B(3pm)

📦

version 5.42.3 · 10 versions

💅 B(3pm) — The Perl Compiler Backend ✨

🌸 NAME 🌸

B - The Perl Compiler Backend

✨ SYNOPSIS ✨

use B;

💖 DESCRIPTION 💖

Okay bestie, so the B module is the whole set of classes that lets a Perl program dig into its own inner workings — like, literally peek at its own guts. It’s the module used to implement the “backends” of the Perl compiler. You do not need to know this module to use the compiler: check the O module for the user-visible part. The B module is for people who want to write new compiler backends. This documentation assumes you already know a fair amount about perl’s internals, including stuff like SVs, OPs, and the internal symbol table and syntax tree of a program. 🫶

🦋 OVERVIEW 🦋

The B module has a bunch of utility functions for asking the current Perl interpreter what’s up; usually these functions return objects from the B::SV and B::OP classes, or their derived classes. Those classes then define methods so you can ask the resulting objects about their own internal state. 💅

🛠️ Utility Functions 🛠️

The B module exports a variety of functions: some are simple utility functions, and others give a Perl program 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 you can call 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. 💀

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
IsBOOL — Returns true if the SV is a boolean (true or false). You can then use TRUE to check if the value is true or false.

my $something = ( 1 == 1 ) # boolean true
             || ( 1 == 0 ) # boolean false
             || 42         # IV true
             || 0;         # IV false
my $sv = B::svref_2object(\$something);

say q[Not a boolean value]
    if ! $sv->IsBOOL;

say q[This is a boolean with value: true]
    if   $sv->IsBOOL && $sv->TRUE_nomg;

say q[This is a boolean with value: false]
    if   $sv->IsBOOL && ! $sv->TRUE_nomg;

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.
TRUE — Returns a boolean indicating whether Perl would evaluate the SV as true or false.

⚠️ Warning: this call performs 'get' magic. If you only want to check the nature of this SV use TRUE_nomg helper. 🫶

This is an alias for SvTRUE($sv). ✨

TRUE_nomg — okay so this checks if the value is true, and it does not perform 'get' magic. It returns a boolean showing whether Perl would evaluate the SV as true or false. This is an alias for SvTRUE_nomg($sv). 💖

🩷 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 — 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 — returns the internal length field, which consists of the number of internal bytes, not necessarily the number of logical characters.
LEN — 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 — 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 — returns TRUE if the GP field of the GV is NULL.
NAME
SAFENAME — 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

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: 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 — returns the op name as a string (e.g. add, rv2av).
ppaddr — returns the function name as a string (e.g. PL_ppaddr[OP_ADD], PL_ppaddr[OP_RV2AV]).
desc — 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) — 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) — 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

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 — a B::PADNAME method. The source gives no extra description here, so no invented lore, bestie. 💅
PVX — another B::PADNAME method. Same deal: name only, no extra claims. ✨
LEN — a B::PADNAME method. That's the whole entry in the source. 🎀
REFCNT — a B::PADNAME method. No extra details provided. 🦋
GEN — a B::PADNAME method. Again, just the name in the original. 🌸
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. They're 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. 🦋
IsUndef — returns a boolean value to check if the padname is PL_padname_undef. ⭐

🦋 $B::overlay 🦋

Okay so the optree is read-only, but there's an overlay facility, and it lets you override what values the various B::*OP methods return for a particular op. Set $B::overlay to reference a two-deep hash: indexed by OP address, then method name. Whenever 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@sable.ox.ac.uk"

And that's the whole page, bestie 💖 — perl v5.42.3, dated 2026-11-03, and it's B(3pm). Slay. ✨

📦 where this page comes from

alpine 3.24: perl-doc 5.42.3-r0 · Artistic-1.0-Perl OR GPL-1.0-or-later
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash-1017 on 2026-11-04: a language model's rewrite of the original, under its terms