Perl error: can't call method "encpass" on an undefined value - WHY?

What is the problem with the following code as "root" user:

chpwd.pl

#!/usr/bin/perl
use Unix::PasswdFile;

use strict;
use warnings;

$pw = new Unix::PasswdFile "/etc/passwd";
$pw->passwd("johndoe", $pw->encpass("newpass"));
$pw->commit();
undef $pw;

Whenever I try to execute the above script, I get the following error:

Can't call method "encpass" on an undefined value at /var/www/chpwd.pl line 5

I confirm the file /etc/passwd is not empty or symbolic link, it has ownership of

rw-r--r-- root:root 1022 passwd

User johndoe do exist in the system.

Please help. Thanks.

Trace Output (partial):

Unix::ConfigFile::mode(/usr/local/share/perl/5.10.1/Unix/ConfigFile.pm:152):
152:        my $this = shift;
Unix::ConfigFile::mode(/usr/local/share/perl/5.10.1/Unix/ConfigFile.pm:153):
153:        return $this->{mode} unless @_;
Unix::ConfigFile::new(/usr/local/s开发者_Python百科hare/perl/5.10.1/Unix/ConfigFile.pm:64):
64:     $this->lock() or return undef;
Unix::ConfigFile::lock(/usr/local/share/perl/5.10.1/Unix/ConfigFile.pm:165):
165:        my $this = shift;
Unix::ConfigFile::lock(/usr/local/share/perl/5.10.1/Unix/ConfigFile.pm:167):
167:        return 1 if ($this->locking eq "none");
Unix::ConfigFile::locking(/usr/local/share/perl/5.10.1/Unix/ConfigFile.pm:127):
127:        my $this = shift;
Unix::ConfigFile::locking(/usr/local/share/perl/5.10.1/Unix/ConfigFile.pm:129):
129:        return $this->{locking} unless @_;
Unix::ConfigFile::lock(/usr/local/share/perl/5.10.1/Unix/ConfigFile.pm:168):
168:        return 0 if $this->{locked};
Unix::ConfigFile::lock(/usr/local/share/perl/5.10.1/Unix/ConfigFile.pm:169):
169:        if ($this->locking eq "flock") {
Unix::ConfigFile::locking(/usr/local/share/perl/5.10.1/Unix/ConfigFile.pm:127):
127:        my $this = shift;
Unix::ConfigFile::locking(/usr/local/share/perl/5.10.1/Unix/ConfigFile.pm:129):
129:        return $this->{locking} unless @_;
Unix::ConfigFile::locking(/usr/local/share/perl/5.10.1/Unix/ConfigFile.pm:127):
127:        my $this = shift;
Unix::ConfigFile::locking(/usr/local/share/perl/5.10.1/Unix/ConfigFile.pm:129):
129:        return $this->{locking} unless @_;
Unix::ConfigFile::lock(/usr/local/share/perl/5.10.1/Unix/ConfigFile.pm:174):
174:        my $fh = new IO::File $this->lockfile, O_CREAT|O_EXCL|O_RDWR;
Unix::ConfigFile::lockfile(/usr/local/share/perl/5.10.1/Unix/ConfigFile.pm:145):
145:        my $this = shift;
Unix::ConfigFile::lockfile(/usr/local/share/perl/5.10.1/Unix/ConfigFile.pm:146):
146:        @_ ? $this->{lockfile} = shift : $this->{lockfile};
IO::File::new(/usr/lib/perl/5.10/IO/File.pm:35):
35:     my $type = shift;
IO::File::new(/usr/lib/perl/5.10/IO/File.pm:36):
36:     my $class = ref($type) || $type || "IO::File";
IO::File::new(/usr/lib/perl/5.10/IO/File.pm:37):
37:     @_ >= 0 && @_ <= 3
38:     or croak "usage: new $class [FILENAME [,MODE [,PERMS]]]";
IO::File::new(/usr/lib/perl/5.10/IO/File.pm:39):
39:     my $fh = $class->SUPER::new();
IO::Handle::new(/usr/lib/perl/5.10/IO/Handle.pm:54):
54:     my $class = ref($_[0]) || $_[0] || "IO::Handle";
IO::Handle::new(/usr/lib/perl/5.10/IO/Handle.pm:55):
55:     @_ == 1 or croak "usage: new $class";
IO::Handle::new(/usr/lib/perl/5.10/IO/Handle.pm:56):
56:     my $io = gensym;
Symbol::gensym(/usr/share/perl/5.10/Symbol.pm:23):
23:     my $name = "GEN" . $genseq++;
Symbol::gensym(/usr/share/perl/5.10/Symbol.pm:24):
24:     my $ref = \*{$genpkg . $name};
Symbol::gensym(/usr/share/perl/5.10/Symbol.pm:24):
24:     my $ref = \*{$genpkg . $name};
Symbol::gensym(/usr/share/perl/5.10/Symbol.pm:25):
25:     delete $$genpkg{$name};
Symbol::gensym(/usr/share/perl/5.10/Symbol.pm:26):
26:     $ref;
IO::Handle::new(/usr/lib/perl/5.10/IO/Handle.pm:57):
57:     bless $io, $class;
IO::File::new(/usr/lib/perl/5.10/IO/File.pm:40):
40:     if (@_) {
IO::File::new(/usr/lib/perl/5.10/IO/File.pm:41):
41:     $fh->open(@_)
42:         or return undef;
IO::File::open(/usr/lib/perl/5.10/IO/File.pm:52):
52:     @_ >= 2 && @_ <= 4 or croak 'usage: $fh->open(FILENAME [,MODE [,PERMS]])';
IO::File::open(/usr/lib/perl/5.10/IO/File.pm:53):
53:     my ($fh, $file) = @_;
IO::File::open(/usr/lib/perl/5.10/IO/File.pm:54):
54:     if (@_ > 2) {
IO::File::open(/usr/lib/perl/5.10/IO/File.pm:55):
55:     my ($mode, $perms) = @_[2, 3];
IO::File::open(/usr/lib/perl/5.10/IO/File.pm:56):
56:     if ($mode =~ /^\d+$/) {
IO::File::open(/usr/lib/perl/5.10/IO/File.pm:57):
57:         defined $perms or $perms = 0666;
IO::File::open(/usr/lib/perl/5.10/IO/File.pm:58):
58:         return sysopen($fh, $file, $mode, $perms);
Unix::ConfigFile::lock(/usr/local/share/perl/5.10.1/Unix/ConfigFile.pm:175):
175:        return 0 unless defined($fh);
Unix::ConfigFile::new(/usr/local/share/perl/5.10.1/Unix/ConfigFile.pm:104):
104:        my $this = shift;
Unix::ConfigFile::new(/usr/local/share/perl/5.10.1/Unix/ConfigFile.pm:106):
106:        $this->unlock() or croak "Can't unlock file: $!";
Unix::ConfigFile::unlock(/usr/local/share/perl/5.10.1/Unix/ConfigFile.pm:184):
184:        my $this = shift;
Unix::ConfigFile::unlock(/usr/local/share/perl/5.10.1/Unix/ConfigFile.pm:193):
193:        return 1 if ($this->locking eq "none");
Unix::ConfigFile::locking(/usr/local/share/perl/5.10.1/Unix/ConfigFile.pm:127):
127:        my $this = shift;
Unix::ConfigFile::locking(/usr/local/share/perl/5.10.1/Unix/ConfigFile.pm:129):
129:        return $this->{locking} unless @_;
Unix::ConfigFile::unlock(/usr/local/share/perl/5.10.1/Unix/ConfigFile.pm:194):
194:        return 0 unless $this->{locked};
Carp::croak(/usr/share/perl/5.10/Carp.pm:44):
44: sub croak   { die  shortmess @_ }
Carp::shortmess(/usr/share/perl/5.10/Carp.pm:29):
29: sub shortmess { goto &shortmess_jmp }
Carp::shortmess_real(/usr/share/perl/5.10/Carp/Heavy.pm:51):
51:     local @CARP_NOT = caller();
Carp::shortmess_real(/usr/share/perl/5.10/Carp/Heavy.pm:52):
52:     shortmess_heavy(@_);
Carp::shortmess_heavy(/usr/share/perl/5.10/Carp/Heavy.pm:241):
241:      return longmess_heavy(@_) if $Verbose;

