Delphi DLL - thread safe

Question

I have a Delphi DLL and I want to load it in my app inside a thread (more than one, to be exactly). The DLL just creates a object, then it uses it and destroys it. For that

Answer 1

The most common pitfall is use of global variables. So long as you don't use any global variables (or properly synchronise access to the ones you do use) you will be a long way towards thread safety.

IsMultiThread is used by, for example, the memory manager to make optimisations in the single threaded case. Personally I don't feel this is a worthwhile optimisation these days since practically all useful code has threads of some description. I'd just set IsMultiThread to True at the start of your DLL, e.g. in the begin/end block of your DLL .dpr file, or in one of your unit's initialization sections which amounts to the same thing.

To answer your question directly, the instance of IsMultiThread in your DLL will not be set true unless you create a thread in that DLL. Since you create the threads in your EXE you need to do it yourself in the DLL.

More generally, it's simply impossible to say much about thread safety of your code without knowing what it does and what you actually mean by thread safe. The latter point may sound odd, but what I'm referring to is the issue discussed in Eric Lippert's famous What is this thing you call "thread safe"? article.