perldebguts.1

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.\" ========================================================================
.\"
.IX Title "PERLDEBGUTS 1"
.TH PERLDEBGUTS 1 "2007-12-18" "perl v5.10.0" "Perl Programmers Reference Guide"
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.SH "NAME"
perldebguts \- Guts of Perl debugging
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
This is not the \fIperldebug\fR\|(1) manpage, 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.
.SH "Debugger Internals"
.IX Header "Debugger Internals"
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 \fIperl \-Dxxx\fR command described in perlrun, which is
usable only if a special Perl is built per the instructions in the
\&\fI\s-1INSTALL\s0\fR podpage in the Perl source tree.
.PP
For example, whenever you call Perl's built-in \f(CW\*(C`caller\*(C'\fR function
from the package \f(CW\*(C`DB\*(C'\fR, the arguments that the corresponding stack
frame was called with are copied to the \f(CW@DB::args\fR array.  These
mechanisms are enabled by calling Perl with the \fB\-d\fR switch.
Specifically, the following additional features are enabled
(cf. \*(L"$^P\*(R" in perlvar):
.IP "\(bu" 4
Perl inserts the contents of \f(CW$ENV{PERL5DB}\fR (or \f(CW\*(C`BEGIN {require
\&\*(Aqperl5db.pl\*(Aq}\*(C'\fR if not present) before the first line of your program.
.IP "\(bu" 4
Each array \f(CW\*(C`@{"_<$filename"}\*(C'\fR holds the lines of \f(CW$filename\fR for a
file compiled by Perl.  The same is also true for \f(CW\*(C`eval\*(C'\fRed strings
that contain subroutines, or which are currently being executed.
The \f(CW$filename\fR for \f(CW\*(C`eval\*(C'\fRed strings looks like \f(CW\*(C`(eval 34)\*(C'\fR.
Code assertions in regexes look like \f(CW\*(C`(re_eval 19)\*(C'\fR.
.Sp
Values in this array are magical in numeric context: they compare
equal to zero only if the line is not breakable.
.IP "\(bu" 4
Each hash \f(CW\*(C`%{"_<$filename"}\*(C'\fR contains breakpoints and actions keyed
by line number.  Individual entries (as opposed to the whole hash)
are settable.  Perl only cares about Boolean true here, although
the values used by \fIperl5db.pl\fR have the form
\&\f(CW"$break_condition\e0$action"\fR.
.Sp
The same holds for evaluated strings that contain subroutines, or
which are currently being executed.  The \f(CW$filename\fR for \f(CW\*(C`eval\*(C'\fRed strings
looks like \f(CW\*(C`(eval 34)\*(C'\fR or  \f(CW\*(C`(re_eval 19)\*(C'\fR.
.IP "\(bu" 4
Each scalar \f(CW\*(C`${"_<$filename"}\*(C'\fR contains \f(CW"_<$filename"\fR.  This is
also the case for evaluated strings that contain subroutines, or
which are currently being executed.  The \f(CW$filename\fR for \f(CW\*(C`eval\*(C'\fRed
strings looks like \f(CW\*(C`(eval 34)\*(C'\fR or \f(CW\*(C`(re_eval 19)\*(C'\fR.
.IP "\(bu" 4
After each \f(CW\*(C`require\*(C'\fRd file is compiled, but before it is executed,
\&\f(CW\*(C`DB::postponed(*{"_<$filename"})\*(C'\fR is called if the subroutine
\&\f(CW\*(C`DB::postponed\*(C'\fR exists.  Here, the \f(CW$filename\fR is the expanded name of
the \f(CW\*(C`require\*(C'\fRd file, as found in the values of \f(CW%INC\fR.
.IP "\(bu" 4
After each subroutine \f(CW\*(C`subname\*(C'\fR is compiled, the existence of
\&\f(CW$DB::postponed{subname}\fR is checked.  If this key exists,
\&\f(CW\*(C`DB::postponed(subname)\*(C'\fR is called if the \f(CW\*(C`DB::postponed\*(C'\fR subroutine
also exists.
.IP "\(bu" 4
A hash \f(CW%DB::sub\fR is maintained, whose keys are subroutine names
and whose values have the form \f(CW\*(C`filename:startline\-endline\*(C'\fR.
\&\f(CW\*(C`filename\*(C'\fR has the form \f(CW\*(C`(eval 34)\*(C'\fR for subroutines defined inside
\&\f(CW\*(C`eval\*(C'\fRs, or \f(CW\*(C`(re_eval 19)\*(C'\fR for those within regex code assertions.
.IP "\(bu" 4
When the execution of your program reaches a point that can hold a
breakpoint, the \f(CW\*(C`DB::DB()\*(C'\fR subroutine is called if any of the variables
\&\f(CW$DB::trace\fR, \f(CW$DB::single\fR, or \f(CW$DB::signal\fR is true.  These variables
are not \f(CW\*(C`local\*(C'\fRizable.  This feature is disabled when executing
inside \f(CW\*(C`DB::DB()\*(C'\fR, including functions called from it 
unless \f(CW\*(C`$^D & (1<<30)\*(C'\fR is true.
.IP "\(bu" 4
When execution of the program reaches a subroutine call, a call to
\&\f(CW&DB::sub\fR(\fIargs\fR) is made instead, with \f(CW$DB::sub\fR holding the
name of the called subroutine. (This doesn't happen if the subroutine
was compiled in the \f(CW\*(C`DB\*(C'\fR package.)
