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B(3pm)                 Perl Programmers Reference Guide                 B(3pm)




NAME

       B - The Perl Compiler Backend


SYNOPSIS

               use B;


DESCRIPTION

       The "B" module supplies classes which allow a Perl program to delve
       into its own innards.  It is the module used to implement the
       "backends" of the Perl compiler.  Usage of the compiler does not
       require knowledge of this module: see the O module for the user-visible
       part.  The "B" module is of use to those who want to write new compiler
       backends.  This documentation assumes that the reader knows a fair
       amount about perl's internals including such things as SVs, OPs and the
       internal symbol table and syntax tree of a program.


OVERVIEW

       The "B" module contains a set of utility functions for querying the
       current state of the Perl interpreter; typically these functions return
       objects from the B::SV and B::OP classes, or their derived classes.
       These classes in turn define methods for querying the resulting objects
       about their own internal state.


Utility Functions

       The "B" module exports a variety of functions: some are simple utility
       functions, others provide a Perl program with a way to get an initial
       "handle" on an internal object.

   Functions Returning "B::SV", "B::AV", "B::HV", and "B::CV" objects
       For descriptions of the class hierarchy of these objects and the
       methods that can be called on them, see below, "OVERVIEW OF CLASSES"
       and "SV-RELATED CLASSES".

       sv_undef
           Returns the SV object corresponding to the C variable "sv_undef".

       sv_yes
           Returns the SV object corresponding to the C variable "sv_yes".

       sv_no
           Returns the SV object corresponding to the C variable "sv_no".

       svref_2object(SVREF)
           Takes a reference to any Perl value, and turns the referred-to
           value into an object in the appropriate B::OP-derived or
           B::SV-derived class.  Apart from functions such as "main_root",
           this is the primary way to get an initial "handle" on an internal
           perl data structure which can then be followed with the other
           access methods.

           The returned object will only be valid as long as the underlying
           OPs and SVs continue to exist.  Do not attempt to use the object
           after the underlying structures are freed.

       amagic_generation
           Returns the SV object corresponding to the C variable
           "amagic_generation".  As of Perl 5.18, this is just an alias to
           "PL_na", so its value is meaningless.

       init_av
           Returns the AV object (i.e. in class B::AV) representing INIT
           blocks.

       check_av
           Returns the AV object (i.e. in class B::AV) representing CHECK
           blocks.

       unitcheck_av
           Returns the AV object (i.e. in class B::AV) representing UNITCHECK
           blocks.

       begin_av
           Returns the AV object (i.e. in class B::AV) representing BEGIN
           blocks.

       end_av
           Returns the AV object (i.e. in class B::AV) representing END
           blocks.

       comppadlist
           Returns the PADLIST object (i.e. in class B::PADLIST) of the global
           comppadlist.  In Perl 5.16 and earlier it returns an AV object
           (class B::AV).

       regex_padav
           Only when perl was compiled with ithreads.

       main_cv
           Return the (faked) CV corresponding to the main part of the Perl
           program.

   Functions for Examining the Symbol Table
       walksymtable(SYMREF, METHOD, RECURSE, PREFIX)
           Walk the symbol table starting at SYMREF and call METHOD on each
           symbol (a B::GV object) visited.  When the walk reaches package
           symbols (such as "Foo::") it invokes RECURSE, passing in the symbol
           name, and only recurses into the package if that sub returns true.

           PREFIX is the name of the SYMREF you're walking.

           For example:

             # Walk CGI's symbol table calling print_subs on each symbol.
             # Recurse only into CGI::Util::
             walksymtable(\%CGI::, 'print_subs',
                          sub { $_[0] eq 'CGI::Util::' }, 'CGI::');

           print_subs() is a B::GV method you have declared.  Also see "B::GV
           Methods", below.

   Functions Returning "B::OP" objects or for walking op trees
       For descriptions of the class hierarchy of these objects and the
       methods that can be called on them, see below, "OVERVIEW OF CLASSES"
       and "OP-RELATED CLASSES".

       main_root
           Returns the root op (i.e. an object in the appropriate
           B::OP-derived class) of the main part of the Perl program.

       main_start
           Returns the starting op of the main part of the Perl program.

       walkoptree(OP, METHOD)
           Does a tree-walk of the syntax tree based at OP and calls METHOD on
           each op it visits.  Each node is visited before its children.  If
           "walkoptree_debug" (see below) has been called to turn debugging on
           then the method "walkoptree_debug" is called on each op before
           METHOD is called.

       walkoptree_debug(DEBUG)
           Returns the current debugging flag for "walkoptree".  If the
           optional DEBUG argument is non-zero, it sets the debugging flag to
           that.  See the description of "walkoptree" above for what the
           debugging flag does.

