perldebug - Perl debugging
First of all, have you tried using the-w switch?
"As soon as we started programming, we found to our surprise that it wasn't as easy to get programs right as we had thought. Debugging had to be discovered. I can remember the exact instant when I realized that a large part of my life from then on was going to be spent in finding mistakes in my own programs."
--Maurice Wilkes, 1949
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 42In Perl, the debugger is not a separate program as 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 Perl library file containing the debugger itself.
The program will haltright 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'sabout 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 its own state information.)
Leading white space before a command would cause the debugger to think it'sNOT a debugger command but for Perl, so be careful not to do that.
The debugger understands the following commands:
Prints out a help message.
If you supply another debugger command as an argument to theh command, it prints out the description for just that command. The special argument ofh h produces a more compact help listing, designed to fit together on one screen.
If the output of theh command (or any command, for that matter) scrolls past your screen, either precede the command with a leading pipe symbol so it's run through your pager, as in
DB> |hYou may change the pager which is used viaO pager=... command.
Same asprint {$DB::OUT} expr in the current package. In particular, because this is just Perl's ownprint function, this means that nested data structures and objects are not dumped, unlike with thex command.
TheDB::OUT filehandle is opened to/dev/tty, regardless of where STDOUT may be redirected to.
Evaluates its expression in list context and dumps out the result in a pretty-printed fashion. Nested data structures are printed out recursively, unlike theprint function.
The details of printout are governed by multipleOptions.
Display all (or some) variables in package (defaulting to themain package) using a data pretty-printer (hashes show their keys and values so you see what's what, control characters are made printable, etc.). Make sure you don't put the type specifier (like$) there, just the symbol names, like this:
V DB filename lineUse~pattern and!pattern for positive and negative regexps.
Nested data structures are printed out in a legible fashion, unlike theprint function.
The details of printout are governed by multipleOptions.
Same asV currentpackage [vars].
Produce a stack backtrace. See below for details on its output.
Single step. Executes until it reaches the beginning of another statement, descending into subroutine calls. If an expression is supplied that includes function calls, it too will be single-stepped.
Next. Executes over subroutine calls, until it reaches 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.
Repeat lastn ors command.
Continue, optionally inserting a one-time-only breakpoint at the specified line or subroutine.
List next window of lines.
Listincr+1 lines starting atmin.
List linesmin throughmax.l - is synonymous to-.
List a single line.
List first window of lines from subroutine.
List previous window of lines.
List window (a few lines) around the current line.
Return debugger pointer to the last-executed line and print it out.
Switch to viewing a different file or eval statement. Iffilename is not a full filename as found in values of %INC, it is considered as a regexp.
Search forwards for pattern; final / is optional.
Search backwards for pattern; final ? is optional.
List all breakpoints and actions.
List subroutine names [not] matching pattern.
Toggle trace mode (see alsoAutoTraceOption).
Trace through execution of expr. For example:
$ perl -de 42Stack dump during die enabled outside of evals.Loading DB routines from perl5db.pl patch level 0.94Emacs support available.Enter h or `h h' for help.main::(-e:1): 0 DB<1> sub foo { 14 } DB<2> sub bar { 3 } DB<3> t print foo() * bar()main::((eval 172):3): print foo() + bar();main::foo((eval 168):2):main::bar((eval 170):2):42or, with theOptionframe=2 set,
DB<4> O f=2 frame = '2' DB<5> t print foo() * bar()3: foo() * bar()entering main::foo 2: sub foo { 14 };exited main::fooentering main::bar 2: sub bar { 3 };exited main::bar42Set a breakpoint. If line is omitted, sets a breakpoint on the line that is about to be executed. If a condition is specified, it's evaluated each time the statement is reached and a breakpoint is taken only if the condition is true. Breakpoints may be set on only lines that begin an executable statement. Conditions don't useif:
b 237 $x > 30b 237 ++$count237 < 11b 33 /pattern/iSet a breakpoint at the first line of the named subroutine.
Set breakpoint at first line of subroutine after it is compiled.
Set breakpoint at the first executed line of the file. Filename should be a full name as found in values of %INC.
Sets breakpoint at the first statement executed after the subroutine is compiled.
Delete a breakpoint at the specified line. If line is omitted, deletes the breakpoint on the line that is about to be executed.
Delete all installed breakpoints.
Set an action to be done before the line is executed. The sequence of steps taken by the debugger is
1. check for a breakpoint at this line2. print the line if necessary (tracing)3. do any actions associated with that line4. prompt user if at a breakpoint or in single-step5. evaluate lineFor example, this will print out $foo every time line 53 is passed:
a 53 print "DB FOUND $foo\n"Delete all installed actions.
Add a global watch-expression.
Delete all watch-expressions.
Set or query values of options. val defaults to 1. opt can be abbreviated. Several options can be listed.
recallCommand,ShellBangThe characters used to recall command or spawn shell. By default, these are both set to!.
pagerProgram to use for output of pager-piped commands (those beginning with a| character.) By default,$ENV{PAGER} will be used.
tkRunningRun Tk while prompting (with ReadLine).
signalLevel,warnLevel,dieLevelLevel of verbosity. By default the debugger is in a sane verbose mode, thus it will print backtraces on all the warnings and die-messages which are going to be printed out, and will print a message when interesting uncaught signals arrive.
