in reply to Re^2: $? set to strange values on failure under Win32
in thread $? set to strange values on failure under Win32

What results do you get on vista from this version? 

#! perl -slw
use strict;

undef $^E;
undef $!;
print 'Successful command returning 1';
system 'c:/perl/bin/perl.exe -e"sleep 5; exit 1"';
print "![$!] ?[@{[$?, ':', $?>>8]}] E[$^E]";

undef $^E;
undef $!;
print "\nNonexistent command, attempted direct but fallback to via cmd
+";
system 'c:/doesNotExists.exe';
print "![$!] ?[@{[$?, ':', $?>>8]}] E[$^E]";

undef $^E;
undef $!;
print "\nNonexistent via cmd because 'shell chars'; sending stderr >nu
+ll";
system 'c:/doesNotExists.exe 2>null';
print "![$!] ?[@{[$?, ':', $?>>8]}] E[$^E]";

__END__
C:\test>junk
Use of uninitialized value in undef operator at C:\test\junk.pl line 4
+.
Successful command returning 1
![] ?[256 : 1] E[]
Use of uninitialized value in undef operator at C:\test\junk.pl line 1
+0.

Nonexistent command, attempted direct but fallback to via cmd
'c:/doesNotExists.exe' is not recognized as an internal or external co
+mmand,
operable program or batch file.
![No such file or directory] ?[256 : 1] E[The system cannot find the f
+ile specified]
Use of uninitialized value in undef operator at C:\test\junk.pl line 1
+6.

Nonexistent via cmd because 'shell chars'; sending stderr >null
![] ?[256 : 1] E[The system cannot find the file specified]

[download]

Aside: That Use of uninitialized value in undef operator ... wins the

BrowserUk Award with Special Commendation" in the category of "Most Useless Warning of the Week Month Year Ever".

And that's in a highly contested category. :)

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.
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Re^4: $? set to strange values on failure under Win32
by syphilis (Archbishop) on Jul 07, 2007 at 11:56 UTC
    What results do you get on vista from this version?

    It makes no difference on Vista (apart from occurrences of Use of uninitialized value in undef operator ... :-)

    However, on Windows 2000 (and probably XP, too) it has the desired effect. As long as the op does an undef($^E); before running the system command, I don't envisage any problem with your proposed solution on XP and Windows 2000.

    Here's a copy'n'paste of what I get on Vista: 
    C:\_32\pscrpt>perl try.pl
Use of uninitialized value in undef operator at try.pl line 4.
Successful command returning 1
![] ?[256 : 1] E[The specified image file did not contain a resource s
+ection]
Use of uninitialized value in undef operator at try.pl line 10.

Nonexistent command, attempted direct but fallback to via cmd
'c:/doesNotExists.exe' is not recognized as an internal or external co
+mmand,
operable program or batch file.
![No such file or directory] ?[256 : 1] E[There are no more files]
Use of uninitialized value in undef operator at try.pl line 16.

Nonexistent via cmd because 'shell chars'; sending stderr >null
![] ?[256 : 1] E[There are no more files]

    [download]
    I tried a couple of different builds of perl-5.8.8 on Vista (being sure to ammend the path to perl appropriately each time) and the output doesn't change.

    Anyway, pjf did specify "XP".

    Cheers,
    Rob
[reply]
[d/l]
[select]

      BrowserUk/Rob, your responses are gold. Your answers are exactly the sort of information I was after.

      However it appears that $^E isn't yet my saviour. Under my Windows XP system I get the following results (uninitalized warnings trimmed): 

      #! perl -slw
use strict;

undef $^E;
undef $!;
print 'Successful command returning 1';
system 'c:/perl/bin/perl.exe -e"sleep 5; exit 1"';
print "![$!] ?[@{[$?, ':', $?>>8]}] E[$^E]";

undef $^E;
undef $!;
print "\nNonexistent command, attempted direct but fallback to via cmd
+";
system 'c:/doesNotExists.exe';
print "![$!] ?[@{[$?, ':', $?>>8]}] E[$^E]";

undef $^E;
undef $!;
print "\nNonexistent via cmd because 'shell chars'; sending stderr >nu
+ll";
system 'c:/doesNotExists.exe 2>null';
print "![$!] ?[@{[$?, ':', $?>>8]}] E[$^E]";
__END__
Successful command returning 1
![] ?[256 : 1] E[]

Nonexistent command, attempted direct but fallback to via cmd
'c:/doesNotExists.exe' is not recognized as an internal or external co
+mmand, operable program or batch file.
![No such file or directory] ?[256 : 1] E[]

Nonexistent via cmd because 'shell chars'; sending stderr >null
![] ?[256 : 1] E[]

      [download]

      It appears that redirecting stderr makes an unsuccessful command appear to be a successful command. When an error does occur, it's found inside $! and not $^E, which seems to never be set. Oh my!

      Unfortunately this leaves me with the same tangle as Perl itself has in emulating system() under Windows. Ideally I want to preserve the following behaviours:

      1. Single argument call always goes via the shell
      2. Multi argument call never goes via the shell
      3. It's possible to tell when a command didn't start (ostensibly via $? being set to -1)

      Using the shell means I don't have a reliable way of determining failure to start a command. Not using the shell means I may have to potentially go searching through $ENV{PATH} by hand.

      I guess this is an excellent opportunity for me to go searching for that discussion by tye, and potentially go looking through Perl's source for system() under Windows. I suspect my final result will make heavy use of Win32:: modules to provide the required behaviour.

      Many thanks again!

      Update: Working with Win32::Process is showing great promise.

      Paul Fenwick
      Perl Training Australia
[reply]
[d/l]
        When an error does occur, it's found inside $! and not $^E

        Could $! then be used (instead of $^E) to determine the info you need ? Looking at what you, BrowserUk, and I have posted, it seems that $! might be all you need to consider (so long as it's undeffed before the system call). Or am I missing something ?

        Cheers,
        Rob
        Update: I have no problem with Paul's preference for Win32::Process. The variablility in $^E and $!, wrt to system(), doesn't inspire a lot of confidence.
[reply]

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