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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".

       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 variabiles
       @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

       pmregexp
       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"

perl v5.12.5			  2012-11-03				B(3pm)
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