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Re: kern/49017: vfork does not suspend all threads

The following reply was made to PR kern/49017; it has been noted by GNATS.

From: Robert Elz <kre%munnari.OZ.AU@localhost>
To: (Christos Zoulas)
Subject: Re: kern/49017: vfork does not suspend all threads
Date: Thu, 06 Apr 2017 21:15:08 +0700

     Date:        Thu, 6 Apr 2017 07:16:36 -0400
     From: (Christos Zoulas)
     Message-ID:  <>
   | It is much more restricted than that. For example if the child
   | continues and returns from its current stack frame to other higher
   | up frames, it makes changes to them so when the parent resumes it
   | finds an inconsistent state.
 Just the same as setjmp() ... no question but that there are restrictions
 after a vfork(), but they're not as strict (not nearly as strict) as async
 signal safe (which needs to be prepared to execute with the "parent" in
 any state at all - here we know the parent is blocked in vfork() so is
 not currently doing anything.)
   | |  would be a "complete the fork" sys call, which would turn the
   | |  vfork() into a fork()
   | The difficulty with that is that the child needs to know that it has been
   | vforked instead of forked... So the child needs code to handle both cases
   | in general which is annoying.
 Huh?   The code is
 	if (vfork() == 0) {
 		/* here I am the child */
 How does the child possibly not know it has been vforked?   What I
 suggested (as some kind of dream, not a serious proposal, or not without
 trying it anyway) would be to allow
 	if (vfork() == 0) {
 		/* child, with parent blocked and shared mem */
 		/* code here, probably tests to decide what next */
 		/* now parent is unblocked, and child has its own addr space */
 (and no, I don't think "vfork_into_fork()" is a sensible name...)
 Joerg Sonnenberger <> said:
   | The problem is that many of the functions outside the "async signal safe"
   | category are exactly allowed to such things.
 Sure, but there are plenty that are, much more than what is safe in a
 signal handler.
 There's no question but that using vfork() requires some care (or very
 simple operations) but that's not a reason to restrict it more than what
 it already is.
 For the PR that started all this, my suggestion would be to simply fix the
 man page, and close it.

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