To disable this behaviour, set these values to 0. IfdieLevel is 2, then the messages which will be caught by surroundingeval are also printed.
AutoTraceTrace mode (similar tot command, but can be put intoPERLDB_OPTS).
LineInfoFile or pipe to print line number info to. If it is a pipe (say,|visual_perl_db), then a short, "emacs like" message is used.
inhibit_exitIf 0, allowsstepping off the end of the script.
PrintRetaffects printing of return value afterr command.
ornamentsaffects screen appearance of the command line (seeTerm::ReadLine).
frameaffects printing messages on entry and exit from subroutines. Ifframe & 2 is false, messages are printed on entry only. (Printing on exit may be useful if inter(di)spersed with other messages.)
Ifframe & 4, arguments to functions are printed as well as the context and caller info. Ifframe & 8, overloadedstringify andtiedFETCH are enabled on the printed arguments. Ifframe & 16, the return value from the subroutine is printed as well.
The length at which the argument list is truncated is governed by the next option:
maxTraceLenlength at which the argument list is truncated whenframe option's bit 4 is set.
The following options affect what happens withV,X, andx commands:
arrayDepth,hashDepthPrint only first N elements ('' for all).
compactDump,veryCompactChange style of array and hash dump. IfcompactDump, short array may be printed on one line.
globPrintWhether to print contents of globs.
DumpDBFilesDump arrays holding debugged files.
DumpPackagesDump symbol tables of packages.
DumpReusedDump contents of "reused" addresses.
quote,HighBit,undefPrintChange style of string dump. Default value ofquote isauto, one can enable either double-quotish dump, or single-quotish by setting it to" or'. By default, characters with high bit set are printedas is.
UsageOnlyvery rudimentally per-package memory usage dump. Calculates total size of strings in variables in the package.
During startup options are initialized from$ENV{PERLDB_OPTS}. You can put additional initialization optionsTTY,noTTY,ReadLine, andNonStop there.
Example rc file:
&parse_options("NonStop=1 LineInfo=db.out AutoTrace");The script will run without human intervention, putting trace information into the filedb.out. (If you interrupt it, you would better resetLineInfo to something "interactive"!)
TTYThe TTY to use for debugging I/O.
noTTYIf set, goes inNonStop mode, and would not connect to a TTY. If interrupt (or if control goes to debugger via explicit setting of $DB::signal or $DB::single from the Perl script), connects to a TTY specified by theTTY option at startup, or to a TTY found at runtime usingTerm::Rendezvous module of your choice.
This module should implement a methodnew which returns an object with two methods:IN andOUT, returning two filehandles to use for debugging input and output correspondingly. Methodnew may inspect an argument which is a value of$ENV{PERLDB_NOTTY} at startup, or is"/tmp/perldbtty$$" otherwise.
ReadLineIf false, readline support in debugger is disabled, so you can debug ReadLine applications.
NonStopIf set, debugger goes into noninteractive 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="N f=2" perl -d myprogramwill run the scriptmyprogram without human intervention, printing out the call tree with entry and exit points. Note thatN f=2 is equivalent toNonStop=1 frame=2. Note also that at the moment when this documentation was written all the options to the debugger could be uniquely abbreviated by the first letter (with exception ofDump* options).
Other examples may include
$ PERLDB_OPTS="N f A L=listing" perl -d myprogram- runs script noninteractively, printing info on each entry into a subroutine and each executed line into the filelisting. (If you interrupt it, you would better resetLineInfo to something "interactive"!)
$ env "PERLDB_OPTS=R=0 TTY=/dev/ttyc" perl -d myprogrammay be useful for debugging a program which usesTerm::ReadLine itself. Do not forget detach shell from the TTY in the window which corresponds to/dev/ttyc, say, by issuing a command like
$ sleep 1000000See"Debugger Internals" below for more details.
Set an action (Perl command) to happen before every debugger prompt. A multi-line command may be entered by backslashing the newlines. Ifcommand is missing, resets the list of actions.
Add an action (Perl command) to happen before every debugger prompt. A multi-line command may be entered by backslashing the newlines.
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. Ifcommand is missing, resets the list of actions.
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.
Set an action (debugger command) to happen before every debugger prompt. A multi-line command may be entered by backslashing the newlines. Ifcommand is missing, resets the list of actions.
Add an action (debugger command) to happen before every debugger prompt. A multi-line command may be entered by backslashing the newlines.
Redo a previous command (default previous command).
Redo number'th-to-last command.
Redo last command that started with pattern. SeeO recallCommand, too.
Run cmd in a subprocess (reads from DB::IN, writes to DB::OUT) SeeO shellBang too.
Display last n commands. Only commands longer than one character are listed. If number is omitted, lists them all.
Quit. ("quit" doesn't work for this.) This is the only supported way to exit the debugger, though typingexit twice may do it too.
Set anOptioninhibit_exit to 0 if you want to be able tostep off the end the script. You may also need to set$finished to 0 at some moment if you want to step through global destruction.
Restart the debugger byexecing a new session. It tries to maintain your history across this, but internal settings and command line options may be lost.
Currently the following setting are preserved: history, breakpoints, actions, debuggerOptions, and the following command line options:-w,-I, and-e.
