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Old 6th February 2014
thirdm thirdm is offline
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Default double checks in pkg-config, openbsd

I'm looking at /usr/bin/pkg-config on OpenBSD. Is there a reason to write this ...

Code:
if (defined($ENV{PKG_CONFIG_LIBDIR}) && $ENV{PKG_CONFIG_LIBDIR}) {
	@PKGPATH = split(/:/, $ENV{PKG_CONFIG_LIBDIR});
} elsif (defined($ENV{PKG_CONFIG_PATH}) && $ENV{PKG_CONFIG_PATH}) {
	unshift(@PKGPATH, split(/:/, $ENV{PKG_CONFIG_PATH}));
}
... instead of this ...

Code:
if ($ENV{PKG_CONFIG_LIBDIR}) {
	@PKGPATH = split(/:/, $ENV{PKG_CONFIG_LIBDIR});
} elsif ($ENV{PKG_CONFIG_PATH}) {
	unshift(@PKGPATH, split(/:/, $ENV{PKG_CONFIG_PATH}));
}
?


As far as I can tell, in boolean context a hash entry that doesn't exist will yield false, as will an entry that's there but undef, as will one that's there but one of the other false values (0, "", "0"). Neither does it seem like referring to a non-existent element will either cause a warning or autovivify anything.
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Old 6th February 2014
jggimi's Avatar
jggimi jggimi is offline
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Disclaimer. I don't speak perl, and cannot answer the why question. I can, however, provide you with who, so that you can ask them directly, or send them both a patch for their consideration.

The original development was by Marc Espie (espie@), at revision 1.16:
Code:
Revision 1.16: download - view: text, markup, annotated - select for diffs
Thu Dec 14 10:23:34 2006 UTC (7 years, 1 month ago) by espie
Branches: MAIN
Diff to: previous 1.15: preferred, coloured
Changes since revision 1.15: +9 -7 lines
add PKG_CONFIG_LIBDIR, okay bernd@
Here is that patch:
Code:
--- src/usr.bin/pkg-config/pkg-config    2006/12/10 10:58:41    1.15
+++ src/usr.bin/pkg-config/pkg-config    2006/12/14 10:23:34    1.16
@@ -1,5 +1,5 @@
 #!/usr/bin/perl
-# $OpenBSD: pkg-config,v 1.15 2006/12/10 10:58:41 espie Exp $
+# $OpenBSD: pkg-config,v 1.16 2006/12/14 10:23:34 espie Exp $
 
 #$CSK: pkgconfig.pl,v 1.39 2006/11/27 16:26:20 ckuethe Exp $
 # Copyright (c) 2006 Chris Kuethe <ckuethe@openbsd.org>
@@ -24,17 +24,19 @@ use OpenBSD::PkgConfig;
 
 my @PKGPATH = qw(/usr/local/lib/pkgconfig /usr/X11R6/lib/pkgconfig );
 
-if (defined($ENV{'PKG_CONFIG_PATH'}) && $ENV{'PKG_CONFIG_PATH'}) {
-    push(@PKGPATH, split /:/, $ENV{'PKG_CONFIG_PATH'});
+if (defined($ENV{PKG_CONFIG_LIBDIR}) && $ENV{PKG_CONFIG_LIBDIR}) {
+    @PKGPATH = split /:/, $ENV{PKG_CONFIG_LIBDIR};
+} elsif (defined($ENV{PKG_CONFIG_PATH}) && $ENV{PKG_CONFIG_PATH}) {
+    push(@PKGPATH, split /:/, $ENV{PKG_CONFIG_PATH});
 }
 
 my $logfile = '';
-if (defined($ENV{'PKG_CONFIG_LOGFILE'}) && $ENV{'PKG_CONFIG_LOGFILE'}) {
-    $logfile = $ENV{'PKG_CONFIG_LOGFILE'};
+if (defined($ENV{PKG_CONFIG_LOGFILE}) && $ENV{PKG_CONFIG_LOGFILE}) {
+    $logfile = $ENV{PKG_CONFIG_LOGFILE};
 }
 
 my $allow_uninstalled = 
-    defined $ENV{'PKG_CONFIG_DISABLE_UNINSTALLED'} ? 0 : 1;
+    defined $ENV{PKG_CONFIG_DISABLE_UNINSTALLED} ? 0 : 1;
 my $found_uninstalled = 0;
 
 my $version = 0.19; # pretend to be this version of pkgconfig
@@ -45,7 +47,7 @@ my $variables = {};
 my $D = 0; # debug flag
 
