Writing Your Own Debugger

Environment Variables

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-script

Another 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).

Debugger Internal Variables

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".

Debugger Customization Functions

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.

Compile-time Output

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" POS

"floating" STRING "at" POS1..POS2

See above.

"matching floating/anchored"

Which substring to check first.

"minlen"

The minimal length of the match.

"stclass" TYPE

Type of first matching node.

"noscan"

Don't scan for the found substrings.

"isall"

Means that the optimizer information is all that the regular expression contains, and thus one does not need to enter the regex engine at all.

"GPOS"

Set if the pattern contains "\G".

"plus"

Set if the pattern starts with a repeated char (as in "x+y").

"implicit"

Set if the pattern starts with ".*".

"with eval"

Set if the pattern contain eval-groups, such as "(?{ code })" and "(??{ code })".

"anchored(TYPE)"

If the pattern may match only at a handful of places, with "TYPE" being "SBOL", "MBOL", or "GPOS". See the table below.

If a substring is known to match at end-of-line only, it may be followed by "$", as in "floating 'k'$".

The optimizer-specific information is used to avoid entering (a slow) regex engine on strings that will not definitely match. If the "isall" flag is set, a call to the regex engine may be avoided even when the optimizer found an appropriate place for the match.

Above the optimizer section is the list of nodes of the compiled form of the regex. Each line has format

" "id: TYPE OPTIONAL-INFO (next-id)