Run debugger command, piping DB::OUT to current pager.
Same as|dbcmd but DB::OUT is temporarilyselected as well. Often used with commands that would otherwise produce long output, such as
|V mainDefine a command alias, like
= quit qor list current aliases.
Execute command as a Perl statement. A missing semicolon will be supplied.
The expression is evaluated, and the methods which may be applied to the result are listed.
The methods which may be applied to objects in thepackage are listed.
The debugger prompt is something like
DB<8>or even
DB<<17>>where that number is the command number, which you'd use to access with the builtincsh-like history mechanism, e.g.,!17 would repeat command number 17. The number of angle brackets indicates the 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 out the result of a function call that itself also has a breakpoint, or you step into an expression vias/n/t expression command.
If you want to enter a multi-line command, such as a subroutine definition with several statements, or a format, you may 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: }okokokokNote that this business of escaping a newline is specific to interactive commands typed into the debugger.
Here's an example of what a stack backtrace viaT 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 4The left-hand character up there tells whether the function was called in a scalar or list context (we bet you can tell which is which). What that says is that you were in the functionmain::infested when you ran the stack dump, and that it was called in a scalar context from line 10 of the fileAmbulation.pm, but without any arguments at all, meaning it was called as&infested. The next stack frame shows that the functionAmbulation::legs was called in a list context from thecamel_flea file with four arguments. The last stack frame shows thatmain::pests was called in a scalar context, also fromcamel_flea, but from line 4.
Note that if you executeT command from inside an activeuse statement, the backtrace will contain bothrequire frame and aneval) frame.
Listing given via different flavors ofl command looks like this:
DB<<13>> l101: @i{@i} = ();102:b @isa{@i,$pack} = ()103 if(exists $i{$prevpack} || exists $isa{$pack});104 }105106 next107==> if(exists $isa{$pack});108109:a if ($extra-- > 0) {110: %isa = ($pack,1);Note that the breakable lines are marked with:, lines with breakpoints are marked byb, with actions bya, and the next executed line is marked by==>.
Whenframe option is set, debugger would print entered (and optionally exited) subroutines in different styles.
What follows is the start of the listing of
env "PERLDB_OPTS=f=n N" perl -d -Vfor different values ofn:
entering main::BEGIN entering Config::BEGIN Package lib/Exporter.pm. Package lib/Carp.pm. Package lib/Config.pm. entering Config::TIEHASH entering Exporter::import entering Exporter::exportentering Config::myconfig entering Config::FETCH entering Config::FETCH entering Config::FETCH entering Config::FETCHentering main::BEGIN entering Config::BEGIN Package lib/Exporter.pm. Package lib/Carp.pm. exited Config::BEGIN Package lib/Config.pm. entering Config::TIEHASH exited Config::TIEHASH entering Exporter::import entering Exporter::export exited Exporter::export exited Exporter::importexited main::BEGINentering Config::myconfig entering Config::FETCH exited Config::FETCH entering Config::FETCH exited Config::FETCH entering Config::FETCHin $=main::BEGIN() from /dev/nul:0 in $=Config::BEGIN() from lib/Config.pm:2 Package lib/Exporter.pm. Package lib/Carp.pm. Package lib/Config.pm. in $=Config::TIEHASH('Config') from lib/Config.pm:644 in $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/nul:0 in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from liin @=Config::myconfig() from /dev/nul:0 in $=Config::FETCH(ref(Config), 'package') from lib/Config.pm:574 in $=Config::FETCH(ref(Config), 'baserev') from lib/Config.pm:574 in $=Config::FETCH(ref(Config), 'PATCHLEVEL') from lib/Config.pm:574 in $=Config::FETCH(ref(Config), 'SUBVERSION') from lib/Config.pm:574 in $=Config::FETCH(ref(Config), 'osname') from lib/Config.pm:574 in $=Config::FETCH(ref(Config), 'osvers') from lib/Config.pm:574in $=main::BEGIN() from /dev/nul:0 in $=Config::BEGIN() from lib/Config.pm:2 Package lib/Exporter.pm. Package lib/Carp.pm. out $=Config::BEGIN() from lib/Config.pm:0 Package lib/Config.pm. in $=Config::TIEHASH('Config') from lib/Config.pm:644 out $=Config::TIEHASH('Config') from lib/Config.pm:644 in $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/nul:0 in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/ out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/ out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/nul:0out $=main::BEGIN() from /dev/nul:0in @=Config::myconfig() from /dev/nul:0 in $=Config::FETCH(ref(Config), 'package') from lib/Config.pm:574 out $=Config::FETCH(ref(Config), 'package') from lib/Config.pm:574 in $=Config::FETCH(ref(Config), 'baserev') from lib/Config.pm:574 out $=Config::FETCH(ref(Config), 'baserev') from lib/Config.pm:574 in $=Config::FETCH(ref(Config), 'PATCHLEVEL') from lib/Config.pm:574 out $=Config::FETCH(ref(Config), 'PATCHLEVEL') from lib/Config.pm:574 in $=Config::FETCH(ref(Config), 'SUBVERSION') from lib/Config.pm:574in $=main::BEGIN() from /dev/nul:0 in $=Config::BEGIN() from lib/Config.pm:2 Package lib/Exporter.pm. Package lib/Carp.pm. out $=Config::BEGIN() from lib/Config.pm:0 Package lib/Config.pm. in $=Config::TIEHASH('Config') from lib/Config.pm:644 out $=Config::TIEHASH('Config') from lib/Config.pm:644 in $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/nul:0 