=head1 NAME
+X<debug> X<debugger>
perldebug - Perl debugging
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
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
(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
=over 12
+=item h
+X<debugger command, h>
+
+Prints out a summary help message
+
=item h [command]
-Prints out a help message.
+Prints out a help message for the given debugger command.
-If you supply another debugger command as an argument to the C<h> command,
-it prints out the description for just that command. The special
-argument of C<h h> produces a more compact help listing, designed to fit
-together on one screen.
+=item h h
-If the output of the C<h> command (or any command, for that matter) scrolls
+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
+ DB> |h h
+
+You may change the pager which is used via C<o pager=...> command.
-You may change the pager which is used via C<O pager=...> command.
=item p expr
+X<debugger command, p>
Same as C<print {$DB::OUT} expr> in the current package. In particular,
because this is just Perl's own C<print> function, this means that nested
The C<DB::OUT> filehandle is opened to F</dev/tty>, regardless of
where STDOUT may be redirected to.
-=item x expr
+=item x [maxdepth] expr
+X<debugger command, x>
-Evaluates its expression in list context and dumps out the result
-in a pretty-printed fashion. Nested data structures are printed out
-recursively, unlike the real C<print> function in Perl.
+Evaluates its expression in list context and dumps out the result in a
+pretty-printed fashion. Nested data structures are printed out
+recursively, unlike the real C<print> function in Perl. When dumping
+hashes, you'll probably prefer 'x \%h' rather than 'x %h'.
See L<Dumpvalue> if you'd like to do this yourself.
The output format is governed by multiple options described under
L<"Configurable Options">.
+If the C<maxdepth> is included, it must be a numeral I<N>; the value is
+dumped only I<N> levels deep, as if the C<dumpDepth> option had been
+temporarily set to I<N>.
+
=item V [pkg [vars]]
+X<debugger command, V>
-Display all (or some) variables in package (defaulting to C<main>)
+Display all (or some) variables in package (defaulting to C<main>)
using a data pretty-printer (hashes show their keys and values so
you see what's what, control characters are made printable, etc.).
Make sure you don't put the type specifier (like C<$>) there, just
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
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).
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.
be a variable that contains a code reference.
=item -
+X<debugger command, ->
List previous window of lines.
-=item w [line]
+=item v [line]
+X<debugger command, v>
-List window (a few lines) around the current line.
+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
+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:
Search backwards for pattern; final ? is optional.
The search is case-insensitive by default.
-=item L
+=item L [abw]
+X<debugger command, L>
-List all breakpoints and actions.
+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 I<line> is omitted, set a
-breakpoint on the line about to be executed. If a condition
+Set a breakpoint before the given line. If a condition
is specified, it's evaluated each time the statement is reached: a
breakpoint is taken only if the condition is true. Breakpoints may
only be set on lines that begin an executable statement. Conditions
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 d [line]
+=item B line
+X<breakpoint>
+X<debugger command, B>
-Delete a breakpoint from the specified I<line>. If I<line> is omitted, deletes
-the breakpoint from the line about to be executed.
+Delete a breakpoint from the specified I<line>.
-=item D
+=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.
a 53 print "DB FOUND $foo\n"
-=item a [line]
+=item A line
+X<debugger command, A>
-Delete an action from the specified line. If I<line> is omitted, delete
-the action on the line that is about to be executed.
+Delete an action from the specified line.
-=item A
+=item A *
+X<debugger command, A>
Delete all installed actions.
-=item W expr
+=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. B<WARNING>: It is far
-too easy to destroy your watch expressions by accidentally omitting
-the I<expr>.
+is, because they're supposed to be obvious.
+
+=item W expr
+X<debugger command, W>
-=item W
+Delete watch-expression
+
+=item W *
+X<debugger command, W>
Delete all watch-expressions.
-=item O booloption ...
+=item o
+X<debugger command, o>
+
+Display all options
+
+=item o booloption ...
+X<debugger command, o>
Set each listed Boolean option to the value C<1>.
-=item O anyoption? ...
+=item o anyoption? ...
+X<debugger command, o>
Print out the value of one or more options.
-=item O option=value ...
+=item o option=value ...
+X<debugger command, o>
Set the value of one or more options. If the value has internal
-whitespace, it should be quoted. For example, you could set C<O
+whitespace, it should be quoted. For example, you could set C<o
pager="less -MQeicsNfr"> to call B<less> with those specific options.
You may use either single or double quotes, but if you do, you must
escape any embedded instances of same sort of quote you began with,
as well as any escaping any escapes that immediately precede that
quote but which are not meant to escape the quote itself. In other
words, you follow single-quoting rules irrespective of the quote;
-eg: C<O option='this isn\'t bad'> or C<O option="She said, \"Isn't
+eg: C<o option='this isn\'t bad'> or C<o option="She said, \"Isn't
it?\"">.
