PERLDEBUG(1) Perl Programmers Reference Guide PERLDEBUG(1)
NAME
perldebug - Perl debugging
DESCRIPTION
First of all, have you tried using the -w switch?
If you're new to the Perl debugger, you may prefer to read perldebtut, which is a tutorial introduction to the
debugger.
The Perl Debugger
If you invoke Perl with the -d switch, your script runs under the Perl source debugger. This works like an
interactive Perl environment, prompting for debugger commands that let you examine source code, set
breakpoints, get stack backtraces, change the values of variables, etc. This is so convenient that you often
fire up the debugger all by itself just to test out Perl constructs interactively to see what they do. For
example:
$ perl -d -e 42
In Perl, the debugger is not a separate program the way it usually is in the typical compiled environment.
Instead, the -d flag tells the compiler to insert source information into the parse trees it's about to hand
off to the interpreter. That means your code must first compile correctly for the debugger to work on it.
Then when the interpreter starts up, it preloads a special Perl library file containing the debugger.
The program will halt right before the first run-time executable statement (but see below regarding compile-
time statements) and ask you to enter a debugger command. Contrary to popular expectations, whenever the
debugger halts and shows you a line of code, it always displays the line it's about to execute, rather than
the one it has just executed.
Any command not recognized by the debugger is directly executed ("eval"'d) as Perl code in the current
package. (The debugger uses the DB package for keeping its own state information.)
Note that the said "eval" is bound by an implicit scope. As a result any newly introduced lexical variable or
any modified capture buffer content is lost after the eval. The debugger is a nice environment to learn Perl,
but if you interactively experiment using material which should be in the same scope, stuff it in one line.
For any text entered at the debugger prompt, leading and trailing whitespace is first stripped before further
processing. If a debugger command coincides with some function in your own program, merely precede the
function with something that doesn't look like a debugger command, such as a leading ";" or perhaps a "+", or
by wrapping it with parentheses or braces.
Calling the Debugger
There are several ways to call the debugger:
perl -d program_name
On the given program identified by "program_name".
perl -d -e 0
Interactively supply an arbitrary "expression" using "-e".
perl -d:Ptkdb program_name
Debug a given program via the "Devel::Ptkdb" GUI.
perl -dt threaded_program_name
Debug a given program using threads (experimental).
Debugger Commands
You may change the pager which is used via "o pager=..." command.
p expr Same as "print {$DB::OUT} expr" in the current package. In particular, because this is just
Perl's own "print" function, this means that nested data structures and objects are not dumped,
unlike with the "x" command.
The "DB::OUT" filehandle is opened to /dev/tty, regardless of where STDOUT may be redirected to.
x [maxdepth] expr
Evaluates its expression in list context and dumps out the result in a pretty-printed fashion.
Nested data structures are printed out recursively, unlike the real "print" function in Perl.
When dumping hashes, you'll probably prefer 'x \%h' rather than 'x %h'. See Dumpvalue if you'd
like to do this yourself.
The output format is governed by multiple options described under "Configurable Options".
If the "maxdepth" is included, it must be a numeral N; the value is dumped only N levels deep, as
if the "dumpDepth" option had been temporarily set to N.
V [pkg [vars]]
Display all (or some) variables in package (defaulting to "main") using a data pretty-printer
(hashes show their keys and values so you see what's what, control characters are made printable,
etc.). Make sure you don't put the type specifier (like "$") there, just the symbol names, like
this:
V DB filename line
Use "~pattern" and "!pattern" for positive and negative regexes.
This is similar to calling the "x" command on each applicable var.
X [vars] Same as "V currentpackage [vars]".
y [level [vars]]
Display all (or some) lexical variables (mnemonic: "mY" variables) in the current scope or level
scopes higher. You can limit the variables that you see with vars which works exactly as it does
for the "V" and "X" commands. Requires the "PadWalker" module version 0.08 or higher; will warn
if this isn't installed. Output is pretty-printed in the same style as for "V" and the format is
controlled by the same options.
T Produce a stack backtrace. See below for details on its output.
s [expr] Single step. Executes until the beginning of another statement, descending into subroutine calls.
