the format.
STRLEN is an integer type (Size_t, usually defined as size_t in
-config.h) guaranteed to be large enough to represent the size of
+config.h) guaranteed to be large enough to represent the size of
any string that perl can handle.
The C<sv_set*()> functions are not generic enough to operate on values
(offset OK) to signal to other functions that the offset hack is in
effect, and it puts the number of bytes chopped off into the IV field
of the SV. It then moves the PV pointer (called C<SvPVX>) forward that
-many bytes, and adjusts C<SvCUR> and C<SvLEN>.
+many bytes, and adjusts C<SvCUR> and C<SvLEN>.
Hence, at this point, the start of the buffer that we allocated lives
at C<SvPVX(sv) - SvIV(sv)> in memory and the PV pointer is pointing
These will tell you if you truly have an integer, double, or string pointer
stored in your SV. The "p" stands for private.
+The are various ways in which the private and public flags may differ.
+For example, a tied SV may have a valid underlying value in the IV slot
+(so SvIOKp is true), but the data should be accessed via the FETCH
+routine rather than directly, so SvIOK is false. Another is when
+numeric conversion has occured and precision has been lost: only the
+private flag is set on 'lossy' values. So when an NV is converted to an
+IV with loss, SvIOKp, SvNOKp and SvNOK will be set, while SvIOK wont be.
+
In general, though, it's best to use the C<Sv*V> macros.
=head2 Working with AVs
bool sv_derived_from(SV* sv, const char* name);
-To check if you've got an object derived from a specific class you have
+To check if you've got an object derived from a specific class you have
to write:
if (sv_isobject(sv) && sv_derived_from(sv, class)) { ... }
However, if you mortalize a variable twice, the reference count will
later be decremented twice.
-You should be careful about creating mortal variables. Strange things
-can happen if you make the same value mortal within multiple contexts,
-or if you make a variable mortal multiple times.
+"Mortal" SVs are mainly used for SVs that are placed on perl's stack.
+For example an SV which is created just to pass a number to a called sub
+is made mortal to have it cleaned up automatically when stack is popped.
+Similarly results returned by XSUBs (which go in the stack) are often
+made mortal.
To create a mortal variable, use the functions:
SV* sv_2mortal(SV*)
SV* sv_mortalcopy(SV*)
-The first call creates a mortal SV, the second converts an existing
+The first call creates a mortal SV (with no value), the second converts an existing
SV to a mortal SV (and thus defers a call to C<SvREFCNT_dec>), and the
third creates a mortal copy of an existing SV.
+Because C<sv_newmortal> gives the new SV no value,it must normally be given one
+via C<sv_setpv>, C<sv_setiv> etc. :
+
+ SV *tmp = sv_newmortal();
+ sv_setiv(tmp, an_integer);
+
+As that is multiple C statements it is quite common so see this idiom instead:
+
+ SV *tmp = sv_2mortal(newSViv(an_integer));
+
+
+You should be careful about creating mortal variables. Strange things
+can happen if you make the same value mortal within multiple contexts,
+or if you make a variable mortal multiple times. Thinking of "Mortalization"
+as deferred C<SvREFCNT_dec> should help to minimize such problems.
+For example if you are passing an SV which you I<know> has high enough REFCNT
+to survive its use on the stack you need not do any mortalization.
+If you are not sure then doing an C<SvREFCNT_inc> and C<sv_2mortal>, or
+making a C<sv_mortalcopy> is safer.
The mortal routines are not just for SVs -- AVs and HVs can be
made mortal by passing their address (type-casted to C<SV*>) to the
The sv_magic function uses C<how> to determine which, if any, predefined
"Magic Virtual Table" should be assigned to the C<mg_virtual> field.
See the "Magic Virtual Table" section below. The C<how> argument is also
-stored in the C<mg_type> field.
+stored in the C<mg_type> field. The value of C<how> should be chosen
+from the set of macros C<PERL_MAGIC_foo> found perl.h. Note that before
+these macros were added, perl internals used to directly use character
+literals, so you may occasionally come across old code or documentation
+referrring to 'U' magic rather than C<PERL_MAGIC_uvar> for example.
The C<obj> argument is stored in the C<mg_obj> field of the C<MAGIC>
structure. If it is not the same as the C<sv> argument, the reference
count of the C<obj> object is incremented. If it is the same, or if
-the C<how> argument is "#", or if it is a NULL pointer, then C<obj> is
-merely stored, without the reference count being incremented.
+the C<how> argument is C<PERL_MAGIC_arylen>", or if it is a NULL pointer,
+then C<obj> is merely stored, without the reference count being incremented.
