B
B - The Perl Compiler
use B;
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.
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.
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 "inheritance". 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).
FLAGS & SVf_IVisUV. Perhaps you want the
int_value method instead?
IV in that it returns the correct
value regardless of whether it's stored signed or
unsigned.
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.
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.
B::OP, B::UNOP, B::BINOP, B::LOGOP, B::LISTOP, B::PMOP, B::SVOP, B::PADOP, B::PVOP, B::CVOP, 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". Access methods correspond to the underlying C structre field names, with the leading "class indication" prefix removed (op_).
B
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.
sv_undef.
sv_yes.
sv_no.
amagic_generation.
walkoptree_debug (q.v.) has been called to turn debugging on then
the method walkoptree_debug is called on each op before METHOD is
called.
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.
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.
# Only recurse into CGI::Util::
walksymtable(\%CGI::, 'print_subs', sub { $_[0] eq 'CGI::Util::' },
'CGI::');
print_subs() is a B::GV method you have declared.
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.
-c command-line option. Obviously, this
is only useful in a BEGIN block or else the flag is set too late.
Malcolm Beattie, mbeattie@sable.ox.ac.uk