NAME
perldebug - Perl debugging
DESCRIPTION
First of all, have you tried using the -w switch?
If youre 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 its 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 its about to execute, rather than the one it has just executed.
Any command not recognized by the debugger is directly executed (
evald) as Perl code in the current package. (The debugger uses the DB package for keeping its own state information.)
Note that the said
evalis 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 doesnt 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 expressionusing -e. |
perl -d:Ptkdb program_name |
Debug a given program via the Devel::PtkdbGUI. |
perl -dt threaded_program_name | Debug a given program using threads (experimental). |
Debugger Commands
The interactive debugger understands the following commands:
h | Prints out a summary help message |
h [command] | Prints out a help message for the given debugger command. |
h h |
The special argument of h hproduces the entire help page, which is quite long. If the output of the h hcommand (or any command, for that matter) scrolls past your screen, precede the command with a leading pipe symbol so that its run through your pager, as in
DB> |h h
You may change the pager which is used via o pager=...command. |
p expr |
Same as print {$DB::OUT} exprin the current package. In particular, because this is just Perls own xcommand. The DB::OUTfilehandle 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 %h. See Dumpvalue if youd like to do this yourself. The output format is governed by multiple options described under Configurable Options. If the maxdepthis included, it must be a numeral N; the value is dumped only N levels deep, as if the dumpDepthoption 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 whats what, control characters are made printable, etc.). Make sure you dont put the type specifier (like $) there, just the symbol names, like this:
V DB filename line
Use ~patternand !patternfor positive and negative regexes. This is similar to calling the xcommand on each applicable var. |
X [vars] |
Same as V currentpackage [vars]. |
y [level [vars]] |
Display all (or some) lexical variables (mnemonic: mYvariables) 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 Vand Xcommands. Requires the PadWalkermodule version 0.08 or higher; will warn if this isnt installed. Output is pretty-printed in the same style as for Vand 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 PrintRetoption is set (default). |
<CR> |
Repeat last nor scommand. |
c [line|sub] | Continue, optionally inserting a one-time-only breakpoint at the specified line or subroutine. |
l | List next window of lines. |
l min+incr |
List incr+1lines starting at min. |
l min-max |
List lines minthrough max. l -is synonymous to -. |
l line | List a single line. |
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 evalstatement. If filename is not a full pathname found in the values of %INC, it is considered a regex.
evaled strings (when accessible) are considered to be filenames: f (eval 7)and f eval 7\baccess the body of the 7th evaled string (in the order of execution). The bodies of the currently executed evaland of evaled 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 |
Toggle trace mode (see also the AutoTraceoption). |
t expr |
Trace through execution of expr. 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, its 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
dont use if:
b 237 $x > 30
b 237 ++$count237 < 11
b 33 /pattern/i
|
b subname [condition] | Set a breakpoint before the first line of the named subroutine. subname may be a variable containing a code reference (in this case condition is not supported). |
b postpone subname [condition] | Set a breakpoint at first line of subroutine after it is compiled. |
b load filename |
Set a breakpoint before the first executed line of the filename,
which should be a full pathname found amongst the %INCvalues. |
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. |
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 $fooevery 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. We hope you know what one of these is, because theyre supposed to be obvious. |
W expr | Delete watch-expression |
W * | Delete all watch-expressions. |
o | Display all options |
o booloption ... |
Set each listed Boolean option to the value 1. |
o anyoption? ... | Print out the value of one or more options. |
o option=value ... |
Set the value of one or more options. If the value has internal
whitespace, it should be quoted. For example, you could set o pager="less -MQeicsNfr"to call less with those specific options. You may use either single or double quotes, but if you do, you must escape any embedded instances of same sort of quote you began with, as well as any escaping any escapes that immediately precede 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 bador o option="She said, \"Isnt it?\"". For historical reasons, the =valueis optional, but defaults to 1 only where it is safe to do sothat is, mostly for Boolean options. It is always better to assign a specific value using =. The optioncan 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 youve just given a command to return to executing the script. A multi-line command may be entered by backslashing the newlines (we bet you couldnt have guessed this by now). |
> * | Delete all post-prompt Perl command actions. |
>> command | Adds an action (Perl command) to happen after the prompt when youve 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 thats 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 numberth 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 users current shell (well, their $ENV{SHELL}variable) will be used, which can interfere with proper interpretation of exit status or signal and coredump information. |
source file |
Read and execute debugger commands from file.