For historical reasons, the C<=value> is optional, but defaults to
1 only where it is safe to do so--that is, mostly for Boolean
options. It is always better to assign a specific value using C<=>.
The C<option> can be abbreviated, but for clarity probably should
-not be. Several options can be set together. See L<"Configurable Options">
+not be. Several options can be set together. See L<"Configurable Options">
for a list of these.
-=item < ?
+=item < ?
+X<< debugger command, < >>
List out all pre-prompt Perl command actions.
=item < [ command ]
+X<< debugger command, < >>
Set an action (Perl command) to happen before every debugger prompt.
-A multi-line command may be entered by backslashing the newlines.
-B<WARNING> If C<command> is missing, all actions are wiped out!
+A multi-line command may be entered by backslashing the newlines.
+
+=item < *
+X<< debugger command, < >>
+
+Delete all pre-prompt Perl command actions.
=item << command
+X<< debugger command, << >>
Add an action (Perl command) to happen before every debugger prompt.
A multi-line command may be entered by backwhacking the newlines.
=item > ?
+X<< debugger command, > >>
List out post-prompt Perl command actions.
=item > command
+X<< debugger command, > >>
Set an action (Perl command) to happen after the prompt when you've
just given a command to return to executing the script. A multi-line
command may be entered by backslashing the newlines (we bet you
-couldn't've guessed this by now). B<WARNING> If C<command> is
-missing, all actions are wiped out!
+couldn't've guessed this by now).
+
+=item > *
+X<< debugger command, > >>
+
+Delete all post-prompt Perl command actions.
=item >> command
+X<<< debugger command, >> >>>
Adds an action (Perl command) to happen after the prompt when you've
just given a command to return to executing the script. A multi-line
command may be entered by backslashing the newlines.
=item { ?
+X<debugger command, {>
List out pre-prompt debugger commands.
=item { [ command ]
Set an action (debugger command) to happen before every debugger prompt.
-A multi-line command may be entered in the customary fashion.
-B<WARNING> If C<command> is missing, all actions are wiped out!
+A multi-line command may be entered in the customary fashion.
Because this command is in some senses new, a warning is issued if
you appear to have accidentally entered a block instead. If that's
-what you mean to do, write it as with C<;{ ... }> or even
+what you mean to do, write it as with C<;{ ... }> or even
C<do { ... }>.
+=item { *
+X<debugger command, {>
+
+Delete all pre-prompt debugger commands.
+
=item {{ command
+X<debugger command, {{>
Add an action (debugger command) to happen before every debugger prompt.
A multi-line command may be entered, if you can guess how: see above.
=item ! number
+X<debugger command, !>
Redo a previous command (defaults to the previous command).
=item ! -number
+X<debugger command, !>
Redo number'th previous command.
=item ! pattern
+X<debugger command, !>
Redo last command that started with pattern.
-See C<O recallCommand>, too.
+See C<o recallCommand>, too.
=item !! cmd
+X<debugger command, !!>
Run cmd in a subprocess (reads from DB::IN, writes to DB::OUT) See
-C<O shellBang>, also. Note that the user's current shell (well,
+C<o shellBang>, also. Note that the user's current shell (well,
their C<$ENV{SHELL}> variable) will be used, which can interfere
with proper interpretation of exit status or signal and coredump
information.
+=item source file
+X<debugger command, source>
+
+Read and execute debugger commands from I<file>.
+I<file> may itself contain C<source> commands.
+
=item H -number
+X<debugger command, H>
Display last n commands. Only commands longer than one character are
listed. If I<number> is omitted, list them all.
=item q or ^D
+X<debugger command, q>
+X<debugger command, ^D>
Quit. ("quit" doesn't work for this, unless you've made an alias)
This is the only supported way to exit the debugger, though typing
C<exit> twice might work.
Set the C<inhibit_exit> option to 0 if you want to be able to step
-off the end the script. You may also need to set $finished to 0
+off the end the script. You may also need to set $finished to 0
if you want to step through global destruction.
=item R
+X<debugger command, R>
Restart the debugger by C<exec()>ing a new session. We try to maintain
your history across this, but internal settings and command-line options
options B<-w>, B<-I>, and B<-e>.
=item |dbcmd
+X<debugger command, |>
Run the debugger command, piping DB::OUT into your current pager.
=item ||dbcmd
+X<debugger command, ||>
Same as C<|dbcmd> but DB::OUT is temporarily C<select>ed as well.
=item = [alias value]
+X<debugger command, =>
Define a command alias, like
Perl debugger, use a leading semicolon, too.