in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/E out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/E out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/nul:0out $=main::BEGIN() from /dev/nul:0in @=Config::myconfig() from /dev/nul:0 in $=Config::FETCH('Config=HASH(0x1aa444)', 'package') from lib/Config.pm:574 out $=Config::FETCH('Config=HASH(0x1aa444)', 'package') from lib/Config.pm:574 in $=Config::FETCH('Config=HASH(0x1aa444)', 'baserev') from lib/Config.pm:574 out $=Config::FETCH('Config=HASH(0x1aa444)', 'baserev') from lib/Config.pm:574in $=CODE(0x15eca4)() from /dev/null:0 in $=CODE(0x182528)() from lib/Config.pm:2 Package lib/Exporter.pm. out $=CODE(0x182528)() from lib/Config.pm:0 scalar context return from CODE(0x182528): undef Package lib/Config.pm. in $=Config::TIEHASH('Config') from lib/Config.pm:628 out $=Config::TIEHASH('Config') from lib/Config.pm:628 scalar context return from Config::TIEHASH: empty hash in $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0 in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/Exporter.pm:171 out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/Exporter.pm:171 scalar context return from Exporter::export: '' out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0 scalar context return from Exporter::import: ''In all the cases indentation of lines shows the call tree, if bit 2 offrame is set, then a line is printed on exit from a subroutine as well, if bit 4 is set, then the arguments are printed as well as the caller info, if bit 8 is set, the arguments are printed even if they are tied or references, if bit 16 is set, the return value is printed as well.
When a package is compiled, a line like this
Package lib/Carp.pm.is printed with proper indentation.
If you have any compile-time executable statements (code within a BEGIN block or ause statement), these willNOT be stopped by debugger, althoughrequires will (and compile-time statements can be traced withAutoTrace option set inPERLDB_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 the value 2, it's equivalent to having just typed then command, whereas a value of 1 means thes command. The$DB::trace variable should be set to 1 to simulate having typed thet command.
Another way to debug compile-time code is to start debugger, set a breakpoint onload of some module thusly
DB<7> b load f:/perllib/lib/Carp.pmWill stop on load of `f:/perllib/lib/Carp.pm'.and restart debugger byR command (if possible). One can useb compile subname for the same purpose.
Most probably you do not want to modify the debugger, it contains enough hooks to satisfy most needs. You may change the behaviour of debugger from the debugger itself, usingOptions, from the command line viaPERLDB_OPTS environment variable, and fromcustomization 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\$/';One changes options from.perldb file via calls like this one;
parse_options("NonStop=1 LineInfo=db.out AutoTrace=1 frame=2");(the code is executed in the packageDB). Note that.perldb is processed before processingPERLDB_OPTS. If.perldb defines the subroutineafterinit, it is called after all the debugger initialization ends..perldb may be contained in the current directory, or in theLOGDIR/HOME directory.
If you want to modify the debugger, copyperl5db.pl from the Perl library to another name and modify it as necessary. You'll also want to set yourPERL5DB environment variable to say something like this:
BEGIN { require "myperl5db.pl" }As the last resort, one can usePERL5DB to customize debugger by directly setting internal variables or calling debugger functions.
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 GNUreadline(3) provides. Look for these in themodules/by-module/Term directory on CPAN.
A rudimentary command line completion is also available. Unfortunately, the names of lexical variables are not available for completion.
If you have GNUemacs 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 is also delivered with a start file for makingemacs act like a syntax-directed editor that understands (some of) Perl's syntax. Look in theemacs directory of the Perl source distribution.
(Historically, a similar setup for interacting withvi and the X11 window system had also been available, but at the time of this writing, no debugger support forvi currently exists.)
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. One of the most popular alternative debuggers for Perl isDProf, the Perl profiler. As of this writing,DProf is not included with the standard Perl distribution, but it is expected to be included soon, for certain values of "soon".
Meanwhile, you can fetch the Devel::Dprof module from CPAN. Assuming it's properly installed on your system, to profile your Perl program in the filemycode.pl, just type:
perl -d:DProf mycode.plWhen the script terminates the profiler will dump the profile information to a file calledtmon.out. A tool likedprofpp (also supplied with the Devel::DProf package) can be used to interpret the information which is in that profile.
When you call thecaller function (see"caller" in perlfunc) from the package DB, Perl sets the array @DB::args to contain the arguments the corresponding stack frame was called with.
If perl is run with-d option, the following additional features are enabled (cf."$^P" in perlvar):
Perl inserts the contents of$ENV{PERL5DB} (orBEGIN {require 'perl5db.pl'} if not present) before the first line of the application.
The array@{"_<$filename"} is the line-by-line contents of $filename for all the compiled files. Same forevaled strings which contain subroutines, or which are currently executed. The$filename forevaled strings looks like(eval 34).