   Miscellaneous Utility Functions
       ppname(OPNUM)
           Return the PP function name (e.g. "pp_add") of op number OPNUM.

       hash(STR)
           Returns a string in the form "0x..." representing the value of the
           internal hash function used by perl on string STR.

       cast_I32(I)
           Casts I to the internal I32 type used by that perl.

       minus_c
           Does the equivalent of the "-c" command-line option.  Obviously,
           this is only useful in a BEGIN block or else the flag is set too
           late.

       cstring(STR)
           Returns a double-quote-surrounded escaped version of STR which can
           be used as a string in C source code.

       perlstring(STR)
           Returns a double-quote-surrounded escaped version of STR which can
           be used as a string in Perl source code.

       safename(STR)
           This function returns the string with the first character modified
           if it is a control character.  It converts it to ^X format first,
           so that "\cG" becomes "^G".  This is used internally by
           B::GV::SAFENAME, but you can call it directly.

       class(OBJ)
           Returns the class of an object without the part of the classname
           preceding the first "::".  This is used to turn "B::UNOP" into
           "UNOP" for example.

       threadsv_names
           This used to provide support for the old 5.005 threading module. It
           now does nothing.

   Exported utility variables
       @optype
             my $op_type = $optype[$op_type_num];

           A simple mapping of the op type number to its type (like 'COP' or
           'BINOP').

       @specialsv_name
             my $sv_name = $specialsv_name[$sv_index];

           Certain SV types are considered 'special'.  They're represented by
           B::SPECIAL and are referred to by a number from the specialsv_list.
           This array maps that number back to the name of the SV (like
           'Nullsv' or '&PL_sv_undef').


OVERVIEW OF CLASSES

       The C structures used by Perl's internals to hold SV and OP information
       (PVIV, AV, HV, ..., OP, SVOP, UNOP, ...) are modelled on a class
       hierarchy and the "B" module gives access to them via a true object
       hierarchy.  Structure fields which point to other objects (whether
       types of SV or types of OP) are represented by the "B" module as Perl
       objects of the appropriate class.

       The bulk of the "B" module is the methods for accessing fields of these
       structures.

       Note that all access is read-only.  You cannot modify the internals by
       using this module.  Also, note that the B::OP and B::SV objects created
       by this module are only valid for as long as the underlying objects
       exist; their creation doesn't increase the reference counts of the
       underlying objects.  Trying to access the fields of a freed object will
       give incomprehensible results, or worse.

   SV-RELATED CLASSES
       B::IV, B::NV, B::RV, B::PV, B::PVIV, B::PVNV, B::PVMG, B::BM (5.9.5 and
       earlier), B::PVLV, B::AV, B::HV, B::CV, B::GV, B::FM, B::IO.  These
       classes correspond in the obvious way to the underlying C structures of
       similar names.  The inheritance hierarchy mimics the underlying C
       "inheritance".  For the 5.10.x branch, (ie 5.10.0, 5.10.1 etc) this is:

                                  B::SV
                                    |
                       +------------+------------+------------+
                       |            |            |            |
                     B::PV        B::IV        B::NV        B::RV
                         \         /           /
                          \       /           /
                           B::PVIV           /
                                \           /
                                 \         /
                                  \       /
                                   B::PVNV
                                      |
                                      |
                                   B::PVMG
                                      |
                          +-----+-----+-----+-----+
                          |     |     |     |     |
                        B::AV B::GV B::HV B::CV B::IO
                                |           |
                                |           |
                             B::PVLV      B::FM

       For 5.9.0 and earlier, PVLV is a direct subclass of PVMG, and BM is
       still present as a distinct type, so the base of this diagram is

                                      |
                                      |
                                   B::PVMG
                                      |
                   +------+-----+-----+-----+-----+-----+
                   |      |     |     |     |     |     |
                B::PVLV B::BM B::AV B::GV B::HV B::CV B::IO
                                                  |
                                                  |
                                                B::FM

       For 5.11.0 and later, B::RV is abolished, and IVs can be used to store
       references, and a new type B::REGEXP is introduced, giving this
       structure:

                                  B::SV
                                    |
                       +------------+------------+
                       |            |            |
                     B::PV        B::IV        B::NV
                         \         /           /
                          \       /           /
                           B::PVIV           /
                                \           /
                                 \         /
                                  \       /
                                   B::PVNV
                                      |
                                      |
                                   B::PVMG
                                      |
                  +-------+-------+---+---+-------+-------+
                  |       |       |       |       |       |
                B::AV   B::GV   B::HV   B::CV   B::IO B::REGEXP
                          |               |
                          |               |
                       B::PVLV          B::FM

       Access methods correspond to the underlying C macros for field access,
       usually with the leading "class indication" prefix removed (Sv, Av, Hv,
       ...).  The leading prefix is only left in cases where its removal would
       cause a clash in method name.  For example, "GvREFCNT" stays as-is
       since its abbreviation would clash with the "superclass" method
       "REFCNT" (corresponding to the C function "SvREFCNT").

   B::SV Methods
       REFCNT
       FLAGS
       object_2svref
           Returns a reference to the regular scalar corresponding to this
           B::SV object.  In other words, this method is the inverse operation
           to the svref_2object() subroutine.  This scalar and other data it
           points at should be considered read-only: modifying them is neither
           safe nor guaranteed to have a sensible effect.

   B::IV Methods
       IV  Returns the value of the IV, interpreted as a signed integer.  This
           will be misleading if "FLAGS & SVf_IVisUV".  Perhaps you want the
           "int_value" method instead?

       IVX
       UVX
       int_value
           This method returns the value of the IV as an integer.  It differs
           from "IV" in that it returns the correct value regardless of
           whether it's stored signed or unsigned.

       needs64bits
       packiv

   B::NV Methods
       NV
       NVX
       COP_SEQ_RANGE_LOW
       COP_SEQ_RANGE_HIGH
           These last two are only valid for pad name SVs.  They only existed
           in the B::NV class before Perl 5.22.  In 5.22 they were moved to
           the B::PADNAME class.

   B::RV Methods
       RV

   B::PV Methods
       PV  This method is the one you usually want.  It constructs a string
           using the length and offset information in the struct: for ordinary
           scalars it will return the string that you'd see from Perl, even if
           it contains null characters.

       RV  Same as B::RV::RV, except that it will die() if the PV isn't a
           reference.

       PVX This method is less often useful.  It assumes that the string
           stored in the struct is null-terminated, and disregards the length
           information.

           It is the appropriate method to use if you need to get the name of
           a lexical variable from a padname array.  Lexical variable names
           are always stored with a null terminator, and the length field
           (CUR) is overloaded for other purposes and can't be relied on here.

       CUR This method returns the internal length field, which consists of
           the number of internal bytes, not necessarily the number of logical
           characters.

       LEN This method returns the number of bytes allocated (via malloc) for
           storing the string.  This is 0 if the scalar does not "own" the
           string.

   B::PVMG Methods
       MAGIC
       SvSTASH

   B::MAGIC Methods
       MOREMAGIC
       precomp
           Only valid on r-magic, returns the string that generated the
           regexp.

       PRIVATE
       TYPE
       FLAGS
       OBJ Will die() if called on r-magic.

       PTR
       REGEX
           Only valid on r-magic, returns the integer value of the REGEX
           stored in the MAGIC.

   B::PVLV Methods
       TARGOFF
       TARGLEN
       TYPE
       TARG

   B::BM Methods
       USEFUL
       PREVIOUS
       RARE
       TABLE

   B::REGEXP Methods
       REGEX
       precomp
       qr_anoncv
       compflags
           The last two were added in Perl 5.22.

   B::GV Methods
       is_empty
           This method returns TRUE if the GP field of the GV is NULL.

       NAME
       SAFENAME
           This method returns the name of the glob, but if the first
           character of the name is a control character, then it converts it
           to ^X first, so that *^G would return "^G" rather than "\cG".

           It's useful if you want to print out the name of a variable.  If
           you restrict yourself to globs which exist at compile-time then the
           result ought to be unambiguous, because code like "${"^G"} = 1" is
           compiled as two ops - a constant string and a dereference (rv2gv) -
           so that the glob is created at runtime.

           If you're working with globs at runtime, and need to disambiguate
           *^G from *{"^G"}, then you should use the raw NAME method.

       STASH
       SV
       IO
       FORM
       AV
       HV
       EGV
       CV
       CVGEN
       LINE
       FILE
       FILEGV
       GvREFCNT
       FLAGS
       GPFLAGS
           This last one is present only in perl 5.22.0 and higher.

   B::IO Methods
       B::IO objects derive from IO objects and you will get more information
       from the IO object itself.

