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



NAME
       B - The Perl Compiler

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(1,2) 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 ta-
       ble and syntax tree of a program.

OVERVIEW
       The "B" module contains a set(7,n,1 builtins) 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(1,8) 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 hierachy 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(1,8) 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(2,5)
           methods.

       amagic_generation
           Returns the SV object corresponding to the C variable "amagic_gen-
           eration".

       init_av
           Returns the AV object (i.e. in(1,8) class B::AV) representing INIT
           blocks.

       check_av
           Returns the AV object (i.e. in(1,8) class B::AV) representing CHECK
           blocks.

       begin_av
           Returns the AV object (i.e. in(1,8) class B::AV) representing BEGIN
           blocks.

       end_av
           Returns the AV object (i.e. in(1,8) class B::AV) representing END
           blocks.

       comppadlist
           Returns the AV object (i.e. in(1,8) class B::AV) of the global comp-
           padlist.

       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(1,8) the symbol
           name, and only recurses into the package if(3,n) 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 hierachy 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(1,8) 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 debug-
           ging 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(3,n) in(1,8) the form "0x..." representing the value of the
           internal hash function used by perl on string(3,n) 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(1,8) a BEGIN block or else the flag is set(7,n,1 builtins) too
           late.

       cstring(STR)
           Returns a double-quote-surrounded escaped version(1,3,5) of STR which can
           be used as a string(3,n) in(1,8) C source code.

       perlstring(STR)
           Returns a double-quote-surrounded escaped version(1,3,5) of STR which can
           be used as a string(3,n) in(1,8) 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.

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 hier-
       archy and the "B" module gives access(2,5) to them via a true object hierar-
       chy. 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(2,5) is read-only.  You cannot modify the internals by
       using this module.

       SV-RELATED CLASSES

       B::IV, B::NV, B::RV, B::PV, B::PVIV, B::PVNV, B::PVMG, B::BM, B::PVLV,
       B::AV, B::HV, B::CV, B::GV, B::FM, B::IO. These classes correspond in(1,8)
       the obvious way to the underlying C structures of similar names. The
       inheritance hierarchy mimics the underlying C "inheritance":

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

       Access methods correspond to the underlying C macros for field access(2,5),
       usually with the leading "class indication" prefix removed (Sv, Av, Hv,
       ...). The leading prefix is only left in(1,8) cases where its removal would
       cause a clash in(1,8) 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(3,n) "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(1,8) 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(3,n)
           using the length and offset information in(1,8) the struct: for ordinary
           scalars it will return the string(3,n) that you'd see from Perl, even if(3,n)
           it contains null characters.

       RV  Same as B::RV::RV, except that it will die() if(3,n) the PV isn't a ref-
           erence.

       PVX This method is less(1,3) often useful. It assumes that the string(3,n) stored
           in(1,8) the struct is null-terminated, and disregards the length infor-
           mation.

           It is the appropriate method to use if(3,n) 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(3,n) that generated the reg-
           exp.

       PRIVATE
       TYPE
       FLAGS
       OBJ Will die() if(3,n) called on r-magic.

       PTR
       REGEX
           Only valid on r-magic, returns the integer value of the REGEX
           stored in(1,8) 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(3,n) the GP field of the GV is NULL.

       NAME
       SAFENAME
           This method returns the name of the glob(1,3,7,n), but if(3,n) the first charac-
           ter 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(3,n) 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(3,n) and a dereference (rv2gv) -
           so that the glob(1,3,7,n) 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(3x,7,8,3x cbreak) 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(3,n) the IoIFP of the object is equal to the handle whose name
           was passed as argument ( i.e. $io->IsSTD('stderr') is true if(3,n)
           IoIFP($io) == PerlIO_stdin() ).

       B::AV Methods


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

       AvFLAGS

       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
       PMROOT
       ARRAY

       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(1,8) the obvious way to the underlying C struc-
       tures of similar names. The inheritance hierarchy mimics the underlying
       C "inheritance":

                                        B::OP
                                          |
                          +---------------+--------+--------+
                          |               |        |        |
                       B::UNOP          B::SVOP B::PADOP  B::COP
                        ,'  `-.
                       /       `--.
                  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


       next
       sibling
       name
           This returns the op name as a string(3,n) (e.g. "add", "rv2av").

       ppaddr
           This returns the function name as a string(3,n) (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
       seq
       flags
       private

       B::UNOP METHOD


       first

       B::BINOP METHOD

       last

       B::LOGOP METHOD


       other

       B::LISTOP METHOD

       children

       B::PMOP Methods


       pmreplroot
       pmreplstart
       pmnext
       pmregexp
       pmflags
       pmdynflags
       pmpermflags
       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(1,n)
       cop_seq
       arybase
       line
       warnings
       io

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



perl v5.8.5                       2001-09-21                              B(3)

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