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.\" ========================================================================
.TH B 3 "2001-09-21" "perl v5.8.8" "Perl Programmers Reference Guide"
The \f(CW\*(C`B\*(C'\fR module supplies classes which allow a Perl program to delve
into its own innards. It is the module used to implement the
\&\*(L"backends\*(R" of the Perl compiler. Usage of the compiler does not
require knowledge of this module: see the \fIO\fR module for the
user-visible part. The \f(CW\*(C`B\*(C'\fR 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
The \f(CW\*(C`B\*(C'\fR 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.
.IX Header "Utility Functions"
The \f(CW\*(C`B\*(C'\fR module exports a variety of functions: some are simple
utility functions, others provide a Perl program with a way to
get an initial \*(L"handle\*(R" on an internal object.
.ie n .Sh "Functions Returning ""B::SV""\fP, \f(CW""B::AV""\fP, \f(CW""B::HV""\fP, and \f(CW""B::CV"" objects"
.el .Sh "Functions Returning \f(CWB::SV\fP, \f(CWB::AV\fP, \f(CWB::HV\fP, and \f(CWB::CV\fP objects"
.IX Subsection "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, \*(L"\s-1OVERVIEW\s0 \s-1OF\s0 \s-1CLASSES\s0\*(R" and \*(L"\s-1SV\-RELATED\s0 \s-1CLASSES\s0\*(R".
Returns the \s-1SV\s0 object corresponding to the C variable \f(CW\*(C`sv_undef\*(C'\fR.
Returns the \s-1SV\s0 object corresponding to the C variable \f(CW\*(C`sv_yes\*(C'\fR.
Returns the \s-1SV\s0 object corresponding to the C variable \f(CW\*(C`sv_no\*(C'\fR.
.IP "svref_2object(\s-1SVREF\s0)" 4
.IX Item "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 \f(CW\*(C`main_root\*(C'\fR, this is the primary
way to get an initial \*(L"handle\*(R" 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.
.IP "amagic_generation" 4
.IX Item "amagic_generation"
Returns the \s-1SV\s0 object corresponding to the C variable \f(CW\*(C`amagic_generation\*(C'\fR.
Returns the \s-1AV\s0 object (i.e. in class B::AV) representing \s-1INIT\s0 blocks.
Returns the \s-1AV\s0 object (i.e. in class B::AV) representing \s-1CHECK\s0 blocks.
Returns the \s-1AV\s0 object (i.e. in class B::AV) representing \s-1BEGIN\s0 blocks.
Returns the \s-1AV\s0 object (i.e. in class B::AV) representing \s-1END\s0 blocks.
Returns the \s-1AV\s0 object (i.e. in class B::AV) of the global comppadlist.
Only when perl was compiled with ithreads.
Return the (faked) \s-1CV\s0 corresponding to the main part of the Perl
.Sh "Functions for Examining the Symbol Table"
.IX Subsection "Functions for Examining the Symbol Table"
.IP "walksymtable(\s-1SYMREF\s0, \s-1METHOD\s0, \s-1RECURSE\s0, \s-1PREFIX\s0)" 4
.IX Item "walksymtable(SYMREF, METHOD, RECURSE, PREFIX)"
Walk the symbol table starting at \s-1SYMREF\s0 and call \s-1METHOD\s0 on each
symbol (a B::GV object) visited. When the walk reaches package
symbols (such as \*(L"Foo::\*(R") it invokes \s-1RECURSE\s0, passing in the symbol
name, and only recurses into the package if that sub returns true.
\&\s-1PREFIX\s0 is the name of the \s-1SYMREF\s0 you're walking.
\& # Walk CGI's symbol table calling print_subs on each symbol.
\& # Recurse only into CGI::Util::
\& walksymtable(\e%CGI::, 'print_subs', sub { $_[0] eq 'CGI::Util::' },
\&\fIprint_subs()\fR is a B::GV method you have declared. Also see \*(L"B::GV Methods\*(R", below.
.ie n .Sh "Functions Returning ""B::OP"" objects or for walking op trees"
.el .Sh "Functions Returning \f(CWB::OP\fP objects or for walking op trees"
.IX Subsection "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, \*(L"\s-1OVERVIEW\s0 \s-1OF\s0 \s-1CLASSES\s0\*(R" and \*(L"\s-1OP\-RELATED\s0 \s-1CLASSES\s0\*(R".
Returns the root op (i.e. an object in the appropriate B::OP\-derived
class) of the main part of the Perl program.
Returns the starting op of the main part of the Perl program.