 {
-    my $d = $ENV{'PKG_CONFIG_TOP_BUILD_DIR'};
+    my $d = $ENV{PKG_CONFIG_TOP_BUILD_DIR};
     if (defined $d) {
         $variables->{pc_top_builddir} = $d;
     } else {
The revision you question was 1.66, by Jasper Lievisse Adriaanse (jasper@):
Code:
Revision 1.66: download - view: text, markup, annotated - select for diffs
Thu Jun 16 08:38:30 2011 UTC (2 years, 7 months ago) by jasper
Branches: MAIN
Diff to: previous 1.65: preferred, coloured
Changes since revision 1.65: +18 -15 lines
- finally unconfuse emacs by using parentheses for split()
And here's that patch:
Code:
--- src/usr.bin/pkg-config/pkg-config    2011/06/16 09:33:54    1.65
+++ src/usr.bin/pkg-config/pkg-config    2011/06/16 09:38:30    1.66
@@ -1,5 +1,5 @@
 #!/usr/bin/perl
-# $OpenBSD: pkg-config,v 1.65 2011/06/16 08:33:54 jasper Exp $
+# $OpenBSD: pkg-config,v 1.66 2011/06/16 08:38:30 jasper Exp $
 # $CSK: pkgconfig.pl,v 1.39 2006/11/27 16:26:20 ckuethe Exp $
 
 # Copyright (c) 2006 Chris Kuethe <ckuethe@openbsd.org>
@@ -27,9 +27,9 @@ use OpenBSD::PkgConfig;
 my @PKGPATH = qw(/usr/lib/pkgconfig /usr/local/lib/pkgconfig /usr/X11R6/lib/pkgconfig);
 
 if (defined($ENV{PKG_CONFIG_LIBDIR}) && $ENV{PKG_CONFIG_LIBDIR}) {
-    @PKGPATH = split /:/, $ENV{PKG_CONFIG_LIBDIR};
+    @PKGPATH = split(/:/, $ENV{PKG_CONFIG_LIBDIR});
 } elsif (defined($ENV{PKG_CONFIG_PATH}) && $ENV{PKG_CONFIG_PATH}) {
-    unshift(@PKGPATH, split /:/, $ENV{PKG_CONFIG_PATH});
+    unshift(@PKGPATH, split(/:/, $ENV{PKG_CONFIG_PATH}));
 }
 
 my $logfile = '';
@@ -124,7 +124,7 @@ my $rc = 0;
 {
 my $p = join(' ', @ARGV);
 $p =~ s/^\s+//;
-@ARGV = split /\,?\s+/, $p;
+@ARGV = split(/\,?\s+/, $p);
 }
 
 if ($mode{myminvers}) {
@@ -211,9 +211,9 @@ if ($mode{variable}) {
 my $dep_cfg_list = simplify_and_reverse($cfg_full_list);
 
 if ($mode{cflags} || $mode{libs} || $mode{variable}) {
-    push @vlist, do_cflags($dep_cfg_list) if $mode{cflags};
-    push @vlist, do_libs($dep_cfg_list) if $mode{libs};
-    print join(' ', @vlist), "\n" if $rc == 0;
+    push @vlist, do_cflags($dep_cfg_list) if $mode{cflags};
+    push @vlist, do_libs($dep_cfg_list) if $mode{libs};
+    print join(' ', @vlist), "\n" if $rc == 0;
 }
 
 exit $rc;
@@ -256,7 +256,9 @@ sub handle_config
         }
     };
 
-    if (defined $mode{cflags} or $mode{static} or $mode{printrequiresprivate}) {
+    if (defined $mode{cflags}
+        or ($mode{static} && $mode{libs})
+        or $mode{printrequiresprivate}) {
         &$get_props("Requires.private");
     }
     &$get_props("Requires");
@@ -458,7 +460,8 @@ sub do_libs
         my $l = $configs{$pkg}->get_property('Libs', $variables);
         push(@$libs, @$l) if defined $l;
     }
-    
+
+    # Get the linker path directives (-L).
     my $a = OpenBSD::PkgConfig->compress($libs,
         sub {
             local $_ = shift;
@@ -476,7 +479,7 @@ sub do_libs
 
