perldebtut.pod

MSYS在windows下模拟了一个类unix的终端

  	DB<3>

Useful, huh?  You can eval nearly anything in there, and experiment with bits
of code or regexes until the cows come home:

	DB<3> @data = qw(this that the other atheism leather theory scythe)

	DB<4> p 'saw -> '.($cnt += map { print "\t:\t$_\n" } grep(/the/, sort @data))
	atheism
	leather
	other
	scythe
	the
	theory  
	saw -> 6

If you want to see the command History, type an 'B<H>':

	DB<5> H
	4: p 'saw -> '.($cnt += map { print "\t:\t$_\n" } grep(/the/, sort @data))
	3: @data = qw(this that the other atheism leather theory scythe)
	2: x $obj
	1: $obj = bless({'unique_id'=>'123', 'attr'=>
	{'col' => 'black', 'things' => [qw(this that etc)]}}, 'MY_class')
	DB<5>

And if you want to repeat any previous command, use the exclamation: 'B<!>':

	DB<5> !4
	p 'saw -> '.($cnt += map { print "$_\n" } grep(/the/, sort @data))
	atheism
	leather
	other
	scythe
	the
	theory  
	saw -> 12

For more on references see L<perlref> and L<perlreftut>


=head1 Stepping through code

Here's a simple program which converts between Celsius and Fahrenheit, it too
has a problem:

	#!/usr/bin/perl -w
	use strict;

	my $arg = $ARGV[0] || '-c20';

	if ($arg =~ /^\-(c|f)((\-|\+)*\d+(\.\d+)*)$/) {
		my ($deg, $num) = ($1, $2);
		my ($in, $out) = ($num, $num);
		if ($deg eq 'c') {
			$deg = 'f';
			$out = &c2f($num);
		} else {
			$deg = 'c';
			$out = &f2c($num);
		}
		$out = sprintf('%0.2f', $out);
		$out =~ s/^((\-|\+)*\d+)\.0+$/$1/;
		print "$out $deg\n";
	} else {
		print "Usage: $0 -[c|f] num\n";
	}
	exit;

	sub f2c {
		my $f = shift;
		my $c = 5 * $f - 32 / 9;
		return $c;
	}

	sub c2f {
		my $c = shift;
		my $f = 9 * $c / 5 + 32;
		return $f;
	}


For some reason, the Fahrenheit to Celsius conversion fails to return the
expected output.  This is what it does:

	> temp -c0.72
	33.30 f

	> temp -f33.3
	162.94 c

Not very consistent!  We'll set a breakpoint in the code manually and run it
under the debugger to see what's going on.  A breakpoint is a flag, to which
the debugger will run without interruption, when it reaches the breakpoint, it
will stop execution and offer a prompt for further interaction.  In normal
use, these debugger commands are completely ignored, and they are safe - if a
little messy, to leave in production code.

	my ($in, $out) = ($num, $num);
	$DB::single=2; # insert at line 9!
	if ($deg eq 'c') 
		...

	> perl -d temp -f33.3
	Default die handler restored.

	Loading DB routines from perl5db.pl version 1.07
	Editor support available.

	Enter h or `h h' for help, or `man perldebug' for more help.

	main::(temp:4): my $arg = $ARGV[0] || '-c100';     

We'll simply continue down to our pre-set breakpoint with a 'B<c>':

  	DB<1> c
	main::(temp:10):                if ($deg eq 'c') {   

Followed by a window command to see where we are:

	DB<1> w
	7:              my ($deg, $num) = ($1, $2);
	8:              my ($in, $out) = ($num, $num);
	9:              $DB::single=2;
	10==>           if ($deg eq 'c') {
	11:                     $deg = 'f';
	12:                     $out = &c2f($num);
	13              } else {
	14:                     $deg = 'c';
	15:                     $out = &f2c($num);
	16              }                             

And a print to show what values we're currently using:

	DB<1> p $deg, $num
	f33.3

We can put another break point on any line beginning with a colon, we'll use
line 17 as that's just as we come out of the subroutine, and we'd like to
pause there later on:

	DB<2> b 17

There's no feedback from this, but you can see what breakpoints are set by
using the list 'L' command:

	DB<3> L
	temp:
 		17:            print "$out $deg\n";
   		break if (1)     

Note that to delete a breakpoint you use 'd' or 'D'.

