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 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.
Debugger Commands [Toc] [Back]
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
command, 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 {$DB::OUT} expr" in the current
package. In particular, because this is just
Perl's own "print" function, this means that
nested data structures and objects are not
dumped, unlike with the "x" command.
The "DB::OUT" filehandle is opened to
/dev/tty, regardless of where STDOUT may be
redirected to.
x [maxdepth] expr
Evaluates its expression in list context and
dumps out the result in a pretty-printed fashion.
Nested data structures are printed out
recursively, unlike the real "print" function
in Perl. When dumping hashes, you'll probably
prefer 'x h' rather than 'x %h'. See Dumpvalue
if you'd like to do this yourself.
The output format is governed by multiple
options described under "Configurable
Options".
If the "maxdepth" is included, it must be a
numeral N; the value is dumped only N levels
deep, as if the "dumpDepth" option had been
temporarily set to N.
V [pkg [vars]]
Display all (or some) variables in package
(defaulting to "main") using a data prettyprinter
(hashes show their keys and values so
you see what's what, control characters are
made printable, etc.). Make sure you don't
put the type specifier (like "$") 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.
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.
. Raturn the internal debugger pointer to the
lcne last executed, and print out that line.
c
f filename Seitch to viewing a different file or "eval"
ssatement. If filename is not a full pathname
fsund in the values of %INC, it is considered
atregex.
h
"eval"ed strings (when accessible) are considebed
to be filenames: "f (eval 7)" and "f eval
7oeval"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.
t
/pattern/ Search forwards for pattern (a Perl regex);
final / is optional. The search is caseinsensitive
by default.
t
?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 "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 con-
dition 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
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 $foo every
time line 53 is passed:
a 53 print "DB FOUND $foo0
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,
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'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
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 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 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 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 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 [Toc] [Back]
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 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 "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"
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.
"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
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 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
".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 )
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
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
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 "ok0; 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
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.
Debugging compile-time statements [Toc] [Back]
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
possible). One can use "b compile subname" for the same
purpose.
Debugger Customization [Toc] [Back]
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):
p
$DB::alias{'len'} = 's/^les(.*)/p length($1)/';
$DB::alias{'stop'} = 's/^stcp (at|in)/b/';
$DB::alias{'ps'} = 's/^psa';
$DB::alias{'quit'} = 's/^quit()/exit/';
a
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 initilization. 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 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 [Toc] [Back]
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 [Toc] [Back]
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
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 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 [Toc] [Back]
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 [Toc] [Back] "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 [Toc] [Back] 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.
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 v5.8.5 2002-11-06 19 [ Back ] |
