=head1 DESCRIPTION
This document attempts to describe how to use the Perl API, as well as
-to provide some info on the basic workings of the Perl core. It is far
-from complete and probably contains many errors. Please refer any
+to provide some info on the basic workings of the Perl core. It is far
+from complete and probably contains many errors. Please refer any
questions or comments to the author below.
=head1 Variables
Additionally, there is the UV, which is simply an unsigned IV.
Perl also uses two special typedefs, I32 and I16, which will always be at
-least 32-bits and 16-bits long, respectively. (Again, there are U32 and U16,
+least 32-bits and 16-bits long, respectively. (Again, there are U32 and U16,
as well.) They will usually be exactly 32 and 16 bits long, but on Crays
they will both be 64 bits.
value (UV), a double (NV), a string (PV), and another scalar (SV).
("PV" stands for "Pointer Value". You might think that it is misnamed
because it is described as pointing only to strings. However, it is
-possible to have it point to other things. For example, inversion
-lists, used in regular expression data structures, are scalars, each
-consisting of an array of UVs which are accessed through PVs. But,
+possible to have it point to other things. For example, it could point
+to an array of UVs. But,
using it for non-strings requires care, as the underlying assumption of
much of the internals is that PVs are just for strings. Often, for
-example, a trailing NUL is tacked on automatically. The non-string use
+example, a trailing C<NUL> is tacked on automatically. The non-string use
is documented only in this paragraph.)
The seven routines are:
SV* newSVpvf(const char*, ...);
SV* newSVsv(SV*);
-C<STRLEN> is an integer type (Size_t, usually defined as size_t in
+C<STRLEN> is an integer type (C<Size_t>, usually defined as C<size_t> in
F<config.h>) guaranteed to be large enough to represent the size of
any string that perl can handle.
In the unlikely case of a SV requiring more complex initialization, you
can create an empty SV with newSV(len). If C<len> is 0 an empty SV of
type NULL is returned, else an SV of type PV is returned with len + 1 (for
-the NUL) bytes of storage allocated, accessible via SvPVX. In both cases
+the C<NUL>) bytes of storage allocated, accessible via SvPVX. In both cases
the SV has the undef value.
SV *sv = newSV(0); /* no storage allocated */
void sv_setpvn(SV*, const char*, STRLEN)
void sv_setpvf(SV*, const char*, ...);
void sv_vsetpvfn(SV*, const char*, STRLEN, va_list *,
- SV **, I32, bool *);
+ SV **, Size_t, bool *);
void sv_setsv(SV*, SV*);
Notice that you can choose to specify the length of the string to be
allow Perl to calculate the length by using C<sv_setpv> or by specifying
0 as the second argument to C<newSVpv>. Be warned, though, that Perl will
determine the string's length by using C<strlen>, which depends on the
-string terminating with a NUL character, and not otherwise containing
+string terminating with a C<NUL> character, and not otherwise containing
NULs.
The arguments of C<sv_setpvf> are processed like C<sprintf>, and the
the format.
The C<sv_set*()> functions are not generic enough to operate on values
-that have "magic". See L<Magic Virtual Tables> later in this document.
+that have "magic". See L</Magic Virtual Tables> later in this document.
-All SVs that contain strings should be terminated with a NUL character.
-If it is not NUL-terminated there is a risk of
+All SVs that contain strings should be terminated with a C<NUL> character.
+If it is not C<NUL>-terminated there is a risk of
core dumps and corruptions from code which passes the string to C
-functions or system calls which expect a NUL-terminated string.
-Perl's own functions typically add a trailing NUL for this reason.
+functions or system calls which expect a C<NUL>-terminated string.
+Perl's own functions typically add a trailing C<NUL> for this reason.
Nevertheless, you should be very careful when you pass a string stored
in an SV to a C function or system call.
used in this case. But that can be quite inefficient because C<PL_na> must
be accessed in thread-local storage in threaded Perl. In any case, remember
that Perl allows arbitrary strings of data that may both contain NULs and
-might not be terminated by a NUL.
+might not be terminated by a C<NUL>.
Also remember that C doesn't allow you to safely say C<foo(SvPV(s, len),
-len);>. It might work with your compiler, but it won't work for everyone.
+len);>. It might work with your
+compiler, but it won't work for everyone.