If an expression is supplied that includes function calls, it too will be single-stepped.
n [expr] Next. Executes over subroutine calls, until the beginning of the next statement. If an
expression is supplied that includes function calls, those functions will be executed with stops
before each statement.
r Continue until the return from the current subroutine. Dump the return value if the "PrintRet"
option is set (default).
<CR> Repeat last "n" or "s" command.
l subname List first window of lines from subroutine. subname may be a variable that contains a code
reference.
- List previous window of lines.
v [line] View a few lines of code around the current line.
. Return the internal debugger pointer to the line last executed, and print out that line.
f filename Switch to viewing a different file or "eval" statement. If filename is not a full pathname found
in the values of %INC, it is considered a regex.
"eval"ed strings (when accessible) are considered to be filenames: "f (eval 7)" and "f eval 7\b"
access the body of the 7th "eval"ed string (in the order of execution). The bodies of the
currently executed "eval" and of "eval"ed strings that define subroutines are saved and thus
accessible.
/pattern/ Search forwards for pattern (a Perl regex); final / is optional. The search is case-insensitive
by default.
?pattern? Search backwards for pattern; final ? is optional. The search is case-insensitive by default.
L [abw] List (default all) actions, breakpoints and watch expressions
S [[!]regex]
List subroutine names [not] matching the regex.
t [n] Toggle trace mode (see also the "AutoTrace" option). Optional argument is the maximum number of
levels to trace below the current one; anything deeper than that will be silent.
t [n] expr Trace through execution of "expr". Optional first argument is the maximum number of levels to
trace below the current one; anything deeper than that will be silent. See "Frame Listing Output
Examples" in perldebguts for examples.
b Sets breakpoint on current line
b [line] [condition]
Set a breakpoint before the given line. If a condition is specified, it's evaluated each time the
statement is reached: a breakpoint is taken only if the condition is true. Breakpoints may only
be set on lines that begin an executable statement. Conditions don't use "if":
b 237 $x > 30
b 237 ++$count237 < 11
b 33 /pattern/i
If the line number is ".", sets a breakpoint on the current line:
b . $n > 100
b [file]:[line] [condition]
Set a breakpoint before the given line in a (possibly different) file. If a condition is
specified, it's evaluated each time the statement is reached: a breakpoint is taken only if the
condition is true. Breakpoints may only be set on lines that begin an executable statement.
Conditions don't use "if":
Set a breakpoint before the first executed line of the filename, which should be a full pathname
found amongst the %INC values.
b compile subname
Sets a breakpoint before the first statement executed after the specified subroutine is compiled.
B line Delete a breakpoint from the specified line.
B * Delete all installed breakpoints.
disable [file]:[line]
Disable the breakpoint so it won't stop the execution of the program. Breakpoints are enabled by
default and can be re-enabled using the "enable" command.
disable [line]
Disable the breakpoint so it won't stop the execution of the program. Breakpoints are enabled by
default and can be re-enabled using the "enable" command.
This is done for a breakpoint in the current file.
enable [file]:[line]
Enable the breakpoint so it will stop the execution of the program.
enable [line]
Enable the breakpoint so it will stop the execution of the program.
This is done for a breakpoint in the current file.
a [line] command
Set an action to be done before the line is executed. If line is omitted, set an action on the
line about to be executed. The sequence of steps taken by the debugger is
1. check for a breakpoint at this line
2. print the line if necessary (tracing)
3. do any actions associated with that line
4. prompt user if at a breakpoint or in single-step
5. evaluate line
For example, this will print out $foo every time line 53 is passed:
a 53 print "DB FOUND $foo\n"
A line Delete an action from the specified line.
A * Delete all installed actions.
w expr Add a global watch-expression. Whenever a watched global changes the debugger will stop and
display the old and new values.
W expr Delete watch-expression
W * Delete all watch-expressions.
o Display all options.
that quote but which are not meant to escape the quote itself. In other words, you follow single-
quoting rules irrespective of the quote; eg: "o option='this isn\'t bad'" or "o option="She said,
\"Isn't it?\""".
For historical reasons, the "=value" is optional, but defaults to 1 only where it is safe to do
so--that is, mostly for Boolean options. It is always better to assign a specific value using
"=". The "option" can be abbreviated, but for clarity probably should not be. Several options
can be set together. See "Configurable Options" for a list of these.
