X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=blobdiff_plain;f=pod%2Fperldebguts.pod;h=6bc2c3781cecf2a57e070a07ebe0032af414758d;hb=9af228c62a22d61074ac942be277a5f0b4bd7aff;hp=c46dfd99df56e93a2b7e2fe5e4d56522f0acfdcc;hpb=848805551b0f28dd093c1130af7aa30922a60349;p=p5sagit%2Fp5-mst-13.2.git diff --git a/pod/perldebguts.pod b/pod/perldebguts.pod index c46dfd9..6bc2c37 100644 --- a/pod/perldebguts.pod +++ b/pod/perldebguts.pod @@ -5,9 +5,10 @@ perldebguts - Guts of Perl debugging =head1 DESCRIPTION This is not the perldebug(1) manpage, which tells you how to use -the debugger. This manpage describes low-level details ranging -between difficult and impossible for anyone who isn't incredibly -intimate with Perl's guts to understand. Caveat lector. +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. =head1 Debugger Internals @@ -18,10 +19,11 @@ usable only if a special Perl is built per the instructions in the F podpage in the Perl source tree. For example, whenever you call Perl's built-in C function -from the package DB, the arguments that the corresponding stack -frame was called with are copied to the C<@DB::args> array. The -general mechanism is enabled by calling Perl with the B<-d> switch, the -following additional features are enabled (cf. L): +from the package C, the arguments that the corresponding stack +frame was called with are copied to the C<@DB::args> array. These +mechanisms are enabled by calling Perl with the B<-d> switch. +Specifically, the following additional features are enabled +(cf. L): =over 4 @@ -33,10 +35,10 @@ Perl inserts the contents of C<$ENV{PERL5DB}> (or C holds the lines of $filename for a -file compiled by Perl. The same for Ced strings that contain -subroutines, or which are currently being executed. The $filename -for Ced strings looks like C<(eval 34)>. Code assertions -in regexes look like C<(re_eval 19)>. +file compiled by Perl. The same is also true for Ced strings +that contain subroutines, or which are currently being executed. +The $filename for Ced strings looks like C<(eval 34)>. +Code assertions in regexes look like C<(re_eval 19)>. Values in this array are magical in numeric context: they compare equal to zero only if the line is not breakable. @@ -84,8 +86,8 @@ Cs, or C<(re_eval 19)> for those within regex code assertions. =item * When the execution of your program reaches a point that can hold a -breakpoint, the C subroutine is called any of the variables -$DB::trace, $DB::single, or $DB::signal is true. These variables +breakpoint, the C subroutine is called if any of the variables +C<$DB::trace>, C<$DB::single>, or C<$DB::signal> is true. These variables are not Cizable. This feature is disabled when executing inside C, including functions called from it unless C<< $^D & (1<<30) >> is true. @@ -94,38 +96,42 @@ unless C<< $^D & (1<<30) >> is true. When execution of the program reaches a subroutine call, a call to C<&DB::sub>(I) is made instead, with C<$DB::sub> holding the -name of the called subroutine. This doesn't happen if the subroutine +name of the called subroutine. (This doesn't happen if the subroutine was compiled in the C package.) =back Note that if C<&DB::sub> needs external data for it to work, no -subroutine call is possible until this is done. For the standard -debugger, the C<$DB::deep> variable (how many levels of recursion -deep into the debugger you can go before a mandatory break) gives -an example of such a dependency. +subroutine call is possible without it. As an example, the standard +debugger's C<&DB::sub> depends on the C<$DB::deep> variable +(it defines how many levels of recursion deep into the debugger you can go +before a mandatory break). If C<$DB::deep> is not defined, subroutine +calls are not possible, even though C<&DB::sub> exists. =head2 Writing Your Own Debugger -The minimal working debugger consists of one line +=head3 Environment Variables + +The C 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 {} -which you could even fit into the C environment -variable: +It can easily be defined like this: $ PERL5DB="sub DB::DB {}" perl -d your-script -although it doesn't do anything that tells you it's working... -Another brief debugger, slightly more useful, could be created +Another brief debugger, slightly more useful, can be created with only the line: sub DB::DB {print ++$i; scalar } -This debugger would print the sequential number of encountered -statement, and would wait for you to hit a newline before continuing. +This debugger prints a number which increments for each statement +encountered and waits for you to hit a newline before continuing +to the next statement. -The following debugger is quite functional: +The following debugger is actually useful: { package DB; @@ -133,39 +139,79 @@ The following debugger is quite functional: sub sub {print ++$i, " $sub\n"; &$sub} } -It prints the sequential number of subroutine call and the name of the -called subroutine. Note that C<&DB::sub> should be compiled into the -package C. +It prints the sequence number of each subroutine call and the name of the +called subroutine. Note that C<&DB::sub> is being compiled into the +package C through the use of the C directive. -At the start, the debugger reads your rc file (F<./.perldb> or -F<~/.perldb> under Unix), which can set important options. This file may -define a subroutine C<&afterinit> to be executed after the debugger is -initialized. +When it starts, the debugger reads your rc file (F<./.perldb> or +F<~/.perldb> under Unix), which can set important options. +(A subroutine (C<&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 parses this as the remainder of a C -line as one might enter at the debugger prompt. +environment variable and uses it to set debugger options. The +contents of this variable are treated as if they were the argument +of an C debugger command (q.v. in L). + +=head3 Debugger internal variables +In addition to the file and subroutine-related variables mentioned above, +the debugger also maintains various magical internal variables. -The debugger also maintains magical internal variables, such as -C<@DB::dbline>, C<%DB::dbline>, which are aliases for -C<@{"::_ C<%{"::_. Here C -is the currently selected file, either explicitly chosen with the +=over 4 + +=item * + +C<@DB::dbline> is an alias for C<@{"::_, which +holds the lines of the currently-selected file (compiled by Perl), either +explicitly chosen with the debugger's C 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. + +=item * + +C<%DB::dbline>, is an alias for C<%{"::_, which +contains breakpoints and actions keyed by line number in +the currently-selected file, either explicitly chosen with the debugger's C command, or implicitly by flow of execution. -Some functions are provided to simplify customization. See -L for description of options parsed by -C. The function C skips the specified number of frames and returns a list -containing information about the calling frames (all of them, if -C is missing). Each entry is reference to a hash with -keys C (either C<.>, C<$>, or C<@>), C (subroutine +As previously noted, individual entries (as opposed to the whole hash) +are settable. Perl only cares about Boolean true here, although +the values used by F have the form +C<"$break_condition\0$action">. + +=back + +=head3 Debugger customization functions + +Some functions are provided to simplify customization. + +=over 4 + +=item * + +See L for description of options parsed by +C parses debugger options; see +L for a description of options recognized. + +=item * + +C skips the specified number of frames +and returns a list containing information about the calling frames (all +of them, if C is missing). Each entry is reference to a hash +with keys C (either C<.>, C<$>, or C<@>), C (subroutine name, or info about C), C (C or a reference to an array), C, and C. -The function C prints +=item * + +C prints formatted info about caller frames. The last two functions may be convenient as arguments to C<< < >>, C<< << >> commands. +=back + Note that any variables and functions that are not documented in this manpages (or in L) are considered for internal use only, and as such are subject to change without notice. @@ -194,9 +240,9 @@ information. For example, contrast this expression trace: main::bar((eval 170):2): 42 -with this one, once the Cption C has been set: +with this one, once the Cption C has been set: - DB<4> O f=2 + DB<4> o f=2 frame = '2' DB<5> t print foo() * bar() 3: foo() * bar() @@ -703,12 +749,21 @@ eightfold increase. This means that the compiled form of reasonable about eight times more space in memory than the code took on disk. -There are two Perl-specific ways to analyze memory usage: -$ENV{PERL_DEBUG_MSTATS} and B<-DL> command-line switch. The first -is available only if Perl is compiled with Perl's malloc(); the -second only if Perl was built with C<-DDEBUGGING>. See the -instructions for how to do this in the F podpage at -the top level of the Perl source tree. +The B<-DL> command-line switch is obsolete since circa Perl 5.6.0 +(it was available only if Perl was built with C<-DDEBUGGING>). +The switch was used to track Perl's memory allocations and possible +memory leaks. These days the use of malloc debugging tools like +F or F is suggested instead. See also +L. + +One way to find out how much memory is being used by Perl data +structures is to install the Devel::Size module from CPAN: it gives +you the minimum number of bytes required to store a particular data +structure. Please be mindful of the difference between the size() +and total_size(). + +If Perl has been compiled using Perl's malloc you can analyze Perl +memory usage by setting the $ENV{PERL_DEBUG_MSTATS}. =head2 Using C<$ENV{PERL_DEBUG_MSTATS}> @@ -813,154 +868,6 @@ never touched. =back -=head2 Example of using B<-DL> switch - -Below