| PERLDEBUG(1) | Perl Programmers Reference Guide | PERLDEBUG(1) |
perldebug - Perl debugging
First of all, have you tried using "use strict;" and "use warnings;"?
If you're new to the Perl debugger, you may prefer to read perldebtut, which is a tutorial introduction to the debugger.
If you're looking for the nitty gritty details of how the debugger is implemented, you may prefer to read perldebguts.
For in-depth technical usage details, see perl5db.pl, the documentation of the debugger itself.
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 the way 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 special Perl library file containing the debugger.
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 keeping its own state information.)
Note that the said "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 ";" or perhaps a "+", or by wrapping it with parentheses or braces.
There are several ways to call the debugger:
The interactive debugger understands the following commands:
If the output of the "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 "o pager=..." command.
The "DB::OUT" filehandle is opened to /dev/tty, regardless of where STDOUT may be redirected to.
The output format is governed by multiple options described under "Configurable Options".
If the "maxdepth" is included, it must be a numeral N; the value is dumped only N levels deep, as if the "dumpDepth" option had been temporarily set to N.
V DB filename line
Use "~pattern" and "!pattern" for positive and negative regexes.
This is similar to calling the "x" command on each applicable var.
"eval"ed strings (when accessible) are considered to be filenames: "f (eval 7)" and "f eval 7\b" access the body of the 7th "eval"ed string (in the order of execution). The bodies of the currently executed "eval" and of "eval"ed strings that define subroutines are saved and thus accessible.
b 237 $x > 30
b 237 ++$count237 < 11
b 33 /pattern/i
If the line number is ".", sets a breakpoint on the current line:
b . $n > 100
b lib/MyModule.pm:237 $x > 30
b /usr/lib/perl5/site_perl/CGI.pm:100 ++$count100 < 11
This is done for a breakpoint in the current file.
This is done for a breakpoint in the current file.
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"
For historical reasons, the "=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 "=". The "option" can be abbreviated, but for clarity probably should not be. Several options can be set together. See "Configurable Options" for a list of these.
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 ";{ ... }" or even "do { ... }".
Set the "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 if you want to step through global destruction.
The following setting are currently preserved: history, breakpoints, actions, debugger options, and the Perl command-line options -w, -I, and -e.
= quit q
or list current aliases.
On systems traditionally bereft of a usable man command, the debugger invokes perldoc. Occasionally this determination is incorrect due to recalcitrant vendors or rather more felicitously, to enterprising users. If you fall into either category, just manually set the $DB::doccmd variable to whatever viewer to view the Perl documentation on your system. This may be set in an rc file, or through direct assignment. We're still waiting for a working example of something along the lines of:
$DB::doccmd = 'netscape -remote http://something.here/';
The debugger has numerous options settable using the "o" command, either interactively or from the environment or an rc file. The file is named ./.perldb or ~/.perldb under Unix with /dev/tty, perldb.ini otherwise.
To disable this default safe mode, set these values to something higher than 0. At a level of 1, you get backtraces upon receiving any kind of warning (this is often annoying) or exception (this is often valuable). Unfortunately, the debugger cannot discern fatal exceptions from non-fatal ones. If "dieLevel" is even 1, then your non-fatal exceptions are also traced and unceremoniously altered if they came from "eval'ed" strings or from any kind of "eval" within modules you're attempting to load. If "dieLevel" is 2, the debugger doesn't care where they came from: It usurps your exception handler and prints out a trace, then modifies all exceptions with its own embellishments. This may perhaps be useful for some tracing purposes, but tends to hopelessly destroy any program that takes its exception handling seriously.
If "frame & 4", arguments to functions are printed, plus context and caller info. If "frame & 8", overloaded "stringify" and "tie"d "FETCH" is enabled on the printed arguments. If "frame & 16", the return value from the subroutine is printed.
The length at which the argument list is truncated is governed by the next option:
The following options affect what happens with "V", "X", and "x" commands:
After the rc file is read, the debugger reads the $ENV{PERLDB_OPTS} 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 "TTY", "noTTY", "ReadLine", and "NonStop" there.
If your rc file contains:
parse_options("NonStop=1 LineInfo=db.out AutoTrace");
then your script will run without human intervention, putting trace information into the file db.out. (If you interrupt it, you'd better reset "LineInfo" to /dev/tty if you expect to see anything.)
This module should implement a method named "new" that returns an object with two methods: "IN" and "OUT". These should return filehandles to use for debugging input and output correspondingly. The "new" method should inspect an argument containing the value of $ENV{PERLDB_NOTTY} at startup, or "$ENV{HOME}/.perldbtty$$" otherwise. This file is not inspected for proper ownership, so security hazards are theoretically possible.
Here's an example of using the $ENV{PERLDB_OPTS} variable:
$ PERLDB_OPTS="NonStop frame=2" perl -d myprogram
That will run the script myprogram without human intervention, printing out the call tree with entry and exit points. Note that "NonStop=1 frame=2" is equivalent to "N f=2", and that originally, options could be uniquely abbreviated by the first letter (modulo the "Dump*" options). It is nevertheless recommended that you always spell them out in full for legibility and future compatibility.
Other examples include
$ 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 listing. (If you interrupt it, you would better reset "LineInfo" to something "interactive"!)
