Rhandom has asked for the wisdom of the Perl Monks concerning the following question:

Is there a penalty for using the DB package. As in the example of the caller function taken from Carp. 
my @a;
while ( do { { package DB; @a = caller($i++) } } ) {
  my @args = @DB::args;  # odd results for some data arguments
}

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It seems that this would give a penalty since there is now a copy of all arguments in @DB::args - unless @DB::args are just special pointers to the actual data with magic lookups upon access. Anyway - anybody have insight?

my @a=qw(random brilliant braindead); print $a[rand(@a)];

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Re: package DB penalty?
by hv (Prior) on May 15, 2003 at 00:54 UTC

    The @DB::args array is populated by copying the pointers from the actual @_ array, so the cost is a pretty negligible fraction of a caller() invocation - grep for the reference to PL_dbargs in pp_ctl.c to find the 20-odd line block of code involved (within pp_caller()).

    I should stress that this is not intended for everyday use, and would suspect that overly imaginative use of this facility is likely to lead you to coredumps and JAPHs.

    Hugo
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      Thank you. This is exactly the type of answer I was looking for.

      I agree with the "not intended for everyday use." Simple assignment of @args = @DB::args or @args = map {Carp::format_arg($_)} @DB::args can cause a blow up. A case we see a lot is when there is a single argument with a value of undef. It happens occasionally - but it does happen. Kind of frustrating when a cluck that is supposed to debug info takes the entire script down. Wish I had an easily duplicatable code snippet for you - but usually there is a bit of a caller trace involved.

      Thanks again.

      my @a=qw(random brilliant braindead); print $a[rand(@a)];
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Re: package DB penalty?
by bobn (Chaplain) on May 15, 2003 at 01:52 UTC
    Even if @args had been a deep copy, rather than a shallow one, of @DB::args, it stopped existing after the while {} loop ended, since it was lexical.

    Hence, no penalty whatsoever.


    Bob Niederman, http://bob-n.com
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      OOOPS!

      The variable goes out of scope, but, at least under Linux (redhat 7.2), the process still holds onto the RAM.

      Never mind.

      Bob Niederman, http://bob-n.com
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Re: package DB penalty?
by PodMaster (Abbot) on May 14, 2003 at 23:57 UTC
    You're not using the DB package (DB - programmatic interface to the Perl debugging API).

    You code snippet makes no sense. Where does @DB::args come from? What are you doing with @DB::a? What are you doing with my @a? my @args?

    @foo = @bar; copies @bar into the variable @foo. Read perldoc perldata.

    update: ah, I see. Still the @a part makes little sense, as does the concern about penalty.

    MJD says you can't just make shit up and expect the computer to know what you mean, retardo!
    I run a Win32 PPM repository for perl 5.6x+5.8x. I take requests.
    ** The Third rule of perl club is a statement of fact: pod is sexy.

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      The reference to @DB::args comes from perldoc -f caller.

      Note that the OP's snippet comes from the Carp module as shipped with an oldish version of perl; that effect is achieved with rather different code in more recent versions (but using the same underlying mechanism) within Carp::caller_info() in Carp::Heavy.

      Hugo
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        Interesting - I didn't know they had changed it on 5.8.0. Perl 5.6.0 still used it the same old way. The new way still actually uses the exact same mechanism - just using different programming technique.

        my @a=qw(random brilliant braindead); print $a[rand(@a)];
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