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Manpages PERLDEBGUTSSection: Perl Programmers Reference Guide (1)Updated: 2005-03-28 Index Return to Main Contents NAMEperldebguts - Guts of Perl debuggingDESCRIPTIONThis 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.Debugger InternalsPerl 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 podpage 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. The general mechanisms is enabled by calling Perl with the -d switch, the following additional features are enabled (cf. ``$^P'' in perlvar):
Note that if &DB::sub needs external data for it to work, no subroutine call is possible until this is done. For the standard debugger, the $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. Writing Your Own DebuggerThe minimal working debugger consists of one line
sub DB::DB {}which is quite handy as contents of "PERL5DB" environment variable:
$ PERL5DB="sub DB::DB {}" perl -d your-scriptAnother brief debugger, slightly more useful, could be created with only the line:
sub DB::DB {print ++$i; scalar <STDIN>}This debugger would print the sequential number of encountered statement, and would wait for you to hit a newline before continuing. The following debugger is quite functional:
{ package DB; sub DB {} sub sub {print ++$i, " $sub\n"; &$sub} }It prints the sequential number of subroutine call and the name of the called subroutine. Note that &DB::sub should be compiled into the package "DB". At the start, the debugger reads your rc file (./.perldb or ~/.perldb under Unix), which can set important options. This file may define a subroutine &afterinit to be executed after the debugger is initialized. After the rc file is read, the debugger reads the PERLDB_OPTS environment variable and parses this as the remainder of a "O ..." line as one might enter at the debugger prompt. The debugger also maintains magical internal variables, such as @DB::dbline, %DB::dbline, which are aliases for "@{"::_<current_file"}" "%{"::_<current_file"}". Here "current_file" is the currently selected file, either explicitly chosen with the debugger's "f" command, or implicitly by flow of execution. Some functions are provided to simplify customization. See ``Options'' in perldebug for description of options parsed by "DB::parse_options(string)". The function "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". The function "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. Frame Listing Output ExamplesThe "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 "O"ption "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 use various values of "n" are shown to give you a feel for the difference between settings. Long those it may be, this is not a complete listing, but only excerpts.
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. Debugging regular expressionsThere 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. Otherwise, one can "use re 'debug'", which has effects at compile time and run time. It is not lexically scoped. Compile-time outputThe debugging output at compile time looks like this:
compiling RE `[bc]d(ef*g)+h[ij]k$' size 43 first at 1 1: ANYOF(11) 11: EXACT <d>(13) 13: CURLYX {1,32767}(27) 15: OPEN1(17) 17: EXACT <e>(19) 19: STAR(22) 20: EXACT <f>(0) 22: EXACT <g>(24) 24: CLOSE1(26) 26: WHILEM(0) 27: NOTHING(28) 28: EXACT <h>(30) 30: ANYOF(40) 40: EXACT <k>(42) 42: EOL(43) 43: END(0) anchored `de' at 1 floating `gh' at 3..2147483647 (checking floating) stclass `ANYOF' minlen 7The first line shows the pre-compiled form of the regex. The second shows the size of the compiled form (in arbitrary units, usually 4-byte words) and the label id of the first node that does a match. The last line (split into two lines above) contains optimizer information. In the example shown, the optimizer found that the match should contain a substring "de" at offset 1, plus substring "gh" at some offset between 3 and infinity. Moreover, when checking for these substrings (to abandon impossible matches quickly), Perl will check for the substring "gh" before checking for the substring "de". The optimizer may also use the knowledge that the match starts (at the "first" id) with a character class, and the match cannot be shorter than 7 chars. The fields of interest which may appear in the last line are
If a substring is known to match at end-of-line only, it may be followed by "$", as in "floating `k'$". The optimizer-specific info is used to avoid entering (a slow) regex engine on strings that will not definitely match. If "isall" flag is set, a call to the regex engine may be avoided even when the optimizer found an appropriate place for the match. The rest of the output contains the list of nodes of the compiled form of the regex. Each line has format " "id: TYPE OPTIONAL-INFO (next-id) Types of nodesHere are the possible types, with short descriptions:
# TYPE arg-description [num-args] [longjump-len] DESCRIPTION # Exit points END no End of program. SUCCEED no Return from a subroutine, basically. # Anchors: BOL no Match "" at beginning of line. MBOL no Same, assuming multiline. SBOL no Same, assuming singleline. EOS no Match "" at end of string. EOL no Match "" at end of line. MEOL no Same, assuming multiline. SEOL no Same, assuming singleline. BOUND no Match "" at any word boundary BOUNDL no Match "" at any word boundary NBOUND no Match "" at any word non-boundary NBOUNDL no Match "" at any word non-boundary GPOS no Matches where last m//g left off. # [Special] alternatives ANY no Match any one character (except newline). SANY no Match any one character. ANYOF sv Match character in (or not in) this class. ALNUM no Match any alphanumeric character ALNUML no Match any alphanumeric char in locale NALNUM no Match any non-alphanumeric character NALNUML no Match any non-alphanumeric char in locale SPACE no Match any whitespace character SPACEL no Match any whitespace char in locale NSPACE no Match any non-whitespace character NSPACEL no Match any non-whitespace char in locale DIGIT no Match any numeric character NDIGIT no Match any non-numeric character # BRANCH The set of branches constituting a single choice are hooked # together with their "next" pointers, since precedence prevents # anything being concatenated to any individual branch. The # "next" pointer of the last BRANCH in a choice points to the # thing following the whole choice. This is also where the # final "next" pointer of each individual branch points; each # branch starts with the operand node of a BRANCH node. # BRANCH node Match this alternative, or the next... # BACK Normal "next" pointers all implicitly point forward; BACK # exists to make loop structures possible. # not used BACK no Match "", "next" ptr points backward. # Literals EXACT sv Match this string (preceded by length). EXACTF sv Match this string, folded (prec. by length). EXACTFL sv Match this string, folded in locale (w/len). # Do nothing NOTHING no Match empty string. # A variant of above which delimits a group, thus stops optimizations TAIL no Match empty string. Can jump here from outside. # STAR,PLUS '?', and complex '*' and '+', are implemented as circular # BRANCH structures using BACK. Simple cases (one character # per match) are implemented with STAR and PLUS for speed # and to minimize recursive plunges. # STAR node Match this (simple) thing 0 or more times. PLUS node Match this (simple) thing 1 or more times. CURLY sv 2 Match this simple thing {n,m} times. CURLYN no 2 Match next-after-this simple thing # {n,m} times, set parens. CURLYM no 2 Match this medium-complex thing {n,m} times. CURLYX sv 2 Match this complex thing {n,m} times. # This terminator creates a loop structure for CURLYX WHILEM no Do curly processing and see if rest matches. # OPEN,CLOSE,GROUPP ...are numbered at compile time. OPEN num 1 Mark this point in input as start of #n. CLOSE num 1 Analogous to OPEN. REF num 1 Match some already matched string REFF num 1 Match already matched string, folded REFFL num 1 Match already matched string, folded in loc. # grouping assertions IFMATCH off 1 2 Succeeds if the following matches. UNLESSM off 1 2 Fails if the following matches. SUSPEND off 1 1 "Independent" sub-regex. IFTHEN off 1 1 Switch, should be preceded by switcher . GROUPP num 1 Whether the group matched. # Support for long regex LONGJMP off 1 1 Jump far away. BRANCHJ off 1 1 BRANCH with long offset. # The heavy worker EVAL evl 1 Execute some Perl code. # Modifiers MINMOD no Next operator is not greedy. LOGICAL no Next opcode should set the flag only. # This is not used yet RENUM off 1 1 Group with independently numbered parens. # This is not really a node, but an optimized away piece of a "long" node. # To simplify debugging output, we mark it as if it were a node OPTIMIZED off Placeholder for dump. Run-time outputFirst of all, when doing a match, one may get no run-time output even if debugging is enabled. This means that the regex engine was never entered and that all of the job was therefore done by the optimizer.If the regex engine was entered, the output may look like this:
Matching `[bc]d(ef*g)+h[ij]k$' against `abcdefg__gh__' Setting an EVAL scope, savestack=3 2 <ab> <cdefg__gh_> | 1: ANYOF 3 <abc> <defg__gh_> | 11: EXACT <d> 4 <abcd> <efg__gh_> | 13: CURLYX {1,32767} 4 <abcd> <efg__gh_> | 26: WHILEM 0 out of 1..32767 cc=effff31c 4 <abcd> <efg__gh_> | 15: OPEN1 4 <abcd> <efg__gh_> | 17: EXACT <e> 5 <abcde> <fg__gh_> | 19: STAR EXACT <f> can match 1 times out of 32767... Setting an EVAL scope, savestack=3 6 <bcdef> <g__gh__> | 22: EXACT <g> 7 <bcdefg> <__gh__> | 24: CLOSE1 7 <bcdefg> <__gh__> | 26: WHILEM 1 out of 1..32767 cc=effff31c Setting an EVAL scope, savestack=12 7 <bcdefg> <__gh__> | 15: OPEN1 7 <bcdefg> <__gh__> | 17: EXACT <e> restoring \1 to 4(4)..7 failed, try continuation... 