perldebguts - Guts of Perl debugging
This is not perldebug, which tells you how to use the debugger. This manpage describes low-level details concerning the debugger's internals, which range from difficult to impossible to understand for anyone who isn't incredibly intimate with Perl's guts. Caveat lector.
Perl has special debugging hooks at compile-time and run-time used to create debugging environments. These hooks are not to be confused with the perl -Dxxx command described in perlrun, which is usable only if a special Perl is built per the instructions in the INSTALL file in the Perl source tree.
For example, whenever you call Perl's built-in caller function from the package DB, the arguments that the corresponding stack frame was called with are copied to the @DB::args array. These mechanisms are enabled by calling Perl with the -d switch. Specifically, the following additional features are enabled (cf. "$^P" in perlvar):
Perl inserts the contents of $ENV{PERL5DB} (or BEGIN {require 'perl5db.pl'} if not present) before the first line of your program.
If the -dt switch is supplied, PERL5DB_THREADED=1 is set in the environment. perl5db.pl or your custom debugger can use this to enable threading support.
Each array @{"_<$filename"} holds the lines of $filename for a file compiled by Perl. The same is also true for evaled strings that contain subroutines, or which are currently being executed. The $filename for evaled strings looks like (eval 34).
Values in this array are magical in numeric context: they compare equal to zero only if the line is not breakable.
Each hash %{"_<$filename"} contains breakpoints and actions keyed by line number. Individual entries (as opposed to the whole hash) are settable. Perl only cares about Boolean true here, although the values used by perl5db.pl have the form "$break_condition\0$action".
The same holds for evaluated strings that contain subroutines, or which are currently being executed. The $filename for evaled strings looks like (eval 34).
Each scalar ${"_<$filename"} contains $filename. This is also the case for evaluated strings that contain subroutines, or which are currently being executed. The $filename for evaled strings looks like (eval 34).
After each required file is compiled, but before it is executed, DB::postponed(*{"_<$filename"}) is called if the subroutine DB::postponed exists. Here, the $filename is the expanded name of the required file, as found in the values of %INC.
After each subroutine subname is compiled, the existence of $DB::postponed{subname} is checked. If this key exists, DB::postponed(subname) is called if the DB::postponed subroutine also exists.
A hash %DB::sub is maintained, whose keys are subroutine names and whose values have the form filename:startline-endline. filename has the form (eval 34) for subroutines defined inside evals.
When the execution of your program reaches a point that can hold a breakpoint, the DB::DB() subroutine is called if any of the variables $DB::trace, $DB::single, or $DB::signal is true. These variables are not localizable. This feature is disabled when executing inside DB::DB(), including functions called from it unless $^D & (1<<30) is true.
When execution of the program reaches a subroutine call, a call to &DB::sub(args) is made instead, with $DB::sub set to identify the called subroutine. (This doesn't happen if the calling subroutine was compiled in the DB package.) $DB::sub normally holds the name of the called subroutine, if it has a name by which it can be looked up. Failing that, $DB::sub will hold a reference to the called subroutine. Either way, the &DB::sub subroutine can use $DB::sub as a reference by which to call the called subroutine, which it will normally want to do.
If the call is to an lvalue subroutine, and &DB::lsub is defined &DB::lsub(args) is called instead, otherwise falling back to &DB::sub(args).
When execution of the program uses goto to enter a non-XS subroutine and the 0x80 bit is set in $^P, a call to &DB::goto is made, with $DB::sub set to identify the subroutine being entered. The call to &DB::goto does not replace the goto; the requested subroutine will still be entered once &DB::goto has returned. $DB::sub normally holds the name of the subroutine being entered, if it has one. Failing that, $DB::sub will hold a reference to the subroutine being entered. Unlike when &DB::sub is called, it is not guaranteed that $DB::sub can be used as a reference to operate on the subroutine being entered.
Note that if &DB::sub needs external data for it to work, no subroutine call is possible without it. As an example, the standard debugger's &DB::sub depends on the $DB::deep variable (it defines how many levels of recursion deep into the debugger you can go before a mandatory break). If $DB::deep is not defined, subroutine calls are not possible, even though &DB::sub exists.
The PERL5DB environment variable can be used to define a debugger. For example, the minimal "working" debugger (it actually doesn't do anything) consists of one line:
sub DB::DB {}It can easily be defined like this:
$ PERL5DB="sub DB::DB {}" perl -d your-scriptAnother brief debugger, slightly more useful, can be created with only the line:
sub DB::DB {print ++$i; scalar <STDIN>}This debugger prints a number which increments for each statement encountered and waits for you to hit a newline before continuing to the next statement.
The following debugger is actually useful:
{
package DB;
sub DB {}
sub sub {print ++$i, " $sub\n"; &$sub}
}It prints the sequence number of each subroutine call and the name of the called subroutine. Note that &DB::sub is being compiled into the package DB through the use of the package directive.
When it starts, the debugger reads your rc file (./.perldb or ~/.perldb under Unix), which can set important options. (A subroutine (&afterinit) can be defined here as well; it is executed after the debugger completes its own initialization.)
