man > B(3perl)

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::PV, B::PVIV, B::PVNV, B::PVMG, 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":

                               B::SV
                                 |
                    +------------+------------+
                    |            |            |
                  B::PV        B::IV        B::NV
                   /  \         /           /
                  /    \       /           /
            B::INVLIST  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::INVLIST Methods
    prev_index
        Returns the cache result of previous invlist_search() (internal usage)

    is_offset
        Returns a boolean value (0 or 1) to know if the invlist is using an offset. When false the
        list begins with the code point U+0000. When true the list begins with the following
        elements.

    array_len
        Returns an integer with the size of the array used to define the invlist.

    get_invlist_array
        This method returns a list of integers representing the array used by the invlist. Note:
        this cannot be used while in middle of iterating on an invlist and croaks.

  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.

  B::CV Methods
    STASH
    START
    ROOT
    GV
    FILE
    DEPTH
    PADLIST
        Returns a B::PADLIST object.

    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

  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
    pmflags
    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
    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 is a B::PADNAMELIST containing the names. 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 AT sable.uk"