PERLDEBUG(1) Perl Programmers Reference Guide PERLDEBUG(1)
perldebug - Perl debugging
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
.
If you invoke Perl with the -d switch, your script runs
under the Perl source debugger. This works like an interac-
tive 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 inter-
preter. 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 contain-
ing the debugger.
The program will halt right before the first run-time exe-
cutable statement (but see below regarding compile-time
statements) and ask you to enter a debugger command. Con-
trary 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 exe-
cuted.
Any command not recognized by the debugger is directly exe-
cuted ("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 modi-
fied 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.
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For any text entered at the debugger prompt, leading and
trailing whitespace is first stripped before further pro-
cessing. If a debugger command coincides with some function
in your own program, merely precede the function with some-
thing that doesn't look like a debugger command, such as a
leading ";" or perhaps a "+", or by wrapping it with
parentheses or braces.
Debugger Commands
The 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 h" produces the
entire help page, which is quite long.
If the output of the "h h" command (or any com-
mand, for that matter) scrolls past your screen,
precede the command with a leading pipe symbol
so that it's 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 {$DB::OUT} expr" in the current
package. In particular, because this is just
Perl's own "print" function, this means that
nested data structures and objects are not
dumped, unlike with the "x" command.
The "DB::OUT" filehandle is opened to /dev/tty,
regardless of where STDOUT may be redirected to.
x [maxdepth] expr
Evaluates its expression in list context and
dumps out the result in a pretty-printed
fashion. Nested data structures are printed out
recursively, unlike the real "print" function in
Perl. When dumping hashes, you'll probably
prefer 'x \%h' rather than 'x %h'. See Dumpvalue
if you'd like to do this yourself.
The output format is governed by multiple
options described under "Configurable Options".
If the "maxdepth" is included, it must be a
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numeral N; the value is dumped only N levels
deep, as if the "dumpDepth" option had been tem-
porarily set to N.
V [pkg [vars]]
Display all (or some) variables in package
(defaulting to "main") using a data pretty-
printer (hashes show their keys and values so
you see what's what, control characters are made
printable, etc.). Make sure you don't put the
type specifier (like "$") 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 vari-
ables 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. Out-
put 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 expres-
sion is supplied that includes function calls,
those functions will be executed with stops
before each statement.
r Continue until the return from the current sub-
routine. Dump the return value if the "PrintRet"
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option is set (default).
<CR> Repeat last "n" or "s" command.
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+1" lines starting at "min".
l min-max List lines "min" through "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 "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 con-
sidered 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
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S [[!]regex]
List subroutine names [not] matching the regex.
t Toggle trace mode (see also the "AutoTrace"
option).
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, 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
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 condi-
tion 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 %INC values.
b compile subname
Sets a breakpoint before the first statement
executed after the specified subroutine is com-
piled.
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 exe-
cuted. If line is omitted, set an action on the
line about to be executed. The sequence of steps
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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. We hope you know
what one of these is, because they're 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 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
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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 new-
lines (we bet you couldn't've 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 new-
lines.
{ ? List out pre-prompt debugger commands.
{ [ command ]
Set an action (debugger command) to happen
before every debugger prompt. A multi-line com-
mand may be entered in the customary fashion.
Because this command is in some senses new, a
warning is issued if you appear to have acciden-
tally 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
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before every debugger prompt. A multi-line com-
mand may be entered, if you can guess how: see
above.
! number Redo a previous command (defaults to the previ-
ous 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 proper interpretation of exit
status or signal and coredump information.
source file Read and execute debugger commands from file.
file may itself contain "source" commands.
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" doesn't work for this, unless
you've made an alias) This is the only supported
way to exit the debugger, though typing "exit"
twice might work.
Set the "inhibit_exit" option to 0 if you want
to be able to step off the end the script. You
may also need to set $finished to 0 if you want
to step through global destruction.
R Restart the debugger by "exec()"ing a new ses-
sion. 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.