file may itself contain sourcecommands. |
H -number | Display last n commands. Only commands longer than one character are listed. If number is omitted, list them all. |
q or ^D |
Quit. (quit doesnt work for this, unless youve made an alias)
This is the only supported way to exit the debugger, though typing
exittwice might work. Set the inhibit_exitoption to 0 if you want to be able to step off the end the script. You may also need to set $finishedto 0 if you want to step through global destruction. |
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 |dbcmdbut DB::OUT is temporarily selected 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 | Displays all loaded modules and their versions |
man [manpage] |
Despite its name, this calls your systems default documentation
viewer on the given page, or on the viewer itself if manpage is
omitted. If that viewer is man, the current Configinformation is used to invoke man using the proper MANPATH or -M manpath option. Failed lookups of the form XXXthat match known manpages of the form perlXXX will be retried. This lets you type man debugor man opfrom 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::doccmdvariable to whatever viewer to view the Perl documentation on your system. This may be set in an rc file, or through direct assignment. Were 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
ocommand, 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 pager. |
tkRunning | Run Tk while prompting (with ReadLine). |
signalLevel, warnLevel, dieLevel |
Level of verbosity. By default, the debugger leaves your exceptions
and warnings alone, because altering them can break correctly running
programs. It will attempt to print a message when uncaught INT, BUS, or
SEGV signals arrive. (But see the mention of signals in BUGS below.)
To disable this default safe mode, set these values to something higher than 0. At a level of 1, you get backtraces upon receiving any kind of warning (this is often annoying) or exception (this is often valuable). Unfortunately, the debugger cannot discern fatal exceptions from non-fatal ones. If dieLevelis even 1, then your non-fatal exceptions are also traced and unceremoniously altered if they came from evaledstrings or from any kind of evalwithin modules youre attempting to load. If dieLevelis 2, the debugger doesnt 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 tcommand, 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 vior emacshooks, or the dddgraphical debugger. |
inhibit_exit | If 0, allows stepping off the end of the script. |
PrintRet |
Print return value after rcommand 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 & 2is 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 stringifyand tied FETCHis 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 frameoptions bit 4 is set. |
windowSize | Change the size of code list window (default is 10 lines). |
V,
X, and
xcommands:
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. |
DumpPackages | Dump symbol tables of packages. |
DumpReused | Dump contents of reused addresses. |
quote, HighBit, undefPrint |
Change the style of string dump. The default value for quoteis auto; one can enable double-quotish or single-quotish format by setting it to "or , respectively. By default, characters with their high bit set are printed verbatim. |
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 modules file scope, or lost in closures. |
$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
NonStopthere.
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, youd better reset
LineInfoto /dev/tty if you expect to see anything.)
TTY | The TTY to use for debugging I/O. |
noTTY |
If set, the debugger goes into NonStopmode and will not connect to a TTY. If interrupted (or if control goes to the debugger via explicit setting of $DB::signalor $DB::singlefrom the Perl script), it connects to a TTY specified in the TTYoption at startup, or to a tty found at runtime using the Term::Rendezvousmodule of your choice. This module should implement a method named newthat returns an object with two methods: INand OUT. These should return filehandles to use for debugging input and output correspondingly. The newmethod 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::signalor $DB::single. |
$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=2is 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
LineInfoto 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 )
which may be useful for debugging a program that uses
Term::ReadLineitself. Do not forget to detach your shell from the TTY in the window that corresponds to /dev/ttyXX, say, by issuing a command like
$ sleep 1000000
See Debugger Internals in perldebguts for details.
Debugger input/output
Prompt |
The debugger prompt is something like
DB<8>
or even
DB<<17>>
where that number is the command number, and which youd use to access with the built-in csh-like history mechanism. For example, !17would 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 youd 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 expressioncommand. |
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.