       For example:

         $gvio = B::svref_2object(\*main::stdin)->IO;
         $IO = $gvio->object_2svref();
         $fd = $IO->fileno();

       LINES
       PAGE
       PAGE_LEN
       LINES_LEFT
       TOP_NAME
       TOP_GV
       FMT_NAME
       FMT_GV
       BOTTOM_NAME
       BOTTOM_GV
       SUBPROCESS
       IoTYPE
           A character symbolizing the type of IO Handle.

             -     STDIN/OUT
             I     STDIN/OUT/ERR
             <     read-only
             >     write-only
             a     append
             +     read and write
             s     socket
             |     pipe
             I     IMPLICIT
             #     NUMERIC
             space closed handle
             \0    closed internal handle

       IoFLAGS
       IsSTD
           Takes one argument ( 'stdin' | 'stdout' | 'stderr' ) and returns
           true if the IoIFP of the object is equal to the handle whose name
           was passed as argument; i.e., $io->IsSTD('stderr') is true if
           IoIFP($io) == PerlIO_stderr().

   B::AV Methods
       FILL
       MAX
       ARRAY
       ARRAYelt
           Like "ARRAY", but takes an index as an argument to get only one
           element, rather than a list of all of them.

       OFF This method is deprecated if running under Perl 5.8, and is no
           longer present if running under Perl 5.9

       AvFLAGS
           This method returns the AV specific flags.  In Perl 5.9 these are
           now stored in with the main SV flags, so this method is no longer
           present.

   B::CV Methods
       STASH
       START
       ROOT
       GV
       FILE
       DEPTH
       PADLIST
           Returns a B::PADLIST object under Perl 5.18 or higher, or a B::AV
           in earlier versions.

       OUTSIDE
       OUTSIDE_SEQ
       XSUB
       XSUBANY
           For constant subroutines, returns the constant SV returned by the
           subroutine.

       CvFLAGS
       const_sv
       NAME_HEK
           Returns the name of a lexical sub, otherwise "undef".

   B::HV Methods
       FILL
       MAX
       KEYS
       RITER
       NAME
       ARRAY
       PMROOT
           This method is not present if running under Perl 5.9, as the PMROOT
           information is no longer stored directly in the hash.

   OP-RELATED CLASSES
       "B::OP", "B::UNOP", "B::UNOP_AUX", "B::BINOP", "B::LOGOP", "B::LISTOP",
       "B::PMOP", "B::SVOP", "B::PADOP", "B::PVOP", "B::LOOP", "B::COP",
       "B::METHOP".

       These classes correspond in the obvious way to the underlying C
       structures of similar names.  The inheritance hierarchy mimics the
       underlying C "inheritance":

                                        B::OP
                                          |
                          +----------+---------+--------+-------+---------+
                          |          |         |        |       |         |
                       B::UNOP    B::SVOP  B::PADOP  B::COP  B::PVOP  B::METHOP
                          |
                      +---+---+---------+
                      |       |         |
                  B::BINOP  B::LOGOP  B::UNOP_AUX
                      |
                      |
                  B::LISTOP
                      |
                  +---+---+
                  |       |
               B::LOOP   B::PMOP

       Access methods correspond to the underlying C structure field names,
       with the leading "class indication" prefix ("op_") removed.

   B::OP Methods
       These methods get the values of similarly named fields within the OP
       data structure.  See top of "op.h" for more info.

       next
       sibling
       parent
           Returns the OP's parent. If it has no parent, or if your perl
           wasn't built with "-DPERL_OP_PARENT", returns NULL.

           Note that the global variable $B::OP::does_parent is undefined on
           older perls that don't support the "parent" method, is defined but
           false on perls that support the method but were built without
           "-DPERL_OP_PARENT", and is true otherwise.

       name
           This returns the op name as a string (e.g. "add", "rv2av").

       ppaddr
           This returns the function name as a string (e.g.
           "PL_ppaddr[OP_ADD]", "PL_ppaddr[OP_RV2AV]").

       desc
           This returns the op description from the global C PL_op_desc array
           (e.g. "addition" "array deref").

       targ
       type
       opt
       flags
       private
       spare

   B::UNOP Method
       first

   B::UNOP_AUX Methods (since 5.22)
       aux_list(cv)
           This returns a list of the elements of the op's aux data structure,
           or a null list if there is no aux. What will be returned depends on
           the object's type, but will typically be a collection of "B::IV",
           "B::GV", etc. objects. "cv" is the "B::CV" object representing the
           sub that the op is contained within.

       string(cv)
           This returns a textual representation of the object (likely to b
           useful for deparsing and debugging), or an empty string if the op
           type doesn't support this. "cv" is the "B::CV" object representing
           the sub that the op is contained within.