< ? List out all pre-prompt Perl command actions.
< [ command ]
Set an action (Perl command) to happen before every debugger prompt. A multi-line command may be
entered by backslashing the newlines.
< * Delete all pre-prompt Perl command actions.
<< command Add an action (Perl command) to happen before every debugger prompt. A multi-line command may be
entered by backwhacking the newlines.
> ? List out post-prompt Perl command actions.
> command Set an action (Perl command) to happen after the prompt when you've just given a command to return
to executing the script. A multi-line command may be entered by backslashing the newlines (we bet
you couldn't have guessed this by now).
> * Delete all post-prompt Perl command actions.
>> command Adds an action (Perl command) to happen after the prompt when you've just given a command to
return to executing the script. A multi-line command may be entered by backslashing the newlines.
{ ? List out pre-prompt debugger commands.
{ [ command ]
Set an action (debugger command) to happen before every debugger prompt. A multi-line command may
be entered in the customary fashion.
Because this command is in some senses new, a warning is issued if you appear to have accidentally
entered a block instead. If that's what you mean to do, write it as with ";{ ... }" or even "do {
... }".
{ * Delete all pre-prompt debugger commands.
{{ command Add an action (debugger command) to happen before every debugger prompt. A multi-line command may
be entered, if you can guess how: see above.
! number Redo a previous command (defaults to the previous command).
! -number Redo number'th previous command.
! pattern Redo last command that started with pattern. See "o recallCommand", too.
!! cmd Run cmd in a subprocess (reads from DB::IN, writes to DB::OUT) See "o shellBang", also. Note that
the user's current shell (well, their $ENV{SHELL} variable) will be used, which can interfere with
R Restart the debugger by "exec()"ing a new session. We try to maintain your history across this,
but internal settings and command-line options may be lost.
The following setting are currently preserved: history, breakpoints, actions, debugger options,
and the Perl command-line options -w, -I, and -e.
|dbcmd Run the debugger command, piping DB::OUT into your current pager.
||dbcmd Same as "|dbcmd" but DB::OUT is temporarily "select"ed as well.
= [alias value]
Define a command alias, like
= quit q
or list current aliases.
command Execute command as a Perl statement. A trailing semicolon will be supplied. If the Perl
statement would otherwise be confused for a Perl debugger, use a leading semicolon, too.
m expr List which methods may be called on the result of the evaluated expression. The expression may
evaluated to a reference to a blessed object, or to a package name.
M Display all loaded modules and their versions.
man [manpage]
Despite its name, this calls your system's default documentation viewer on the given page, or on
the viewer itself if manpage is omitted. If that viewer is man, the current "Config" information
is used to invoke man using the proper MANPATH or -M manpath option. Failed lookups of the form
"XXX" that match known manpages of the form perlXXX will be retried. This lets you type "man
debug" or "man op" from the debugger.
On systems traditionally bereft of a usable man command, the debugger invokes perldoc.
Occasionally this determination is incorrect due to recalcitrant vendors or rather more
felicitously, to enterprising users. If you fall into either category, just manually set the
$DB::doccmd variable to whatever viewer to view the Perl documentation on your system. This may
be set in an rc file, or through direct assignment. We're still waiting for a working example of
something along the lines of:
$DB::doccmd = 'netscape -remote http://something.here/';
Configurable Options
The debugger has numerous options settable using the "o" command, either interactively or from the environment
or an rc file. (./.perldb or ~/.perldb under Unix.)
"recallCommand", "ShellBang"
The characters used to recall command or spawn shell. By default, both are set to "!", which is
unfortunate.
"pager" Program to use for output of pager-piped commands (those beginning with a "|" character.) By
default, $ENV{PAGER} will be used. Because the debugger uses your current terminal
characteristics for bold and underlining, if the chosen pager does not pass escape sequences
through unchanged, the output of some debugger commands will not be readable when sent through the
ones. If "dieLevel" is even 1, then your non-fatal exceptions are also traced and unceremoniously
altered if they came from "eval'ed" strings or from any kind of "eval" within modules you're
attempting to load. If "dieLevel" is 2, the debugger doesn't care where they came from: It
usurps your exception handler and prints out a trace, then modifies all exceptions with its own
embellishments. This may perhaps be useful for some tracing purposes, but tends to hopelessly
destroy any program that takes its exception handling seriously.
