Perl Debugging Tools

1
Perl Debugging Tools

• warn() die()
• DataDumper
• BDeparse
• __DATA__
• perldebug
• ptkdb

2
warn LIST

• Concatenates LIST, and prints result to STDERR.
  Program flow then continues uninterrupted.
• If LIST is empty, uses the default message
  "Warning Something's wrong"
• if last element of LIST does not end in a
  newline, appends "at filename, line line"

3
die LIST

• Like warn, prints the concatenated list to
  STDERR. Program then exits, with the current
  value of ! as its exit code.
• (! is an error code in numeric context, and the
  corresponding error message in string context)
• Like warn, will print the file name and line
  number (and input line number, if applicable), if
  the last element of LIST does not end with a
  newline.

4
Special Tokens

• In your internal debugging messages, it's often
  helpful to know what line (and sometimes what
  file) program execution was at when the message
  was printed.
• Two tokens __FILE__ and __LINE__
• Hold the filename and linenumber, respectively.
• Note These are NOT variables, so do not try to
  interpolate them!!

5
Token Markers

• The special marker __END__ can be used to give an
  absolute end to your code.
• The parser will not look at any text below this
  marker. Type anything you wish here.
• The marker __DATA__ is used as a
  pseudo-filehandle
• Within your code, use DATA as an open input
  filehandle.
• ltDATAgt reads from text below the __DATA__ marker.
• Useful when debugging code, to avoid continuously
  editing input file
• Very helpful when emailing for help
• You can send me a small test script that shows
  your problem, without saying "I have a text file
  that has . . . "
• hint, hint. . .

6
DataDumper

• Very useful function for seeing exactly what is
  contained in a variable
• To use, first import the module
• use DataDumper
• Then, call Dumper with a reference to the
  structure you want to investigate, and print the
  results
• print Dumper(\bigHash)
• for now, just accept the fact that a reference is
  created by prepending a \ to the variable name
• perldoc DataDumper for more info

7
Useful command line switches

• -w
• obsolete. use warnings instead
• -c
• Simply check your program for compilation errors,
  without bothering to actually run the program.
• Very useful for large programs, or when machine
  resources are tight
• -e
• Evaluate a "one-liner", and return
• perl -e 'a 42 print "\a a\n"'

8
More Useful Switches

• -n Automatically open each file on the command
  line and loop through.
• perl ne 'print unless _ lt 10' .txt
• !/usr/bin/env perl for (_at_ARGV) if (!open my
  f, 'lt', _) warn "Can't open _ !"
  else while (ltfgt) print unless _ lt
  10

9
One step further

• -p Automatically open each file on the command
  line and loop through, printing each line.
• perl pe '_substr(_,0,10)' .bat gtout.txt
• !/usr/bin/env perl for (_at_ARGV) if (!open my
  f, 'lt', _) warn "Can't open _ !"
  else while (ltfgt) _ substr(_, 0,
  10) continue print

10
Just two more. . .

• -i used with p or n to edit files "inline"
• perl pi.bkp -e '_substr(_,0,10)' .bat
• All .bat files are backed up as .bat.bkp, and
  the original .bat files are edited in place.
• -M use a module from the command line
• perl MDataDumper e' print Dumper(\f)'
• for more information on all command line
  switches, see perldoc perlrun

11
The Perl Debugger

• Command-line, interactive debugging program,
  written (of course) in Perl.
• Start the debugger by supplying the -d switch
• perl -d myfile.pl
• This isn't actually a separate program. Instead,
  -d inserts debugger library code, then executes
  your program as normal, halting before first
  run-time executable statement.

12
now what?

• The debugger halts just before the first run-time
  executable statement, and prints that statement
  to the screen, giving the prompt
• DBlt1gt
• It then waits for you to type a debugger command
  or a perl statement to be evaluated
• Maybe most important command is h h
• compact listing of debugger commands
• Also simply h
• expanded listing of debugger commands
• precede any command with to page the output

13
Debugger commands

• s Single-step, entering functions
• n Single-step over functions
• ltENTERgt repeat the last s or n command
• r run to end of current subroutine
• b set breakpoints
• d or D delete breakpoints
• c continue until next breakpoint
• L list breakpoints
• T trace the stack calls
• w set watch variables
• p or x print or examine variables
• l list code

14
Debugger

• Those are just a quick summary of the available
  commands. For more information, use h or h h
  within the debugger, or readperldoc perldebug

15
A Better (?) Debugger

• ptkdb
• (Perl ToolKit DeBugger)
• Graphical debugger, written in Perl, making use
  of Perl/Tk
• On a Unix machine in an Xwindows environment, use
  CPAN
• perl MCPAN e'install DEVELptkdb'
• On Windows, using Activestate, use PPM
• ppm
• search ptkdb
• install ltgt
• if search yields no results, run this first, then
  try again
• repository add BdP http//www.bribes.org/perl/ppm

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Running ptkdb

• perl dptkdb file.pl
• creates a GUI window, with your code on the left
  (with line numbers)
• On right, panel of expressions, breakpoints, and
  subroutines
• click on any line number to set break point
• click the breakpoints tab to give a break point a
  condition (ex i 5)
• Use buttons at top to step in/over, run to next
  breakpoint, or run to cursor.
• Expressions panel auto-updates after every
  executed line.
• Hover mouse over a variable to popup its value

17
BDeparse

• The B modules are different than most. They
  exist for digging into the perl internals. They
  are called via the O module.
• Just nod your head. It's easier.
• To run a script through Deparseperl MODeparse
  file.pl

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Deparsing

• The result of running your script through
  BDeparse is code that more closely approximates
  what Perl sees once it's finished optimizing and
  blowing away the syntactic sugar.
• perl MODeparse e'print "_ gt f_\n" for
  sort fa cmp fb keys f'
• foreach _ (sort fa cmp fb keys f)
  print "_ gt f_\n"

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Why Deparse?

• Deparse is great for those situations in which
  you start to believe that Perl isn't seeing your
  code the same way you are.
• One common occurrence is with precedence.
• Provide a p option, and Deparse will use
  parentheses to explicitly show every level of
  precedence.
• perl MODeparse,-p file.pl
• For more information, perldoc BDeparse