Break this sort of statement up into separate assignments:
SV *s;
which will determine if more memory needs to be allocated. If so, it will
call the function C<sv_grow>. Note that C<SvGROW> can only increase, not
decrease, the allocated memory of an SV and that it does not automatically
-add space for the trailing NUL byte (perl's own string functions typically do
+add space for the trailing C<NUL> byte (perl's own string functions typically do
C<SvGROW(sv, len + 1)>).
+If you want to write to an existing SV's buffer and set its value to a
+string, use SvPV_force() or one of its variants to force the SV to be
+a PV. This will remove any of various types of non-stringness from
+the SV while preserving the content of the SV in the PV. This can be
+used, for example, to append data from an API function to a buffer
+without extra copying:
+
+ (void)SvPVbyte_force(sv, len);
+ s = SvGROW(sv, len + needlen + 1);
+ /* something that modifies up to needlen bytes at s+len, but
+ modifies newlen bytes
+ eg. newlen = read(fd, s + len, needlen);
+ ignoring errors for these examples
+ */
+ s[len + newlen] = '\0';
+ SvCUR_set(sv, len + newlen);
+ SvUTF8_off(sv);
+ SvSETMAGIC(sv);
+
+If you already have the data in memory or if you want to keep your
+code simple, you can use one of the sv_cat*() variants, such as
+sv_catpvn(). If you want to insert anywhere in the string you can use
+sv_insert() or sv_insert_flags().
+
+If you don't need the existing content of the SV, you can avoid some
+copying with:
+
+ SvPVCLEAR(sv);
+ s = SvGROW(sv, needlen + 1);
+ /* something that modifies up to needlen bytes at s, but modifies
+ newlen bytes
+ eg. newlen = read(fd, s. needlen);
+ */
+ s[newlen] = '\0';
+ SvCUR_set(sv, newlen);
+ SvPOK_only(sv); /* also clears SVf_UTF8 */
+ SvSETMAGIC(sv);
+
+Again, if you already have the data in memory or want to avoid the
+complexity of the above, you can use sv_setpvn().
+
+If you have a buffer allocated with Newx() and want to set that as the
+SV's value, you can use sv_usepvn_flags(). That has some requirements
+if you want to avoid perl re-allocating the buffer to fit the trailing
+NUL:
+
+ Newx(buf, somesize+1, char);
+ /* ... fill in buf ... */
+ buf[somesize] = '\0';
+ sv_usepvn_flags(sv, buf, somesize, SV_SMAGIC | SV_HAS_TRAILING_NUL);
+ /* buf now belongs to perl, don't release it */
+
If you have an SV and want to know what kind of data Perl thinks is stored
in it, you can use the following macros to check the type of SV you have.
yourself. The third function processes its arguments like C<sprintf> and
appends the formatted output. The fourth function works like C<vsprintf>.
You can specify the address and length of an array of SVs instead of the
-va_list argument. The fifth function extends the string stored in the first
+va_list argument. The fifth function
+extends the string stored in the first
SV with the string stored in the second SV. It also forces the second SV
to be interpreted as a string.
The C<sv_cat*()> functions are not generic enough to operate on values that
-have "magic". See L<Magic Virtual Tables> later in this document.
+have "magic". See L</Magic Virtual Tables> later in this document.
If you know the name of a scalar variable, you can get a pointer to its SV
by using the following:
The scalar C<undef> value is stored in an SV instance called C<PL_sv_undef>.
-Its address can be used whenever an C<SV*> is needed. Make sure that
-you don't try to compare a random sv with C<&PL_sv_undef>. For example
+Its address can be used whenever an C<SV*> is needed. Make sure that
+you don't try to compare a random sv with C<&PL_sv_undef>. For example
when interfacing Perl code, it'll work correctly for:
foo(undef);
So to repeat always use SvOK() to check whether an sv is defined.
Also you have to be careful when using C<&PL_sv_undef> as a value in
-AVs or HVs (see L<AVs, HVs and undefined values>).
+AVs or HVs (see L</AVs, HVs and undefined values>).
There are also the two values C<PL_sv_yes> and C<PL_sv_no>, which contain
boolean TRUE and FALSE values, respectively. Like C<PL_sv_undef>, their
first line and all will be well.
To free an SV that you've created, call C<SvREFCNT_dec(SV*)>. Normally this
-call is not necessary (see L<Reference Counts and Mortality>).
