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Newlib's posix_spawn has been taken from FreeBSD. The code relies on BSD-specific behaviour of vfork, namely the fact that vfork blocks the parent until the child exits or calls execve as well as the fact that the child shares parent memory in non-COW mode. This behaviour can't be emulated by Cygwin. Cygwin's vfork is equivalent to fork. This is POSIX-compliant, but it's lacking BSD's vfork ingrained synchronization of the parent to wait for the child calling execve, or the chance to just write a variable and the parent will see the result. So this requires a Cygwin-specific solution. The core function of posix_spawn, called do_posix_spawn is now implemented twice, once using the BSD method, and once for Cygwin using Windows synchronization under the hood waiting for the child to call execve and signalling errors upstream. The Windows specifics are hidden inside Cygwin, so newlib only calls internal Cygwin functions. Signed-off-by: Corinna Vinschen <corinna@vinschen.de>