.PP
Note that if \f(CW&DB::sub\fR needs external data for it to work, no
subroutine call is possible without it. As an example, the standard
debugger's \f(CW&DB::sub\fR depends on the \f(CW$DB::deep\fR variable
(it defines how many levels of recursion deep into the debugger you can go
before a mandatory break).  If \f(CW$DB::deep\fR is not defined, subroutine
calls are not possible, even though \f(CW&DB::sub\fR exists.
.Sh "Writing Your Own Debugger"
.IX Subsection "Writing Your Own Debugger"
\fIEnvironment Variables\fR
.IX Subsection "Environment Variables"
.PP
The \f(CW\*(C`PERL5DB\*(C'\fR environment variable can be used to define a debugger.
For example, the minimal \*(L"working\*(R" debugger (it actually doesn't do anything)
consists of one line:
.PP
.Vb 1
\&  sub DB::DB {}
.Ve
.PP
It can easily be defined like this:
.PP
.Vb 1
\&  $ PERL5DB="sub DB::DB {}" perl \-d your\-script
.Ve
.PP
Another brief debugger, slightly more useful, can be created
with only the line:
.PP
.Vb 1
\&  sub DB::DB {print ++$i; scalar <STDIN>}
.Ve
.PP
This debugger prints a number which increments for each statement
encountered and waits for you to hit a newline before continuing
to the next statement.
.PP
The following debugger is actually useful:
.PP
.Vb 5
\&  {
\&    package DB;
\&    sub DB  {}
\&    sub sub {print ++$i, " $sub\en"; &$sub}
\&  }
.Ve
.PP
It prints the sequence number of each subroutine call and the name of the
called subroutine.  Note that \f(CW&DB::sub\fR is being compiled into the
package \f(CW\*(C`DB\*(C'\fR through the use of the \f(CW\*(C`package\*(C'\fR directive.
.PP
When it starts, the debugger reads your rc file (\fI./.perldb\fR or
\&\fI~/.perldb\fR under Unix), which can set important options.
(A subroutine (\f(CW&afterinit\fR) can be defined here as well; it is executed
after the debugger completes its own initialization.)
.PP
After the rc file is read, the debugger reads the \s-1PERLDB_OPTS\s0
environment variable and uses it to set debugger options. The
contents of this variable are treated as if they were the argument
of an \f(CW\*(C`o ...\*(C'\fR debugger command (q.v. in \*(L"Options\*(R" in perldebug).
.PP
\fIDebugger internal variables In addition to the file and subroutine-related variables mentioned above, the debugger also maintains various magical internal variables.\fR
.IX Subsection "Debugger internal variables In addition to the file and subroutine-related variables mentioned above, the debugger also maintains various magical internal variables."
.IP "\(bu" 4
\&\f(CW@DB::dbline\fR is an alias for \f(CW\*(C`@{"::_<current_file"}\*(C'\fR, which
holds the lines of the currently-selected file (compiled by Perl), either
explicitly chosen with the debugger's \f(CW\*(C`f\*(C'\fR command, or implicitly by flow
of execution.
.Sp
Values in this array are magical in numeric context: they compare
equal to zero only if the line is not breakable.
.IP "\(bu" 4
\&\f(CW%DB::dbline\fR, is an alias for \f(CW\*(C`%{"::_<current_file"}\*(C'\fR, which
contains breakpoints and actions keyed by line number in
the currently-selected file, either explicitly chosen with the
debugger's \f(CW\*(C`f\*(C'\fR command, or implicitly by flow of execution.
.Sp
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 \fIperl5db.pl\fR have the form
\&\f(CW"$break_condition\e0$action"\fR.
.PP
\fIDebugger customization functions\fR
.IX Subsection "Debugger customization functions"
.PP
Some functions are provided to simplify customization.
.IP "\(bu" 4
See \*(L"Configurable Options\*(R" in perldebug for a description of options parsed by
\&\f(CW\*(C`DB::parse_options(string)\*(C'\fR.
.IP "\(bu" 4
\&\f(CW\*(C`DB::dump_trace(skip[,count])\*(C'\fR skips the specified number of frames
and returns a list containing information about the calling frames (all
of them, if \f(CW\*(C`count\*(C'\fR is missing).  Each entry is reference to a hash
with keys \f(CW\*(C`context\*(C'\fR (either \f(CW\*(C`.\*(C'\fR, \f(CW\*(C`$\*(C'\fR, or \f(CW\*(C`@\*(C'\fR), \f(CW\*(C`sub\*(C'\fR (subroutine
name, or info about \f(CW\*(C`eval\*(C'\fR), \f(CW\*(C`args\*(C'\fR (\f(CW\*(C`undef\*(C'\fR or a reference to
an array), \f(CW\*(C`file\*(C'\fR, and \f(CW\*(C`line\*(C'\fR.
.IP "\(bu" 4
\&\f(CW\*(C`DB::print_trace(FH, skip[, count[, short]])\*(C'\fR prints
formatted info about caller frames.  The last two functions may be
convenient as arguments to \f(CW\*(C`<\*(C'\fR, \f(CW\*(C`<<\*(C'\fR commands.
.PP
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.
.SH "Frame Listing Output Examples"
.IX Header "Frame Listing Output Examples"
The \f(CW\*(C`frame\*(C'\fR option can be used to control the output of frame 
information.  For example, contrast this expression trace:
.PP
.Vb 2
\& $ perl \-de 42
\& Stack dump during die enabled outside of evals.
\&
\& Loading DB routines from perl5db.pl patch level 0.94
\& Emacs support available.