+call is not necessary (see L</Reference Counts and Mortality>).
=head2 Offsets
Perl provides the function C<sv_chop> to efficiently remove characters
from the beginning of a string; you give it an SV and a pointer to
somewhere inside the PV, and it discards everything before the
-pointer. The efficiency comes by means of a little hack: instead of
+pointer. The efficiency comes by means of a little hack: instead of
actually removing the characters, C<sv_chop> sets the flag C<OOK>
(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>.
+effect, and it moves the PV pointer (called C<SvPVX>) forward
+by the number of bytes chopped off, and adjusts C<SvCUR> and C<SvLEN>
+accordingly. (A portion of the space between the old and new PV
+pointers is used to store the count of chopped bytes.)
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
into the middle of this allocated storage.
-This is best demonstrated by example:
+This is best demonstrated by example. Normally copy-on-write will prevent
+the substitution from operator from using this hack, but if you can craft a
+string for which copy-on-write is not possible, you can see it in play. In
+the current implementation, the final byte of a string buffer is used as a
+copy-on-write reference count. If the buffer is not big enough, then
+copy-on-write is skipped. First have a look at an empty string:
- % ./perl -Ilib -MDevel::Peek -le '$a="12345"; $a=~s/.//; Dump($a)'
- SV = PVIV(0x8128450) at 0x81340f0
+ % ./perl -Ilib -MDevel::Peek -le '$a=""; $a .= ""; Dump $a'
+ SV = PV(0x7ffb7c008a70) at 0x7ffb7c030390
REFCNT = 1
- FLAGS = (POK,OOK,pPOK)
- IV = 1 (OFFSET)
- PV = 0x8135781 ( "1" . ) "2345"\0
- CUR = 4
- LEN = 5
-
-Here the number of bytes chopped off (1) is put into IV, and
-C<Devel::Peek::Dump> helpfully reminds us that this is an offset. The
+ FLAGS = (POK,pPOK)
+ PV = 0x7ffb7bc05b50 ""\0
+ CUR = 0
+ LEN = 10
+
+Notice here the LEN is 10. (It may differ on your platform.) Extend the
+length of the string to one less than 10, and do a substitution:
+
+ % ./perl -Ilib -MDevel::Peek -le '$a=""; $a.="123456789"; $a=~s/.//; \
+ Dump($a)'
+ SV = PV(0x7ffa04008a70) at 0x7ffa04030390
+ REFCNT = 1
+ FLAGS = (POK,OOK,pPOK)
+ OFFSET = 1
+ PV = 0x7ffa03c05b61 ( "\1" . ) "23456789"\0
+ CUR = 8
+ LEN = 9
+
+Here the number of bytes chopped off (1) is shown next as the OFFSET. The
portion of the string between the "real" and the "fake" beginnings is
shown in parentheses, and the values of C<SvCUR> and C<SvLEN> reflect
-the fake beginning, not the real one.
+the fake beginning, not the real one. (The first character of the string
+buffer happens to have changed to "\1" here, not "1", because the current
+implementation stores the offset count in the string buffer. This is
+subject to change.)
Something similar to the offset hack is performed on AVs to enable
efficient shifting and splicing off the beginning of the array; while
C<AvARRAY> points to the first element in the array that is visible from
-Perl, C<AvALLOC> points to the real start of the C array. These are
+Perl, C<AvALLOC> points to the real start of the C array. These are
usually the same, but a C<shift> operation can be carried out by
increasing C<AvARRAY> by one and decreasing C<AvFILL> and C<AvMAX>.
Again, the location of the real start of the C array only comes into
-play when freeing the array. See C<av_shift> in F<av.c>.
+play when freeing the array.
See C<av_shift> in F<av.c>.
=head2 What's Really Stored in an SV?
stored in your SV. The "p" stands for private.
There 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
+For example, in perl 5.16 and earlier 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. (In perl 5.18 onwards, tied scalars use
+the flags the same way as untied scalars.) Another is when
numeric conversion has occurred and precision has been lost: only the
-private flag is set on 'lossy' values. So when an NV is converted to an
+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.
The second method both creates the AV and initially populates it with SVs:
- AV* av_make(I32 num, SV **ptr);
+ AV* av_make(SSize_t num, SV **ptr);
The second argument points to an array containing C<num> C<SV*>'s. Once the
AV has been created, the SVs can be destroyed, if so desired.
void av_push(AV*, SV*);
SV* av_pop(AV*);
SV* av_shift(AV*);
- void av_unshift(AV*, I32 num);
+ void av_unshift(AV*, SSize_t num);
These should be familiar operations, with the exception of C<av_unshift>.
