylib.dll","myFunction")()

    verb
        stop()
            runn = FALSE

        start()
            runn = TRUE
            while(runn)
                src.doCalls(rand(1000, 13000))
                sleep(1)


Basically I am doing each tick a random amount of calls.
Up to 12000 calls per tick is what my pc can barely handle.
After using the stop verb, memory stays allocated.
Even a restart of the server (by using the host command) doesn't free the memory.
Feb 22 2011, 4:18 am
In response to Amsel
That is distinctly strange. I suggest you report a BYOND bug on the bug tracker, including the DM source code and the DLL source code you've shown us on the forums. If those cause that memory behaviour, it's either a bug in BYOND or it's a bug in the documentation (because you have to do something crazy like del the returned string or clean up argv and it's not documented).
Feb 22 2011, 5:03 am
In response to Jp
Feb 22 2011, 6:46 am (Edited by moderator on Feb 22 2011, 9:04 am)
In response to Jp
I tested a few things and now i'm realy confused. This is the real [expletive deleted], maybe I'll try to get more info tomorrow.