--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 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 its own state information.)
Leading white space before a command would cause the debugger to think it's NOT a debugger command but for Perl, so be careful not to do that.
If you supply another debugger command as an argument to the h command, it prints out the description for just that command. The special argument of h h produces a more compact help listing, designed to fit together on one screen.
If the output the h 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 via O pager=... command.
The DB::OUT filehandle is opened to /dev/tty, regardless of
where STDOUT may be redirected to.
The details of printout are governed by multiple Options.
V DB filename lineUse ~pattern and !pattern for positive and negative regexps.
Nested data structures are printed out in a legible fashion, unlike the print function.
The details of printout are governed by multiple Options.
$ perl -de 42 Stack dump during die enabled outside of evals.
Loading DB routines from perl5db.pl patch level 0.94 Emacs 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 the Option frame=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::foo entering main::bar 2: sub bar { 3 }; exited main::bar 42
b 237 $x > 30 b 237 ++$count237 < 11 b 33 /pattern/i
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 lineFor example, this will print out $foo every time line 53 is passed:
a 53 print "DB FOUND $foo\n"
To disable this behaviour, set these values to 0. If dieLevel is 2,
then the messages which will be caught by surrounding eval are also
printed.
If frame & 4, arguments to functions are printed as well as the context and caller info. If frame & 8, overloaded stringify and tied FETCH are enabled on the printed arguments. If frame & 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:
The following options affect what happens with V, X, and x
commands:
During startup options are initialized from $ENV{PERLDB_OPTS}. You can put additional initialization options TTY, noTTY, ReadLine, and NonStop there.
Example rc file:
&parse_options("NonStop=1 LineInfo=db.out AutoTrace");The script will run without human intervention, putting trace information into the file db.out. (If you interrupt it, you would better reset LineInfo to something ``interactive''!)
This module should implement a method new which returns an object
with two methods: IN and OUT, returning two filehandles to use
for debugging input and output correspondingly. Method new may
inspect an argument which is a value of $ENV{PERLDB_NOTTY} at
startup, or is "/tmp/perldbtty$$" otherwise.
Here's an example of using the $ENV{PERLDB_OPTS} variable:
$ PERLDB_OPTS="N f=2" perl -d myprogramwill run the script myprogram without human intervention, printing out the call tree with entry and exit points. Note that N f=2 is equivalent to NonStop=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 of Dump* 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 file listing. (If you interrupt it, you would better reset LineInfo to something ``interactive''!)
$ env "PERLDB_OPTS=R=0 TTY=/dev/ttyc" perl -d myprogrammay be useful for debugging a program which uses Term::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 the section on Debugger Internals below for more details.
Set an Option inhibit_exit to 0 if you want to be able to step
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.
Currently the following setting are preserved: history, breakpoints,
actions, debugger Options, and the following command line
options: -w, -I, and -e.
|V main
= quit qor list current aliases.
DB<8>or even
DB<<17>>where that number is the command number, which you'd use to access with the builtin csh-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 via s/n/t expression command.
DB<1> for (1..4) { \ cont: print "ok\n"; \ cont: } ok ok ok okNote that this business of escaping a newline is specific to interactive commands typed into the debugger.
$ = 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 function main::infested when you ran the stack dump, and that it was called in a 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 a list context from the camel_flea file with four arguments. The last stack frame shows that main::pests was called in a scalar context, also from camel_flea, but from line 4.
Note that if you execute T command from inside an active use
statement, the backtrace will contain both the require entry in the perlfunc manpage
frame and an the section on eval EXPR in the perlfunc manpage) frame.
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);Note that the breakable lines are marked with :, lines with breakpoints are marked by b, with actions by a, and the next executed line is marked by ==>.
What follows is the start of the listing of
env "PERLDB_OPTS=f=n N" perl -d -Vfor different values of n:
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::export entering Config::myconfig entering Config::FETCH entering Config::FETCH entering Config::FETCH entering Config::FETCH
entering 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::import exited main::BEGIN entering Config::myconfig entering Config::FETCH exited Config::FETCH entering Config::FETCH exited Config::FETCH entering Config::FETCH
in $=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 li in @=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:574
in $=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:0 out $=main::BEGIN() from /dev/nul:0 in @=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:574
in $=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:0 out $=main::BEGIN() from /dev/nul:0 in @=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:574
in $=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 of frame 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.
$DB::single = 1;If you set $DB::single to the value 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 debugger, set a breakpoint on load of some module thusly
DB<7> b load f:/perllib/lib/Carp.pm Will stop on load of `f:/perllib/lib/Carp.pm'.and restart debugger by R command (if possible). One can use b compile subname for the same purpose.
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 package DB). Note that .perldb is processed before processing PERLDB_OPTS. If .perldb defines the subroutine afterinit, it is called after all the debugger initialization ends. .perldb may be contained in the current directory, or in the LOGDIR/HOME directory.
If you want to modify the debugger, copy perl5db.pl from the Perl library to another name and modify it as necessary. You'll also want to set your PERL5DB environment variable to say something like this:
BEGIN { require "myperl5db.pl" }As the last resort, one can use PERL5DB to customize debugger by directly setting internal variables or calling debugger functions.
A rudimentary command line completion is also available. Unfortunately, the names of lexical variables are not available for completion.
Perl is also delivered 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.
(Historically, a similar setup for interacting with vi and the X11 window system had also been available, but at the time of this writing, no debugger support for vi currently exists.)
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 file mycode.pl, just type:
perl -d:DProf mycode.plWhen the script terminates the profiler will dump the profile information to a file called tmon.out. A tool like dprofpp (also supplied with the Devel::DProf package) can be used to interpret the information which is in that profile.
If perl is run with -d option, the following additional features
are enabled (cf. the section on $^P in the perlvar manpage):
Same for evaluated strings which contain subroutines, or which are
currently executed. The $filename for evaled strings looks like
(eval 34).
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 $DB::deep (how many levels of recursion deep into the debugger you can go before a mandatory break) gives an example of such a dependency.
The minimal working debugger consists of one line
sub DB::DB {}which is quite handy as contents of PERL5DB environment variable:
env "PERL5DB=sub DB::DB {}" perl -d your-scriptAnother (a little bit more useful) minimal debugger can be created with the only line being
sub DB::DB {print ++$i; scalar <STDIN>}This debugger would print the sequential number of encountered statement, and would wait for your CR 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 package DB.
After the rc file is read, the debugger reads environment variable PERLDB_OPTS and parses it as a rest of O ... line in debugger prompt.
It also maintains magical internal variables, such as @DB::dbline, %DB::dbline, which are aliases for C<@{"::_<current_file"}> C<%{"::_<current_file"}>. Here current_file is the currently selected (with the debugger's f command, or by flow of execution) file.
Some functions are provided to simplify customization. See the section on Debugger Customization for description of DB::parse_options(string). The function DB::dump_trace(skip[, count]) skips the specified number of frames, and returns an array containing info about the caller frames (all if count is missing). Each entry is a hash with keys context ($ or @), sub (subroutine name, or info about eval), args (undef or a reference to an array), file, and line.
The function DB::print_trace(FH, skip[, count[, short]]) prints formatted info about caller frames. The last two functions may be convenient as arguments to <, << commands.
If you alter your @_ arguments in a subroutine (such as with shift or pop, the stack backtrace will not show the original values.