Heres 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 |
Heres an example of what a stack backtrace via Tcommand 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 $and @meaning scalar or list contexts respectively, and .meaning void context (which is actually a sort of scalar context). The display above says that you were in the function main::infestedwhen you ran the stack dump, and that it was called in scalar context from line 10 of the file Ambulation.pm, but without any arguments at all, meaning it was called as &infested. The next stack frame shows that the function Ambulation::legswas called in list context from the camel_flea file with four arguments. The last stack frame shows that main::pestswas called in scalar context, also from camel_flea, but from line 4. If you execute the Tcommand from inside an active usestatement, the backtrace will contain both a requireframe and an eval) frame. |
Line Listing Format |
This shows the sorts of output the lcommand can produce:
DB<<13>> l
101: @i{@i} = ();
102:b @isa{@i,$pack} = ()
103 if(exists $i{$prevpack} || exists $isa{$pack});
104 }
105
106 next
107==> if(exists $isa{$pack});
108
109:a if ($extra-- > 0) {
110: %isa = ($pack,1);
Breakable lines are marked with :. Lines with breakpoints are marked by band those with actions by a. The line thats about to be executed is marked by ==>. Please be aware that code in debugger listings may not look the same as your original source code. Line directives and external source filters can alter the code before Perl sees it, causing code to move from its original positions or take on entirely different forms. |
Frame listing |
When the frameoption 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
usestatements), these will not be stopped by debugger, although
requires and INIT blocks will, and compile-time statements can be traced with
AutoTraceoption 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::singleto 2, its equivalent to having just typed the
ncommand, whereas a value of 1 means the
scommand. The
$DB::tracevariable should be set to 1 to simulate having typed the
tcommand.
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
Rcommand (if possible). One can use
b compile subnamefor the same purpose.
Debugger Customization
The debugger probably contains enough configuration hooks that you wont ever have to modify it yourself. You may change the behaviour of debugger from within the debugger using its
ocommand, from the command line via the
PERLDB_OPTSenvironment 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
@DB::typeaheadis not a supported interface and is subject to change in future releases.
If you want to modify the debugger, copy perl5db.pl from the Perl library to another name and hack it to your hearts content. Youll then want to set your
PERL5DBenvironment variable to say something like this:
BEGIN { require "myperl5db.pl" }
As a last resort, you could also use
PERL5DBto customize the debugger by directly setting internal variables or calling debugger functions.
Note that any variables and functions that are not documented in this document (or in perldebguts) are considered for internal use only, and as such are subject to change without notice.
Readline Support
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, you will have full editing capabilities much like 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. Unfortunately, the names of lexical variables are not available for completion.
Editor Support for Debugging
If you have the FSFs 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.
Perl comes with a start file for making emacs act like a syntax-directed editor that understands (some of) Perls syntax. Look in the emacs directory of the Perl source distribution.
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 tools 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 dont program your Perl as a C programmer might.
The Perl Profiler
If you wish to supply an alternative debugger for Perl to run, just invoke your script with a colon and a package argument given to the -d flag. The most popular alternative debuggers for Perl is the Perl profiler. Devel::DProf is now included with the standard Perl distribution. To profile your Perl program in the file mycode.pl, just type:
$ perl -d:DProf mycode.pl
When the script terminates the profiler will dump the profile information to a file called tmon.out. A tool like dprofpp, also supplied with the standard Perl distribution, can be used to interpret the information in that profile.
Debugging regular expressions
use re debugenables 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 Perls 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, didnt you?
perldebtut, perldebguts, re, DB, Devel::DProf, dprofpp, Dumpvalue, and perlrun.
When debugging a script that uses #! and is thus normally found in
$PATH, the -S option causes perl to search
$PATHfor it, so you dont have to type the path or
which $scriptname.
$ perl -Sd foo.pl
BUGS
You cannot get stack frame information or in any fashion debug functions that were not compiled by Perl, such as those from C or C++ extensions.
If you alter your
@_arguments in a subroutine (such as with
shiftor
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 youre in a slow syscall (like
waiting,
accepting, or
reading from your keyboard or a socket) and havent set up your own
$SIG{INT}handler, then you wont be able to CTRL-C your way back to the debugger, because the debuggers own
$SIG{INT}handler doesnt understand that it needs to raise an exception to longjmp(3) out of slow syscalls.