Other examples include (using standard shell syntax to show environment variable settings):
$ ( PERLDB_OPTS="NonStop frame=1 AutoTrace LineInfo=tperl.out"
perl -d myprogram )
which may be useful for debugging a program that uses "Term::ReadLine" itself. Do not forget to detach your shell from the TTY in the window that corresponds to /dev/ttyXX, say, by issuing a command like
$ sleep 1000000
See "Debugger Internals" in perldebguts for details.
DB<8>
or even
DB<<17>>
where that number is the command number, and which you'd use to access with the built-in csh-like history mechanism. For example, "!17" would repeat command number 17. The depth of the angle brackets indicates the nesting 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 the result of a function call that itself 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 indicates the context in which the function was called, with "$" and "@" meaning scalar or list contexts respectively, and "." meaning void context (which is actually a sort of scalar context). The display above says that you were in the function "main::infested" when you ran the stack dump, and that it was called in scalar context from line 10 of the file Ambulation.pm, but without any arguments at all, meaning it was called as &infested. The next stack frame shows that the function "Ambulation::legs" was called in list context from the camel_flea file with four arguments. The last stack frame shows that "main::pests" was called in scalar context, also from camel_flea, but from line 4.
If you execute the "T" command from inside an active "use" statement, the backtrace will contain both a "require" frame and an "eval" frame.
DB<<13>> l
101: @i{@i} = ();
102:b @isa{@i,$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);
Breakable lines are marked with ":". Lines with breakpoints are marked by "b" and those with actions by "a". The line that's about to be executed is marked by "==>".
Please be aware that code in debugger listings may not look the same as your original source code. Line directives and external source filters can alter the code before Perl sees it, causing code to move from its original positions or take on entirely different forms.
If you have compile-time executable statements (such as code within BEGIN, UNITCHECK and CHECK blocks or "use" statements), these will not be stopped by debugger, although "require"s and INIT blocks will, and compile-time statements can be traced with the "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 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 the debugger, set a breakpoint on the load of some module:
DB<7> b load f:/perllib/lib/Carp.pm Will stop on load of 'f:/perllib/lib/Carp.pm'.
and then restart the debugger using the "R" command (if possible). One can use "b compile subname" for the same purpose.
The debugger probably contains enough configuration hooks that you won't ever have to modify it yourself. You may change the behaviour of the debugger from within the debugger using its "o" command, from the command line via the "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/';
You can change options from .perldb by using 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", that function is called after debugger initialization ends. .perldb may be contained in the current directory, or in the home directory. Because this file is sourced in by Perl and may contain arbitrary commands, for security reasons, 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 .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 perl5db.pl from the Perl library to another name and hack it to your heart's content. You'll then want to set your "PERL5DB" environment variable to say something like this:
BEGIN { require "myperl5db.pl" }
As a last resort, you could also use "PERL5DB" to customize the debugger by directly setting internal variables or calling debugger functions.
Note that any variables and functions that are not documented in this document (or in perldebguts) are considered for internal use only, and as such are subject to change without notice.
As shipped, the only command-line history supplied is a simplistic one that checks for leading exclamation points. However, if you install the Term::ReadKey and Term::ReadLine modules from CPAN (such as Term::ReadLine::Gnu, Term::ReadLine::Perl, ...) you will have full editing capabilities much like those GNU readline(3) provides. Look for these in the modules/by-module/Term directory on CPAN. These do not support normal vi command-line editing, however.
A rudimentary command-line completion is also available, including lexical variables in the current scope if the "PadWalker" module is installed.
Without Readline support you may see the symbols "^[[A", "^[[C", "^[[B", "^[[D"", "^H", ... when using the arrow keys and/or the backspace key.
If you have the GNU's version of emacs installed on your system, it can interact with the Perl debugger to provide an integrated software development environment reminiscent of its interactions with C debuggers.
Recent versions of Emacs come with a start file for making emacs act like a syntax-directed editor that understands (some of) Perl's syntax. See perlfaq3.
Users of vi should also look into vim and gvim, the mousey 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.
If you wish to supply an alternative debugger for Perl to run, invoke your script with a colon and a package argument given to the -d flag. Perl's alternative debuggers include a Perl profiler, Devel::NYTProf, which is available separately as a CPAN distribution. To profile your Perl program in the file mycode.pl, just type:
$ perl -d:NYTProf mycode.pl
When the script terminates the profiler will create a database of the profile information that you can turn into reports using the profiler's tools. See <perlperf> for details.
"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 "Debugging Regular Expressions" in perldebguts.
Perl contains internal support for reporting its own memory usage, but this is a fairly advanced concept that requires some understanding of how memory allocation works. See "Debugging Perl Memory Usage" in perldebguts for the details.
You do have "use strict" and "use warnings" enabled, don't you?
perldebtut, perldebguts, perl5db.pl, re, DB, Devel::NYTProf, Dumpvalue, and 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 "which $scriptname".
$ perl -Sd foo.pl
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 "shift" or "pop"), the stack backtrace will not show the original values.
The debugger does not currently work in conjunction with the -W command-line switch, because it itself is not free of warnings.
If you're in a slow syscall (like "wait"ing, "accept"ing, or "read"ing from your keyboard or a socket) and haven't set up your own $SIG{INT} handler, then you won't be able to CTRL-C your way back to the debugger, because the debugger's own $SIG{INT} handler doesn't understand that it needs to raise an exception to longjmp(3) out of slow syscalls.
| 2022-02-19 | perl v5.34.1 |