I need to build a DLL capable to load other DLLs at runtime; these other DLLs have a rather intimate relationship with the main DLL (this is Python and extensions), so they must have a common runtime. The main DLL must be a single file that can be simply copied on the target machine. The auxiliary DLLs will be placed in a different directory.
So: “common runtime” means no static linking; “single file + simple copy” rules out the shared MS redistributables, especially when coupled with “different directories”.
I only see the following options: link all DLLs against msvcrt.dll; embed current msvcrtXX into the main DLL and re-export all its symbols; use the msvcrtXX of the host application. To me the first looks the simplest, because it’s a common need and there are many web pages explaining how to go about it. How would you approach this?
I suggest you re-architecture so that your plug-in DLL (the one the host application loads explicitly) contains nothing but proxy functions plus the file management logic. This DLL could use a static runtime, or no runtime at all.
Put the actual functionality (and in particular all code that needs to share a runtime with the Python extensions) in a separate (“primary”) DLL and put this DLL, as well as the runtime of your choice, in the same directory as the Python extensions.
The primary DLL and the runtime could be zipped to the plug-in DLL, along with the core extensions. (I presume this is within the scope of the runtime’s redistributable license, since it’s basically the same as the way most installers work, but you should check for yourself.)
The calls to the plug-in will be slightly less efficient since they have to go through the proxy DLL, but the performance difference probably won’t be measurable. 🙂