we show how to analyse memory usage by - - do 'lib/auto/POSIX/autosplit.ix'; - -The file in question contains a header and 146 lines similar to - - sub getcwd; - -B: The discussion below supposes 32-bit architecture. In -newer releases of Perl, memory usage of the constructs discussed -here is greatly improved, but the story discussed below is a real-life -story. This story is mercilessly terse, and assumes rather more than cursory -knowledge of Perl internals. Type space to continue, `q' to quit. -(Actually, you just want to skip to the next section.) - -Here is the itemized list of Perl allocations performed during parsing -of this file: - - !!! "after" at test.pl line 3. - Id subtot 4 8 12 16 20 24 28 32 36 40 48 56 64 72 80 80+ - 0 02 13752 . . . . 294 . . . . . . . . . . 4 - 0 54 5545 . . 8 124 16 . . . 1 1 . . . . . 3 - 5 05 32 . . . . . . . 1 . . . . . . . . - 6 02 7152 . . . . . . . . . . 149 . . . . . - 7 02 3600 . . . . . 150 . . . . . . . . . . - 7 03 64 . -1 . 1 . . 2 . . . . . . . . . - 7 04 7056 . . . . . . . . . . . . . . . 7 - 7 17 38404 . . . . . . . 1 . . 442 149 . . 147 . - 9 03 2078 17 249 32 . . . . 2 . . . . . . . . - - -To see this list, insert two C statements around the call: - - warn('!'); - do 'lib/auto/POSIX/autosplit.ix'; - warn('!!! "after"'); - -and run it with Perl's B<-DL> option. The first warn() will print -memory allocation info before parsing the file and will memorize -the statistics at this point (we ignore what it prints). The second -warn() prints increments with respect to these memorized data. This -is the printout shown above. - -Different Is on the left correspond to different subsystems of -the perl interpreter. They are just the first argument given to -the perl memory allocation API named New(). To find what C<9 03> -means, just B the perl source for C<903>. You'll find it in -F, function savepvn(). (I know, you wonder why we told you -to B and then gave away the answer. That's because grepping -the source is good for the soul.) This function is used to store -a copy of an existing chunk of memory. Using a C debugger, one can -see that the function was called either directly from gv_init() or -via sv_magic(), and that gv_init() is called from gv_fetchpv()--which -was itself called from newSUB(). Please stop to catch your breath now. - -B: To reach this point in the debugger and skip the calls to -savepvn() during the compilation of the main program, you should -set a C breakpoint -in Perl_warn(), continue until this point is reached, and I set -a C breakpoint in Perl_savepvn(). Note that you may need to skip a -handful of Perl_savepvn() calls that do not correspond to mass production -of CVs (there are more C<903> allocations than 146 similar lines of -F). Note also that C prefixes are -added by macroization code in perl header files to avoid conflicts -with external libraries. - -Anyway, we see that C<903> ids correspond to creation of globs, twice -per glob - for glob name, and glob stringification magic. - -Here are explanations for other Is above: - -=over 4 - -=item C<717> - -Creates bigger C structures. In the case above, it -creates 3 Cs per subroutine, one for a list of lexical variable -names, one for a scratchpad (which contains lexical variables and -C), and one for the array of scratchpads needed for -recursion. - -It also creates a C and a C per subroutine, all called from -start_subparse(). - -=item C<002> - -Creates a C array corresponding to the C of scratchpads and the -scratchpad itself. The first fake entry of this scratchpad is -created though the subroutine itself is not defined yet. - -It also creates C arrays to keep data for the stash. This is one HV, -but it grows; thus, there are 4 big allocations: the big chunks are not -freed, but are kept as additional arenas for C allocations. - -=item C<054> - -Creates a C for the name of the glob for the subroutine. This -name is a key in a I. - -Big allocations with this I correspond to allocations of new -arenas to keep C. - -=item C<602> - -Creates a C for the glob for the subroutine. - -=item C<702> - -Creates the C for the glob for the subroutine. - -=item C<704> - -Creates I which keep SVs. - -=back - -=head2 B<-DL> details - -If Perl is run with B<-DL> option, then warn()s that start with `!' -behave specially. They print a list of I of memory -allocations, and statistics of allocations of different sizes for -these categories. - -If warn() string starts with - -=over 4 - -=item C - -print changed categories only, print the differences in counts of allocations. - -=item C - -print grown categories only; print the absolute values of counts, and totals. - -=item C - -print nonempty categories, print the absolute values of counts and totals. - -=back - -=head2 Limitations of B<-DL> statistics - -If an extension or external library does not use the Perl API to -allocate memory, such allocations are not counted. - =head1 SEE ALSO L,