perldebug.pod

MSYS在windows下模拟了一个类unix的终端

=head1 NAME

perldebug - Perl debugging

=head1 DESCRIPTION

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

=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:

    $ 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.)

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 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, precede the command with a leading pipe symbol so
that 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 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 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 real C<print> function in Perl.
See L<Dumpvalue> if you'd like to do this yourself.

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

=item V [pkg [vars]]

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]

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 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 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

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]

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.  I<subname> may
be a variable that contains a code reference.

=item -

List previous window of lines.

=item w [line]

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

=item .

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

=item f filename

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.

=item ?pattern?

Search backwards for pattern; final ? is optional.

=item L

List all breakpoints and actions.

=item S [[!]regex]

List subroutine names [not] matching the regex.

=item t

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

=item t expr

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

=item b [line] [condition]

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
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]

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]

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

=item b load filename

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

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

=item d [line]

Delete a breakpoint from the specified I<line>.  If I<line> is omitted, deletes
the breakpoint from the line 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.  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]

Delete an action from the specified line.  If I<line> is omitted, delete
the action on the line that is about to be executed.

=item A

Delete all installed actions.

=item W expr

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>.

=item W

Delete all watch-expressions.

=item O booloption ...

Set each listed Boolean option to the value C<1>.

=item O anyoption? ...

Print out the value of one or more options.

=item O option=value ...

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
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
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"> 
for a list of these.

=item < ? 

List out all pre-prompt Perl command actions.

=item < [ command ]