After the rc file is read, the debugger reads the PERLDB_OPTS environment variable and uses it to set debugger options. The contents of this variable are treated as if they were the argument of an o ... debugger command (q.v. in "Configurable Options" in perldebug).
In addition to the file and subroutine-related variables mentioned above, the debugger also maintains various magical internal variables.
@DB::dbline is an alias for @{"::_<current_file"}, which holds the lines of the currently-selected file (compiled by Perl), either explicitly chosen with the debugger's f command, or implicitly by flow of execution.
Values in this array are magical in numeric context: they compare equal to zero only if the line is not breakable.
%DB::dbline is an alias for %{"::_<current_file"}, which contains breakpoints and actions keyed by line number in the currently-selected file, either explicitly chosen with the debugger's f command, or implicitly by flow of execution.
As previously noted, individual entries (as opposed to the whole hash) are settable. Perl only cares about Boolean true here, although the values used by perl5db.pl have the form "$break_condition\0$action".
Some functions are provided to simplify customization.
See "Configurable Options" in perldebug for a description of options parsed by DB::parse_options(string).
DB::dump_trace(skip[,count]) skips the specified number of frames and returns a list containing information about the calling frames (all of them, if count is missing). Each entry is reference to a hash with keys context (either ., $, or @), sub (subroutine name, or info about eval), args (undef or a reference to an array), file, and line.
DB::print_trace(FH, skip[, count[, short]]) prints formatted info about caller frames. The last two functions may be convenient as arguments to <, << commands.
Note that any variables and functions that are not documented in this manpages (or in perldebug) are considered for internal use only, and as such are subject to change without notice.
The frame option can be used to control the output of frame information. For example, contrast this expression trace:
$ 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):
42with this one, once the option frame=2 has been 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
42By way of demonstration, we present below a laborious listing resulting from setting your PERLDB_OPTS environment variable to the value f=n N, and running perl -d -V from the command line. Examples using various values of n are shown to give you a feel for the difference between settings. Long though it may be, this is not a complete listing, but only excerpts.
entering main::BEGIN
entering Config::BEGIN
Package lib/Exporter.pm.
Package lib/Carp.pm.
Package lib/Config.pm.
entering Config::TIEHASH
entering Exporter::import
entering Exporter::export
entering Config::myconfig
entering Config::FETCH
entering Config::FETCH
entering Config::FETCH
entering Config::FETCHentering main::BEGIN
entering Config::BEGIN
Package lib/Exporter.pm.
Package lib/Carp.pm.
exited Config::BEGIN
Package lib/Config.pm.
entering Config::TIEHASH
exited Config::TIEHASH
entering Exporter::import
entering Exporter::export
exited Exporter::export
exited Exporter::import
exited main::BEGIN
entering Config::myconfig
entering Config::FETCH
exited Config::FETCH
entering Config::FETCH
exited Config::FETCH
entering Config::FETCHin $=main::BEGIN() from /dev/null:0
in $=Config::BEGIN() from lib/Config.pm:2
Package lib/Exporter.pm.
Package lib/Carp.pm.
Package lib/Config.pm.
in $=Config::TIEHASH('Config') from lib/Config.pm:644
in $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from li
in @=Config::myconfig() from /dev/null:0
in $=Config::FETCH(ref(Config), 'package') from lib/Config.pm:574
in $=Config::FETCH(ref(Config), 'baserev') from lib/Config.pm:574
in $=Config::FETCH(ref(Config), 'PERL_VERSION') from lib/Config.pm:574
in $=Config::FETCH(ref(Config), 'PERL_SUBVERSION') from lib/Config.pm:574
in $=Config::FETCH(ref(Config), 'osname') from lib/Config.pm:574
in $=Config::FETCH(ref(Config), 'osvers') from lib/Config.pm:574in $=main::BEGIN() from /dev/null:0
in $=Config::BEGIN() from lib/Config.pm:2
Package lib/Exporter.pm.
Package lib/Carp.pm.
out $=Config::BEGIN() from lib/Config.pm:0
Package lib/Config.pm.
in $=Config::TIEHASH('Config') from lib/Config.pm:644
out $=Config::TIEHASH('Config') from lib/Config.pm:644
in $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/
out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/
out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
out $=main::BEGIN() from /dev/null:0
in @=Config::myconfig() from /dev/null:0
in $=Config::FETCH(ref(Config), 'package') from lib/Config.pm:574
out $=Config::FETCH(ref(Config), 'package') from lib/Config.pm:574
in $=Config::FETCH(ref(Config), 'baserev') from lib/Config.pm:574
out $=Config::FETCH(ref(Config), 'baserev') from lib/Config.pm:574
in $=Config::FETCH(ref(Config), 'PERL_VERSION') from lib/Config.pm:574
out $=Config::FETCH(ref(Config), 'PERL_VERSION') from lib/Config.pm:574
in $=Config::FETCH(ref(Config), 'PERL_SUBVERSION') from lib/Config.pm:574in $=main::BEGIN() from /dev/null:0
in $=Config::BEGIN() from lib/Config.pm:2
Package lib/Exporter.pm.
