Re: [PATCH] [BZ #18433] Check file access/existence before forking.

[Szabolcs Nagy <szabolcs dot nagy at arm dot com>]

On 18/09/15 16:19, Alexander Monakov wrote:
> On Fri, 18 Sep 2015, Szabolcs Nagy wrote:
>
> > On 18/09/15 15:44, Carlos O'Donell wrote:
> > > Note that it is possible to use vfork in certain conditions,
> > > and we do in glibc. So one should not entirely dismiss vfork,
> > > but that's slightly off topic.
> >
> > are you sure?
> >
> > i think all use of vfork is invalid c: the compiler can
> > spill registers on the stack then in the child clobber
> > them, then after vfork returns in the parent the
> > clobbered registers are restored breaking the expectations
> > of the compiler. (this can break independently of how
> > the c code around vfork looks like).
>
> The same argument applies to setjmp, and the solution in GCC is the
> same for both: GCC internally recognizes vfork, setjmp, and a few other
> functions as functions that "return twice".
>
> (after all, if it was broken like you describe, one would expect to witness
> such breakage in practice, for instance with GNU Make prior to 4.0)

that's surprising: setjmp is standard c
and it is a macro so it cannot be called
through a function pointer.. this is not
true for vfork so it's not clear why gcc
recognizes it.

> Alexander