The vast majority of programs that fork are doing fork() followed almost immediately by exec(), to the extent that on macOS for example a process is only really considered safe for exec() after fork() happens. Pretty much nothing else is considered safe.
Yeah; that would be my assumption too. I worked one time on a significant project that benefit from fork() without exec() and it was a monstrous pain - only if you own every single line of code in your project, have centralized resource management, and have no significant library dependencies should you ever consider doing this.
Yeah, you can't depend on pthreads or pthread mutexes (they're not defined as being fork safe).
The entirety of Foundation (so presumably anything in Swift) is not fork safe either.
To be clear: "not fork safe" in this case means "severely constrained environment": e.g. you can do things liker limits, set up pipes, etc but good luck with much more. I guess morally similar to the restrictions you have in a signal handler, albeit with different restrictions.