The hash%{"_<$filename"} contains breakpoints and action (it is keyed by line number), and individual entries are settable (as opposed to the whole hash). Only true/false is important to Perl, though the values used byperl5db.pl have the form"$break_condition\0$action". Values are magical in numeric context: they are zeros if the line is not breakable.
Same for evaluated strings which contain subroutines, or which are currently executed. The $filename forevaled strings looks like(eval 34).
The scalar${"_<$filename"} contains"_<$filename". Same for evaluated strings which contain subroutines, or which are currently executed. The $filename forevaled strings looks like(eval 34).
After eachrequired file is compiled, but before it is executed,DB::postponed(*{"_<$filename"}) is called (if subroutineDB::postponed exists). Here the $filename is the expanded name of therequired file (as found in values of %INC).
After each subroutinesubname is compiled existence of$DB::postponed{subname} is checked. If this key exists,DB::postponed(subname) is called (if subroutineDB::postponed exists).
A hash%DB::sub is maintained, with keys being subroutine names, values having the formfilename:startline-endline.filename has the form(eval 31) for subroutines defined insideevals.
When execution of the application reaches a place that can have a breakpoint, a call toDB::DB() is performed if any one of variables $DB::trace, $DB::single, or $DB::signal is true. (Note that these variables are notlocalizable.) This feature is disabled when the control is insideDB::DB() or functions called from it (unless$^D & (1<<30)).
When execution of the application reaches a subroutine call, a call to&DB::sub(args) is performed instead, with$DB::sub being the name of the called subroutine. (Unless the subroutine is compiled in the packageDB.)
Note that if&DB::sub needs some external data to be setup for it to work, no subroutine call is possible until this is done. For the standard debugger$DB::deep (how many levels of recursion deep into the debugger you can go before a mandatory break) gives an example of such a dependency.
The minimal working debugger consists of one line
sub DB::DB {}which is quite handy as contents ofPERL5DB environment variable:
env "PERL5DB=sub DB::DB {}" perl -d your-scriptAnother (a little bit more useful) minimal debugger can be created with the only line being
sub DB::DB {print ++$i; scalar <STDIN>}This debugger would print the sequential number of encountered statement, and would wait for yourCR to continue.
The following debugger is quite functional:
{ package DB; sub DB {} sub sub {print ++$i, " $sub\n"; &$sub}}It prints the sequential number of subroutine call and the name of the called subroutine. Note that&DB::sub should be compiled into the packageDB.
At the start, the debugger reads your rc file (./.perldb or~/.perldb under Unix), which can set important options. This file may define a subroutine&afterinit to be executed after the debugger is initialized.
After the rc file is read, the debugger reads environment variable PERLDB_OPTS and parses it as a rest ofO ... line in debugger prompt.
It also maintains magical internal variables, such as@DB::dbline,%DB::dbline, which are aliases for@{"::_<current_file"}%{"::_<current_file"}. Herecurrent_file is the currently selected (with the debugger'sf command, or by flow of execution) file.
Some functions are provided to simplify customization. See"Debugger Customization" for description ofDB::parse_options(string). The functionDB::dump_trace(skip[, count]) skips the specified number of frames, and returns a list containing info about the caller frames (all ifcount is missing). Each entry is a hash with keyscontext ($ or@),sub (subroutine name, or info about eval),args (undef or a reference to an array),file, andline.
The functionDB::print_trace(FH, skip[, count[, short]]) prints formatted info about caller frames. The last two functions may be convenient as arguments to<,<< commands.
You did try the-w switch, didn't you?
You cannot get the stack frame information or otherwise debug functions that were not compiled by Perl, such as C or C++ extensions.
If you alter your @_ arguments in a subroutine (such as withshift orpop, the stack backtrace will not show the original values.
Perl isvery frivolous with memory. There is a saying that to estimate memory usage of Perl, assume a reasonable algorithm of allocation, and multiply your estimages by 10. This is not absolutely true, but may give you a good grasp of what happens.
Say, an integer cannot take less than 20 bytes of memory, a float cannot take less than 24 bytes, a string cannot take less than 32 bytes (all these examples assume 32-bit architectures, the result are much worse on 64-bit architectures). If a variable is accessed in two of three different ways (which require an integer, a float, or a string), the memory footprint may increase by another 20 bytes. A sloppy malloc() implementation will make these numbers yet more.
On the opposite end of the scale, a declaration like
sub foo;may take (on some versions of perl) up to 500 bytes of memory.
Off-the-cuff anecdotal estimates of a code bloat give a factor around 8. This means that the compiled form of reasonable (commented indented etc.) code will take approximately 8 times more than the disk space the code takes.
There are two Perl-specific ways to analyze the memory usage: $ENV{PERL_DEBUG_MSTATS} and-DL switch. First one is available only if perl is compiled with Perl's malloc(), the second one only if Perl compiled with-DDEBUGGING (as with giving-D optimise=-g option toConfigure).
