Question

in my Windows C++ program I have a few dependencies on DLLs (coming with drivers of input devices). I don’t actually load the DLLs myself, but the drivers provide (small) .lib library that I statically link against (and I assume it is those libraries that make sure the DLLs are present in the system and loads them). I’m writing an application that can take input from a series of video cameras. At run-time, the user chooses which one to use. Currently my problem is that my routines that query whether a camera is connected already require the functionality of the camera being present on the system. I.e. let’s say there is camera model A and B, the user has to install the drivers for A and B, even if he knows he just owns model B. The user has to do this, as otherwise my program won’t even start (then when it started it will of course tell the user which of the two cameras are actually connected).

I’d like to know whether there is any possibility, at run-time, to determine which of the DLLs are present, and for those that aren’t, somehow disable loading even the static (and, thus, dynamic) component.

So basically my problem is that you cannot do if(DLL was found){ #include "source that includes header using functions defined in lib which loads DLL"}

Answer 1

I think using the DELAYLOAD linker flag may provide the functionality required. It would allow linking with the .lib files but would only attempt to load the DLL if it is used:

link.exe … /DELAYLOAD:cameraA.dll /DELAYLOAD:cameraB.dll Delayimp.lib

The code would be structured something similar to:

if (/* user selected A */)
{
    // Use camera A functions, resulting in load of cameraA's DLL.
}
else
{
    // Use camera B functions, resulting in load of cameraB's DLL.
}

From Linker Support for Delay-Loaded DLLs
:

Beginning with Visual C++ 6.0, when statically linking with a DLL, the
linker provides options to delay load the DLL until the program calls
a function in that DLL.

An application can delay load a DLL using the /DELAYLOAD (Delay Load Import)
linker option with a helper function (default implementation provided by
Visual C++). The helper function will load the DLL at run time by calling
LoadLibrary and GetProcAddress for you.

You should consider delay loading a DLL if: 

- Your program may not call a function in the DLL.
- A function in the DLL may not get called until late in your program's
 execution.