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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".

       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 AV object (i.e. in class B::AV) of the global comppadlist.

       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.

       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
           In a perl compiled for threads, this returns a list of the special per-thread threadsv
           variables.

   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

   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 (SvCUR) is overloaded for other purposes and can't be relied on
           here.

   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::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

   B::IO Methods
       LINES
       PAGE
       PAGE_LEN
       LINES_LEFT
       TOP_NAME
       TOP_GV
       FMT_NAME
       FMT_GV
       BOTTOM_NAME
       BOTTOM_GV
       SUBPROCESS
       IoTYPE
       IoFLAGS
       IsSTD
           Takes one arguments ( '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_stdin() ).

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

       CvFLAGS
       const_sv

   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::BINOP", "B::LOGOP", "B::LISTOP", "B::PMOP", "B::SVOP", "B::PADOP",
       "B::PVOP", "B::LOOP", "B::COP".

       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::BINOP     B::LOGOP
                      |
                      |
                  B::LISTOP
                    ,' `.
                   /     \
               B::LOOP B::PMOP

       Access methods correspond to the underlying C structre 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
       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::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.

   B::SVOP METHOD
       sv
       gv

   B::PADOP METHOD
       padix

   B::PVOP METHOD
       pv

   B::LOOP Methods
       redoop
       nextop
       lastop

   B::COP Methods
       label
       stash
       stashpv
       file
       cop_seq
       arybase
       line
       warnings
       io
       hints
       hints_hash

AUTHOR

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