From mike@fysh.org

This is a bug report for perl from mike@​fysh.org,
generated with the help of perlbug 1.26 running under perl 5.00503.

The following code does not generate a warning​:

#!/usr/bin/perl -w

$cheese{"cake"}=undef;

However, the following code does​:

#!/usr/bin/perl -w

use DB_File;

tie %cheese, 'DB_File', "fred";

$cheese{"cake"}=undef;

I get a "Use of uninitalized variable" warning, which is a shame, because it
makes -w unusable in my particular case (I'm creating millions of undefined
hash elements). The 'exists' versus 'defined' functionality appears to be
working correctly; it's the superfluous warning that causes me a problem.

Cheers,
Mike.

Site configuration information for perl 5.00503​:

Configured by torin at Mon Mar 27 07​:50​:39 PST 2000.

Summary of my perl5 (5.0 patchlevel 5 subversion 3) configuration​:
  Platform​:
  osname=linux, osvers=2.2.14, archname=i386-linux
  uname='linux perv 2.2.14 #1 sat jan 15 20​:48​:59 pst 2000 i686 unknown '
  hint=recommended, useposix=true, d_sigaction=define
  usethreads=undef useperlio=undef d_sfio=undef
  Compiler​:
  cc='cc', optimize='-O2 ', gccversion=2.95.2 20000220 (Debian GNU/Linux)
  cppflags='-Dbool=char -DHAS_BOOL -D_REENTRANT -DDEBIAN -I/usr/local/include'
  ccflags ='-Dbool=char -DHAS_BOOL -D_REENTRANT -DDEBIAN -I/usr/local/include'
  stdchar='char', d_stdstdio=undef, usevfork=false
  intsize=4, longsize=4, ptrsize=4, doublesize=8
  d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=12
  alignbytes=4, usemymalloc=n, prototype=define
  Linker and Libraries​:
  ld='cc', ldflags =' -L/usr/local/lib'
  libpth=/usr/local/lib /lib /usr/lib
  libs=-lnsl -lndbm -lgdbm -ldbm -ldb -ldl -lm -lc -lposix -lcrypt
  libc=, so=so, useshrplib=false, libperl=libperl.a
  Dynamic Linking​:
  dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-rdynamic'
  cccdlflags='-fPIC', lddlflags='-shared -L/usr/local/lib'

Locally applied patches​:
 

@​INC for perl 5.00503​:
  /usr/lib/perl5/5.005/i386-linux
  /usr/lib/perl5/5.005
  /usr/local/lib/site_perl/i386-linux
  /usr/local/lib/site_perl
  /usr/lib/perl5
  .

Environment for perl 5.00503​:
  HOME=/home/mike
  LANG (unset)
  LANGUAGE (unset)
  LD_LIBRARY_PATH (unset)
  LOGDIR (unset)
  PATH=/home/mike/bin/​:/usr/local/bin​:/usr/bin/​:/bin/​:/usr/bin/X11​:/usr/openwin/bin/xview​:/usr/openwin/bin​:/usr/ucb​:/usr/etc​:/usr/local/bin​:/diskB/local/tran/bin​:/usr/local/java/bin/​:/usr/local/lib/xemacs/site-lisp/tm/methods​:/usr/local/teTeX/bin​:/usr/local/netpbm-bin/bin​:/usr/lib/teTeX/bin
  PERL_BADLANG (unset)
  SHELL=/usr/bin/zsh

From [Unknown Contact. See original ticket]

Mike Ashton <mike@​fysh.org> wrote

use DB_File;

tie %cheese, 'DB_File', "fred";

$cheese{"cake"}=undef;

That doesn't work. DB_File only supports strings as hash values, not
undef or references or ... . This reflects a limitation of the
underlying Berkeley DB.

I get a "Use of uninitalized variable" warning, which is a shame, because it
makes -w unusable in my particular case (I'm creating millions of undefined
hash elements).

To avoid the warning, write instead

  $cheese{"cake"}='';

If you really need to distinguish between undef and '', you're out of
luck. You'll need to redesign your data representation.

Mike Guy

From [Unknown Contact. See original ticket]

#!/usr/bin/perl -w
use DB_File;
tie %cheese, 'DB_File', "fred";
$cheese{"cake"}=undef;

I get a "Use of uninitalized variable" warning, ...

Just how do you expect DB_File to store undef in a file?

For both in-memory and on-disk hashes, a give key will exist or not exist.
For in-memory hashes, the value can be the null string, a non-null string,
or undef. For on-disk hashes, the value can be null or non-null.

The docs for DB_File say nothing about the ability to store a value of
undef (as opposed to "" which is a defined() value). My guess is that the
warnings you are seeing are the result of DB_File converting the value of
undef to "" when the key & value are stored on disk.
  -Joe

From @ask

Not a bug.

--
ask bjoern hansen, http​://askbjoernhansen.com/ !try; do();
Develooper LLC, http​://develooper.com/ $stuff->better;

@ask - Status changed from 'open' to 'resolved'