This routine adds C<num> elements at the front of the array with the C<undef>
Here are some other functions:
- I32 av_top_index(AV*);
- SV** av_fetch(AV*, I32 key, I32 lval);
- SV** av_store(AV*, I32 key, SV* val);
+ SSize_t av_top_index(AV*);
+ SV** av_fetch(AV*, SSize_t key, I32 lval);
+ SV** av_store(AV*, SSize_t key, SV* val);
The C<av_top_index> function returns the highest index value in an array (just
like $#array in Perl). If the array is empty, -1 is returned. The
void av_clear(AV*);
void av_undef(AV*);
- void av_extend(AV*, I32 key);
+ void av_extend(AV*, SSize_t key);
The C<av_clear> function deletes all the elements in the AV* array, but
does not actually delete the array itself. The C<av_undef> function will
This returns NULL if the variable does not exist.
-See L<Understanding the Magic of Tied Hashes and Arrays> for more
+See L</Understanding the Magic of Tied Hashes and Arrays> for more
information on how to use the array access functions on tied arrays.
=head2 Working with HVs
of perl, and the return value may change per invocation, so the value
is only valid for the duration of a single perl process.
-See L<Understanding the Magic of Tied Hashes and Arrays> for more
+See L</Understanding the Magic of Tied Hashes and Arrays> for more
information on how to use the hash access functions on tied hashes.
+=for apidoc Amh|void|PERL_HASH|U32 hash|char *key|STRLEN klen
+
=head2 Hash API Extensions
Beginning with version 5.004, the following functions are also supported:
=head2 AVs, HVs and undefined values
-Sometimes you have to store undefined values in AVs or HVs. Although
-this may be a rare case, it can be tricky. That's because you're
+Sometimes you have to store undefined values in AVs or HVs. Although
+this may be a rare case, it can be tricky. That's because you're
used to using C<&PL_sv_undef> if you need an undefined SV.
For example, intuition tells you that this XS code:
my @av;
$av[0] = undef;
-Unfortunately, this isn't true. AVs use C<&PL_sv_undef> as a marker
+Unfortunately, this isn't true. In perl 5.18 and earlier, AVs use C<&PL_sv_undef> as a marker
for indicating that an array element has not yet been initialized.
Thus, C<exists $av[0]> would be true for the above Perl code, but
-false for the array generated by the XS code.
+false for the array generated by the XS code. In perl 5.20, storing
+&PL_sv_undef will create a read-only element, because the scalar
+&PL_sv_undef itself is stored, not a copy.
-Other problems can occur when storing C<&PL_sv_undef> in HVs:
+Similar problems can occur when storing C<&PL_sv_undef> in HVs:
hv_store( hv, "key", 3, &PL_sv_undef, 0 );
Modification of non-creatable hash value attempted
In perl 5.8.0, C<&PL_sv_undef> was also used to mark placeholders
-in restricted hashes. This caused such hash entries not to appear
+in restricted hashes. This caused such hash entries not to appear
when iterating over the hash or when checking for the keys
with the C<hv_exists> function.
You can run into similar problems when you store C<&PL_sv_yes> or
-C<&PL_sv_no> into AVs or HVs. Trying to modify such elements
+C<&PL_sv_no> into AVs or HVs. Trying to modify such elements
will give you the following error:
Modification of a read-only value attempted
The most useful types that will be returned are:
- < SVt_PVAV Scalar
SVt_PVAV Array
SVt_PVHV Hash
SVt_PVCV Code
SVt_PVGV Glob (possibly a file handle)
+Any numerical value returned which is less than SVt_PVAV will be a scalar
+of some form.
+
See L<perlapi/svtype> for more details.
=head2 Blessed References and Class Objects
The C<sv> argument must be a reference value. The C<stash> argument
specifies which class the reference will belong to. See
-L<Stashes and Globs> for information on converting class names into stashes.
+L</Stashes and Globs> for information on converting class names into stashes.