     if ($mode{libs} & 1) {
         my $b = OpenBSD::PkgConfig->rcompress($libs,
-            sub { shift =~ m/^-l/; });
+                sub { shift =~ m/^-l/; });
         return ($a, $b);
     } else {
         return $a;
@@ -533,7 +536,7 @@ Usage: $0 [options]
 --print-provides - print all the modules the given package provides
 --print-requires - print all the modules the given package requires
 --print-requires-private - print all the private modules the given package requires
---silence-errors - don't print error messages in case of error
+--silence-errors - don\'t print error messages in case of error
 --atleast-pkgconfig-version [version] - require a certain version of pkgconfig
 --cflags package [versionspec] [package [versionspec]]
 --cflags-only-I - only output -Iincludepath flags
@@ -560,8 +563,8 @@ sub self_version
     my ($v) = @_;
     my (@a, @b);
 
-    @a = split /\./, $v;
-    @b = split /\./, $version;
+    @a = split(/\./, $v);
+    @b = split(/\./, $version);
 
     if (($b[0] >= $a[0]) && ($b[1] >= $a[1])) {
         return 0;
@@ -593,8 +596,8 @@ sub compare
         $suffix_b[1] = $2;
     }
 
-    my @a = split /\./, $a;
-    my @b = split /\./, $b;
+    my @a = split(/\./, $a);
+    my @b = split(/\./, $b);
 
     while (@a && @b) { #so long as both lists have something
         if (!(@suffix_a || @suffix_b)) {
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  #3   (View Single Post)  
Old 6th February 2014
thirdm thirdm is offline
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The part that nagged me wasn't that there was an elsif, it was that it said...

if (defined(A) && A) ...

instead of just

if (A)

I think, for a check like this in boolean context, if it is the case that $ENV{SHELLVAR} is not in the hash or in the hash with an undefined value or in the hash with a blank or zero value, it's all the same as far as the condition and "if ($ENV{SHELLVAR})" covers it. My experiments seem to confirm this, but I could be missing something. Perl is pretty complicated when you try to analyze the dwim stuff.

It seems to have been this way since the first version by Chris Kuethe. Good of you to look to the history. I should have done that.

I'm a little wary sending this to misc. I've got this intuition this might be something that's really annoying for me to point out (though apparently not so annoying not to bother you guys with it ... sorry). The last person I want to annoy with novice Perl questions is Marc Espie. Pretty sure there's no practical harm or performance problem to the double check, it just bugs me reading it.

Maybe I could try the local Perl mongers see if they know why someone would write it this way.

Last edited by thirdm; 6th February 2014 at 07:18 PM. Reason: clarification
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Old 6th February 2014
J65nko J65nko is offline
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Right now, I don't have time to work on this anymore, but maybe somebody else has

A shell test program aptly called testit that sets the environment and calls that Perl snippet with some diagnostics:
Code:
#!/bin/sh

pkg_path='PKG_PATH=/home/p1:scp://j65nko@pkg_bulk.my.local:ftp://ftp.nluug.nl/pub/OpenBSD'
pkg_config_libdir='PKG_CONFIG_LIBDIR=/usr/bin/local/lib:/sbin/lib:/usr/sbin/lib'

env -i ${pkg_path} ${pkg_config_libdir}  ./wonder_why.pl
The wonder_why.pl program:
Code:
#!/usr/bin/perl

use warnings;
use strict;

my @PKGPATH;

sub show_env {
   print "The environment:\n===============\n";
   foreach my $key (sort keys %ENV) {
     print $key, '=', $ENV{$key}, "\n";
   } 
   print "===============\n\n";
}

sub pr_array {
    my $listref = shift;
    print "\n---- $listref -----\n";
    for (@$listref) {
       print;
       print " \n";
    }
    print "---- end of $listref -----\n";
}


sub orig {

        defined($ENV{PKG_CONFIG_LIBDIR}) ?  print "YES defined\n" : print "NOT defined\n"; 
        $ENV{PKG_CONFIG_LIBDIR} ? print "YES bare\n" : print "NO bare\n";

        if (defined($ENV{PKG_CONFIG_LIBDIR}) && $ENV{PKG_CONFIG_LIBDIR}) {
                print "\nPKG_CONFIG_LIBDIR test succeeded\n";
                @PKGPATH = split(/:/, $ENV{PKG_CONFIG_LIBDIR});
                pr_array(\@PKGPATH);
        } elsif (defined($ENV{PKG_CONFIG_PATH}) && $ENV{PKG_CONFIG_PATH}) {
                print "\nPKG_CONFIG_PATH test succeeded\n";
                unshift(@PKGPATH, split(/:/, $ENV{PKG_CONFIG_PATH}));
                pr_array(\@PKGPATH);
        }

