Big picture on shared libraries when they go wrong, how?

[Gregor Mückl]

On Wednesday, 8 May 2024 at 03:08:15 UTC, Walter Bright wrote:
> Thanks for writing this.
>
> Are you writing solely about DLLs on Windows? They don't have 
> much in common with shared libraries on OSX and Posix.

That is confusing me as well. DLLs share concepts with shared 
libraries on other platforms, but they have subtle differences. 
The ones that come to my mind:

- Shared libraries export everything by default. DLLs export 
nothing by default. This relates to the non-standard 
declspec(dllexport) declaration supported by MSVC to mark 
exported symbols.

- Unix system linkers take shared libraries as input files 
directly. Windows linkers require import libraries. These import 
libraries contain thunks that jump to the real code in the DLL. 
Those thunks can be avoided if the compiler knows a symbol comes 
from a DLL. This is why declspec(dllimport) exists in MSVC (as a 
performance optimization).

- DllMain() is a Windows only construct. If it is present, it is 
invoked for a lot of different events (PROCESS_ATTACH, 
THREAD_ATTACH...). Some Unix/Posix OSes support callbacks for 
loading/unloading libraries at most. The mechanisms are not 
equivalent.

- And then there are all the funny ways in which static 
initialization in C++ can break in combination with Unix shared 
libraries. There are some fun, really opaque pitfalls like static 
constructors getting executed multiple times (and at times when 
you probably woudldn't expect). I don't think the same is true on 
Windows.

These differences result in a number of things that are different 
in one model and not the other. On Unix, it's legal to have name 
collisions between symbols exported from different libraries. 
Typically, the first encountered symbol wins. This allows 
mechanisms like LD_PRELOAD to work and and use a program with a 
replacement malloc() implementation, for example. There is no 
Windows equivalent for this. You'd have to provide a shim DLL in 
the search path that provides all symbols.