7 <bcdefg> <__gh__> | 27: NOTHING 7 <bcdefg> <__gh__> | 28: EXACT <h> failed... failed...The most significant information in the output is about the particular node of the compiled regex that is currently being tested against the target string. The format of these lines is " "STRING-OFFSET <PRE-STRING> <POST-STRING> |ID: TYPE The TYPE info is indented with respect to the backtracking level. Other incidental information appears interspersed within. Debugging Perl memory usagePerl is a profligate wastrel when it comes to memory use. There is a saying that to estimate memory usage of Perl, assume a reasonable algorithm for memory allocation, multiply that estimate by 10, and while you still may miss the mark, at least you won't be quite so astonished. This is not absolutely true, but may provide a good grasp of what happens.Assume that an integer cannot take less than 20 bytes of memory, a float cannot take less than 24 bytes, a string cannot take less than 32 bytes (all these examples assume 32-bit architectures, the result are quite a bit worse on 64-bit architectures). If a variable is accessed in two of three different ways (which require an integer, a float, or a string), the memory footprint may increase yet another 20 bytes. A sloppy malloc(3) implementation can inflate these numbers dramatically. On the opposite end of the scale, a declaration like
sub foo;may take up to 500 bytes of memory, depending on which release of Perl you're running. Anecdotal estimates of source-to-compiled code bloat suggest an eightfold increase. This means that the compiled form of reasonable (normally commented, properly indented etc.) code will take 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 -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 "-DDEBUGGING". See the instructions for how to do this in the INSTALL podpage at the top level of the Perl source tree. Using $ENV{PERL_DEBUG_MSTATS}If your perl is using Perl's malloc() and was compiled with the necessary switches (this is the default), then it will print memory usage statistics after compiling your code when "$ENV{PERL_DEBUG_MSTATS} > 1", and before termination of the program when "$ENV{PERL_DEBUG_MSTATS} >= 1". The report format is similar to the following example:
$ PERL_DEBUG_MSTATS=2 perl -e "require Carp" Memory allocation statistics after compilation: (buckets 4(4)..8188(8192) 14216 free: 130 117 28 7 9 0 2 2 1 0 0 437 61 36 0 5 60924 used: 125 137 161 55 7 8 6 16 2 0 1 74 109 304 84 20 Total sbrk(): 77824/21:119. Odd ends: pad+heads+chain+tail: 0+636+0+2048. Memory allocation statistics after execution: (buckets 4(4)..8188(8192) 30888 free: 245 78 85 13 6 2 1 3 2 0 1 315 162 39 42 11 175816 used: 265 176 1112 111 26 22 11 27 2 1 1 196 178 1066 798 39 Total sbrk(): 215040/47:145. Odd ends: pad+heads+chain+tail: 0+2192+0+6144.It is possible to ask for such a statistic at arbitrary points in your execution using the mstat() function out of the standard Devel::Peek module. Here is some explanation of that format:
Example of using -DL switchBelow 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;WARNING: 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 "warn('!...')" statements around the call:
warn('!'); do 'lib/auto/POSIX/autosplit.ix'; warn('!!! "after"');and run it with Perl's -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 Ids 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 "9 03" means, just grep the perl source for 903. You'll find it in util.c, function savepvn(). (I know, you wonder why we told you to grep 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. NOTE: 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 then 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 903 allocations than 146 similar lines of lib/auto/POSIX/autosplit.ix). Note also that "Perl_" prefixes are added by macroization code in perl header files to avoid conflicts with external libraries. Anyway, we see that 903 ids correspond to creation of globs, twice per glob - for glob name, and glob stringification magic. Here are explanations for other Ids above:
-DL detailsIf Perl is run with -DL option, then warn()s that start with `!' behave specially. They print a list of categories of memory allocations, and statistics of allocations of different sizes for these categories.If warn() string starts with
Limitations of -DL statisticsIf an extension or external library does not use the Perl API to allocate memory, such allocations are not counted.SEE ALSOperldebug, perlguts, perlrun re, and Devel::Dprof.
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