trap does not work in a trap function?

trap does not work in a trap function?

[Dominik Vogt]

[-- Attachment #1: Type: text/plain, Size: 393 bytes --]

The attached small script behaves in an unexpected way (at least to me):

  $ ./sigtest
  shutdown
  shutdown

Why is the shutdown function called again by exit when the EXIT trap was just removed?  This looks like the trap builtin does not properly work from within a trap handler.
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Re: trap does not work in a trap function?

[Dominik Vogt]

> The attached small script behaves in an unexpected way (at least to me):

Hmpf, the attachment has been stripped from the mail somwhere
on the way:

-- BEGIN sigtest
#!/usr/bin/zsh

shutdown () {
	echo shutdown
	trap - EXIT
	exit 77
}

trap shutdown EXIT
shutdown
-- END sigtest

>   $ ./sigtest
>   shutdown
>   shutdown
> 
> Why is the shutdown function called again by exit when the EXIT trap was
> just removed?  This looks like the trap builtin does not properly work from
> within a trap handler.

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Re: trap does not work in a trap function?

[Peter Stephenson]

On Thu, 09 Sep 2010 10:17:33 +0200
"Dominik Vogt" <dominik.vogt@gmx.de> wrote:
> shutdown () {
> 	echo shutdown
> 	trap - EXIT
> 	exit 77
> }
> 
> trap shutdown EXIT
> shutdown
> -- END sigtest
> 
> >   $ ./sigtest
> >   shutdown
> >   shutdown
> > 
> > Why is the shutdown function called again by exit when the EXIT
> > trap was just removed?  This looks like the trap builtin does not
> > properly work from within a trap handler.

The EXIT trap you removed was the one for the "shutdown" function.  In zsh,
EXIT is used with functions as well as scripts.

You can do what you want like this:

shutdown () {
  echo shutdown
  trap 'trap - EXIT' EXIT
  exit 77
}

unless shutdown is itself called from within a function, in which case
you're removing the EXIT trap for that.

From the documentation for the "trap" builtin:

  If  sig  is  0 or EXIT and the trap statement is executed inside
  the body of a function, then the command arg is  executed  after
  the  function completes.  The value of $? at the start of execu-
  tion is the exit status of the shell or the return status of the
  function exiting.  If sig is 0 or EXIT and the trap statement is
  not executed inside the body of a function, then the command arg
  is executed when the shell terminates.

-- 
Peter Stephenson <pws@csr.com>            Software Engineer
Tel: +44 (0)1223 692070                   Cambridge Silicon Radio Limited
Churchill House, Cambridge Business Park, Cowley Road, Cambridge, CB4 0WZ, UK


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Re: trap does not work in a trap function?

[Mikael Magnusson]

On 9 September 2010 10:57, Peter Stephenson <Peter.Stephenson@csr.com> wrote:
> On Thu, 09 Sep 2010 10:17:33 +0200
> "Dominik Vogt" <dominik.vogt@gmx.de> wrote:
>> shutdown () {
>>       echo shutdown
>>       trap - EXIT
>>       exit 77
>> }
>>
>> trap shutdown EXIT
>> shutdown
>> -- END sigtest
>>
>> >   $ ./sigtest
>> >   shutdown
>> >   shutdown
>> >
>> > Why is the shutdown function called again by exit when the EXIT
>> > trap was just removed?  This looks like the trap builtin does not
>> > properly work from within a trap handler.
>
> The EXIT trap you removed was the one for the "shutdown" function.  In zsh,
> EXIT is used with functions as well as scripts.
>
> You can do what you want like this:
>
> shutdown () {
>  echo shutdown
>  trap 'trap - EXIT' EXIT
>  exit 77
> }
>
> unless shutdown is itself called from within a function, in which case
> you're removing the EXIT trap for that.

Could you also use "SIGEXIT() shutdown" and "unfunction SIGEXIT"
instead, regardless of the amount of function nesting? (It seems to
work when I try it).

-- 
Mikael Magnusson

Re: trap does not work in a trap function?