$ENV{PERL_DEBUG_MSTATS}If your perl is using Perl's malloc(), and compiled with correct switches (this is the default), then it will print memory usage statistics after compiling your code (if$ENV{PERL_DEBUG_MSTATS} > 1), and before termination of the script (if$ENV{PERL_DEBUG_MSTATS} >= 1). The report format is similar to one in the following example:
env PERL_DEBUG_MSTATS=2 perl -e "require Carp" Memory allocation statistics after compilation: (buckets 4(4)..8188(8192) 14216 free: 130 117 28 7 9 0 2 2 1 0 0437 61 36 0 5 60924 used: 125 137 161 55 7 8 6 16 2 0 1 74 109 304 84 20 Total sbrk(): 77824/21:119. Odd ends: pad+heads+chain+tail: 0+636+0+2048. Memory allocation statistics after execution: (buckets 4(4)..8188(8192) 30888 free: 245 78 85 13 6 2 1 3 2 0 1315 162 39 42 11 175816 used: 265 176 1112 111 26 22 11 27 2 1 1196 178 1066 798 39 Total sbrk(): 215040/47:145. Odd ends: pad+heads+chain+tail: 0+2192+0+6144.It is possible to ask for such a statistic at arbitrary moment by usind Devel::Peek::mstats() (module Devel::Peek is available on CPAN).
Here is the explanation of different parts of the format:
buckets SMALLEST(APPROX)..GREATEST(APPROX)Perl's malloc() uses bucketed allocations. Every request is rounded up to the closest bucket size available, and a bucket of these size is taken from the pool of the buckets of this size.
The above line describes limits of buckets currently in use. Each bucket has two sizes: memory footprint, and the maximal size of user data which may be put into this bucket. Say, in the above example the smallest bucket is both sizes 4. The biggest bucket has usable size 8188, and the memory footprint 8192.
With debugging Perl some buckets may have negative usable size. This means that these buckets cannot (and will not) be used. For greater buckets the memory footprint may be one page greater than a power of 2. In such a case the corresponding power of two is printed instead in theAPPROX field above.
The following 1 or 2 rows of numbers correspond to the number of buckets of each size betweenSMALLEST andGREATEST. In the first row the sizes (memory footprints) of buckets are powers of two (or possibly one page greater). In the second row (if present) the memory footprints of the buckets are between memory footprints of two buckets "above".
Say, with the above example the memory footprints are (with current algorith)
free: 8 16 32 64 128 256 512 1024 2048 4096 8192 4 12 24 48 80With non-DEBUGGING perl the buckets starting from128-long ones have 4-byte overhead, thus 8192-long bucket may take up to 8188-byte-long allocations.
Total sbrk(): SBRKed/SBRKs:CONTINUOUSThe first two fields give the total amount of memory perl sbrk()ed, and number of sbrk()s used. The third number is what perl thinks about continuity of returned chunks. As far as this number is positive, malloc() will assume that it is probable that sbrk() will provide continuous memory.
The amounts sbrk()ed by external libraries is not counted.
pad: 0The amount of sbrk()ed memory needed to keep buckets aligned.
heads: 2192While memory overhead of bigger buckets is kept inside the bucket, for smaller buckets it is kept in separate areas. This field gives the total size of these areas.
chain: 0malloc() may want to subdivide a bigger bucket into smaller buckets. If only a part of the deceased-bucket is left non-subdivided, the rest is kept as an element of a linked list. This field gives the total size of these chunks.
tail: 6144To minimize amount of sbrk()s malloc() asks for more memory. This field gives the size of the yet-unused part, which is sbrk()ed, but never touched.
Below we show how to analyse memory usage by
do 'lib/auto/POSIX/autosplit.ix';The file in question contains a header and 146 lines similar to
sub getcwd ;Note:the discussion below supposes 32-bit architecture. In the newer versions of perl the memory usage of the constructs discussed here is much improved, but the story discussed below is a real-life story. This story is very terse, and assumes more than cursory knowledge of Perl internals.
Here is the itemized list of Perl allocations performed during parsing of this file:
!!! "after" at test.pl line 3. Id subtot 4 8 12 16 20 24 28 32 36 40 48 56 64 72 80 80+ 0 02 13752 . . . . 294 . . . . . . . . . . 4 0 54 5545 . . 8 124 16 . . . 1 1 . . . . . 3 5 05 32 . . . . . . . 1 . . . . . . . . 6 02 7152 . . . . . . . . . . 149 . . . . . 7 02 3600 . . . . . 150 . . . . . . . . . . 7 03 64 . -1 . 1 . . 2 . . . . . . . . . 7 04 7056 . . . . . . . . . . . . . . . 7 7 17 38404 . . . . . . . 1 . . 442 149 . . 147 . 9 03 2078 17 249 32 . . . . 2 . . . . . . . .To see this list insert twowarn('!...') statements around the call:
warn('!');do 'lib/auto/POSIX/autosplit.ix';warn('!!! "after"');and run it with-DL option. The first warn() will print memory allocation info before the parsing of the file, and will memorize the statistics at this point (we ignore what it prints). The second warn() will print increments w.r.t. this memorized statistics. This is the above printout.
DifferentIds on the left correspond to different subsystems of perl interpreter, they are just first argument given to perl memory allocation API New(). To find what9 03 meansgrep the perl source for903. You will see that it isutil.c, function savepvn(). This function is used to store a copy of existing chunk of memory. Using C debugger, one can see that it is called either directly from gv_init(), or via sv_magic(), and gv_init() is called from gv_fetchpv() - which is called from newSUB().