.IP "walkoptree(\s-1OP\s0, \s-1METHOD\s0)" 4
.IX Item "walkoptree(OP, METHOD)"
Does a tree-walk of the syntax tree based at \s-1OP\s0 and calls \s-1METHOD\s0 on
each op it visits. Each node is visited before its children. If
\&\f(CW\*(C`walkoptree_debug\*(C'\fR (see below) has been called to turn debugging on then
the method \f(CW\*(C`walkoptree_debug\*(C'\fR is called on each op before \s-1METHOD\s0 is
.IP "walkoptree_debug(\s-1DEBUG\s0)" 4
.IX Item "walkoptree_debug(DEBUG)"
Returns the current debugging flag for \f(CW\*(C`walkoptree\*(C'\fR. If the optional
\&\s-1DEBUG\s0 argument is non\-zero, it sets the debugging flag to that. See
the description of \f(CW\*(C`walkoptree\*(C'\fR above for what the debugging flag
.Sh "Miscellaneous Utility Functions"
.IX Subsection "Miscellaneous Utility Functions"
.IP "ppname(\s-1OPNUM\s0)" 4
Return the \s-1PP\s0 function name (e.g. \*(L"pp_add\*(R") of op number \s-1OPNUM\s0.
Returns a string in the form \*(L"0x...\*(R" representing the value of the
internal hash function used by perl on string \s-1STR\s0.
Casts I to the internal I32 type used by that perl.
Does the equivalent of the \f(CW\*(C`\-c\*(C'\fR command-line option. Obviously, this
is only useful in a \s-1BEGIN\s0 block or else the flag is set too late.
.IP "cstring(\s-1STR\s0)" 4
Returns a double-quote-surrounded escaped version of \s-1STR\s0 which can
be used as a string in C source code.
.IP "perlstring(\s-1STR\s0)" 4
.IX Item "perlstring(STR)"
Returns a double-quote-surrounded escaped version of \s-1STR\s0 which can
be used as a string in Perl source code.
.IP "class(\s-1OBJ\s0)" 4
Returns the class of an object without the part of the classname
preceding the first \f(CW"::"\fR. This is used to turn \f(CW"B::UNOP"\fR into
\&\f(CW"UNOP"\fR for example.
.IX Item "threadsv_names"
In a perl compiled for threads, this returns a list of the special
per-thread threadsv variables.
.SH "OVERVIEW OF CLASSES"
.IX Header "OVERVIEW OF CLASSES"
The C structures used by Perl's internals to hold \s-1SV\s0 and \s-1OP\s0
information (\s-1PVIV\s0, \s-1AV\s0, \s-1HV\s0, ..., \s-1OP\s0, \s-1SVOP\s0, \s-1UNOP\s0, ...) are modelled on a
class hierarchy and the \f(CW\*(C`B\*(C'\fR module gives access to them via a true
object hierarchy. Structure fields which point to other objects
(whether types of \s-1SV\s0 or types of \s-1OP\s0) are represented by the \f(CW\*(C`B\*(C'\fR
module as Perl objects of the appropriate class.
The bulk of the \f(CW\*(C`B\*(C'\fR module is the methods for accessing fields of
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.
.Sh "SV-RELATED \s-1CLASSES\s0"
.IX Subsection "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
inheritance hierarchy mimics the underlying C \*(L"inheritance\*(R". For 5.9.1
\& +--------------+----------+------------+
\& B::PV B::IV B::NV B::RV
\& +-----+----+------+-----+-----+
\& B::BM B::AV B::GV B::HV B::CV B::IO
For 5.9.0 and earlier, \s-1PVLV\s0 is a direct subclass of \s-1PVMG\s0, so the base
\& +------+-----+----+------+-----+-----+
\& B::PVLV B::BM B::AV B::GV B::HV B::CV B::IO
Access methods correspond to the underlying C macros for field access,
usually with the leading \*(L"class indication\*(R" 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, \f(CW\*(C`GvREFCNT\*(C'\fR stays
as-is since its abbreviation would clash with the \*(L"superclass\*(R" method
\&\f(CW\*(C`REFCNT\*(C'\fR (corresponding to the C function \f(CW\*(C`SvREFCNT\*(C'\fR).
.IX Subsection "B::SV Methods"
Returns a reference to the regular scalar corresponding to this
B::SV object. In other words, this method is the inverse operation
to the \fIsvref_2object()\fR 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.
.IX Subsection "B::IV Methods"
Returns the value of the \s-1IV\s0, \fIinterpreted as
a signed integer\fR. This will be misleading
if \f(CW\*(C`FLAGS & SVf_IVisUV\*(C'\fR. Perhaps you want the
\&\f(CW\*(C`int_value\*(C'\fR method instead?
This method returns the value of the \s-1IV\s0 as an integer.
It differs from \f(CW\*(C`IV\*(C'\fR in that it returns the correct
value regardless of whether it's stored signed or
.IX Subsection "B::NV Methods"
.IX Subsection "B::RV Methods"
.IX Subsection "B::PV Methods"
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.