Types of Nodes

Here are the current 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.
 # Line Start Anchors:
 SBOL             no         Match "" at beginning of line: /^/, /\A/
 MBOL             no         Same, assuming multiline: /^/m
 # Line End Anchors:
 SEOL             no         Match "" at end of line: /$/
 MEOL             no         Same, assuming multiline: /$/m
 EOS              no         Match "" at end of string: /\z/
 # Match Start Anchors:
 GPOS             no         Matches where last m//g left off.
 # Word Boundary Opcodes:
 BOUND            no         Like BOUNDA for non-utf8, otherwise match
                             "" between any Unicode \w\W or \W\w
 BOUNDL           no         Like BOUND/BOUNDU, but \w and \W are
                             defined by current locale
 BOUNDU           no         Match "" at any boundary of a given type
                             using Unicode rules
 BOUNDA           no         Match "" at any boundary between \w\W or
                             \W\w, where \w is [_a-zA-Z0-9]
 NBOUND           no         Like NBOUNDA for non-utf8, otherwise match
                             "" between any Unicode \w\w or \W\W
 NBOUNDL          no         Like NBOUND/NBOUNDU, but \w and \W are
                             defined by current locale
 NBOUNDU          no         Match "" at any non-boundary of a given
                             type using using Unicode rules
 NBOUNDA          no         Match "" betweeen any \w\w or \W\W, where
                             \w is [_a-zA-Z0-9]
 # [Special] alternatives:
 REG_ANY          no         Match any one character (except newline).
 SANY             no         Match any one character.
 ANYOF            sv 1       Match character in (or not in) this class,
                             single char match only
 ANYOFD           sv 1       Like ANYOF, but /d is in effect
 ANYOFL           sv 1       Like ANYOF, but /l is in effect
 ANYOFM           byte 1     Like ANYOF, but matches an invariant byte
                             as determined by the mask and arg
 # POSIX Character Classes:
 POSIXD           none       Some [[:class:]] under /d; the FLAGS field
                             gives which one
 POSIXL           none       Some [[:class:]] under /l; the FLAGS field
                             gives which one
 POSIXU           none       Some [[:class:]] under /u; the FLAGS field
                             gives which one
 POSIXA           none       Some [[:class:]] under /a; the FLAGS field
                             gives which one
 NPOSIXD          none       complement of POSIXD, [[:^class:]]
 NPOSIXL          none       complement of POSIXL, [[:^class:]]
 NPOSIXU          none       complement of POSIXU, [[:^class:]]
 NPOSIXA          none       complement of POSIXA, [[:^class:]]
 ASCII            none       [[:ascii:]]
 NASCII           none       [[:^ascii:]]
 CLUMP            no         Match any extended grapheme cluster
                             sequence
 # Alternation
 # 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...
 # Literals
 EXACT            str        Match this string (preceded by length).
 EXACTL           str        Like EXACT, but /l is in effect (used so
                             locale-related warnings can be checked
                             for).
 EXACTF           str        Match this non-UTF-8 string (not guaranteed
                             to be folded) using /id rules (w/len).
 EXACTFL          str        Match this string (not guaranteed to be
                             folded) using /il rules (w/len).
 EXACTFU          str        Match this string (folded iff in UTF-8,
                             length in folding doesn't change if not in
                             UTF-8) using /iu rules (w/len).
 EXACTFAA         str        Match this string (not guaranteed to be
                             folded) using /iaa rules (w/len).
 EXACTFU_SS       str        Match this string (folded iff in UTF-8,
                             length in folding may change even if not in
                             UTF-8) using /iu rules (w/len).
 EXACTFLU8        str        Rare circumstances: like EXACTFU, but is
                             under /l, UTF-8, folded, and everything in
                             it is above 255.
 EXACTFAA_NO_TRIE str        Match this string (which is not trie-able;
                             not guaranteed to be folded) using /iaa
                             rules (w/len).
 # Do nothing types
 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.
 # Loops
 # STAR,PLUS    '?', and complex '*' and '+', are implemented as
 #               circular BRANCH structures.  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       Capture next-after-this simple thing
 CURLYM           no 2       Capture 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.
 # Buffer related
 # OPEN,CLOSE,GROUPP     ...are numbered at compile time.
 OPEN             num 1      Mark this point in input as start of #n.
 CLOSE            num 1      Close corresponding OPEN of #n.
 SROPEN           none       Same as OPEN, but for script run
 SRCLOSE          none       Close preceding SROPEN
 REF              num 1      Match some already matched string
 REFF             num 1      Match already matched string, folded using
                             native charset rules for non-utf8
 REFFL            num 1      Match already matched string, folded in
                             loc.
 REFFU            num 1      Match already matched string, folded using
                             unicode rules for non-utf8
 REFFA            num 1      Match already matched string, folded using
                             unicode rules for non-utf8, no mixing
                             ASCII, non-ASCII
 # Named references.  Code in regcomp.c assumes that these all are after
 # the numbered references
 NREF             no-sv 1    Match some already matched string
 NREFF            no-sv 1    Match already matched string, folded using
                             native charset rules for non-utf8
 NREFFL           no-sv 1    Match already matched string, folded in
                             loc.
 NREFFU           num 1      Match already matched string, folded using
                             unicode rules for non-utf8
 NREFFA           num 1      Match already matched string, folded using
                             unicode rules for non-utf8, no mixing
                             ASCII, non-ASCII
 # Support for long RE
 LONGJMP          off 1 1    Jump far away.
 BRANCHJ          off 1 1    BRANCH with long offset.
 # Special Case Regops
 IFMATCH          off 1 2    Succeeds if the following matches.
 UNLESSM          off 1 2    Fails if the following matches.
 SUSPEND          off 1 1    "Independent" sub-RE.
 IFTHEN           off 1 1    Switch, should be preceded by switcher.
 GROUPP           num 1      Whether the group matched.
 # The heavy worker
 EVAL             evl/flags  Execute some Perl code.
                  2L
 # 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.
 # Trie Related
 # Behave the same as A|LIST|OF|WORDS would. The '..C' variants
 # have inline charclass data (ascii only), the 'C' store it in the
 # structure.
 TRIE             trie 1     Match many EXACT(F[ALU]?)? at once.
                             flags==type
 TRIEC            trie       Same as TRIE, but with embedded charclass
                  charclass  data
 AHOCORASICK      trie 1     Aho Corasick stclass. flags==type
 AHOCORASICKC     trie       Same as AHOCORASICK, but with embedded
                  charclass  charclass data
 # Regex Subroutines
 GOSUB            num/ofs 2L recurse to paren arg1 at (signed) ofs arg2
 # Special conditionals
 NGROUPP          no-sv 1    Whether the group matched.
 INSUBP           num 1      Whether we are in a specific recurse.
 DEFINEP          none 1     Never execute directly.
 # Backtracking Verbs
 ENDLIKE          none       Used only for the type field of verbs
 OPFAIL           no-sv 1    Same as (?!), but with verb arg
 ACCEPT           no-sv/num  Accepts the current matched string, with
                  2L         verbar
 # Verbs With Arguments
 VERB             no-sv 1    Used only for the type field of verbs
 PRUNE            no-sv 1    Pattern fails at this startpoint if no-
                             backtracking through this
 MARKPOINT        no-sv 1    Push the current location for rollback by
                             cut.
 SKIP             no-sv 1    On failure skip forward (to the mark)
                             before retrying
 COMMIT           no-sv 1    Pattern fails outright if backtracking
                             through this
 CUTGROUP         no-sv 1    On failure go to the next alternation in
                             the group
 # Control what to keep in $&.
 KEEPS            no         $& begins here.
 # New charclass like patterns
 LNBREAK          none       generic newline pattern
 # SPECIAL  REGOPS
 # 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.
 # Special opcode with the property that no opcode in a compiled program
 # will ever be of this type. Thus it can be used as a flag value that
 # no other opcode has been seen. END is used similarly, in that an END
 # node cant be optimized. So END implies "unoptimizable" and PSEUDO
 # mean "not seen anything to optimize yet".
 PSEUDO           off        Pseudo opcode for internal use.