Package lib/Carp.pm.
out $=Config::BEGIN() from lib/Config.pm:0
Package lib/Config.pm.
in $=Config::TIEHASH('Config') from lib/Config.pm:644
out $=Config::TIEHASH('Config') from lib/Config.pm:644
in $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/E
out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/E
out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
out $=main::BEGIN() from /dev/null:0
in @=Config::myconfig() from /dev/null:0
in $=Config::FETCH('Config=HASH(0x1aa444)', 'package') from lib/Config.pm:574
out $=Config::FETCH('Config=HASH(0x1aa444)', 'package') from lib/Config.pm:574
in $=Config::FETCH('Config=HASH(0x1aa444)', 'baserev') from lib/Config.pm:574
out $=Config::FETCH('Config=HASH(0x1aa444)', 'baserev') from lib/Config.pm:574in $=CODE(0x15eca4)() from /dev/null:0
in $=CODE(0x182528)() from lib/Config.pm:2
Package lib/Exporter.pm.
out $=CODE(0x182528)() from lib/Config.pm:0
scalar context return from CODE(0x182528): undef
Package lib/Config.pm.
in $=Config::TIEHASH('Config') from lib/Config.pm:628
out $=Config::TIEHASH('Config') from lib/Config.pm:628
scalar context return from Config::TIEHASH: empty hash
in $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/Exporter.pm:171
out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/Exporter.pm:171
scalar context return from Exporter::export: ''
out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
scalar context return from Exporter::import: ''In all cases shown above, the line indentation shows the call tree. If bit 2 of frame is set, a line is printed on exit from a subroutine as well. If bit 4 is set, the arguments are printed along with the caller info. If bit 8 is set, the arguments are printed even if they are tied or references. If bit 16 is set, the return value is printed, too.
When a package is compiled, a line like this
Package lib/Carp.pm.is printed with proper indentation.
There are two ways to enable debugging output for regular expressions.
If your perl is compiled with -DDEBUGGING, you may use the -Dr flag on the command line, and -Drv for more verbose information.
Otherwise, one can use re 'debug', which has effects at both compile time and run time. Since Perl 5.9.5, this pragma is lexically scoped.
The debugging output at compile time looks like this:
Compiling REx '[bc]d(ef*g)+h[ij]k$'
size 45 Got 364 bytes for offset annotations.
first at 1
rarest char g at 0
rarest char d at 0
1: ANYOF[bc](12)
12: EXACT <d>(14)
14: CURLYX[0] {1,32767}(28)
16: OPEN1(18)
18: EXACT <e>(20)
20: STAR(23)
21: EXACT <f>(0)
23: EXACT <g>(25)
25: CLOSE1(27)
27: WHILEM[1/1](0)
28: NOTHING(29)
29: EXACT <h>(31)
31: ANYOF[ij](42)
42: EXACT <k>(44)
44: EOL(45)
45: END(0)
anchored 'de' at 1 floating 'gh' at 3..2147483647 (checking floating)
stclass 'ANYOF[bc]' minlen 7
Offsets: [45]
1[4] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 5[1]
0[0] 12[1] 0[0] 6[1] 0[0] 7[1] 0[0] 9[1] 8[1] 0[0] 10[1] 0[0]
11[1] 0[0] 12[0] 12[0] 13[1] 0[0] 14[4] 0[0] 0[0] 0[0] 0[0]
0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 18[1] 0[0] 19[1] 20[0]
Omitting $` $& $' support.The first line shows the pre-compiled form of the regex. The second shows the size of the compiled form (in arbitrary units, usually 4-byte words) and the total number of bytes allocated for the offset/length table, usually 4+size*8. The next line shows the label id of the first node that does a match.
The
anchored 'de' at 1 floating 'gh' at 3..2147483647 (checking floating)
stclass 'ANYOF[bc]' minlen 7 line (split into two lines above) contains optimizer information. In the example shown, the optimizer found that the match should contain a substring de at offset 1, plus substring gh at some offset between 3 and infinity. Moreover, when checking for these substrings (to abandon impossible matches quickly), Perl will check for the substring gh before checking for the substring de. The optimizer may also use the knowledge that the match starts (at the first id) with a character class, and no string shorter than 7 characters can possibly match.
The fields of interest which may appear in this line are
anchored STRING at POSfloating STRING at POS1..POS2See above.
matching floating/anchoredWhich substring to check first.
minlenThe minimal length of the match.
stclass TYPEType of first matching node.
noscanDon't scan for the found substrings.
isallMeans that the optimizer information is all that the regular expression contains, and thus one does not need to enter the regex engine at all.
GPOSSet if the pattern contains \G.
plusSet if the pattern starts with a repeated char (as in x+y).
implicitSet if the pattern starts with .*.
with evalSet if the pattern contain eval-groups, such as (?{ code }) and (??{ code }).