}

sub new {
        if ($ENV{PKG_CONFIG_LIBDIR}) {
            @PKGPATH = split(/:/, $ENV{PKG_CONFIG_LIBDIR});
        } elsif ($ENV{PKG_CONFIG_PATH}) {
            unshift(@PKGPATH, split(/:/, $ENV{PKG_CONFIG_PATH}));
        }
}

show_env() ;
orig();
A test run:
Code:
$ ./testit
                                                                
The environment:
===============
PKG_CONFIG_LIBDIR=/usr/bin/local/lib:/sbin/lib:/usr/sbin/lib
PKG_PATH=/home/p1:scp://j65nko@pkg_bulk.my.local:ftp://ftp.nluug.nl/pub/OpenBSD
===============

YES defined
YES bare

PKG_CONFIG_LIBDIR test succeeded

---- ARRAY(0x1e4f00d83c88) -----
/usr/bin/local/lib 
/sbin/lib 
/usr/sbin/lib 
---- end of ARRAY(0x1e4f00d83c88) -----
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  #5   (View Single Post)  
Old 6th February 2014
ocicat ocicat is offline
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Quote:
Originally Posted by thirdm View Post
As far as I can tell, in boolean context a hash entry that doesn't exist will yield false, as will an entry that's there but undef, as will one that's there but one of the other false values (0, "", "0"). Neither does it seem like referring to a non-existent element will either cause a warning or autovivify anything.
Unfortunately, I don't have as complete an answer as I would like, but I have probably coded similar ambiguities myself, & the following code demonstrates a common issue:
Code:
$ cat ./test.pl                                                                                         
#!/usr/bin/env perl

use strict;
use warnings;
use Data::Dumper;

my %h = ( zero_value => 0, zero_string => '0', empty_string => '', undef_element => undef );

print Data::Dumper->Dump([\%h], [qw/h/]); 

for my $k (keys %h) {
    print "'$k'\t'$h{$k}'\n";
}
print '=' x 4, $/;
for my $k (keys %h) {
    print "'$k'\t'$h{$k}'\n" if defined $h{$k};
}
$ chmod +x ./test.pl
$ ./test.pl                                                                                           
$h = {
       'zero_value' => 0,
       'undef_element' => undef,
       'zero_string' => '0',
       'empty_string' => ''
     };
'zero_value'    '0'
Use of uninitialized value $h{"undef_element"} in concatenation (.) or string at ./autovivify.pl line 12.
'undef_element' ''
'zero_string'   '0'
'empty_string'  ''
====
'zero_value'    '0'
'zero_string'   '0'
'empty_string'  ''
$
I agree that autovivification shouldn't be the issue (& those interested should review the documentation for defined()...), but I have been burnt by undefined hash elements before (as demonstrated by the example above...) which was alleviated by brute force use of defined().

So to further this hand-waving explanation, I would say that the practice is simply stylistic. Plus, I haven't taken the time to fully grok the CVS history.

Last edited by ocicat; 6th February 2014 at 07:40 PM. Reason: remove dangling articles... :)
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Old 6th February 2014
ocicat ocicat is offline
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Quote:
Originally Posted by thirdm View Post
Maybe I could try the local Perl mongers see if they know why someone would write it this way.
My suggestion would be to post to http://perlmonks.org/.
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Old 6th February 2014
thirdm thirdm is offline
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Quote:
Originally Posted by ocicat View Post
My suggestion would be to post to http://perlmonks.org/.
This post is kind of apropos to what I'm getting at: http://www.perlmonks.org/?node_id=680611

A difference between the code you've posted and the pkg-config case is that in your case you only need care if it's undefined before using the hash element. Had you wanted only to print "true" values and written ...

print "'$k'\t'$h{$k}'\n" if defined $h{$k} and $h{$k};

(or how about print "'$k'\t'$h{$k}'\n" if exists $h{$k} and defined $h{$k} and $h{$k}; )

... then I would be wondering why you hadn't simply written

print "'$k'\t'$h{$k}'\n" if $h{$k}

I can understand sprinkling extra defined checks around reflexively from getting yelled at by the Perl warnings. Maybe that's all it's about, plus C programmer instinct to guard against access to an undefined element, analogous to if (p && *p != '\0') ... . In which case, criticizing that practice might be annoying.