/* Still under construction */
int sv_isobject(SV* sv);
The following function tests whether the SV is derived from the specified
-class. SV can be either a reference to a blessed object or a string
-containing a class name. This is the function implementing the
+class. SV can be either a reference to a blessed object or a string
+containing a class name. This is the function implementing the
C<UNIVERSAL::isa> functionality.
bool sv_derived_from(SV* sv, const char* name);
=head2 Reference Counts and Mortality
-Perl uses a reference count-driven garbage collection mechanism. SVs,
+Perl uses a reference count-driven garbage collection mechanism. SVs,
AVs, or HVs (xV for short in the following) start their life with a
reference count of 1. If the reference count of an xV ever drops to 0,
then it will be destroyed and its memory made available for reuse.
-
-This normally doesn't happen at the Perl level unless a variable is
-undef'ed or the last variable holding a reference to it is changed or
-overwritten. At the internal level, however, reference counts can be
-manipulated with the following macros:
+At the most basic internal level, reference counts can be manipulated
+with the following macros:
int SvREFCNT(SV* sv);
SV* SvREFCNT_inc(SV* sv);
void SvREFCNT_dec(SV* sv);
-However, there is one other function which manipulates the reference
-count of its argument. The C<newRV_inc> function, you will recall,
-creates a reference to the specified argument. As a side effect,
-it increments the argument's reference count. If this is not what
-you want, use C<newRV_noinc> instead.
-
-For example, imagine you want to return a reference from an XSUB function.
-Inside the XSUB routine, you create an SV which initially has a reference
-count of one. Then you call C<newRV_inc>, passing it the just-created SV.
-This returns the reference as a new SV, but the reference count of the
-SV you passed to C<newRV_inc> has been incremented to two. Now you
-return the reference from the XSUB routine and forget about the SV.
-But Perl hasn't! Whenever the returned reference is destroyed, the
-reference count of the original SV is decreased to one and nothing happens.
-The SV will hang around without any way to access it until Perl itself
-terminates. This is a memory leak.
-
-The correct procedure, then, is to use C<newRV_noinc> instead of
-C<newRV_inc>. Then, if and when the last reference is destroyed,
-the reference count of the SV will go to zero and it will be destroyed,
-stopping any memory leak.
+(There are also suffixed versions of the increment and decrement macros,
+for situations where the full generality of these basic macros can be
+exchanged for some performance.)
+
+However, the way a programmer should think about references is not so
+much in terms of the bare reference count, but in terms of I<ownership>
+of references. A reference to an xV can be owned by any of a variety
+of entities: another xV, the Perl interpreter, an XS data structure,
+a piece of running code, or a dynamic scope. An xV generally does not
+know what entities own the references to it; it only knows how many
+references there are, which is the reference count.
+
+To correctly maintain reference counts, it is essential to keep track
+of what references the XS code is manipulating. The programmer should
+always know where a reference has come from and who owns it, and be
+aware of any creation or destruction of references, and any transfers
+of ownership. Because ownership isn't represented explicitly in the xV
+data structures, only the reference count need be actually maintained
+by the code, and that means that this understanding of ownership is not
+actually evident in the code. For example, transferring ownership of a
+reference from one owner to another doesn't change the reference count
+at all, so may be achieved with no actual code. (The transferring code
+doesn't touch the referenced object, but does need to ensure that the
+former owner knows that it no longer owns the reference, and that the
+new owner knows that it now does.)
+
+An xV that is visible at the Perl level should not become unreferenced
+and thus be destroyed. Normally, an object will only become unreferenced
+when it is no longer visible, often by the same means that makes it
+invisible. For example, a Perl reference value (RV) owns a reference to
+its referent, so if the RV is overwritten that reference gets destroyed,
+and the no-longer-reachable referent may be destroyed as a result.
+
+Many functions have some kind of reference manipulation as
+part of their purpose. Sometimes this is documented in terms
+of ownership of references, and sometimes it is (less helpfully)
+documented in terms of changes to reference counts. For example, the
+L<newRV_inc()|perlapi/newRV_inc> function is documented to create a new RV
+(with reference count 1) and increment the reference count of the referent
+that was supplied by the caller. This is best understood as creating
+a new reference to the referent, which is owned by the created RV,
+and returning to the caller ownership of the sole reference to the RV.
+The L<newRV_noinc()|perlapi/newRV_noinc> function instead does not
+increment the reference count of the referent, but the RV nevertheless
+ends up owning a reference to the referent. It is therefore implied
+that the caller of C<newRV_noinc()> is relinquishing a reference to the
+referent, making this conceptually a more complicated operation even
+though it does less to the data structures.
