because posix_spawn() in linux often calls fork(). I just looked at the manpage now and it says under some conditions it'd call vfork() instead, but I don't remember that being the case when I last looked at this (6-7 years ago?)
Nowadays, posix_spawn() calls clone() on linux, with the CLONE_VM flag, behaving much like vfork() as far as I can tell.
That means the child and parent process shares the memory (until exec() is performed).
Especially if the parent process is multi-threaded this avoids a whole lot of pagefaults that would occur if using fork() when another thread touches memory, possibly triggering a lot of copy-on-writes in the time window between calling fork() and the child calling exec()