"AutoTrace" Trace mode (similar to "t" command, but can be put into "PERLDB_OPTS").
"LineInfo" File or pipe to print line number info to. If it is a pipe (say, "|visual_perl_db"), then a short
message is used. This is the mechanism used to interact with a slave editor or visual debugger,
such as the special "vi" or "emacs" hooks, or the "ddd" graphical debugger.
"inhibit_exit"
If 0, allows stepping off the end of the script.
"PrintRet" Print return value after "r" command if set (default).
"ornaments" Affects screen appearance of the command line (see Term::ReadLine). There is currently no way to
disable these, which can render some output illegible on some displays, or with some pagers. This
is considered a bug.
"frame" Affects the printing of messages upon entry and exit from subroutines. If "frame & 2" is false,
messages are printed on entry only. (Printing on exit might be useful if interspersed with other
messages.)
If "frame & 4", arguments to functions are printed, plus context and caller info. If "frame & 8",
overloaded "stringify" and "tie"d "FETCH" is enabled on the printed arguments. If "frame & 16",
the return value from the subroutine is printed.
The length at which the argument list is truncated is governed by the next option:
"maxTraceLen"
Length to truncate the argument list when the "frame" option's bit 4 is set.
"windowSize"
Change the size of code list window (default is 10 lines).
The following options affect what happens with "V", "X", and "x" commands:
"arrayDepth", "hashDepth"
Print only first N elements ('' for all).
"dumpDepth" Limit recursion depth to N levels when dumping structures. Negative values are interpreted as
infinity. Default: infinity.
"compactDump", "veryCompact"
Change the style of array and hash output. If "compactDump", short array may be printed on one
line.
"globPrint" Whether to print contents of globs.
"DumpDBFiles"
Dump arrays holding debugged files.
"UsageOnly" Rudimentary per-package memory usage dump. Calculates total size of strings found in variables in
the package. This does not include lexicals in a module's file scope, or lost in closures.
After the rc file is read, the debugger reads the $ENV{PERLDB_OPTS} environment variable and parses this as
the remainder of a "O ..." line as one might enter at the debugger prompt. You may place the initialization
options "TTY", "noTTY", "ReadLine", and "NonStop" there.
If your rc file contains:
parse_options("NonStop=1 LineInfo=db.out AutoTrace");
then your script will run without human intervention, putting trace information into the file db.out. (If you
interrupt it, you'd better reset "LineInfo" to /dev/tty if you expect to see anything.)
"TTY" The TTY to use for debugging I/O.
"noTTY" If set, the debugger goes into "NonStop" mode and will not connect to a TTY. If interrupted (or
if control goes to the debugger via explicit setting of $DB::signal or $DB::single from the Perl
script), it connects to a TTY specified in the "TTY" option at startup, or to a tty found at
runtime using the "Term::Rendezvous" module of your choice.
This module should implement a method named "new" that returns an object with two methods: "IN"
and "OUT". These should return filehandles to use for debugging input and output correspondingly.
The "new" method should inspect an argument containing the value of $ENV{PERLDB_NOTTY} at startup,
or "$ENV{HOME}/.perldbtty$$" otherwise. This file is not inspected for proper ownership, so
security hazards are theoretically possible.
"ReadLine" If false, readline support in the debugger is disabled in order to debug applications that
themselves use ReadLine.
"NonStop" If set, the debugger goes into non-interactive mode until interrupted, or programmatically by
setting $DB::signal or $DB::single.
Here's an example of using the $ENV{PERLDB_OPTS} variable:
$ PERLDB_OPTS="NonStop frame=2" perl -d myprogram
That will run the script myprogram without human intervention, printing out the call tree with entry and exit
points. Note that "NonStop=1 frame=2" is equivalent to "N f=2", and that originally, options could be
uniquely abbreviated by the first letter (modulo the "Dump*" options). It is nevertheless recommended that
you always spell them out in full for legibility and future compatibility.
