MacOS has posix_spawn. See https://developer.apple.com/library/archive/documentation/Sy... (yes, that’s an iOS man page. MacOS has the call, too, but I couldn’t find the man page online and it looks identical to me)

I don’t know how they implemented it, though. Under the hood, it could do the equivalent of a fork/exec pair.

XNU is open source; here’s a link into the middle of the implementation, after it’s copied all the necessary attributes of the parent into the new process structure: https://github.com/apple-oss-distributions/xnu/blob/f6217f89...

XNU's posix_spawn implementation is not fork/exec-based. It does roughly what the API suggests it would do.

NtCreateProcess does not implement a forking model. It is analogous to posix_spawn.

If you pass null for the section handle, it shares pages with the calling process, thus implementing a forking model. Or at least the parts of a forking model that some people erroneously believe are responsible for performance differences.