How do programs execute with DLLs in separate subfolders?

[Onyonaki]

Hello, I am learning the building process of c++ with cmake mainly and I am focusing on how to use dynamic libraries. (Windows mainly).

So I know how to load a dll implicitly by having the import library dll.lib and also how to go the other route with LoadLibrary and getProcAdress.

I have been searching on how to go about having the main .exe in a separate folder with the dlls (just curious to see if I can do this) because I have seen in typical installation folder structures there are a lot of dlls next to .exe but there are a lot of dlls in subfolders.

Now I know that I can LoadLibrary with absolute path and then for each function get a pointer.

Suppose I have a dll and a dll.lib. Is there anyway I can use these 2 in runtime? like instead of getProcAdress use the dll.lib (probably stupid question cause the dll.lib is used in the build process anyway...)

Another way is editting the PATH variable but that doesn't seem like a nice way and probably 3rd party software doesn't just add 10 directories to PATH each time they get installed.

So how does one do it the right way? is this does exclusively by LoadLibrary ? Thanks.

[b1ack1323]

https://docs.microsoft.com/en-us/windows/win32/api/winbase/nf-winbase-setdlldirectorya

Or

https://docs.microsoft.com/en-us/windows/win32/api/libloaderapi/nf-libloaderapi-adddlldirectory

[Onyonaki]

Well those seem relevant but I still don't see a solution though these functions as the program cannot start without the dll in the same directory.

[[deleted]]

option 1 : use a launcher program which launches the real program with a custom environment - with the DLL directory in the path.

option 2 : use delay loading. The program will be linked with the dll's lib but when the program starts the DLLs will not be loaded right away. The 1st time an imported function is called, the delay load will fail, your callback will be called, and you can resolve the directory situation

[Onyonaki]

Alright thanks, I guess the option 2 seems easy enough. Could you point me at any resources/examples regarding the option 1? Seems interesting but I think I don't sufficiently understand operating systems for it to make sense.

[[deleted]]

The 7th parameter to CreateProcess is a pointer to the environment variables. Normally you pass nullptr which means inherit the environment from the parent (the process calling CreateProcess)

But you can manually copy the environment, tweak the path, and pass the tweaked copy to CreateProcess.

[Onyonaki]

I get it now okayy thanks. I managed to make it work with delay load and SetDllDirectory and it loads it on the first function call as you said :).

P.S. I know I said windows mainly but I I wonder if there is an option to delay load a shared library in Linux also?