How do I programmatically find why a DLL fails to load? - c++

Question

My C++ program calls LoadLibraryEx() to load a third party DLL. The result is a null handle - it fails to load. A call to GetLastError() returns zero afterwards which isn't of much use but at least it's not a missing DLL file.
The code goes something like this:
HINSTANCE instance = ::LoadLibraryExW(
path, 0, LOAD_WITH_ALTERED_SEARCH_PATH );
if (instance == 0)
{
DWORD lastError = GetLastError();
LOG( "Failed to load, error code is " +
LastErrorAsString( lastError ));
return E_FAIL;
}
I cannot access that machine - I can only deploy code there and observe logs uploaded into network storage.
How would I programmatically find why the DLL fails to load?

Answer 1

Probably your dll can be found but it has a dependency on a dll that cannot found on the remote machine.
If you cannot use Dependency Walker then you can try to use techniques as described in articles such as this:
How to determine a windows executables DLL dependencies programatically?

Answer 2

Okay, so I checked better - and indeed there was another WinAPI call which caused "last error" to be overwritten after LoadLibraryEx(). It was hidden deep inside several layers of C++ helper objects so I didn't notice it earlier. So it was a bug in the caller code and the real "last error" was non-zero.

Answer 3

It looks like the problem is likely to be third-party software that has installed a hook or other anti-virus measure, the hook might be buggy and not setting the correct last error code.
As a troubleshooting measure, you should try LdrLoadDll instead of LoadLibraryEx.
Note that this is an undocumented internal function, so you might prefer not to use it in production code, but it would be a useful troubleshooting step as it should produce a more useful error code.