DLL swapping in c, am I doing it wrong?

Question

So I would like to be able to change the behaviour of my application during runtime without using any external scripting languages. In order to achive that I tried using DLLs. I have something like:

begin program;
load dll and function pointers;
init_func_ptr();
loop:
    if(compiled_new_version)
    {
        pause threads;
        unload dll;
        overwrite expired dll with new dll;
        load dll and function pointers;
        resume threads;
    }
    update_func_ptr(state);

Initially I didn't have any problems. However once I wrote some actual code I started crashing after reloading the dll. Only function pointers that I'm exporting/reloading manually are 'init' and update'.

Some information about the crashes I'm having. It's crashing in the middle of nowhere, there're bunch of entries in the callstack with the address '0xCDCDCDCD'(worth to mention that this happens when I'm using Visual Studio as the debugger and the application is compiled in debug mode).

What I believe happening is, when I pause the threads, one or more of them are executing some code within the expired dll so when I unload that dll and resume the threads they're causing the crash.

How can I solve this problem?

Don't post pseudo-code, but a [mcve] of your **actual** code. See [ask] — too honest for this site, Sep 17 '16 at 15:31

Paul Ogilvie · Accepted Answer · 2016-09-22T15:37:46.890

2

You may need to write code in your DLL to check whether it is safe to unload the DLL. For example, every function upon entry increments a DLL global variable and decrements it upon exit. Only when it is zero can the DLL be safely unloaded.

You may also need to write code that manages that the DLL is in "shutdown mode" and should no longer be called. For example a wrapper module with wrappers for all exported functions that can pause the calling thread until the DLL is reloaded. Note that this module probably cannot be in the DLL itself.

I suggest you first make a skeleton application for testing the concept that contains the minimal functions for counting, synchronization and loading.

edited Sep 22 '16 at 15:37

answered Sep 17 '16 at 15:20

Paul Ogilvie

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Sadly I have I/O completion threads which wait on GetQueuedCompletionStatus. Would placing these threads inside the executable and making sure that those threads aren't calling any DLL functions while unloading solve the issue? – AcarX Sep 17 '16 at 15:21
1

Seems it would solve the issue: as long as the DLL is not being called, it can be safely unloaded/reloaded. Note that the `0xcdcdccd` is typical of VS debug uninitialzed memory. – Paul Ogilvie Sep 17 '16 at 15:25
Alright thanks for the answer. Will go ahead and give it a try then report back. – AcarX Sep 17 '16 at 15:27
1

Acar, don't forget that upon unloading the DLL, all data of the DLL will be _lost_! – Paul Ogilvie Sep 17 '16 at 15:31
Thanks for the answer. I've done the suggested and no longer crash while reloading. – AcarX Sep 17 '16 at 16:46

DLL swapping in c, am I doing it wrong?

1 Answers1