Re: How to report a death by signal ?

From: Laurent Bercot <>
Date: Wed, 18 Feb 2015 13:27:46 +0100

On 18/02/2015 11:58, Peter Pentchev wrote:
> OK, so the "not using the whole range of valid exit codes" point rules
> out my obvious reply - "do what the shell does - exit 128 + signum".

  Well the shell is happily ignoring the problem, but it doesn't mean
it has solved it. The shell reserves a few exit codes, then does some
best effort, hoping its invoked commands do not step on its feet.
It works because most commands will avoid exiting something > 125,
but it's still a convention, and most importantly, the shell itself
does not follow that convention (it obviously cannot!)
  So, something like sh -c "sh -c foobar" does not report errors
properly: for 126 and 127, there's no way to know if the code belongs
to the inner shell or the outer shell, and for 128+, there's no way
to know if the inner shell or the foobar process got killed.

  Shells get away with it because when they're nested, it's usually
auto-subshell magic and users don't want to know about the inner
shell; but here, I'm trying to solve the problem for execline commands,
and those tend to be nested a lot - so I definitely cannot reserve codes
for the outer command, because the inner command may very well use the
same ones too.

> Now the question is, do you want to solve this problem in general, or do
> you want to solve it for a particular combination of programs, even if
> new programs may be added to that combination in the future, but only
> under certain rules? If it's the former (in general), then, sorry, I
> don't have a satisfactory answer for you, and the fact that the POSIX
> shell still keeps the "exit 128 + signum" behavior mostly means that
> nobody else has come up with a better one, either (or it might be
> available at least as some kind of an option).

  It just means that nobody cares about shell exit codes. Error handling,
if any, is done inside of shell scripts anyway; and in most scripts, a
random signal killing a running command isn't even something people think
about, and I'm sure there are hilarious behaviours hiding in dark corners
of very popular shell scripts, that fortunately remain asleep to this day.

  For execline, however, I cannot use the same casual approach. Execline
scripts live and die by proper exit code reporting, and carelessness may
lead to very obvious breakage.

> Personally, I quite like the idea of some kind of a pipe (be it a
> pipe(2) pair of file descriptors or an AF_UNIX/PF_UNSPEC socketpair or
> some other kind of communication channel based on file descriptors),
> even if it is only unidirectional:

  Oh, don't get me wrong, I'm a fan of child-to-parent communication via
pipes, and I use it wherever applicable. Unfortunately, the child may
be anything here, so I need something generic.

  Thanks for your input !

Received on Wed Feb 18 2015 - 12:27:46 UTC

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