[RFC] Saving the latest errno from syscalls.

[Kostik Belousov]

On Thu, Mar 11, 2010 at 06:15:07PM -0500, Jung-uk Kim wrote:
> On Thursday 11 March 2010 04:55 pm, Marcel Moolenaar wrote:
> > On Mar 11, 2010, at 1:24 PM, Jung-uk Kim wrote:
> > > While I was debugging syscalls, I found a very useful field in
> > > struct thread, td_errno.  It seems it was added for dtrace but it
> > > is only populated on amd64 and i386.  Is the attached patch
> > > acceptable for maintainers of other platforms?
> >
> > Isn't it better to do it in cpu_set_syscall_retval()?
> > That way you catch all cases, plus you can save the
> > translated error as well...
> 
> I just took amd64/i386 as an example and I was not sure whether it was 
> meant to store translated error or not.  Does anyone with DTrace 
> internal knowledge answer the question?

I do not know that much about DTrace, but it seems that setting td_errno
in cpu_set_syscall_retval() is too late. Dtrace has a probe after the
syscall return, and it is called right before cpu_set_syscall_retval()
can be reasonably called. The probe only issued for syscall that goes
into sysent.
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