There is also a function to add magic to an C<HV>:
svt_free Free any extra storage associated with the SV.
For instance, the MGVTBL structure called C<vtbl_sv> (which corresponds
-to an C<mg_type> of '\0') contains:
+to an C<mg_type> of C<PERL_MAGIC_sv>) contains:
{ magic_get, magic_set, magic_len, 0, 0 }
-Thus, when an SV is determined to be magical and of type '\0', if a get
-operation is being performed, the routine C<magic_get> is called. All
-the various routines for the various magical types begin with C<magic_>.
-NOTE: the magic routines are not considered part of the Perl API, and may
-not be exported by the Perl library.
+Thus, when an SV is determined to be magical and of type C<PERL_MAGIC_sv>,
+if a get operation is being performed, the routine C<magic_get> is
+called. All the various routines for the various magical types begin
+with C<magic_>. NOTE: the magic routines are not considered part of
+the Perl API, and may not be exported by the Perl library.
The current kinds of Magic Virtual Tables are:
- mg_type MGVTBL Type of magic
- ------- ------ ----------------------------
- \0 vtbl_sv Special scalar variable
- A vtbl_amagic %OVERLOAD hash
- a vtbl_amagicelem %OVERLOAD hash element
- c (none) Holds overload table (AMT) on stash
- B vtbl_bm Boyer-Moore (fast string search)
- D vtbl_regdata Regex match position data (@+ and @- vars)
- d vtbl_regdatum Regex match position data element
- E vtbl_env %ENV hash
- e vtbl_envelem %ENV hash element
- f vtbl_fm Formline ('compiled' format)
- g vtbl_mglob m//g target / study()ed string
- I vtbl_isa @ISA array
- i vtbl_isaelem @ISA array element
- k vtbl_nkeys scalar(keys()) lvalue
- L (none) Debugger %_<filename
- l vtbl_dbline Debugger %_<filename element
- o vtbl_collxfrm Locale transformation
- P vtbl_pack Tied array or hash
- p vtbl_packelem Tied array or hash element
- q vtbl_packelem Tied scalar or handle
- S vtbl_sig %SIG hash
- s vtbl_sigelem %SIG hash element
- t vtbl_taint Taintedness
- U vtbl_uvar Available for use by extensions
- v vtbl_vec vec() lvalue
- x vtbl_substr substr() lvalue
- y vtbl_defelem Shadow "foreach" iterator variable /
- smart parameter vivification
- * vtbl_glob GV (typeglob)
- # vtbl_arylen Array length ($#ary)
- . vtbl_pos pos() lvalue
- ~ (none) Available for use by extensions
+ mg_type
+ (old-style char and macro) MGVTBL Type of magic
+ -------------------------- ------ ----------------------------
+ \0 PERL_MAGIC_sv vtbl_sv Special scalar variable
+ A PERL_MAGIC_overload vtbl_amagic %OVERLOAD hash
+ a PERL_MAGIC_overload_elem vtbl_amagicelem %OVERLOAD hash element
+ c PERL_MAGIC_overload_table (none) Holds overload table (AMT)
+ on stash
+ B PERL_MAGIC_bm vtbl_bm Boyer-Moore (fast string search)
+ D PERL_MAGIC_regdata vtbl_regdata Regex match position data
+ (@+ and @- vars)
+ d PERL_MAGIC_regdatum vtbl_regdatum Regex match position data
+ element
+ E PERL_MAGIC_env vtbl_env %ENV hash
+ e PERL_MAGIC_envelem vtbl_envelem %ENV hash element
+ f PERL_MAGIC_fm vtbl_fm Formline ('compiled' format)
+ g PERL_MAGIC_regex_global vtbl_mglob m//g target / study()ed string
+ I PERL_MAGIC_isa vtbl_isa @ISA array
+ i PERL_MAGIC_isaelem vtbl_isaelem @ISA array element
+ k PERL_MAGIC_nkeys vtbl_nkeys scalar(keys()) lvalue
+ L PERL_MAGIC_dbfile (none) Debugger %_<filename
+ l PERL_MAGIC_dbline vtbl_dbline Debugger %_<filename element
+ m PERL_MAGIC_mutex vtbl_mutex ???
