perldebug.pod

ARM上的如果你对底层感兴趣

=head1 NAME

perldebug - Perl debugging

=head1 DESCRIPTION

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

=head1 The Perl Debugger

"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."

I<  --Maurice Wilkes, 1949>

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:

    perl -d -e 42

In Perl, the debugger is not a separate program as 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 Perl library file containing the debugger itself.

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 its own state information.)

Leading white space before a command would cause the debugger to think
it's I<NOT> a debugger command but for Perl, so be careful not to do
that.

=head2 Debugger Commands

The debugger understands the following commands:

=over 12

=item h [command]

Prints out a help message.

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.

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

=item p expr

Same as C<print {$DB::OUT} expr> in the current package.  In particular,
because this is just Perl's own B<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 expr

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 C<print> function.

The details of printout are governed by multiple C<O>ptions.

=item V [pkg [vars]]

Display all (or some) variables in package (defaulting to the C<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 C<$>) there, just the symbol
names, like this:

    V DB filename line

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

Nested data structures are printed out in a legible fashion, unlike
the C<print> function.

The details of printout are governed by multiple C<O>ptions.

=item X [vars]

Same as C<V currentpackage [vars]>.

=item T

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

=item s [expr]

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.

=item n [expr]

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.

=item E<lt>CRE<gt>

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

=item c [line|sub]

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

=item 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.

=item -

List previous window of lines.

=item w [line]

List window (a few lines) around the current line.

=item .

Return debugger pointer to the last-executed line and
print it out.

=item f filename

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

=item /pattern/

Search forwards for pattern; final / is optional.

=item ?pattern?

Search backwards for pattern; final ? is optional.

=item L

List all breakpoints and actions.

=item S [[!]pattern]

List subroutine names [not] matching pattern.

=item t

Toggle trace mode (see also C<AutoTrace> C<O>ption).

=item t expr

Trace through execution of expr.  For example:

 $ 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 C<O>ption C<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

=item b [line] [condition]

Set 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 use B<if>:

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

=item b subname [condition]

Set a breakpoint at the first line of the named subroutine.

=item b postpone subname [condition]

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

=item b load filename

Set breakpoint at the first executed line of the file.  Filename should
be a full name as found in values of %INC.

=item b compile subname

Sets breakpoint at the first statement executed after the subroutine
is compiled.

=item d [line]

Delete a breakpoint at the specified line.  If line is omitted, deletes
the breakpoint on the line that is about to be executed.

=item D

Delete all installed breakpoints.

=item a [line] command

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 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

Delete all installed actions.

=item W [expr]

Add a global watch-expression.

=item W

Delete all watch-expressions.

=item O [opt[=val]] [opt"val"] [opt?]...

Set or query values of options.  val defaults to 1.  opt can
be abbreviated.  Several options can be listed.

=over 12

=item C<recallCommand>, C<ShellBang>

The characters used to recall command or spawn shell.  By
default, these are both set to C<!>.

=item C<pager>

Program to use for output of pager-piped commands (those
beginning with a C<|> character.)  By default,
C<$ENV{PAGER}> will be used.

=item C<tkRunning>

Run Tk while prompting (with ReadLine).

=item C<signalLevel>, C<warnLevel>, C<dieLevel>

Level 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.  If C<dieLevel> is 2,
then the messages which will be caught by surrounding C<eval> are also
printed.

=item C<AutoTrace>

Trace mode (similar to C<t> command, but can be put into
C<PERLDB_OPTS>).

=item C<LineInfo>

File or pipe to print line number info to.  If it is a pipe (say,
C<|visual_perl_db>), then a short, "emacs like" message is used.

=item C<inhibit_exit>

If 0, allows I<stepping off> the end of the script.

=item C<PrintRet>

affects printing of return value after C<r> command.

=item C<ornaments>

affects screen appearance of the command line (see L<Term::ReadLine>).

=item C<frame>

affects printing messages on entry and exit from subroutines.  If
C<frame & 2> is false, messages are printed on entry only. (Printing
on exit may be useful if inter(di)spersed with other messages.)

If C<frame & 4>, arguments to functions are printed as well as the
context and caller info.  If C<frame & 8>, overloaded C<stringify> and
C<tie>d C<FETCH> are enabled on the printed arguments. If C<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:

=item C<maxTraceLen>

length at which the argument list is truncated when C<frame> option's
bit 4 is set.

=back

The following options affect what happens with C<V>, C<X>, and C<x>
commands:

=over 12

=item C<arrayDepth>, C<hashDepth>

Print only first N elements ('' for all).

=item C<compactDump>, C<veryCompact>

Change style of array and hash dump.  If C<compactDump>, short array
may be printed on one line.

=item C<globPrint>

Whether to print contents of globs.

=item C<DumpDBFiles>

Dump arrays holding debugged files.

=item C<DumpPackages>

Dump symbol tables of packages.

=item C<DumpReused>

Dump contents of "reused" addresses.

=item C<quote>, C<HighBit>, C<undefPrint>

Change style of string dump.  Default value of C<quote> is C<auto>, one
can enable either double-quotish dump, or single-quotish by setting it
to C<"> or C<'>.  By default, characters with high bit set are printed
I<as is>.

=item C<UsageOnly>

I<very> rudimentally per-package memory usage dump.  Calculates total