STC VS langs

This technique is often used to implement RAII in C. See example in Standard Template Containers. The library delivers STL-like functionality to C.

I'd say it's evil, but quite understandable very commonly used because there are no built-in alternatives in C. I basically never use void* in user-code, simply because there are no need for it when using a templating technique, like in my STC library. Even in the implementation of STC itself, void* is hardly used, if at all.

Study Other Peoples C Code and here's one that is easy to read: https://github.com/stclib/STC/releases

STC

This is more or less how C-ish templates are implemented in STC library.

As the author of a STL-like templated C container library, I had many of the exact same thoughts when implementing the unordered map. In fact, I also changed to many of the suggestions here, rather than consistently following the C++ umap API. E.g.

Sure it is. C misses a proper efficient generic standard/container library, like my https://github.com/stclib/STC, but that is irrelevant.

Smart Template Containers (STC)

Some data structure and algorithm library in C enable the (optional) separation between the interface of the container (which is expanded in your header) and its implementation (which is expanded in your source), like STC.

The code is here. Note that the input filename is hardcoded in the source code.