=item m expr
+X<debugger command, m>
List which methods may be called on the result of the evaluated
-expression. The expression may evaluated to a reference to a
+expression. The expression may evaluated to a reference to a
blessed object, or to a package name.
+=item M
+X<debugger command, M>
+
+Displays all loaded modules and their versions
+
+
=item man [manpage]
+X<debugger command, man>
Despite its name, this calls your system's default documentation
viewer on the given page, or on the viewer itself if I<manpage> is
=head2 Configurable Options
-The debugger has numerous options settable using the C<O> command,
+The debugger has numerous options settable using the C<o> command,
either interactively or from the environment or an rc file.
(./.perldb or ~/.perldb under Unix.)
=over 12
=item C<recallCommand>, C<ShellBang>
+X<debugger option, recallCommand>
+X<debugger option, ShellBang>
The characters used to recall command or spawn shell. By
default, both are set to C<!>, which is unfortunate.
=item C<pager>
+X<debugger option, pager>
Program to use for output of pager-piped commands (those beginning
with a C<|> character.) By default, C<$ENV{PAGER}> will be used.
will not be readable when sent through the pager.
=item C<tkRunning>
+X<debugger option, tkRunning>
Run Tk while prompting (with ReadLine).
=item C<signalLevel>, C<warnLevel>, C<dieLevel>
+X<debugger option, signalLevel> X<debugger option, warnLevel>
+X<debugger option, dieLevel>
Level of verbosity. By default, the debugger leaves your exceptions
and warnings alone, because altering them can break correctly running
destroy any program that takes its exception handling seriously.
=item C<AutoTrace>
+X<debugger option, AutoTrace>
Trace mode (similar to C<t> command, but can be put into
C<PERLDB_OPTS>).
=item C<LineInfo>
+X<debugger option, LineInfo>
File or pipe to print line number info to. If it is a pipe (say,
C<|visual_perl_db>), then a short message is used. This is the
debugger.
=item C<inhibit_exit>
+X<debugger option, inhibit_exit>
If 0, allows I<stepping off> the end of the script.
=item C<PrintRet>
+X<debugger option, PrintRet>
Print return value after C<r> command if set (default).
=item C<ornaments>
+X<debugger option, ornaments>
Affects screen appearance of the command line (see L<Term::ReadLine>).
There is currently no way to disable these, which can render
This is considered a bug.
=item C<frame>
+X<debugger option, frame>
Affects the printing of messages upon entry and exit from subroutines. If
C<frame & 2> is false, messages are printed on entry only. (Printing
next option:
=item C<maxTraceLen>
+X<debugger option, maxTraceLen>
Length to truncate the argument list when the C<frame> option's
bit 4 is set.
+=item C<windowSize>
+X<debugger option, windowSize>
+
+Change the size of code list window (default is 10 lines).
+
=back
The following options affect what happens with C<V>, C<X>, and C<x>
=over 12
=item C<arrayDepth>, C<hashDepth>
+X<debugger option, arrayDepth> X<debugger option, hashDepth>
Print only first N elements ('' for all).
+=item C<dumpDepth>
+X<debugger option, dumpDepth>
+
+Limit recursion depth to N levels when dumping structures.
+Negative values are interpreted as infinity. Default: infinity.
+
=item C<compactDump>, C<veryCompact>
+X<debugger option, compactDump> X<debugger option, veryCompact>
Change the style of array and hash output. If C<compactDump>, short array
may be printed on one line.
=item C<globPrint>
+X<debugger option, globPrint>
Whether to print contents of globs.
=item C<DumpDBFiles>
+X<debugger option, DumpDBFiles>
Dump arrays holding debugged files.
=item C<DumpPackages>
+X<debugger option, DumpPackages>
Dump symbol tables of packages.
=item C<DumpReused>
+X<debugger option, DumpReused>
Dump contents of "reused" addresses.
=item C<quote>, C<HighBit>, C<undefPrint>
+X<debugger option, quote> X<debugger option, HighBit>
+X<debugger option, undefPrint>
Change the style of string dump. The default value for C<quote>
is C<auto>; one can enable double-quotish or single-quotish format
with their high bit set are printed verbatim.
=item C<UsageOnly>
+X<debugger option, UsageOnly>
Rudimentary per-package memory usage dump. Calculates total
size of strings found in variables in the package. This does not
=back
After the rc file is read, the debugger reads the C<$ENV{PERLDB_OPTS}>
-environment variable and parses this as the remainder of a `O ...'
+environment variable and parses this as the remainder of a "O ..."
line as one might enter at the debugger prompt. You may place the
initialization options C<TTY>, C<noTTY>, C<ReadLine>, and C<NonStop>
there.
=over 12
=item C<TTY>
+X<debugger option, TTY>
The TTY to use for debugging I/O.
