[pdpi]

It’s really not. If you’re implementing a language runtime for language X, and the runtime is written in C, then writing the X GC actually means “writing a GC in C, for the subset of C allocations that represent X allocations.”

E.g. Python objects are represented as PyObject values in CPython. Saying that Python is garbage collected is equivalent to saying PyObjects are garbage collected. Sure enough, PyObject contains the ref count necessary for its GC.

[funcDropShadow]

I have to second GP, writing a GC for C is strange. Not because it is written in C, but for C. State of the art efficient and low latency GC pose requirements on the memory layout of allocated objects and often on the code generated to access those objects, i.e. memory fences of different kind. Some GCs like GHC's or some JVM GCs have special features integrated into the programming language semantics. Since, the semantics of C are basically fixed, the GC misses out on potential improvements and is only ever applicable to a strict subset of C programs.

[pdpi]

There's a subtlety here that I think you're missing.

The Hotspot JVM is a C++ program, CPython is a C program that implements the Python language. Both CPython and Hotspot mostly manage their respective memory in the usual styles of their host languages, but they still need some sort of host language representation for their target language objects.

AIUI, this is less obvious with the JVM (where the runtime manages to avoid a lot of host language allocations), but CPython has an explicit PyObject type, and every target language allocation corresponds to a host level allocation in some way. Because the lifecycle of these allocations in C is inextricably linked to the Python-level objects' lifecycle, you don't explicitly allocate/deallocate those manually on an individual level, you let the GC (Refcount in this case) handle that instead. It just turns out that "letting the GC do it" is really just "letting some other logic in my program do it". This means that implementing the Python GC corresponds exactly to implementing GC for PyObject objects at the C level.