I'll look more closely at your script tonight, J65nko. My manager just got after me to make progress on what I'm supposed to be working on.
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Old 6th February 2014
ocicat ocicat is offline
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Quote:
Originally Posted by thirdm View Post
... then I would be wondering why you hadn't simply written

print "'$k'\t'$h{$k}'\n" if $h{$k}
Modify the posted code to the following:
Code:
$ cat test.pl                                                                                         
#!/usr/bin/env perl

use strict;
use warnings;
use Data::Dumper;

my %h = ( zero_value => 0, zero_string => '0', empty_string => '', undef_element => undef );

print Data::Dumper->Dump([\%h], [qw/h/]); 

print '=' x 4, $/;
for my $k (keys %h) {
    print "'$k'\t'$h{$k}'\n";
}

print '=' x 4, $/;
for my $k (keys %h) {
    print "'$k'\t'$h{$k}'\n" if $h{$k};
}
print '=' x 4, $/;
for my $k (keys %h) {
    print "'$k'\t'$h{$k}'\n" if defined $h{$k};
}
$ test.pl                                                                                             
$h = {
       'zero_value' => 0,
       'undef_element' => undef,
       'zero_string' => '0',
       'empty_string' => ''
     };
====
'zero_value'    '0'
Use of uninitialized value $h{"undef_element"} in concatenation (.) or string at ./autovivify.pl line 13.
'undef_element' ''
'zero_string'   '0'
'empty_string'  ''
====
====
'zero_value'    '0'
'zero_string'   '0'
'empty_string'  ''
$
Testing against the element's value all evaluates to false in these cases.

Last edited by ocicat; 6th February 2014 at 08:32 PM.
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Old 7th February 2014
J65nko J65nko is offline
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From the "Perl Cookbook" chapter about hashes by Christiansen and Torkington:

Quote:
... problems caused by confusing existence, definedness, and truth can multiply like rabbits.
They give the following example:
Code:
#!/usr/bin/perl

use warnings;
use strict;

my %age = ();
my $thing;

$age{'Toddler'} = 3;
$age{'Unborn'} = 0;
$age{'Phantasm'} = undef;

foreach $thing('Toddler', 'Unborn', 'Phantasm', 'Relic') { 
   print "$thing: ";
   print "Exists " if exists $age{$thing};
   print "Defined " if defined $age{$thing};
   print "True " if $age{$thing};
   print "\n";
}
The output:
Code:
Toddler: Exists Defined True 
Unborn: Exists Defined 
Phantasm: Exists 
Relic:
A summary of their explanation:

$age{'Unborn'} fails the truth test because the number 0 is one of Perl's false values.

$age{'Phantasm'} only exists because it has been given a value in the hash. Because that value is 'undef' it does not pass the test for definedness.
'undef' is also a Perl false value.
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Old 7th February 2014
thirdm thirdm is offline
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Quote:
Originally Posted by J65nko View Post
Right now, I don't have time to work on this anymore, but maybe somebody else has
wonder_why.pl:
Code:
#!/usr/bin/perl

use warnings;
use strict;
use feature 'say';
my @PKGPATH = '/usr/lib/pkgconfig';

sub show_env {
   say "The environment:\n===============";
   foreach my $key (sort keys %ENV) {
     say $key, '=', $ENV{$key};
   } 
   say "===============\n";
}

sub orig {
        defined($ENV{PKG_CONFIG_LIBDIR}) ?  say "YES defined" : say "NOT defined"; 
        $ENV{PKG_CONFIG_LIBDIR} ? say "YES bare" : say "NO bare";

        if (defined($ENV{PKG_CONFIG_LIBDIR}) && $ENV{PKG_CONFIG_LIBDIR}) {
                say "\nPKG_CONFIG_LIBDIR test succeeded";
                @PKGPATH = split(/:/, $ENV{PKG_CONFIG_LIBDIR});
                { local $, = "\n"; say 'PKGPATH...', @PKGPATH }
        } elsif (defined($ENV{PKG_CONFIG_PATH}) && $ENV{PKG_CONFIG_PATH}) {
                say "\nPKG_CONFIG_PATH test succeeded";
                unshift(@PKGPATH, split(/:/, $ENV{PKG_CONFIG_PATH}));
                { local $, = "\n"; say 'PKGPATH...', @PKGPATH }
        }