Now we'll continue down into our subroutine, this time rather than by line
number, we'll use the subroutine name, followed by the now familiar 'w':

	DB<3> c f2c
	main::f2c(temp:30):             my $f = shift;  

	DB<4> w
	24:     exit;
	25
	26      sub f2c {
	27==>           my $f = shift;
	28:             my $c = 5 * $f - 32 / 9; 
	29:             return $c;
	30      }
	31
	32      sub c2f {
	33:             my $c = shift;   


Note that if there was a subroutine call between us and line 29, and we wanted
to B<single-step> through it, we could use the 'B<s>' command, and to step
over it we would use 'B<n>' which would execute the sub, but not descend into
it for inspection.  In this case though, we simply continue down to line 29:

	DB<4> c 29  
	main::f2c(temp:29):             return $c;

And have a look at the return value:

	DB<5> p $c
	162.944444444444

This is not the right answer at all, but the sum looks correct.  I wonder if
it's anything to do with operator precedence?  We'll try a couple of other
possibilities with our sum:

	DB<6> p (5 * $f - 32 / 9)
	162.944444444444

	DB<7> p 5 * $f - (32 / 9) 
	162.944444444444

	DB<8> p (5 * $f) - 32 / 9
	162.944444444444

	DB<9> p 5 * ($f - 32) / 9
	0.722222222222221

:-) that's more like it!  Ok, now we can set our return variable and we'll
return out of the sub with an 'r':

	DB<10> $c = 5 * ($f - 32) / 9

	DB<11> r
	scalar context return from main::f2c: 0.722222222222221

Looks good, let's just continue off the end of the script:

	DB<12> c
	0.72 c 
	Debugged program terminated.  Use q to quit or R to restart,
  	use O inhibit_exit to avoid stopping after program termination,
  	h q, h R or h O to get additional info.   

A quick fix to the offending line (insert the missing parentheses) in the
actual program and we're finished.


=head1 Placeholder for a, w, t, T

Actions, watch variables, stack traces etc.: on the TODO list.

	a 

	W 

	t 

	T


=head1 REGULAR EXPRESSIONS

Ever wanted to know what a regex looked like?  You'll need perl compiled with
the DEBUGGING flag for this one:

	> perl -Dr -e '/^pe(a)*rl$/i'
	Compiling REx `^pe(a)*rl$'
	size 17 first at 2
	rarest char
	 at 0
	   1: BOL(2)
	   2: EXACTF <pe>(4)
	   4: CURLYN[1] {0,32767}(14)
	   6:   NOTHING(8)
	   8:   EXACTF <a>(0)
	  12:   WHILEM(0)
	  13: NOTHING(14)
	  14: EXACTF <rl>(16)
	  16: EOL(17)
	  17: END(0)
	floating `'$ at 4..2147483647 (checking floating) stclass `EXACTF <pe>'
anchored(BOL) minlen 4
	Omitting $` $& $' support.

	EXECUTING...

	Freeing REx: `^pe(a)*rl$'  

Did you really want to know? :-)
For more gory details on getting regular expressions to work, have a look at
L<perlre>, L<perlretut>, and to decode the mysterious labels (BOL and CURLYN,
etc. above), see L<perldebguts>.


=head1 OUTPUT TIPS

To get all the output from your error log, and not miss any messages via
helpful operating system buffering, insert a line like this, at the start of
your script:

	$|=1;	

To watch the tail of a dynamically growing logfile, (from the command line):

	tail -f $error_log

Wrapping all die calls in a handler routine can be useful to see how, and from
where, they're being called, L<perlvar> has more information:

	BEGIN { $SIG{__DIE__} = sub { require Carp; Carp::confess(@_) } }

Various useful techniques for the redirection of STDOUT and STDERR filehandles
are explained in L<perlopentut> and L<perlfaq8>.


=head1 CGI

Just a quick hint here for all those CGI programmers who can't figure out how
on earth to get past that 'waiting for input' prompt, when running their CGI
script from the command-line, try something like this:

	> perl -d my_cgi.pl -nodebug 

Of course L<CGI> and L<perlfaq9> will tell you more.


=head1 GUIs

The command line interface is tightly integrated with an B<emacs> extension
and there's a B<vi> interface too.  

You don't have to do this all on the command line, though, there are a few GUI
options out there.  The nice thing about these is you can wave a mouse over a
variable and a dump of it's data will appear in an appropriate window, or in a
popup balloon, no more tiresome typing of 'x $varname' :-)

In particular have a hunt around for the following:

B<ptkdb> perlTK based wrapper for the built-in debugger

B<ddd> data display debugger

B<PerlDevKit> and B<PerlBuilder> are NT specific

NB. (more info on these and others would be appreciated).


=head1 SUMMARY

We've seen how to encourage good coding practices with B<use strict> and
B<-w>.  We can run the perl debugger B<perl -d scriptname> to inspect your
data from within the perl debugger with the B<p> and B<x> commands.  You can
walk through your code, set breakpoints with B<b> and step through that code
with B<s> or B<n>, continue with B<c> and return from a sub with B<r>.  Fairly
intuitive stuff when you get down to it.  

There is of course lots more to find out about, this has just scratched the
surface.  The best way to learn more is to use perldoc to find out more about
the language, to read the on-line help (L<perldebug> is probably the next
place to go), and of course, experiment.  


=head1 SEE ALSO

L<perldebug>, 
L<perldebguts>, 
L<perldiag>,
L<dprofpp>,
L<perlrun>


=head1 AUTHOR

Richard Foley <[e-mail]> Copyright (c) 2000


=head1 CONTRIBUTORS

Various people have made helpful suggestions and contributions, in particular:

Ronald J Kimball <[e-mail]>

Hugo van der Sanden <[e-mail]>

Peter Scott <[e-mail]>