Perl Scripting And Developing Blog

You have a subroutine that should set a

Tuesday, January 31st, 2006

You have a subroutine that should set a

Tuesday, January 31st, 2006

You have a subroutine that should set a private variable based on some condition. Not unreasonably, you might code something following this general form: sub ordinal { my $what = shift; my $res = ‘first’ if $what == 1; $res = ’second’ if $what == 2; return $res; } But you’d be wrong. If we run this through perl-lwith for(1..3){ print ordinal($_)}, we get the result: first second second Eh? Shouldn’t that last line be a warning about an undefined value? Regrettably, you’ve fallen afoul of an internal optimization in Perl[4] that keeps the storage assigned to lexical variables around after they’re gone, on the premise that they may be needed again shortly. The my statement has both a compile-time effect (declaring the variable) and a run-time effect (setting it to undef, unless the statement is qualified by a failed condition). [4] That may change in a future version. Never qualify a my statement with a condition. Note that this is not talking about putting my statements inside conditions: if ((my $tag = $element->tag) ne ‘p’) { # Code using $tag } while (my ($key, $value) = each %hash) { … } which is a very good idiom in that it concisely limits the scope of the new variable to precisely the block in which you need it. 10.6 Bringing Some Closure Understanding how lexical variables work is worth the effort. Suppose you have advanced to the stage of understanding closures give yourself a pat on the back and you decide to code a few subroutines that use a private variable that way: { my $protocol = “ftp”; # default

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my %port = (http => 80, https =>

Tuesday, January 31st, 2006

my %port = (http => 80, https => 443, ftp => 79, nntp => 119, gopher => 70); while () { if (my ($url) = m#((w+)://S+[w])#) { $url = ~ s#:$port{$2}/#/# if $port{$2}; print “URL found: $urln”; } } (Yes, there are better ways of isolating URLs from text; this code doesn’t even find more than one per line. But that’s not the point we’re about to make.) Flushed with the cleverness of our code, we feed it the input: The President (http://www.whitehouse.gov:80/president/) said today that he liked ferrets (http://www.ferretcentral.org/), and was considering placing a picture of one (ftp://ftp.optics. rochester.edu:79/pub/pgreene/icons/central-logo-t.gif) on the next version of the national flag; but he thought they were too often confused with gophers (gopher://gopher.tc.umn.edu:70/11/). expecting to see the output:[3] [3] We indented continuation lines in the input and output to make them easier to read. URL found: http://www.whitehouse.gov/president/ URL found: http://www.ferretcentral.org URL found: ftp://ftp.optics.rochester.edu/pub/pgreene/ icons/central-logo-t.gif URL found: gopher://gopher.tc.umn.edu/11 But instead, we see: URL found: 4294967294 URL found: 4294967295 URL found: 4294967294 URL found: 4294967294 If you run this program through Deparse, you’ll see the line $url = ~s[:$port{$2}/][/] if $port{$2}; which gives us just enough of a clue if we haven’t found the typo already. The space we accidentally put inside the regex binding operator has changed its meaning to “$urlis
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my %port = (http => 80, https =>

Tuesday, January 31st, 2006

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The specification of that function says that if

Tuesday, January 31st, 2006

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The specification of that function says that if

Tuesday, January 31st, 2006

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Semantical errors are a way of saying that

Tuesday, January 31st, 2006

An even better choice would be the oroperator introduced in Perl 5, which has such low precedence that you can leave out the parentheses on open: open FH, $file or die “Error opening $file: $!n”; 10.2 Reading Directories This has bitten us more times than we care to admit: readdir() returns the list of filenames in the directory, but they are not qualified by the directory itself. Consider the following program: use strict; use File::stat; my $dir = shift || ‘.’; opendir DIR, $dir or die “Error opening directory $dir: $!n”; while (defined (my $file = readdir DIR)) { print “File $file was last modified on ” . localtime(stat($file)->mtime), “n”; } closedir DIR; It takes a directory as its argument, defaulting to the current directory. This program prints out the names and modification times of all the files in the current directory. If run with no arguments, the program runs fine. If we then run it with an argument (this example is on Unix), it appears to start working and then fails: File . was last modified on Sun Mar 28 14:36:27 1999 File .. was last modified on Tue Aug 3 15:29:12 1999 Can’t call method “mtime” without a package or object reference at mod_times.pl line 9. What happened here? First, we see the error refers to line 9 and that perl attempted to invoke the mtimemethod on something that wasn’t even an object. What was the object? It was the result of the call to stat($file) (we’re using the stat routine from the File::stat module here, instead of the core perl stat routine that returns a list; it saves us from having to look up the index of the mtimeelement).
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for my $sock (@ready) { my ($who, $target)

