[27360] in Perl-Users-Digest
Perl-Users Digest, Issue: 9058 Volume: 10
daemon@ATHENA.MIT.EDU (Perl-Users Digest)
Thu Mar 16 06:05:49 2006
Date: Thu, 16 Mar 2006 03:05:04 -0800 (PST)
From: Perl-Users Digest <Perl-Users-Request@ruby.OCE.ORST.EDU>
To: Perl-Users@ruby.OCE.ORST.EDU (Perl-Users Digest)
Perl-Users Digest Thu, 16 Mar 2006 Volume: 10 Number: 9058
Today's topics:
Re: Determine read/write status of filehandles connecte (Anno Siegel)
Re: Determine read/write status of filehandles connecte <sisyphus1@nomail.afraid.org>
Re: Determine read/write status of filehandles connecte (Anno Siegel)
Re: Great JAPH (was Re: Creating Graphs Dynamically wit <zen13097@zen.co.uk>
Timeout value in Net::FTP seems to have no effect <news@lawshouse.org>
Digest Administrivia (Last modified: 6 Apr 01) (Perl-Users-Digest Admin)
----------------------------------------------------------------------
Date: 16 Mar 2006 08:49:13 GMT
From: anno4000@lublin.zrz.tu-berlin.de (Anno Siegel)
Subject: Re: Determine read/write status of filehandles connected to memory objects.
Message-Id: <dvb8q9$vs$1@mamenchi.zrz.TU-Berlin.DE>
Sisyphus <sisyphus1@nomail.afraid.org> wrote in comp.lang.perl.misc:
> Hi,
>
> On unix, with filehandles connected to normal files, we can query the
> read/write status of the filehandle by examining the return value of the
> fcntl() function:
>
> use Fcntl;
> # some code that creates the open $filehandle.
> my $fmode = fcntl($filehandle, F_GETFL, my $slush = 0);
>
> The value of $fmode will allow us to determine whether the filehandle is
> readonly, writeonly, or readable/writable.
>
> But with perl 5.8, it's possible to create filehandles connected to memory
> objects:
>
> use warnings;
> use strict;
>
> my ($fh1, $fh2, $var1, $var2);
> open $fh1, '>', \$var1 or die $!;
> print $fh1 "hello"; # $var1 contains "hello"
>
> open $fh2, '<', \$var1 or die $!;
> $var2 = <$fh2>; # $var2 contains "hello";
>
> close $fh1 or die $!;
> close $fh2 or die $!;
>
> print $var1, " ", $var2, "\n"; # prints "hello hello"
> __END__
>
> But now the fcntl() function is unable to provide information that I can use
> to determine the read/write status of $fh1and $fh2.
> For both $fh1 and $fh2 the fcntl() function will return undef - and fileno()
> will return -1.
>
> The question:
> How can I determine the read/write status of an open filehandle that's
> connected to a memory object ?
I don't know of a built-in method. You can attempt to write an empty
string to the filehandle and catch the warning if it fails:
sub can_write {
my $fh = shift;
eval {
use warnings FATAL => 'io';
print $fh '';
};
not $@;
}
Alternatively one could try to use 4-arg select().
Anno
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------------------------------
Date: Thu, 16 Mar 2006 21:15:27 +1100
From: "Sisyphus" <sisyphus1@nomail.afraid.org>
Subject: Re: Determine read/write status of filehandles connected to memory objects.
Message-Id: <44193b78$0$10677$afc38c87@news.optusnet.com.au>
"Anno Siegel" wrote
.
.
>
> I don't know of a built-in method. You can attempt to write an empty
> string to the filehandle and catch the warning if it fails:
>
> sub can_write {
> my $fh = shift;
> eval {
> use warnings FATAL => 'io';
> print $fh '';
> };
> not $@;
> }
>
Any script containing that subroutine that was run on pre-5.6 perl would, I
believe, fail to compile - even if can_write() was never going to be called
unless $] >= 5.008. (I know this shouldn't be a consideration, but I was
hoping to do it in a way that wouldn't break pre-5.6 perls, if possible.) Is
there perhaps a workaround to "use warnings FATAL => 'io';" involving
'require warnings;' (and perhaps also 'import') ? I couldn't produce an
incantation that worked.
> Alternatively one could try to use 4-arg select().
>
Yes - otherwise this looks a possibility. Unfortunately, I find the 1-arg
select() confusing enough. I'm not going to specifically request any help
with potential code, as I haven't yet made a serious attempt to come to
terms with the 4-arg version - but if someone likes to provide further
hints/suggestions along those lines, then that's fine by me :-)
Thanks for your insight, Anno.
Now ... back to 'perldoc -f select' ....
Cheers,
Rob
------------------------------
Date: 16 Mar 2006 11:03:11 GMT
From: anno4000@lublin.zrz.tu-berlin.de (Anno Siegel)
Subject: Re: Determine read/write status of filehandles connected to memory objects.
Message-Id: <dvbglf$5dr$1@mamenchi.zrz.TU-Berlin.DE>
Sisyphus <sisyphus1@nomail.afraid.org> wrote in comp.lang.perl.misc:
>
> "Anno Siegel" wrote
> .
> .