=item C<noTTY>
+X<debugger option, noTTY>
If set, the debugger goes into C<NonStop> mode and will not connect to a TTY. If
interrupted (or if control goes to the debugger via explicit setting of
with two methods: C<IN> and C<OUT>. These should return filehandles to use
for debugging input and output correspondingly. The C<new> method should
inspect an argument containing the value of C<$ENV{PERLDB_NOTTY}> at
-startup, or C<"/tmp/perldbtty$$"> otherwise. This file is not
+startup, or C<"$ENV{HOME}/.perldbtty$$"> otherwise. This file is not
inspected for proper ownership, so security hazards are theoretically
possible.
=item C<ReadLine>
+X<debugger option, ReadLine>
If false, readline support in the debugger is disabled in order
to debug applications that themselves use ReadLine.
=item C<NonStop>
+X<debugger option, NonStop>
If set, the debugger goes into non-interactive mode until interrupted, or
programmatically by setting $DB::signal or $DB::single.
Other examples include
- $ PERLDB_OPTS="NonStop frame=2" perl -d myprogram
+ $ PERLDB_OPTS="NonStop LineInfo=listing frame=2" perl -d myprogram
which runs script non-interactively, printing info on each entry
into a subroutine and each executed line into the file named F<listing>.
commands typed into the debugger.
=item Stack backtrace
+X<backtrace> X<stack, backtrace>
Here's an example of what a stack backtrace via C<T> command might
look like:
The debugger probably contains enough configuration hooks that you
won't ever have to modify it yourself. You may change the behaviour
-of debugger from within the debugger using its C<O> command, from
+of debugger from within the debugger using its C<o> command, from
the command line via the C<PERLDB_OPTS> environment variable, and
from customization files.
it must be owned by the superuser or the current user, and writable
by no one but its owner.
+You can mock TTY input to debugger by adding arbitrary commands to
+@DB::typeahead. For example, your F<.perldb> file might contain:
+
+ sub afterinit { push @DB::typeahead, "b 4", "b 6"; }
+
+Which would attempt to set breakpoints on lines 4 and 6 immediately
+after debugger initialization. Note that @DB::typeahead is not a supported
+interface and is subject to change in future releases.
+
If you want to modify the debugger, copy F<perl5db.pl> from the
Perl library to another name and hack it to your heart's content.
You'll then want to set your C<PERL5DB> environment variable to say
Perl distribution was uncertain.
Users of B<vi> should also look into B<vim> and B<gvim>, the mousey
-and windy version, for coloring of Perl keywords.
+and windy version, for coloring of Perl keywords.
Note that only perl can truly parse Perl, so all such CASE tools
fall somewhat short of the mark, especially if you don't program
your Perl as a C programmer might.
=head2 The Perl Profiler
+X<profile> X<profiling> X<profiler>
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
interpret the information in that profile.
=head1 Debugging regular expressions
-
-C<use re 'debug'> enables you to see the gory details of how the
-Perl regular expression engine works. In order to understand this
-typically voluminous output, one must not only have some idea about
-about how regular expression matching works in general, but also
-know how Perl's regular expressions are internally compiled into
-an automaton. These matters are explored in some detail in
+X<regular expression, debugging>
+X<regex, debugging> X<regexp, debugging>
+
+C<use re 'debug'> enables you to see the gory details of how the Perl
+regular expression engine works. In order to understand this typically
+voluminous output, one must not only have some idea about how regular
+expression matching works in general, but also know how Perl's regular
+expressions are internally compiled into an automaton. These matters
+are explored in some detail in
L<perldebguts/"Debugging regular expressions">.
=head1 Debugging memory usage
+X<memory usage>
Perl contains internal support for reporting its own memory usage,
but this is a fairly advanced concept that requires some understanding
You did try the B<-w> switch, didn't you?
+L<perldebtut>,
L<perldebguts>,
L<re>,
L<DB>,
-L<Devel::Dprof>,
+L<Devel::DProf>,
L<dprofpp>,
L<Dumpvalue>,
and
L<perlrun>.
+When debugging a script that uses #! and is thus normally found in
+$PATH, the -S option causes perl to search $PATH for it, so you don't
+have to type the path or C<which $scriptname>.
+
+ $ perl -Sd foo.pl
+
=head1 BUGS
You cannot get stack frame information or in any fashion debug functions
that were not compiled by Perl, such as those from C or C++ extensions.
If you alter your @_ arguments in a subroutine (such as with C<shift>
-or C<pop>, the stack backtrace will not show the original values.
+or C<pop>), the stack backtrace will not show the original values.
The debugger does not currently work in conjunction with the B<-W>
command-line switch, because it itself is not free of warnings.
https://perl5.git.perl.org / perl5.git / blobdiff