[Peter Stephenson]

On Thu, 9 Sep 2010 11:44:49 +0200
Mikael Magnusson <mikachu@gmail.com> wrote:
> Could you also use "SIGEXIT() shutdown" and "unfunction SIGEXIT"
> instead, regardless of the amount of function nesting? (It seems to
> work when I try it).

What's supposed to happen is the trap, however it is set, is saved and
unset during the function, then restored afterwards (before the scope in
which the function's EXIT trap is run, hence the workaround).  So this
isn't supposed to work.  I get the following:

% TRAPEXIT() { echo This is the exit trap; }
% fn() { unfunction TRAPEXIT; }
% fn
fn:unfunction: no such hash table element: TRAPEXIT
% functions TRAPEXIT
TRAPEXIT () {
	echo This is the exit trap
}
% exit
This is the exit trap

-- 
Peter Stephenson <pws@csr.com>            Software Engineer
Tel: +44 (0)1223 692070                   Cambridge Silicon Radio Limited
Churchill House, Cambridge Business Park, Cowley Road, Cambridge, CB4 0WZ, UK


Member of the CSR plc group of companies. CSR plc registered in England and Wales, registered number 4187346, registered office Churchill House, Cambridge Business Park, Cowley Road, Cambridge, CB4 0WZ, United Kingdom

Re: trap does not work in a trap function?

[Mikael Magnusson]

On 9 September 2010 11:56, Peter Stephenson <Peter.Stephenson@csr.com> wrote:
> On Thu, 9 Sep 2010 11:44:49 +0200
> Mikael Magnusson <mikachu@gmail.com> wrote:
>> Could you also use "SIGEXIT() shutdown" and "unfunction SIGEXIT"
>> instead, regardless of the amount of function nesting? (It seems to
>> work when I try it).
>
> What's supposed to happen is the trap, however it is set, is saved and
> unset during the function, then restored afterwards (before the scope in
> which the function's EXIT trap is run, hence the workaround).  So this
> isn't supposed to work.  I get the following:
>
> % TRAPEXIT() { echo This is the exit trap; }
> % fn() { unfunction TRAPEXIT; }
> % fn
> fn:unfunction: no such hash table element: TRAPEXIT
> % functions TRAPEXIT
> TRAPEXIT () {
>        echo This is the exit trap
> }
> % exit
> This is the exit trap

Ah, I had SIG instead of TRAP which obviously didn't work, which had
the side effect of looking like it worked ;).

-- 
Mikael Magnusson

Re: trap does not work in a trap function?

[Dominik Vogt]

> > > Why is the shutdown function called again by exit when the EXIT
> > > trap was just removed?  This looks like the trap builtin does not
> > > properly work from within a trap handler.
> 
> The EXIT trap you removed was the one for the "shutdown" function.  In
> zsh,
> EXIT is used with functions as well as scripts.
> 
> You can do what you want like this:
> 
> shutdown () {
>   echo shutdown
>   trap 'trap - EXIT' EXIT
>   exit 77
> }
> 
> unless shutdown is itself called from within a function, in which case
> you're removing the EXIT trap for that.

Understood.  But then, what is the localtraps option good
for?  I thought it was meant to do exactly what you describe
above.

Ciao

Dominik ^_^  ^_^
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Re: trap does not work in a trap function?

[Peter Stephenson]

On Thu, 09 Sep 2010 14:03:41 +0200
"Dominik Vogt" <dominik.vogt@gmx.de> wrote:
> But then, what is the localtraps option good
> for?  I thought it was meant to do exactly what you describe
> above.

The "special" traps, the ones that don't map directly to signals, are
handled differently from others; in particular because of its relationship
with functions, TRAPEXIT is effectively always local.  (The other special
traps, SIGZERR and SIGDEBUG, don't have this kind of specialness, but they
have others.)

-- 
Peter Stephenson <pws@csr.com>            Software Engineer
Tel: +44 (0)1223 692070                   Cambridge Silicon Radio Limited
Churchill House, Cambridge Business Park, Cowley Road, Cambridge, CB4 0WZ, UK


Member of the CSR plc group of companies. CSR plc registered in England and Wales, registered number 4187346, registered office Churchill House, Cambridge Business Park, Cowley Road, Cambridge, CB4 0WZ, United Kingdom