}

sub new {
        defined($ENV{PKG_CONFIG_LIBDIR}) ?  say "YES defined" : say "NOT defined"; 
        $ENV{PKG_CONFIG_LIBDIR} ? say "YES bare" : say "NO bare";

        if ($ENV{PKG_CONFIG_LIBDIR}) {
	    say "\nPKG_CONFIG_LIBDIR test succeeded";
	    @PKGPATH = split(/:/, $ENV{PKG_CONFIG_LIBDIR});
	    { local $, = "\n"; say 'PKGPATH...', @PKGPATH }
        } elsif ($ENV{PKG_CONFIG_PATH}) {
	    say "\nPKG_CONFIG_PATH test succeeded";
	    unshift(@PKGPATH, split(/:/, $ENV{PKG_CONFIG_PATH}));
	    { local $, = "\n"; say 'PKGPATH...', @PKGPATH }
        }
}

show_env();
die unless @ARGV == 1;
#say 'running ', @ARGV;
no strict 'refs';
$ARGV[0]->();
say "\n";
testit
Code:
#!/bin/sh

pkg_config_libdir='PKG_CONFIG_LIBDIR=/usr/bin/local/lib:/sbin/lib:/usr/sbin/lib'
pkg_config_path='PKG_CONFIG_PATH=/usr/X11R6/lib/pkgconfig:/usr/X11R6/share/pkgconfig';

for fn in orig new; do
    {
	env -i ${pkg_config_libdir} ${pkg_config_path} ./wonder_why.pl $fn
	env -i ${pkg_config_path} ./wonder_why.pl $fn
    } | tee $fn.out
done

if cmp orig.out new.out; then
    echo Result: output identical
else
    diff -u orig.out new.out
fi
Output...
Code:
The environment:
===============
PKG_CONFIG_LIBDIR=/usr/bin/local/lib:/sbin/lib:/usr/sbin/lib
PKG_CONFIG_PATH=/usr/X11R6/lib/pkgconfig:/usr/X11R6/share/pkgconfig
===============

YES defined
YES bare

PKG_CONFIG_LIBDIR test succeeded
PKGPATH...
/usr/bin/local/lib
/sbin/lib
/usr/sbin/lib


The environment:
===============
PKG_CONFIG_PATH=/usr/X11R6/lib/pkgconfig:/usr/X11R6/share/pkgconfig
===============

NOT defined
NO bare

PKG_CONFIG_PATH test succeeded
PKGPATH...
/usr/X11R6/lib/pkgconfig
/usr/X11R6/share/pkgconfig
/usr/lib/pkgconfig


The environment:
===============
PKG_CONFIG_LIBDIR=/usr/bin/local/lib:/sbin/lib:/usr/sbin/lib
PKG_CONFIG_PATH=/usr/X11R6/lib/pkgconfig:/usr/X11R6/share/pkgconfig
===============

YES defined
YES bare

PKG_CONFIG_LIBDIR test succeeded
PKGPATH...
/usr/bin/local/lib
/sbin/lib
/usr/sbin/lib


The environment:
===============
PKG_CONFIG_PATH=/usr/X11R6/lib/pkgconfig:/usr/X11R6/share/pkgconfig
===============

NOT defined
NO bare

PKG_CONFIG_PATH test succeeded
PKGPATH...
/usr/X11R6/lib/pkgconfig
/usr/X11R6/share/pkgconfig
/usr/lib/pkgconfig


Result: output identical

$
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Old 7th February 2014
thirdm thirdm is offline
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Quote:
Originally Posted by J65nko View Post
A summary of their explanation:

$age{'Unborn'} fails the truth test because the number 0 is one of Perl's false values.

$age{'Phantasm'} only exists because it has been given a value in the hash. Because that value is 'undef' it does not pass the test for definedness.
'undef' is also a Perl false value.
And I'm saying that the pkg-config test's purpose is to look for toddlers to put in the pkg-path. That is, it spells out defined and not equal to blank (and also zero, so you can't put in a path of zero -- can't see that as a realistic need though) but if you view the cookbook example as a kind of truth table you can see that the defined test is redundant. It's only necessary to see if $ENV{VAR} is true before using it since not existing and existing as undef both evaluate to false.
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