An "environment block" consists of a null-terminated block of null-terminated strings. Using caml_stat_strdup_to_os doesn't work because it only supports null-terminated strings -- it stops on the first \0 and misses the rest of the block.

ocaml itself deals with this here:

https://github.com/ocaml/ocaml/blob/000d15d4e229e895612177590df08d0364b1cb04/otherlibs/unix/createprocess.c#L122-L127

but an external library like Lwt can't do the same thing. see ocaml/ocaml#11449 re: adding a public API.