Note: to reach this place in debugger and skip all the calls to savepvn during the compilation of the main script, set a C breakpoint in Perl_warn(),continue this point is reached,then set breakpoint in Perl_savepvn(). Note that you may need to skip a handful of Perl_savepvn() which do not correspond to mass production of CVs (there are more903 allocations than 146 similar lines oflib/auto/POSIX/autosplit.ix). Note also thatPerl_ prefixes are added by macroization code in perl header files to avoid conflicts with external libraries.
Anyway, we see that903 ids correspond to creation of globs, twice per glob - for glob name, and glob stringification magic.
Here are explanations for otherIds above:
717is for creation of biggerXPV* structures. In the above case it creates 3AV per subroutine, one for a list of lexical variable names, one for a scratchpad (which contains lexical variables andtargets), and one for the array of scratchpads needed for recursion.
It also creates aGV and aCV per subroutine (all called from start_subparse()).
002Creates C array corresponding to theAV of scratchpads, and the scratchpad itself (the first fake entry of this scratchpad is created though the subroutine itself is not defined yet).
It also creates C arrays to keep data for the stash (this is one HV, but it grows, thus there are 4 big allocations: the big chunks are not freeed, but are kept as additional arenas forSV allocations).
054creates aHEK for the name of the glob for the subroutine (this name is a key in astash).
Big allocations with thisId correspond to allocations of new arenas to keepHE.
602creates aGP for the glob for the subroutine.
702creates theMAGIC for the glob for the subroutine.
704createsarenas which keep SVs.
If Perl is run with-DL option, then warn()s which start with `!' behave specially. They print a list ofcategories of memory allocations, and statistics of allocations of different sizes for these categories.
If warn() string starts with
!!!print changed categories only, print the differences in counts of allocations;
!!print grown categories only; print the absolute values of counts, and totals;
!print nonempty categories, print the absolute values of counts and totals.
If an extension or an external library does not use Perl API to allocate memory, these allocations are not counted.
There are two ways to enable debugging output for regular expressions.
If your perl is compiled with-DDEBUGGING, you may use the-Dr flag on the command line.
Otherwise, one canuse re 'debug', which has effects both at compile time, and at run time (and isnot lexically scoped).
The debugging output for the compile time looks like this:
compiling RE `[bc]d(ef*g)+h[ij]k$' size 43 first at 1 1: ANYOF(11) 11: EXACT <d>(13) 13: CURLYX {1,32767}(27) 15: OPEN1(17) 17: EXACT <e>(19) 19: STAR(22) 20: EXACT <f>(0) 22: EXACT <g>(24) 24: CLOSE1(26) 26: WHILEM(0) 27: NOTHING(28) 28: EXACT <h>(30) 30: ANYOF(40) 40: EXACT <k>(42) 42: EOL(43) 43: END(0) anchored `de' at 1 floating `gh' at 3..2147483647 (checking floating) stclass `ANYOF' minlen 7The first line shows the pre-compiled form of the regexp, and the second shows the size of the compiled form (in arbitrary units, usually 4-byte words) and the labelid of the first node which does a match.
The last line (split into two lines in the above) contains the optimizer info. In the example shown, the optimizer found that the match should contain a substringde at the offset 1, and substringgh at some offset between 3 and infinity. Moreover, when checking for these substrings (to abandon impossible matches quickly) it will check for the substringgh before checking for the substringde. The optimizer may also use the knowledge that the match starts (at thefirstid) with a character class, and the match cannot be shorter than 7 chars.
The fields of interest which may appear in the last line are
anchoredSTRINGatPOSfloatingSTRINGatPOS1..POS2see above;
matching floating/anchoredwhich substring to check first;
minlenthe minimal length of the match;
stclassTYPEThe type of the first matching node.
noscanwhich advises to not scan for the found substrings;
isallwhich says that the optimizer info is in fact all that the regular expression contains (thus one does not need to enter the RE engine at all);
GPOSif the pattern contains\G;
plusif the pattern starts with a repeated char (as inx+y);
implicitif the pattern starts with.*;
with evalif the pattern contain eval-groups (see"(?{ code })" in perlre);
anchored(TYPE)if the pattern may match only at a handful of places (withTYPE beingBOL,MBOL, orGPOS, see the table below).
If a substring is known to match at end-of-line only, it may be followed by$, as infloating `k'$.
The optimizer-specific info is used to avoid entering (a slow) RE engine on strings which will definitely not match. Ifisall flag is set, a call to the RE engine may be avoided even when optimizer found an appropriate place for the match.
