Add _Fork()

Current POSIX standard requires fork() to be async-signal safe.  Neither
our implementation, nor implementations in other operating systems are,
and practically it is impossible to make fork() async-signal safe without
too much efforts.  Also, that would put undue requirement that all atfork
handlers should be async-signal safe as well, which contradicts its main
use.

As result, Austin Group dropped the requirement, and added a new function
_Fork() that should be async-signal safe, but it does not call atfork
handlers.  Basically, _Fork() can be implemented as a raw syscall.

Release of glibc 2.34 added _Fork(), do the same for FreeBSD.
Clarify threading behavior for fork() in the manpage.

Reviewed by:	markj
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks
Differential revision:	https://reviews.freebsd.org/D31378

1 files changed, 1 insertions, 0 deletions

diff --git a/include/unistd.h b/include/unistd.h
index 9fa9bebbc4c0..5f358ad72d9a 100644
--- a/include/unistd.h
+++ b/include/unistd.h
@@ -586,6 +586,7 @@ int	 undelete(const char *);
 int	 unwhiteout(const char *);
 void	*valloc(size_t);			/* obsoleted by malloc() */
 int	 funlinkat(int, const char *, int, int);
+pid_t	 _Fork(void);
 
 #ifndef _OPTRESET_DECLARED
 #define	_OPTRESET_DECLARED