[PATCH v3 2/3] The -z and -Z options print only successful and failing syscalls respectively. However, failure of syscall is only known after syscall return. Thus, we end up with something like this on, e.g., ENOENT:

[Dmitry V. Levin]

On Mon, May 13, 2019 at 04:00:31PM +0000, Kohl, Burkhard wrote:
[...]
> > > diff --git a/strace.1.in b/strace.1.in index 76a74119..6ab95836 100644
> > > --- a/strace.1.in
> > > +++ b/strace.1.in
> > > @@ -771,6 +771,13 @@ Print unabbreviated versions of environment, stat, termios, etc.
> > >  calls.  These structures are very common in calls and so the default
> > > behavior displays a reasonable subset of structure members.  Use  this
> > > option to get all of the gory details.
> > > +.TP
> > > +.B \-z
> > > +Print successful syscalls only.
> >
> > It might be better to be precise here regarding what we mean by "successful" exactly.  Maybe "Print only
> > syscalls that did not return an error code"?
> 
> Seems there are 3 possible outcomes for a syscall:
> - zero/successful
> - non-zero/not successful
> - void

Strictly speaking, there are more than 3 possible outcomes for a syscall:
- syscall returns with an error
- syscall returns without an error
- syscall returns but strace fails to fetch error status (see e.g. <unavailable>)
- syscall does not return because of PTRACE_EVENT_EXIT (see print_event_exit)
- syscall does not return because of process disappearance (e.g. due to execve in a neighbour thread)
- strace detaches after entering syscall

I'm inclined to think that -z/-Z options are not versatile enough
to cover this diversity.

Should we implement a more flexible syntax instead?
For example, we could have a traditional -e qualifier=set syntax,
where set is a subset of
all, successful, failed, exited, unavailable, unfinished, detached.


-- 
ldv
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