XNU has a load of non-standard extensions to posix_spawn for setting parameters. The problem with the API in general is that it’s designed to be able to be possible to implement in userspace and so is a strict subset of the set of things you can do with vfork. The things I actually want from a process creation API on *NIX are:

• Set up all of the security credentials (including things like Capsicum mode and other sandboxing).
• Open some files with the credentials of the new process, but before execve.
• Set up the file descriptor table with a specific layout.
• Set up shared memory regions that exist after execve.
• Don’t inherit any file descriptors that aren’t explicitly passed.

Unfortunately posix_spawn helps with precisely zero of these.