+ o PERL_MAGIC_collxfrm vtbl_collxfrm Locale transformation
+ P PERL_MAGIC_tied vtbl_pack Tied array or hash
+ p PERL_MAGIC_tiedelem vtbl_packelem Tied array or hash element
+ q PERL_MAGIC_tiedscalar vtbl_packelem Tied scalar or handle
+ r PERL_MAGIC_qr vtbl_qr precompiled qr// regex
+ S PERL_MAGIC_sig vtbl_sig %SIG hash
+ s PERL_MAGIC_sigelem vtbl_sigelem %SIG hash element
+ t PERL_MAGIC_taint vtbl_taint Taintedness
+ U PERL_MAGIC_uvar vtbl_uvar Available for use by extensions
+ v PERL_MAGIC_vec vtbl_vec vec() lvalue
+ x PERL_MAGIC_substr vtbl_substr substr() lvalue
+ y PERL_MAGIC_defelem vtbl_defelem Shadow "foreach" iterator
+ variable / smart parameter
+ vivification
+ * PERL_MAGIC_glob vtbl_glob GV (typeglob)
+ # PERL_MAGIC_arylen vtbl_arylen Array length ($#ary)
+ . PERL_MAGIC_pos vtbl_pos pos() lvalue
+ < PERL_MAGIC_backref vtbl_backref ???
+ ~ PERL_MAGIC_ext (none) Available for use by extensions
When an uppercase and lowercase letter both exist in the table, then the
uppercase letter is used to represent some kind of composite type (a list
or a hash), and the lowercase letter is used to represent an element of
-that composite type.
-
-The '~' and 'U' magic types are defined specifically for use by
-extensions and will not be used by perl itself. Extensions can use
-'~' magic to 'attach' private information to variables (typically
-objects). This is especially useful because there is no way for
-normal perl code to corrupt this private information (unlike using
-extra elements of a hash object).
-
-Similarly, 'U' magic can be used much like tie() to call a C function
-any time a scalar's value is used or changed. The C<MAGIC>'s
+that composite type. Some internals code makes use of this case
+relationship.
+
+The C<PERL_MAGIC_ext> and C<PERL_MAGIC_uvar> magic types are defined
+specifically for use by extensions and will not be used by perl itself.
+Extensions can use C<PERL_MAGIC_ext> magic to 'attach' private information
+to variables (typically objects). This is especially useful because
+there is no way for normal perl code to corrupt this private information
+(unlike using extra elements of a hash object).
+
+Similarly, C<PERL_MAGIC_uvar> magic can be used much like tie() to call a
+C function any time a scalar's value is used or changed. The C<MAGIC>'s
C<mg_ptr> field points to a C<ufuncs> structure:
struct ufuncs {
};
When the SV is read from or written to, the C<uf_val> or C<uf_set>
-function will be called with C<uf_index> as the first arg and a
-pointer to the SV as the second. A simple example of how to add 'U'
+function will be called with C<uf_index> as the first arg and a pointer to
+the SV as the second. A simple example of how to add C<PERL_MAGIC_uvar>
magic is shown below. Note that the ufuncs structure is copied by
sv_magic, so you can safely allocate it on the stack.
uf.uf_val = &my_get_fn;
uf.uf_set = &my_set_fn;
uf.uf_index = 0;
- sv_magic(sv, 0, 'U', (char*)&uf, sizeof(uf));
+ sv_magic(sv, 0, PERL_MAGIC_uvar, (char*)&uf, sizeof(uf));
-Note that because multiple extensions may be using '~' or 'U' magic,
-it is important for extensions to take extra care to avoid conflict.
-Typically only using the magic on objects blessed into the same class
-as the extension is sufficient. For '~' magic, it may also be
-appropriate to add an I32 'signature' at the top of the private data
-area and check that.
+Note that because multiple extensions may be using C<PERL_MAGIC_ext>
+or C<PERL_MAGIC_uvar> magic, it is important for extensions to take
+extra care to avoid conflict. Typically only using the magic on
+objects blessed into the same class as the extension is sufficient.
+For C<PERL_MAGIC_ext> magic, it may also be appropriate to add an I32
+'signature' at the top of the private data area and check that.
Also note that the C<sv_set*()> and C<sv_cat*()> functions described
earlier do B<not> invoke 'set' magic on their targets. This must
=head2 Understanding the Magic of Tied Hashes and Arrays
-Tied hashes and arrays are magical beasts of the 'P' magic type.
+Tied hashes and arrays are magical beasts of the C<PERL_MAGIC_tied>
+magic type.
WARNING: As of the 5.004 release, proper usage of the array and hash
access functions requires understanding a few caveats. Some
tie = newRV_noinc((SV*)newHV());
stash = gv_stashpv("MyTie", TRUE);
sv_bless(tie, stash);
- hv_magic(hash, tie, 'P');
+ hv_magic(hash, (GV*)tie, PERL_MAGIC_tied);
RETVAL = newRV_noinc(hash);
OUTPUT:
RETVAL
=item C<SAVEFREESV(SV *sv)>
The refcount of C<sv> would be decremented at the end of
-I<pseudo-block>. This is similar to C<sv_2mortal>, which should (?) be
-used instead.