UPDATE:

The above script runs perfectly on my local machine not in server machine.

Obviously, the constructor returns undef rather than the newly created object. Here is a piece of the source of the constructor (in ConfigFile.pm):

# Get a filehandle
my $fh = new IO::File $this->filename, $this->mode;
return undef unless defined($fh);
$this->fh($fh);

# Do file locking - this must happen before read is called or we could
# end up with stale data in memory
if ($this->mode eq "r") {
  $this->lock("shared") or return undef;
}
else {
  $this->lock() or return undef;
}

# Initialize object structure from the file
if (exists $opt{readopts}) {
  $this->read($this->fh, $opt{readopts}) or return undef;
}
else {
  $this->read($this->fh) or return undef;
}
return $this;

You can trace it for yourself and see in which branch it goes.

You should use

use strict;
use warnings;

Always. Do this, and you may discover errors you did not notice before.

It appears that $pw is uninitialized, which would imply that the new declaration has failed. However, it would seem unlikely that it did, since you should have an /etc/passwd file, and as root you should be able to read it. You can easily check for this with:

die "Failed to read PW" unless (defined $pw);

Is "johndoe" an existing user? If not, I suspect the method passwd would fail.

ETA: Since the trace shows an error with locking, perhaps the file is being used by another process, and therefore can't be locked/unlocked?