Formerly working Matlab Compiler-based DLL suddenly cannot initialize-CodePudding

I created several DLLs with Matlab Compiler 10 years ago, with C wrappers, to make them available with Delphi. Once I got them working, they always worked - until today! The code in the C wrapper initialization function in question is in the code box below; the "Could not initialize library" is printed to the console when I run my Delphi app.

mclmcrInitialize();
if (!mclInitializeApplication(NULL, 0)) {
    fprintf(stderr, "Could not initialize application\n");
}
if (!libMyDllInitialize()) {
    fprintf(stderr, "Could not initialize library\n");
}

The problem is that this has never happened before, over all the probably 10 years since we first wrote these! My machine has the correct version of the 32-bit 2021a MCR installed, as it has for several years; I've installed this on numerous machines from Windows XP up to Windows 10, The DLLs were last built 5 - 7 years ago; anyway, I don't have access to the Matlab compiler anymore. The only thing that has changed is my app, but not anywhere near where this DLL initialization code is called; also, when the problem first happened, my app was working, then didn't -without any changes. Finally, I went back a few days and rebuilt my app, and it still fails.

So I am really stuck, and need some advanced help in debugging DLL startup issues on Windows. I tried looking in the Windows Event Logger, but nothing appears to show up there. Logs to check? A setting in the Registry that somehow got hosed? Wrong phase of the moon? How does one debug loading/initializing a formerly working DLL when forced to treat it as a black box? Help!

CodePudding user response：

How does one debug loading/initializing a formerly working DLL [...]?

I think there is no definitive answer to your question.

This is how we have gone about debugging the loading/initializing of DLLs and applications and may help you:

We regularly work with systems where we have no source code for the DLLs (and often we don't have any source code for the applications either). We experience DLL conflicts quite regularly. When testing why applications don't start as expected we have found the use of Sysinternal's Process Monitor by Mark Russinovich invaluable.

This will show you system level activity. You can filter for your process and then you will see all file, registry, thread and network activity (although thread and network are quite limited). If the DLL has dependencies then the system tries to find those and so you will be able to discover all dependent DLLs and COM interfaces (by seeing the registry lookups for that interface) that it's looking for. Process Monitor will show if the resource is not found or if access is denied.
Slightly more difficult to discover is if one of the dependencies exists but the export table has changed (so the functions have different signatures or export ordinals). There are ways to check that (by looking at the export and import tables) but generally (if you have access to a working environment) it's enough to check the filesize, timestamp (and the VERSIONINFO resource if there is one) between DLLs.