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[perl #120959] @DB::args inconsitently reflects changes to @_

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Jim Avera
January 9, 2014 23:35
[perl #120959] @DB::args inconsitently reflects changes to @_
Message ID:
# New Ticket Created by  Jim Avera 
# Please include the string:  [perl #120959]
# in the subject line of all future correspondence about this issue. 
# <URL: >

This is a bug report for perl from,
generated with the help of perlbug 1.39 running under perl 5.14.2.

When caller() is called from package DB, it sets @DB::args
"to be the arguments with which the subroutine was invoked...
@DB::args will contain modifications the subroutine makes to @_ or
its contents, not the original values at call time" [perlfunc]

However this is not always the case.

If the calling sub (not in package DB) modifies @_ before calling into
package DB, then @DB::args may or may not reflect the changes.
Specifically, "shift @_" is effectively un-done and the removed arg
re-appears in @DB::args, BUT only if no other changes have been made
to @_, such as 'unshift' or 'push'.

Is this a bug, a side-effect of the optimizer, or intentional behavior?

On the surface, it seems like @DB::args should either always get
the current state of @_ (my preference), or always get the args
before any modifications.

[This matters to me because @_ isn't displayed reliably in a
a debug tool -- ]

use strict; use warnings;
sub f(@) {
   shift @_;
   my @a = &DB::test; print "After 'shift', DB::test returned (@a)\n";
sub g(@) {
   pop @_;
   my @a = &DB::test; print "After 'pop', DB::test returned (@a)\n";
sub h(@) {
   shift @_; unshift @_,"subst";
   my @a = &DB::test; print "After 'shift' then 'unshift', DB::test 
returned (@a)\n";
f(1,2,3);  # result is (1,2,3), i.e. shift alone not visible
g(1,2,3);  # result is (1,2)         pop is visible
h(1,2,3);  # result is ("subst",2,3) shift visible if followed by unshift

package DB;
sub test {
   (undef) = (caller(1));
   return @DB::args;

Site configuration information for perl 5.14.2:

Configured by Debian Project at Thu Jul 18 22:04:35 UTC 2013.

Summary of my perl5 (revision 5 version 14 subversion 2) configuration:

     osname=linux, osvers=3.2.0-37-generic, 
     uname='linux roseapple 3.2.0-37-generic #58-ubuntu smp thu jan 24 
15:28:10 utc 2013 x86_64 x86_64 x86_64 gnulinux '
     config_args='-Dusethreads -Duselargefiles -Dccflags=-DDEBIAN 
-D_FORTIFY_SOURCE=2 -g -O2 -fstack-protector --param=ssp-buffer-size=4 
-Wformat -Werror=format-security -Dldflags= -Wl,-Bsymbolic-functions 
-Wl,-z,relro -Dlddlflags=-shared -Wl,-Bsymbolic-functions -Wl,-z,relro 
-Dcccdlflags=-fPIC -Darchname=x86_64-linux-gnu -Dprefix=/usr 
-Dprivlib=/usr/share/perl/5.14 -Darchlib=/usr/lib/perl/5.14 
-Dvendorprefix=/usr -Dvendorlib=/usr/share/perl5 
-Dvendorarch=/usr/lib/perl5 -Dsiteprefix=/usr/local 
-Dsitearch=/usr/local/lib/perl/5.14.2 -Dman1dir=/usr/share/man/man1 
-Dman3dir=/usr/share/man/man3 -Dsiteman1dir=/usr/local/man/man1 
-Dsiteman3dir=/usr/local/man/man3 -Duse64bitint -Dman1ext=1 
-Dman3ext=3perl -Dpager=/usr/bin/sensible-pager -Uafs -Ud_csh -Ud_ualarm 
-Uusesfio -Uusenm -Ui_libutil -DDEBUGGING=-g -Doptimize=-O2 -Duseshrplib -des'
     hint=recommended, useposix=true, d_sigaction=define
     useithreads=define, usemultiplicity=define
     useperlio=define, d_sfio=undef, uselargefiles=define, usesocks=undef
     use64bitint=define, use64bitall=define, uselongdouble=undef
     usemymalloc=n, bincompat5005=undef
     cc='cc', ccflags ='-D_REENTRANT -D_GNU_SOURCE -DDEBIAN 
-fstack-protector -fno-strict-aliasing -pipe -I/usr/local/include 
     optimize='-O2 -g',
     cppflags='-D_REENTRANT -D_GNU_SOURCE -DDEBIAN -fstack-protector 
-fno-strict-aliasing -pipe -I/usr/local/include'
     ccversion='', gccversion='4.8.1', gccosandvers=''
     intsize=4, longsize=8, ptrsize=8, doublesize=8, byteorder=12345678
     d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=16
     ivtype='long', ivsize=8, nvtype='double', nvsize=8, Off_t='off_t', 
     alignbytes=8, prototype=define
   Linker and Libraries:
     ld='cc', ldflags =' -fstack-protector -L/usr/local/lib'
     libpth=/usr/local/lib /lib/x86_64-linux-gnu /lib/../lib 
/usr/lib/x86_64-linux-gnu /usr/lib/../lib /lib /usr/lib
     libs=-lgdbm -lgdbm_compat -ldb -ldl -lm -lpthread -lc -lcrypt
     perllibs=-ldl -lm -lpthread -lc -lcrypt
     libc=, so=so, useshrplib=true,
   Dynamic Linking:
     dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-Wl,-E'
     cccdlflags='-fPIC', lddlflags='-shared -L/usr/local/lib 

Locally applied patches:

@INC for perl 5.14.2:

Environment for perl 5.14.2:
     LOGDIR (unset)
     PERL_BADLANG (unset)
     PERL_MB_OPT=--install_base /home/jima/perl5

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