Following the optimizer information is a dump of the offset/length table, here split across several lines:

  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]

The first line here indicates that the offset/length table contains 45 entries. Each entry is a pair of integers, denoted by "offset[length]". Entries are numbered starting with 1, so entry #1 here is "1[4]" and entry #12 is "5[1]". "1[4]" indicates that the node labeled "1:" (the "1: ANYOF[bc]") begins at character position 1 in the pre-compiled form of the regex, and has a length of 4 characters. "5[1]" in position 12 indicates that the node labeled "12:" (the "12: EXACT <d>") begins at character position 5 in the pre-compiled form of the regex, and has a length of 1 character. "12[1]" in position 14 indicates that the node labeled "14:" (the "14: CURLYX[0] {1,32767}") begins at character position 12 in the pre-compiled form of the regex, and has a length of 1 character---that is, it corresponds to the "+" symbol in the precompiled regex.

"0[0]" items indicate that there is no corresponding node.

Run-time Output

First 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.

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:

"buckets SMALLEST(APPROX)..GREATEST(APPROX)"

Perl's malloc() uses bucketed allocations. Every request is rounded up to the closest bucket size available, and a bucket is taken from the pool of buckets of that size.

The line above describes the limits of buckets currently in use. Each bucket has two sizes: memory footprint and the maximal size of user data that can fit into this bucket. Suppose in the above example that the smallest bucket were size 4. The biggest bucket would have usable size 8188, and the memory footprint would be 8192.

In a Perl built for debugging, some buckets may have negative usable size. This means that these buckets cannot (and will not) be used. For larger buckets, the memory footprint may be one page greater than a power of 2. If so, the corresponding power of two is printed in the "APPROX" field above.

Free/Used

The 1 or 2 rows of numbers following that correspond to the number of buckets of each size between "SMALLEST" and "GREATEST". In the first row, the sizes (memory footprints) of buckets are powers of two--or possibly one page greater. In the second row, if present, the memory footprints of the buckets are between the memory footprints of two buckets "above".

For example, suppose under the previous example, the memory footprints were

   free:    8     16    32    64    128  256 512 1024 2048 4096 8192
           4     12    24    48    80
    

With a non-"DEBUGGING" perl, the buckets starting from 128 have a 4-byte overhead, and thus an 8192-long bucket may take up to 8188-byte allocations.

"Total sbrk(): SBRKed/SBRKs:CONTINUOUS"

The first two fields give the total amount of memory perl sbrk(2)ed (ess-broken? :-) and number of sbrk(2)s used. The third number is what perl thinks about continuity of returned chunks. So long as this number is positive, malloc() will assume that it is probable that sbrk(2) will provide continuous memory.

Memory allocated by external libraries is not counted.

"pad: 0"

The amount of sbrk(2)ed memory needed to keep buckets aligned.

"heads: 2192"

Although memory overhead of bigger buckets is kept inside the bucket, for smaller buckets, it is kept in separate areas. This field gives the total size of these areas.

"chain: 0"

malloc() may want to subdivide a bigger bucket into smaller buckets. If only a part of the deceased bucket is left unsubdivided, the rest is kept as an element of a linked list. This field gives the total size of these chunks.

"tail: 6144"

To minimize the number of sbrk(2)s, malloc() asks for more memory. This field gives the size of the yet unused part, which is sbrk(2)ed, but never touched.