--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 42
In 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> |h
You may change the pager which is used via O pager=... command.
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.
The details of printout are governed by multiple Options.
main
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 line
Use ~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.
V currentpackage [vars].
incr+1 lines starting at min.
min through max. l - is synonymous to -.
filename
is not a full filename as found in values of
%INC, it is considered as a regexp.
$ 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): 42
or, 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 line
For example, this will print out $foo every time line 53 is passed:
a 53 print "DB FOUND $foo\n"
!.
| character.) By default,
$ENV{PAGER} will be used.
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.
PERLDB_OPTS).
|visual_perl_db), then a short, ``emacs like'' message is used.
r command.
frame & 2 is false, messages are printed on entry only. (Printing on exit may be
useful if inter(di)spersed with other messages.)
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:
auto, one can enable either double-quotish dump, or single-quotish by setting
it to " or '. By default, characters with high bit set are printed
as is.
$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''!)
Term::Rendezvous module of your choice.
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.
$ENV{PERLDB_OPTS} variable:
$ PERLDB_OPTS="N f=2" perl -d myprogram
will 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 myprogram
may 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 1000000
See Debugger Internals below for more details.
O recallCommand, too.
O shellBang too.
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.
|dbcmd but
DB::OUT is temporarily selected as well. Often used with commands that would otherwise produce long
output, such as
|V main
= quit q
or 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
ok
Note 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 4
The 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 require
frame and an eval EXPR) 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 -V
for 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: ''
When a package is compiled, a line like this
Package lib/Carp.pm.
is printed with proper indentation.
NOT be stopped by debugger, although requires 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 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.
PERLDB_OPTS environment variable, and from customization files.
You can do some customization by setting up a .perldb file which contains initialization code. For instance, you could make aliases like these (the last one is one people expect to be there):
$DB::alias{'len'} = 's/^len(.*)/p length($1)/';
$DB::alias{'stop'} = 's/^stop (at|in)/b/';
$DB::alias{'ps'} = 's/^ps\b/p scalar /';
$DB::alias{'quit'} = 's/^quit(\s*)/exit\$/';
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.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 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. $^P):
$ENV{PERL5DB} (or BEGIN {require
'perl5db.pl'} if not present) before the first line of the application.
@{"_<$filename"} is the line-by-line contents of $filename for all the compiled
files. Same for evaled strings which contain subroutines, or which are currently executed. The $filename
for evaled strings looks like (eval 34).
%{"_<$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 by perl5db.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 for evaled strings looks like
(eval 34).
${"_<$filename"} contains "_<$filename". Same for evaluated strings which contain subroutines, or which are
currently executed. The $filename for evaled strings looks like (eval
34).
DB::postponed(*{"_<$filename"}) is called (if subroutine
DB::postponed exists). Here the $filename is the expanded name of the required file (as found in values of %INC).
subname is compiled existence of
$DB::postponed{subname} is checked. If this key exists,
DB::postponed(subname) is called (if subroutine DB::postponed
exists).
filename:startline-endline. filename has the form (eval 31) for subroutines defined inside evals.
DB::DB() is performed if any one of variables $DB::trace, $DB::single, or
$DB::signal is true. (Note that these variables are not localizable.) This feature is disabled when the control is inside DB::DB() or functions called from it (unless
$^D & (1<<30)).
DB.)
$DB::deep (how many levels of recursion deep into the debugger you can go before a
mandatory break) gives an example of such a dependency.
The minimal working debugger consists of one line
sub DB::DB {}
which is quite handy as contents of PERL5DB environment variable:
env "PERL5DB=sub DB::DB {}" perl -d your-script
Another (a little bit more useful) minimal debugger can be created with the only line being
sub DB::DB {print ++$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.
&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 of O ... line in debugger prompt.
It also maintains magical internal variables, such as
Some functions are provided to simplify customization. See Debugger Customization for description of
The function
If you alter your
@DB::dbline,
%DB::dbline, which are aliases for @{"::_%{"::_current_file is the currently selected (with the debugger's f command, or by flow of execution) file.
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.
DB::print_trace(FH, skip[, count[, short]]) prints formatted info about caller frames. The last two functions may be
convenient as arguments to <, << commands.
Other resources
You did try the -w switch, didn't you?
BUGS
You cannot get the stack frame information or otherwise debug functions that were not compiled by Perl, such as
C or
C++ extensions.
@_ arguments in a subroutine (such as with shift
or pop, the stack backtrace will not show the original values.