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= [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 state-
ment 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 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 man-
pages 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. Occa-
sionally this determination is incorrect due to
recalcitrant vendors or rather more felici-
tously, 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 assign-
ment. 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"
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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 com-
mands (those beginning with a "|" character.)
By default, $ENV{PAGER} will be used. Because
the debugger uses your current terminal charac-
teristics 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 run-
ning programs. It will attempt to print a mes-
sage 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 "dieLevel" is even 1, then
your non-fatal exceptions are also traced and
unceremoniously altered if they came from
"eval'd" 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"
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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 subrou-
tine is printed.
The length at which the argument list is trun-
cated 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.
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"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 "quote" is "auto"; one can enable
double-quotish or single-quotish format by set-
ting it to """ or "'", respectively. By
default, characters with their high bit set are
printed verbatim.
"UsageOnly" Rudimentary per-package memory usage dump. Cal-
culates 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, put-
ting trace information into the file db.out. (If you inter-
rupt 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:
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"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 secu-
rity 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 interven-
tion, 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 )
which may be useful for debugging a program that uses
"Term::ReadLine" itself. 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
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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
you'd use to access with the built-in csh-like his-
tory 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 exam-
ple, 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 con-
text in which the function was called, with "$" and
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"@" meaning scalar or list contexts respectively,
and "." meaning void context (which is actually a
sort of scalar context). The display above says
that you were in the function "main::infested" when
you ran the stack dump, and that it was called in
scalar context from line 10 of the file
Ambulation.pm, but without any arguments at all,
meaning it was called as &infested. The next stack
frame shows that the function "Ambulation::legs" was
called in list context from the camel_flea file with
four arguments. The last stack frame shows that
"main::pests" was called in scalar context, also
from camel_flea, but from line 4.
If you execute the "T" command from inside an active
"use" statement, the backtrace will contain both a
"require" frame and an "eval") frame.
Line Listing Format
This shows the sorts of output the "l" command can
produce:
DB<<13>> l
101: @i{@i} = ();
102:b @isa{@i,$pack} = ()
103 if(exists $i{$prevpack} || exists $isa{$pack});
104 }
105
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 "b" and those with actions
by "a". The line that's 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 "frame" option is set, the debugger would
print entered (and optionally exited) subroutines in
different styles. See perldebguts for incredibly
long examples of these.
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Debugging compile-time statements
If you have compile-time executable statements (such as code
within BEGIN 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 "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 pos-
sible). One can use "b compile subname" for the same pur-
pose.
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 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");
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PERLDEBUG(1) Perl Programmers Reference Guide PERLDEBUG(1)
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 con-
tained 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 com-
mands 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::typeahead is 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 heart's
content. You'll then want to set your "PERL5DB" environment
variable to say something like this:
BEGIN { require "myperl5db.pl" }
As a last resort, you could also use "PERL5DB" to customize
the debugger by directly setting internal variables or cal-
ling debugger functions.
Note that any variables and functions that are not docu-
mented 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 sup-
port normal vi command-line editing, however.
A rudimentary command-line completion is also available.
Unfortunately, the names of lexical variables are not avail-
able for completion.
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PERLDEBUG(1) Perl Programmers Reference Guide PERLDEBUG(1)
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.
Perl comes with a start file for making emacs act like a
syntax-directed editor that understands (some of) Perl's
syntax. Look in the emacs directory of the Perl source dis-
tribution.
A similar setup by Tom Christiansen for interacting with any
vendor-shipped vi and the X11 window system is also avail-
able. 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, 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 pro-
file your Perl program in the file mycode.pl, just type:
$ perl -d:DProf mycode.pl
When the script terminates the profiler will dump the pro-
file 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.
"use re 'debug'" enables you to see the gory details of how
the Perl regular expression engine works. In order to under-
stand 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.
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PERLDEBUG(1) Perl Programmers Reference Guide PERLDEBUG(1)
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.
You did try the -w switch, didn't 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 $PATH
for it, so you don't have to type the path or "which
$scriptname".
$ perl -Sd foo.pl
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
"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 v5.8.8 2006-06-30 19