Same as B::RV::RV, except that it will \fIdie()\fR if the \s-1PV\s0 isn't
This method is less often useful. It assumes that the string
stored in the struct is null\-terminated, and disregards the
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.
.IX Subsection "B::PVMG Methods"
.IX Subsection "B::MAGIC Methods"
Only valid on r\-magic, returns the string that generated the regexp.
Will \fIdie()\fR if called on r\-magic.
Only valid on r\-magic, returns the integer value of the \s-1REGEX\s0 stored
.IX Subsection "B::PVLV Methods"
.IX Subsection "B::BM Methods"
.IX Subsection "B::GV Methods"
This method returns \s-1TRUE\s0 if the \s-1GP\s0 field of the \s-1GV\s0 is \s-1NULL\s0.
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 \*(L"^G\*(R" rather than \*(L"\ecG\*(R".
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
\&\f(CW\*(C`${"^G"} = 1\*(C'\fR 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 *{\*(L"^G\*(R"}, then you should use the raw \s-1NAME\s0 method.
.IX Subsection "B::IO Methods"
.IP "\s-1LINES_LEFT\s0" 4
.IP "\s-1BOTTOM_NAME\s0" 4
.IP "\s-1SUBPROCESS\s0" 4
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. \f(CW$io\fR\->IsSTD('stderr') is true if
IoIFP($io) == \fIPerlIO_stdin()\fR ).
.IX Subsection "B::AV Methods"
Like \f(CW\*(C`ARRAY\*(C'\fR, but takes an index as an argument to get only one element,
rather than a list of all of them.
.IX Subsection "B::CV Methods"
.IP "\s-1OUTSIDE_SEQ\s0" 4
For constant subroutines, returns the constant \s-1SV\s0 returned by the subroutine.
.IX Subsection "B::HV Methods"
.Sh "OP-RELATED \s-1CLASSES\s0"
.IX Subsection "OP-RELATED CLASSES"
\&\f(CW\*(C`B::OP\*(C'\fR, \f(CW\*(C`B::UNOP\*(C'\fR, \f(CW\*(C`B::BINOP\*(C'\fR, \f(CW\*(C`B::LOGOP\*(C'\fR, \f(CW\*(C`B::LISTOP\*(C'\fR, \f(CW\*(C`B::PMOP\*(C'\fR,
\&\f(CW\*(C`B::SVOP\*(C'\fR, \f(CW\*(C`B::PADOP\*(C'\fR, \f(CW\*(C`B::PVOP\*(C'\fR, \f(CW\*(C`B::LOOP\*(C'\fR, \f(CW\*(C`B::COP\*(C'\fR.
These classes correspond in the obvious way to the underlying C
structures of similar names. The inheritance hierarchy mimics the
underlying C \*(L"inheritance\*(R":
\& +---------------+--------+--------+
\& B::UNOP B::SVOP B::PADOP B::COP
Access methods correspond to the underlying C structre field names,
with the leading \*(L"class indication\*(R" prefix (\f(CW"op_"\fR) removed.
.IX Subsection "B::OP Methods"
These methods get the values of similarly named fields within the \s-1OP\s0
data structure. See top of \f(CW\*(C`op.h\*(C'\fR for more info.
This returns the op name as a string (e.g. \*(L"add\*(R", \*(L"rv2av\*(R").
This returns the function name as a string (e.g. \*(L"PL_ppaddr[\s-1OP_ADD\s0]\*(R",
\&\*(L"PL_ppaddr[\s-1OP_RV2AV\s0]\*(R").
This returns the op description from the global C PL_op_desc array
(e.g. \*(L"addition\*(R" \*(L"array deref\*(R").
.Sh "B::UNOP \s-1METHOD\s0"
.IX Subsection "B::UNOP METHOD"
.Sh "B::BINOP \s-1METHOD\s0"
.IX Subsection "B::BINOP METHOD"
.Sh "B::LOGOP \s-1METHOD\s0"
.IX Subsection "B::LOGOP METHOD"
.Sh "B::LISTOP \s-1METHOD\s0"
.IX Subsection "B::LISTOP METHOD"
.IX Subsection "B::PMOP Methods"
Only when perl was compiled with ithreads.
.Sh "B::SVOP \s-1METHOD\s0"
.IX Subsection "B::SVOP METHOD"
.Sh "B::PADOP \s-1METHOD\s0"
.IX Subsection "B::PADOP METHOD"
.Sh "B::PVOP \s-1METHOD\s0"
.IX Subsection "B::PVOP METHOD"
.IX Subsection "B::LOOP Methods"
.IX Subsection "B::COP Methods"
Malcolm Beattie, \f(CW\*(C`mbeattie@sable.ox.ac.uk\*(C'\fR
projects / OpenSPARC-T2-SAM / blob