> >
> > I don't know of a built-in method. You can attempt to write an empty
> > string to the filehandle and catch the warning if it fails:
> >
> > sub can_write {
> > my $fh = shift;
> > eval {
> > use warnings FATAL => 'io';
> > print $fh '';
> > };
> > not $@;
> > }
> >
>
> Any script containing that subroutine that was run on pre-5.6 perl would, I
> believe, fail to compile - even if can_write() was never going to be called
> unless $] >= 5.008. (I know this shouldn't be a consideration, but I was
> hoping to do it in a way that wouldn't break pre-5.6 perls, if possible.) Is
> there perhaps a workaround to "use warnings FATAL => 'io';" involving
> 'require warnings;' (and perhaps also 'import') ? I couldn't produce an
> incantation that worked.
Use $SIG{ __WARN__} instead, that's been around forever.
sub can_write {
my $fh = shift;
my $warned;
local $SIG{ __WARN__} = sub { $warned ++ };
print $fh '';
return ! $warned;
}
A more specific test if the warning was indeed the text "Filehandle
%s opened only for input" would be wise in both variants.
> > Alternatively one could try to use 4-arg select().
> >
>
> Yes - otherwise this looks a possibility. Unfortunately, I find the 1-arg
> select() confusing enough. I'm not going to specifically request any help
> with potential code, as I haven't yet made a serious attempt to come to
> terms with the 4-arg version - but if someone likes to provide further
> hints/suggestions along those lines, then that's fine by me :-)
Can't be done with select(). You'd need a file number for the handle
you want to test, but fileno( $f) is -1 (invalid) if $f is a "stringy"
filehandle.
Anno
Anno
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------------------------------
Date: 16 Mar 2006 09:49:50 GMT
From: Dave Weaver <zen13097@zen.co.uk>
Subject: Re: Great JAPH (was Re: Creating Graphs Dynamically with Perl)
Message-Id: <441934be$0$5012$db0fefd9@news.zen.co.uk>
A. Sinan Unur <1usa@llenroc.ude.invalid> wrote:
> Abigail <abigail@abigail.nl> wrote in
> news:slrne1eb9n.1v0.abigail@alexandra.abigail.nl:
>
> > #!/opt/perl/bin/perl -- # No trailing newline after the last line!
> > BEGIN{$|=$SIG{__WARN__}=sub{$_=$_[0];y-_- -;print/(.)"$/;seek _,-open(_
> > ,"+<$0"),2;truncate _,tell _;close _;exec$0}}//rekcaH_lreP_rehtona_tsuJ
>
> This is an amazing JAPH. Very instructive.
>
Seconded!
For those having difficulty getting it to work (like me!), you need to
save this to a file and make that file executable. i.e.:
% perl japh.pl # won't work
Jsyntax error at japh.pl line 3, next token ???
Execution of japh.pl aborted due to compilation errors.
% chmod +x japh.pl ; ./japh.pl # will work.
Just another Perl Hacker
------------------------------
Date: Thu, 16 Mar 2006 09:19:20 +0000
From: Henry Law <news@lawshouse.org>
Subject: Timeout value in Net::FTP seems to have no effect
Message-Id: <1142500761.38875.0@doris.uk.clara.net>
I expect I'm doing something wrong, but Googling for [perl "Net::FTP"
timeout problem] shows nothing; nor does perldoc -q ftp. Can someone
point me in the right direction?
(ActiveState Perl, "v5.8.7 built for MSWin32-x86-multi-thread").
I use Net::FTP to pass files around my internal network. Occasionally I
specify a host that isn't up, at which point Net::FTP hangs for a long
time before giving up. "Set a timeout," I thought, but using the
Timeout=>nnn option on the login operation seems to have no effect -
though the documented default of 120 seconds isn't in operation either.
Here's a sample program which should work with any server that
supports anonymous FTP.
------------------------- sample -----------------------------
#! /usr/bin/perl
use strict; use warnings;
use Net::FTP;
my ($host,$timeout) = @ARGV;
exit 0 unless defined $host;
$timeout = 120 unless defined $timeout;
my $start_time = time;
print "Logging in to $host with timeout $timeout seconds\n";
my $ftp = Net::FTP->new( $host, Timeout=>$timeout );
unless (defined $ftp) {
print "Timed out in ",(time - $start_time)," seconds\n";
print "Session to '$host' failed\n";
exit 0;
}
$ftp->login( 'anonymous', 'news@lawshouse.org' );
my $cwd = $ftp->pwd();
print "Current directory at $host is '$cwd'\n";
$ftp->quit();
print "Session finished\n";
-------------------------- sample output follows -----------------
F:\User\PC\NFBack>tryit.pl neptune 5
Logging in to neptune with timeout 5 seconds
Current directory at neptune is '/'
Session finished
F:\User\PC\NFBack>tryit.pl uranus 5
Logging in to uranus with timeout 5 seconds
Timed out in 21 seconds
Session to 'uranus' failed
F:\User\PC\NFBack>tryit.pl uranus 1000
Logging in to uranus with timeout 1000 seconds
Timed out in 21 seconds
Session to 'uranus' failed
----------------------------- end ---------------------------------
--
Henry Law <>< Manchester, England
------------------------------
Date: 6 Apr 2001 21:33:47 GMT (Last modified)
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Subject: Digest Administrivia (Last modified: 6 Apr 01)
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------------------------------
End of Perl-Users Digest V10 Issue 9058
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