   B::BINOP Method
       last

   B::LOGOP Method
       other

   B::LISTOP Method
       children

   B::PMOP Methods
       pmreplroot
       pmreplstart
       pmnext
           Only up to Perl 5.9.4

       pmflags
       extflags
           Since Perl 5.9.5

       precomp
       pmoffset
           Only when perl was compiled with ithreads.

       code_list
           Since perl 5.17.1

       pmregexp
           Added in perl 5.22, this method returns the B::REGEXP associated
           with the op.  While PMOPs do not actually have "pmregexp" fields
           under threaded builds, this method returns the regexp under threads
           nonetheless, for convenience.

   B::SVOP Methods
       sv
       gv

   B::PADOP Method
       padix

   B::PVOP Method
       pv

   B::LOOP Methods
       redoop
       nextop
       lastop

   B::COP Methods
       The "B::COP" class is used for "nextstate" and "dbstate" ops.  As of
       Perl 5.22, it is also used for "null" ops that started out as COPs.

       label
       stash
       stashpv
       stashoff (threaded only)
       file
       cop_seq
       arybase
       line
       warnings
       io
       hints
       hints_hash

   B::METHOP Methods (Since Perl 5.22)
       first
       meth_sv

   PAD-RELATED CLASSES
       Perl 5.18 introduced a new class, B::PADLIST, returned by B::CV's
       "PADLIST" method.

       Perl 5.22 introduced the B::PADNAMELIST and B::PADNAME classes.

   B::PADLIST Methods
       MAX
       ARRAY
           A list of pads.  The first one contains the names.

           The first one is a B::PADNAMELIST under Perl 5.22, and a B::AV
           under earlier versions.  The rest are currently B::AV objects, but
           that could change in future versions.

       ARRAYelt
           Like "ARRAY", but takes an index as an argument to get only one
           element, rather than a list of all of them.

       NAMES
           This method, introduced in 5.22, returns the B::PADNAMELIST.  It is
           equivalent to "ARRAYelt" with a 0 argument.

       REFCNT
       id  This method, introduced in 5.22, returns an ID shared by clones of
           the same padlist.

       outid
           This method, also added in 5.22, returns the ID of the outer
           padlist.

   B::PADNAMELIST Methods
       MAX
       ARRAY
       ARRAYelt
           These two methods return the pad names, using B::SPECIAL objects
           for null pointers and B::PADNAME objects otherwise.

       REFCNT

   B::PADNAME Methods
       PV
       PVX
       LEN
       REFCNT
       FLAGS
           For backward-compatibility, if the PADNAMEt_OUTER flag is set, the
           FLAGS method adds the SVf_FAKE flag, too.

       TYPE
           A B::HV object representing the stash for a typed lexical.

       SvSTASH
           A backward-compatibility alias for TYPE.

       OURSTASH
           A B::HV object representing the stash for 'our' variables.

       PROTOCV
           The prototype CV for a 'my' sub.

       COP_SEQ_RANGE_LOW
       COP_SEQ_RANGE_HIGH
           Sequence numbers representing the scope within which a lexical is
           visible.  Meaningless if PADNAMEt_OUTER is set.

       PARENT_PAD_INDEX
           Only meaningful if PADNAMEt_OUTER is set.

       PARENT_FAKELEX_FLAGS
           Only meaningful if PADNAMEt_OUTER is set.

   $B::overlay
       Although the optree is read-only, there is an overlay facility that
       allows you to override what values the various B::*OP methods return
       for a particular op. $B::overlay should be set to reference a two-deep
       hash: indexed by OP address, then method name. Whenever a an op method
       is called, the value in the hash is returned if it exists. This
       facility is used by B::Deparse to "undo" some optimisations. For
       example:

           local $B::overlay = {};
           ...
           if ($op->name eq "foo") {
               $B::overlay->{$$op} = {
                       name => 'bar',
                       next => $op->next->next,
               };
           }
           ...
           $op->name # returns "bar"
           $op->next # returns the next op but one


AUTHOR

       Malcolm Beattie, "mbeattie@sable.ox.ac.uk"



perl v5.24.0                      2016-03-01                            B(3pm)

perl 5.24 - Generated Thu Nov 3 07:47:50 CDT 2016
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