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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 mod-
       ule: see the O module for the user-visible part. The "B" module is of use to those who want to write new com-
       piler 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; typ-
       ically 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.

       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.

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, 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 inheri-
       tance hierarchy mimics the underlying C "inheritance". For 5.9.1 and later this is:

                                    B::SV
                                      |
                       +--------------+----------+------------+
                       |              |          |            |
                     B::PV          B::IV      B::NV        B::RV
                          \         /          /
                           \       /          /
                            B::PVIV          /
                                \           /
                                 \         /
                                  \       /
                                   B::PVNV
                                      |
                                      |
                                   B::PVMG
                                      |
                           +-----+----+------+-----+-----+
                           |     |    |      |     |     |
                         B::BM 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, 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

       Access methods correspond to the underlying C macros for field access, usually with the leading "class indica-
       tion" 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 "super-
       class" 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_IVi-
           sUV". 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) == Per-
           lIO_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 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::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
       static
       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
       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
       cop_seq
       arybase
       line
       warnings
       io

AUTHOR
       Malcolm Beattie, "mbeattieATsable.uk"



perl v5.8.8                       2001-09-21                              B(3)