Short answer you can't. Nim will keep the free memory for itself in case it needs to use it.
Many libraries have a way to supply it a memory allocator: [https://github.com/nim-lang/Nim/blob/version-1-2/lib/wrappers/openssl.nim#L495](https://github.com/nim-lang/Nim/blob/version-1-2/lib/wrappers/openssl.nim#L495) [https://forum.nim-lang.org/t/4932](https://forum.nim-lang.org/t/4932)