Other examples include
$ PERLDB_OPTS="NonStop LineInfo=listing frame=2" perl -d myprogram
which runs script non-interactively, printing info on each entry into a subroutine and each executed line into
the file named listing. (If you interrupt it, you would better reset "LineInfo" to something "interactive"!)
Other examples include (using standard shell syntax to show environment variable settings):
$ ( PERLDB_OPTS="NonStop frame=1 AutoTrace LineInfo=tperl.out"
perl -d myprogram )
or even
DB<<17>>
where that number is the command number, and which you'd use to access with the built-in csh-like
history mechanism. For example, "!17" would repeat command number 17. The depth of the angle
brackets indicates the nesting depth of the debugger. You could get more than one set of brackets,
for example, if you'd already at a breakpoint and then printed the result of a function call that
itself has a breakpoint, or you step into an expression via "s/n/t expression" command.
Multiline commands
If you want to enter a multi-line command, such as a subroutine definition with several statements or
a format, escape the newline that would normally end the debugger command with a backslash. Here's an
example:
DB<1> for (1..4) { \
cont: print "ok\n"; \
cont: }
ok
ok
ok
ok
Note that this business of escaping a newline is specific to interactive commands typed into the
debugger.
Stack backtrace
Here's an example of what a stack backtrace via "T" command might look like:
$ = main::infested called from file 'Ambulation.pm' line 10
@ = Ambulation::legs(1, 2, 3, 4) called from file 'camel_flea' line 7
$ = main::pests('bactrian', 4) called from file 'camel_flea' line 4
The left-hand character up there indicates the context in which the function was called, with "$" and
"@" meaning scalar or list contexts respectively, and "." meaning void context (which is actually a
sort of scalar context). The display above says that you were in the function "main::infested" when
you ran the stack dump, and that it was called in scalar context from line 10 of the file
Ambulation.pm, but without any arguments at all, meaning it was called as &infested. The next stack
frame shows that the function "Ambulation::legs" was called in list context from the camel_flea file
with four arguments. The last stack frame shows that "main::pests" was called in scalar context, also
from camel_flea, but from line 4.
If you execute the "T" command from inside an active "use" statement, the backtrace will contain both
a "require" frame and an "eval" frame.
Line Listing Format
This shows the sorts of output the "l" command can produce:
DB<<13>> l
101: @i{@i} = ();
102:b @isa{@i,$pack} = ()
103 if(exists $i{$prevpack} || exists $isa{$pack});
104 }
105
Frame listing
When the "frame" option is set, the debugger would print entered (and optionally exited) subroutines
in different styles. See perldebguts for incredibly long examples of these.
Debugging Compile-Time Statements
If you have compile-time executable statements (such as code within BEGIN, UNITCHECK and CHECK blocks or "use"
statements), these will not be stopped by debugger, although "require"s and INIT blocks will, and compile-time
statements can be traced with the "AutoTrace" option set in "PERLDB_OPTS"). From your own Perl code, however,
you can transfer control back to the debugger using the following statement, which is harmless if the debugger
is not running:
$DB::single = 1;
If you set $DB::single to 2, it's equivalent to having just typed the "n" command, whereas a value of 1 means
the "s" command. The $DB::trace variable should be set to 1 to simulate having typed the "t" command.
Another way to debug compile-time code is to start the debugger, set a breakpoint on the load of some module:
DB<7> b load f:/perllib/lib/Carp.pm
Will stop on load of 'f:/perllib/lib/Carp.pm'.
and then restart the debugger using the "R" command (if possible). One can use "b compile subname" for the
same purpose.
Debugger Customization
The debugger probably contains enough configuration hooks that you won't ever have to modify it yourself. You
may change the behaviour of the debugger from within the debugger using its "o" command, from the command line
via the "PERLDB_OPTS" environment variable, and from customization files.
