fts and ftw are glibc wrappers over glibc wrappers that I had to bypass for better performance. They are made for convenience, not for speed.

Also: last time I looked at fts in the context of glibc, it did not support 64 bit file offset builds (-D_FILE_OFFSET_BITS=64).

I was looking at fts because there exists a BSD licensed implementation of nftw in terms of fts; I was researching the possibility of creating a semantically extended/enriched version of nftw, without coding it entirely from scratch. So I plonked that implementation into my program and, lo and behold, error message from glibc's fts header file about not supporting 64 bit file offsets.