+I<pseudo-block>. This is similar to C<sv_2mortal> in that it is also a
+mechanism for doing a delayed C<SvREFCNT_dec>. However, while C<sv_2mortal>
+extends the lifetime of C<sv> until the beginning of the next statement,
+C<SAVEFREESV> extends it until the end of the enclosing scope. These
+lifetimes can be wildly different.
+
+Also compare C<SAVEMORTALIZESV>.
+
+=item C<SAVEMORTALIZESV(SV *sv)>
+
+Just like C<SAVEFREESV>, but mortalizes C<sv> at the end of the current
+scope instead of decrementing its reference count. This usually has the
+effect of keeping C<sv> alive until the statement that called the currently
+live scope has finished executing.
=item C<SAVEFREEOP(OP *op)>
The following API list contains functions, thus one needs to
provide pointers to the modifiable data explicitly (either C pointers,
-or Perlish C<GV *>s). Where the above macros take C<int>, a similar
+or Perlish C<GV *>s). Where the above macros take C<int>, a similar
function takes C<int *>.
=over 4
where C<SP> is the macro that represents the local copy of the stack pointer,
and C<num> is the number of elements the stack should be extended by.
-Now that there is room on the stack, values can be pushed on it using the
-macros to push IVs, doubles, strings, and SV pointers respectively:
+Now that there is room on the stack, values can be pushed on it using C<PUSHs>
+macro. The values pushed will often need to be "mortal" (See L</Reference Counts and Mortality>).
- PUSHi(IV)
- PUSHn(double)
- PUSHp(char*, I32)
- PUSHs(SV*)
+ PUSHs(sv_2mortal(newSViv(an_integer)))
+ PUSHs(sv_2mortal(newSVpv("Some String",0)))
+ PUSHs(sv_2mortal(newSVnv(3.141592)))
And now the Perl program calling C<tzname>, the two values will be assigned
as in:
($standard_abbrev, $summer_abbrev) = POSIX::tzname;
An alternate (and possibly simpler) method to pushing values on the stack is
-to use the macros:
+to use the macro:
- XPUSHi(IV)
- XPUSHn(double)
- XPUSHp(char*, I32)
XPUSHs(SV*)
-These macros automatically adjust the stack for you, if needed. Thus, you
+This macro automatically adjust the stack for you, if needed. Thus, you
do not need to call C<EXTEND> to extend the stack.
-However, see L</Putting a C value on Perl stack>
+
+Despite their suggestions in earlier versions of this document the macros
+C<PUSHi>, C<PUSHn> and C<PUSHp> are I<not> suited to XSUBs which return
+multiple results, see L</Putting a C value on Perl stack>.
For more information, consult L<perlxs> and L<perlxstut>.
On a related note, if you do use C<(X)PUSH[npi]>, then you're going to
need a C<dTARG> in your variable declarations so that the C<*PUSH*>
-macros can make use of the local variable C<TARG>.
+macros can make use of the local variable C<TARG>.
=head2 Scratchpads
The most commonly used of these functions is C<Perl_sv_dump>; it's used
for dumping SVs, AVs, HVs, and CVs. The C<Devel::Peek> module calls
C<sv_dump> to produce debugging output from Perl-space, so users of that
-module should already be familiar with its format.
+module should already be familiar with its format.
C<Perl_op_dump> can be used to dump an C<OP> structure or any of its
derivatives, and produces output similiar to C<perl -Dx>; in fact,
use of macros and subroutine naming conventions.
First problem: deciding which functions will be public API functions and
-which will be private. All functions whose names begin C<S_> are private
+which will be private. All functions whose names begin C<S_> are private
(think "S" for "secret" or "static"). All other functions begin with
"Perl_", but just because a function begins with "Perl_" does not mean it is
-part of the API. (See L</Internal Functions>.) The easiest way to be B<sure> a
-function is part of the API is to find its entry in L<perlapi>.
-If it exists in L<perlapi>, it's part of the API. If it doesn't, and you
-think it should be (i.e., you need it for your extension), send mail via
+part of the API. (See L</Internal Functions>.) The easiest way to be B<sure> a
+function is part of the API is to find its entry in L<perlapi>.
+If it exists in L<perlapi>, it's part of the API. If it doesn't, and you
+think it should be (i.e., you need it for your extension), send mail via
L<perlbug> explaining why you think it should be.
Second problem: there must be a syntax so that the same subroutine
=item M
-This function is part of the experimental development API, and may change
+This function is part of the experimental development API, and may change
or disappear without notice.
=item o
projects / p5sagit/p5-mst-13.2.git / blobdiff