You can do some customization by setting up a .perldb file, which contains initialization code. For instance,
you could make aliases like these (the last one is one people expect to be there):
$DB::alias{'len'} = 's/^len(.*)/p length($1)/';
$DB::alias{'stop'} = 's/^stop (at|in)/b/';
$DB::alias{'ps'} = 's/^ps\b/p scalar /';
$DB::alias{'quit'} = 's/^quit(\s*)/exit/';
You can change options from .perldb by using calls like this one;
parse_options("NonStop=1 LineInfo=db.out AutoTrace=1 frame=2");
The code is executed in the package "DB". Note that .perldb is processed before processing "PERLDB_OPTS". If
.perldb defines the subroutine "afterinit", that function is called after debugger initialization ends.
.perldb may be contained in the current directory, or in the home directory. Because this file is sourced in
by Perl and may contain arbitrary commands, for security reasons, it must be owned by the superuser or the
current user, and writable by no one but its owner.
You can mock TTY input to debugger by adding arbitrary commands to @DB::typeahead. For example, your .perldb
file might contain:
sub afterinit { push @DB::typeahead, "b 4", "b 6"; }
Which would attempt to set breakpoints on lines 4 and 6 immediately after debugger initialization. Note that
Readline Support / History in the Debugger
As shipped, the only command-line history supplied is a simplistic one that checks for leading exclamation
points. However, if you install the Term::ReadKey and Term::ReadLine modules from CPAN (such as
Term::ReadLine::Gnu, Term::ReadLine::Perl, ...) you will have full editing capabilities much like those GNU
readline(3) provides. Look for these in the modules/by-module/Term directory on CPAN. These do not support
normal vi command-line editing, however.
A rudimentary command-line completion is also available, including lexical variables in the current scope if
the "PadWalker" module is installed.
Without Readline support you may see the symbols "^[[A", "^[[C", "^[[B", "^[[D"", "^H", ... when using the
arrow keys and/or the backspace key.
Editor Support for Debugging
If you have the FSF's version of emacs installed on your system, it can interact with the Perl debugger to
provide an integrated software development environment reminiscent of its interactions with C debuggers.
Recent versions of Emacs come with a start file for making emacs act like a syntax-directed editor that
understands (some of) Perl's syntax. See perlfaq3.
A similar setup by Tom Christiansen for interacting with any vendor-shipped vi and the X11 window system is
also available. This works similarly to the integrated multiwindow support that emacs provides, where the
debugger drives the editor. At the time of this writing, however, that tool's eventual location in the Perl
distribution was uncertain.
Users of vi should also look into vim and gvim, the mousey and windy version, for coloring of Perl keywords.
Note that only perl can truly parse Perl, so all such CASE tools fall somewhat short of the mark, especially
if you don't program your Perl as a C programmer might.
The Perl Profiler
If you wish to supply an alternative debugger for Perl to run, invoke your script with a colon and a package
argument given to the -d flag. Perl's alternative debuggers include a Perl profiler, Devel::NYTProf, which is
available separately as a CPAN distribution. To profile your Perl program in the file mycode.pl, just type:
$ perl -d:NYTProf mycode.pl
When the script terminates the profiler will create a database of the profile information that you can turn
into reports using the profiler's tools. See <perlperf> for details.
Debugging Regular Expressions
"use re 'debug'" enables you to see the gory details of how the Perl regular expression engine works. In order
to understand this typically voluminous output, one must not only have some idea about how regular expression
matching works in general, but also know how Perl's regular expressions are internally compiled into an
automaton. These matters are explored in some detail in "Debugging Regular Expressions" in perldebguts.
Debugging Memory Usage
Perl contains internal support for reporting its own memory usage, but this is a fairly advanced concept that
requires some understanding of how memory allocation works. See "Debugging Perl Memory Usage" in perldebguts
for the details.
SEE ALSO
You did try the -w switch, didn't you?
If you alter your @_ arguments in a subroutine (such as with "shift" or "pop"), the stack backtrace will not
show the original values.
The debugger does not currently work in conjunction with the -W command-line switch, because it itself is not
free of warnings.
If you're in a slow syscall (like "wait"ing, "accept"ing, or "read"ing from your keyboard or a socket) and
haven't set up your own $SIG{INT} handler, then you won't be able to CTRL-C your way back to the debugger,
because the debugger's own $SIG{INT} handler doesn't understand that it needs to raise an exception to
longjmp(3) out of slow syscalls.
perl v5.16.3 2013-03-04 PERLDEBUG(1)
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