Tuesday, January 3rd, 2006

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$| = 1; near the beginning of your

Tuesday, January 3rd, 2006

o Does not handle multiple simultaneous connections. o Reads are asynchronous, writes are synchronous. o Messages larger than 64K will be segmented. EndUsage } use vars qw($opt_i $opt_o $opt_1 $opt_v); getopts(’i:o:1:v’) and $opt_o or die usage; my $proxy_port = $opt_i or die usage; my ($server_host, $server_port) = $opt_o =~ /^(.+):(d+)$/ or die usage; my $verbose = 1 if $opt_v; my $only_one = 1 if $opt_1; $SIG{TERM} = $SIG{INT} = $SIG{HUP} = &shutdown; my $proxy = IO::Socket::INET->new(LocalPort => $proxy_port, Type => SOCK_STREAM, Proto => ‘tcp’, Reuse => 1, Listen => 1) or die “Can’t listen on port $proxy_port: $!n”; print “[listening on port $proxy_port]n” if $verbose; my ($client, $server); OUTER: while ($client = $proxy->accept) { my ($client_host, $client_port) = (gethostbyaddr($client->peeraddr, AF_INET) || $client->peerhost, $client->peerport); print “[connection from $client_host on $client_port]n” if $verbose; $server = IO::Socket::INET->new(PeerAddr => $server_host, PeerPort => $server_port, Proto => ‘tcp’, Type => SOCK_STREAM) or die “Can’t connect to $server_host:$server_portn”; print “[connected to server $server_host:$server_port]n” if $verbose; my $selector = IO::Select->new($client, $server); CONNECTION: while (my @ready = $selector->can_read) {
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$| = 1; near the beginning of your

Tuesday, January 3rd, 2006

$| = 1; near the beginning of your program to unbuffer STDOUT so that when you send output, it gets passed to the Web server immediately. 13.7 cgi-test Let’s add to our black bag the following script: #!/usr/bin/perl -Tw use strict; print “Content-type: text/plainnn”; print “$_: $ENV{$_}n” for sort keys %ENV; print “nInput:n”; print ; Use this whenever you have doubts about what a form is really sending to the server. Just make this script the target of the form action, and you’ll get a terse dump of the environment variables and any content that the form sent via POST. 13.8 Eavesdropping Sometimes you’d just like to listen in on the conversation between a browser and a server, like a gossip-hungry neighbor on a party line. Perhaps you’re dealing with browsers whose caching behavior is questionable, and you don’t want to guess what’s really being fetched. You could use a connection-sniffing tool like tcpdump to monitor the traffic between the client and the server, but this generally requires superuser access to put the network interface into “promiscuous” mode. So try instead our proxylog tool: #!/usr/bin/perl -w use strict; use Getopt::Std; use IO::Socket; use IO::Select; sub usage { <<"EndUsage"; $0 lets you snoop on the conversation between a client and server. $0 -i clientport -o serverhost:port [-1] [-v] -i incoming port to listen to -o outgoing port to make connection to -1 only process one client request, then exit -v verbose Caveats:
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Archive for January, 2006

You have a subroutine that should set a

You have a subroutine that should set a

my %port = (http => 80, https =>

my %port = (http => 80, https =>

The specification of that function says that if

The specification of that function says that if

Semantical errors are a way of saying that

for my $sock (@ready) { my ($who, $target)

$| = 1; near the beginning of your

$| = 1; near the beginning of your

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