Pablo’s code looked unfinished to me. As well as missing documentation, I suspect there may be memory leaks and poor error handling.
The two calls above the “fail:” label look like dead code. The “parse_envlist” result appears to be leaked.
I’m curious why you never call “posix_spawn_file_actions_destroy”. I saw on Open BSD <https://man.openbsd.org/posix_spawn_file_actions_init.3> it reclaims memory, and it seems sensible to call it on other platforms as well.
No error checking on any of the “posix_spawn_file_actions_” methods.
If “posix_spawn” fails, the “pid” variable will be returned uninitialized.