perldebug.pod

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=head1 NAME
X<debug> X<debugger>

perldebug - Perl debugging

=head1 DESCRIPTION

First of all, have you tried using the B<-w> switch?


If you're new to the Perl debugger, you may prefer to read
L<perldebtut>, which is a tutorial introduction to the debugger .

=head1 The Perl Debugger

If you invoke Perl with the B<-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:
X<-d>

    $ 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 B<-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 I<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 I<about> to execute, rather than the one it has just executed.

Any command not recognized by the debugger is directly executed
(C<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 C<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 C<;> or perhaps a C<+>, or by wrapping it with parentheses
or braces.

=head2 Calling the debugger

There are several ways to call the debugger:

=over 4

=item perl -d program_name

On the given program identified by C<program_name>.

=item perl -d -e 0 

Interactively supply an arbitrary C<expression> using C<-e>.

=item perl -d:Ptkdb program_name

Debug a given program via the C<Devel::Ptkdb> GUI.

=item perl -dt threaded_program_name

Debug a given program using threads (experimental).

=back

=head2 Debugger Commands

The interactive debugger understands the following commands:

=over 12

=item h
X<debugger command, h>

Prints out a summary help message

=item h [command]

Prints out a help message for the given debugger command.

=item h h

The special argument of C<h h> produces the entire help page, which is quite long.

If the output of the C<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 C<o pager=...> command.


=item p expr
X<debugger command, p>

Same as C<print {$DB::OUT} expr> in the current package.  In particular,
because this is just Perl's own C<print> function, this means that nested
data structures and objects are not dumped, unlike with the C<x> command.

The C<DB::OUT> filehandle is opened to F</dev/tty>, regardless of
where STDOUT may be redirected to.

=item x [maxdepth] expr
X<debugger command, x>

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 C<print> function in Perl.  When dumping
hashes, you'll probably prefer 'x \%h' rather than 'x %h'.
See L<Dumpvalue> if you'd like to do this yourself.

The output format is governed by multiple options described under
L<"Configurable Options">.

If the C<maxdepth> is included, it must be a numeral I<N>; the value is
dumped only I<N> levels deep, as if the C<dumpDepth> option had been
temporarily set to I<N>.

=item V [pkg [vars]]
X<debugger command, V>

Display all (or some) variables in package (defaulting to C<main>)
using a data pretty-printer (hashes show their keys and values so
you see what's what, control characters are made printable, etc.).
Make sure you don't put the type specifier (like C<$>) there, just
the symbol names, like this:

    V DB filename line

Use C<~pattern> and C<!pattern> for positive and negative regexes.

This is similar to calling the C<x> command on each applicable var.

=item X [vars]
X<debugger command, X>

Same as C<V currentpackage [vars]>.

=item y [level [vars]]
X<debugger command, y>

Display all (or some) lexical variables (mnemonic: C<mY> variables)
in the current scope or I<level> scopes higher.  You can limit the
variables that you see with I<vars> which works exactly as it does
for the C<V> and C<X> commands.  Requires the C<PadWalker> module
version 0.08 or higher; will warn if this isn't installed.  Output
is pretty-printed in the same style as for C<V> and the format is
controlled by the same options.

=item T
X<debugger command, T> X<backtrace> X<stack, backtrace>

Produce a stack backtrace.  See below for details on its output.

=item s [expr]
X<debugger command, s> X<step>

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.

=item n [expr]
X<debugger command, n>

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.

=item r
X<debugger command, r>

Continue until the return from the current subroutine.
Dump the return value if the C<PrintRet> option is set (default).

=item <CR>

Repeat last C<n> or C<s> command.

=item c [line|sub]
X<debugger command, c>

Continue, optionally inserting a one-time-only breakpoint
at the specified line or subroutine.

=item l
X<debugger command, l>

List next window of lines.

=item l min+incr

List C<incr+1> lines starting at C<min>.

=item l min-max

List lines C<min> through C<max>.  C<l -> is synonymous to C<->.

=item l line

List a single line.

=item l subname

List first window of lines from subroutine.  I<subname> may
be a variable that contains a code reference.

=item -
X<debugger command, ->

List previous window of lines.

=item v [line]
X<debugger command, v>

View a few lines of code around the current line.

=item .
X<debugger command, .>

Return the internal debugger pointer to the line last
executed, and print out that line.

=item f filename
X<debugger command, f>

Switch to viewing a different file or C<eval> statement.  If I<filename>
is not a full pathname found in the values of %INC, it is considered
a regex.

C<eval>ed strings (when accessible) are considered to be filenames:
C<f (eval 7)> and C<f eval 7\b> access the body of the 7th C<eval>ed string
(in the order of execution).  The bodies of the currently executed C<eval>
and of C<eval>ed strings that define subroutines are saved and thus
accessible.

=item /pattern/

Search forwards for pattern (a Perl regex); final / is optional.
The search is case-insensitive by default.

=item ?pattern?

Search backwards for pattern; final ? is optional.
The search is case-insensitive by default.

=item L [abw]
X<debugger command, L>

List (default all) actions, breakpoints and watch expressions

=item S [[!]regex]
X<debugger command, S>

List subroutine names [not] matching the regex.

=item t
X<debugger command, t>

Toggle trace mode (see also the C<AutoTrace> option).

=item t expr
X<debugger command, t>

Trace through execution of C<expr>.
See L<perldebguts/"Frame Listing Output Examples"> for examples.

=item b
X<breakpoint>
X<debugger command, b>

Sets breakpoint on current line

=item b [line] [condition]
X<breakpoint>
X<debugger command, b>

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 C<if>:

    b 237 $x > 30
    b 237 ++$count237 < 11
    b 33 /pattern/i

=item b subname [condition]
X<breakpoint>
X<debugger command, b>

Set a breakpoint before the first line of the named subroutine.  I<subname> may
be a variable containing a code reference (in this case I<condition>
is not supported).

=item b postpone subname [condition]
X<breakpoint>
X<debugger command, b>

Set a breakpoint at first line of subroutine after it is compiled.

=item b load filename
X<breakpoint>
X<debugger command, b>

Set a breakpoint before the first executed line of the I<filename>,
which should be a full pathname found amongst the %INC values.

=item b compile subname
X<breakpoint>
X<debugger command, b>

Sets a breakpoint before the first statement executed after the specified
subroutine is compiled.

=item B line
X<breakpoint>
X<debugger command, B>

Delete a breakpoint from the specified I<line>.

=item B *
X<breakpoint>
X<debugger command, B>

Delete all installed breakpoints.

=item a [line] command
X<debugger command, a>

Set an action to be done before the line is executed.  If I<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 $foo\n"

=item A line
X<debugger command, A>

Delete an action from the specified line.

=item A *
X<debugger command, A>

Delete all installed actions.

=item w expr
X<debugger command, w>

Add a global watch-expression.  We hope you know what one of these
is, because they're supposed to be obvious.

=item W expr
X<debugger command, W>

Delete watch-expression

=item W *
X<debugger command, W>

Delete all watch-expressions.

=item o
X<debugger command, o>