The rest of the output contains the list ofnodes of the compiled form of the RE. Each line has format
id:TYPEOPTIONAL-INFO (next-id)
Here is the list of possible types with short descriptions:
# TYPE arg-description [num-args] [longjump-len] DESCRIPTION# Exit pointsENDnoEnd of program.SUCCEEDnoReturn from a subroutine, basically.# Anchors:BOLnoMatch "" at beginning of line.MBOLnoSame, assuming multiline.SBOLnoSame, assuming singleline.EOSnoMatch "" at end of string.EOLnoMatch "" at end of line.MEOLnoSame, assuming multiline.SEOLnoSame, assuming singleline.BOUNDnoMatch "" at any word boundaryBOUNDLnoMatch "" at any word boundaryNBOUNDnoMatch "" at any word non-boundaryNBOUNDLnoMatch "" at any word non-boundaryGPOSnoMatches where last m//g left off.# [Special] alternativesANYnoMatch any one character (except newline).SANYnoMatch any one character.ANYOFsvMatch character in (or not in) this class.ALNUMnoMatch any alphanumeric characterALNUMLnoMatch any alphanumeric char in localeNALNUMnoMatch any non-alphanumeric characterNALNUMLnoMatch any non-alphanumeric char in localeSPACEnoMatch any whitespace characterSPACELnoMatch any whitespace char in localeNSPACEnoMatch any non-whitespace characterNSPACELnoMatch any non-whitespace char in localeDIGITnoMatch any numeric characterNDIGITnoMatch any non-numeric character# BRANCHThe set of branches constituting a single choice are hooked#together with their "next" pointers, since precedence prevents#anything being concatenated to any individual branch. The#"next" pointer of the last BRANCH in a choice points to the#thing following the whole choice. This is also where the#final "next" pointer of each individual branch points; each#branch starts with the operand node of a BRANCH node.#BRANCHnodeMatch this alternative, or the next...# BACKNormal "next" pointers all implicitly point forward; BACK#exists to make loop structures possible.# not usedBACKnoMatch "", "next" ptr points backward.# LiteralsEXACTsvMatch this string (preceded by length).EXACTFsvMatch this string, folded (prec. by length).EXACTFLsvMatch this string, folded in locale (w/len).# Do nothingNOTHINGnoMatch empty string.# A variant of above which delimits a group, thus stops optimizationsTAILnoMatch empty string. Can jump here from outside.# STAR,PLUS'?', and complex '*' and '+', are implemented as circular#BRANCH structures using BACK. Simple cases (one character#per match) are implemented with STAR and PLUS for speed#and to minimize recursive plunges.#STARnodeMatch this (simple) thing 0 or more times.PLUSnodeMatch this (simple) thing 1 or more times.CURLYsv 2Match this simple thing {n,m} times.CURLYNno 2Match next-after-this simple thing #{n,m} times, set parenths.CURLYMno 2Match this medium-complex thing {n,m} times.CURLYXsv 2Match this complex thing {n,m} times.# This terminator creates a loop structure for CURLYXWHILEMnoDo curly processing and see if rest matches.# OPEN,CLOSE,GROUPP...are numbered at compile time.OPENnum 1Mark this point in input as start of #n.CLOSEnum 1Analogous to OPEN.REFnum 1Match some already matched stringREFFnum 1Match already matched string, foldedREFFLnum 1Match already matched string, folded in loc.# grouping assertionsIFMATCHoff 1 2Succeeds if the following matches.UNLESSMoff 1 2Fails if the following matches.SUSPENDoff 1 1"Independent" sub-RE.IFTHENoff 1 1Switch, should be preceeded by switcher .GROUPPnum 1Whether the group matched.# Support for long RELONGJMPoff 1 1Jump far away.BRANCHJoff 1 1BRANCH with long offset.# The heavy workerEVALevl 1Execute some Perl code.# ModifiersMINMODnoNext operator is not greedy.LOGICALnoNext opcode should set the flag only.# This is not used yetRENUMoff 1 1Group with independently numbered parens.# This is not really a node, but an optimized away piece of a "long" node.# To simplify debugging output, we mark it as if it were a nodeOPTIMIZEDoffPlaceholder for dump.First of all, when doing a match, one may get no run-time output even if debugging is enabled. this means that the RE engine was never entered, all of the job was done by the optimizer.
If RE engine was entered, the output may look like this:
Matching `[bc]d(ef*g)+h[ij]k$' against `abcdefg__gh__' Setting an EVAL scope, savestack=3 2 <ab> <cdefg__gh_> | 1: ANYOF 3 <abc> <defg__gh_> | 11: EXACT <d> 4 <abcd> <efg__gh_> | 13: CURLYX {1,32767} 4 <abcd> <efg__gh_> | 26: WHILEM0 out of 1..32767 cc=effff31c 4 <abcd> <efg__gh_> | 15: OPEN1 4 <abcd> <efg__gh_> | 17: EXACT <e> 5 <abcde> <fg__gh_> | 19: STAR EXACT <f> can match 1 times out of 32767... Setting an EVAL scope, savestack=3 6 <bcdef> <g__gh__> | 22: EXACT <g> 7 <bcdefg> <__gh__> | 24: CLOSE1 7 <bcdefg> <__gh__> | 26: WHILEM 1 out of 1..32767 cc=effff31c Setting an EVAL scope, savestack=12 7 <bcdefg> <__gh__> | 15: OPEN1 7 <bcdefg> <__gh__> | 17: EXACT <e> restoring \1 to 4(4)..7 failed, try continuation... 7 <bcdefg> <__gh__> | 27: NOTHING 7 <bcdefg> <__gh__> | 28: EXACT <h> failed...failed...The most significant information in the output is about the particularnode of the compiled RE which is currently being tested against the target string. The format of these lines is
STRING-OFFSET <PRE-STRING> <POST-STRING> |ID:TYPE
TheTYPE info is indented with respect to the backtracking level. Other incidental information appears interspersed within.
4 POD Errors
The following errors were encountered while parsing the POD:
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