+
+For example, imagine you want to return a reference from an XSUB
+function. Inside the XSUB routine, you create an SV which initially
+has just a single reference, owned by the XSUB routine. This reference
+needs to be disposed of before the routine is complete, otherwise it
+will leak, preventing the SV from ever being destroyed. So to create
+an RV referencing the SV, it is most convenient to pass the SV to
+C<newRV_noinc()>, which consumes that reference. Now the XSUB routine
+no longer owns a reference to the SV, but does own a reference to the RV,
+which in turn owns a reference to the SV. The ownership of the reference
+to the RV is then transferred by the process of returning the RV from
+the XSUB.
There are some convenience functions available that can help with the
destruction of xVs. These functions introduce the concept of "mortality".
-An xV that is mortal has had its reference count marked to be decremented,
-but not actually decremented, until "a short time later". Generally the
-term "short time later" means a single Perl statement, such as a call to
-an XSUB function. The actual determinant for when mortal xVs have their
-reference count decremented depends on two macros, SAVETMPS and FREETMPS.
-See L<perlcall> and L<perlxs> for more details on these macros.
-
-"Mortalization" then is at its simplest a deferred C<SvREFCNT_dec>.
-However, if you mortalize a variable twice, the reference count will
-later be decremented twice.
-
-"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 it's popped off
-the stack. Similarly, results returned by XSUBs (which are pushed on the
-stack) are often made mortal.
-
-To create a mortal variable, use the functions:
+Much documentation speaks of an xV itself being mortal, but this is
+misleading. It is really I<a reference to> an xV that is mortal, and it
+is possible for there to be more than one mortal reference to a single xV.
+For a reference to be mortal means that it is owned by the temps stack,
+one of perl's many internal stacks, which will destroy that reference
+"a short time later". Usually the "short time later" is the end of
+the current Perl statement. However, it gets more complicated around
+dynamic scopes: there can be multiple sets of mortal references hanging
+around at the same time, with different death dates. Internally, the
+actual determinant for when mortal xV references are destroyed depends
+on two macros, SAVETMPS and FREETMPS. See L<perlcall> and L<perlxs>
+and L</Temporaries Stack> below for more details on these macros.
+
+Mortal references are mainly used for xVs that are placed on perl's
+main stack. The stack is problematic for reference tracking, because it
+contains a lot of xV references, but doesn't own those references: they
+are not counted. Currently, there are many bugs resulting from xVs being
+destroyed while referenced by the stack, because the stack's uncounted
+references aren't enough to keep the xVs alive. So when putting an
+(uncounted) reference on the stack, it is vitally important to ensure that
+there will be a counted reference to the same xV that will last at least
+as long as the uncounted reference. But it's also important that that
+counted reference be cleaned up at an appropriate time, and not unduly
+prolong the xV's life. For there to be a mortal reference is often the
+best way to satisfy this requirement, especially if the xV was created
+especially to be put on the stack and would otherwise be unreferenced.
+
+To create a mortal reference, use the functions:
SV* sv_newmortal()
- SV* sv_2mortal(SV*)
SV* sv_mortalcopy(SV*)
+ SV* sv_2mortal(SV*)
-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. :
+C<sv_newmortal()> creates an SV (with the undefined value) whose sole
+reference is mortal. C<sv_mortalcopy()> creates an xV whose value is a
+copy of a supplied xV and whose sole reference is mortal. C<sv_2mortal()>
+mortalises an existing xV reference: it transfers ownership of a reference
+from the caller to the temps stack. 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);
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 a 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
C<sv_2mortal> or C<sv_mortalcopy> routines.
If the order of C<sv_setiv> and C<sv_setpv> had been reversed, then the
macro C<SvPOK_on> would need to be called instead of C<SvIOK_on>.
+=head2 Read-Only Values
+
+In Perl 5.16 and earlier, copy-on-write (see the next section) shared a
+flag bit with read-only scalars. So the only way to test whether
+C<sv_setsv>, etc., will raise a "Modification of a read-only value" error
+in those versions is:
+
+ SvREADONLY(sv) && !SvIsCOW(sv)
+
+Under Perl 5.18 and later, SvREADONLY only applies to read-only variables,
+and, under 5.20, copy-on-write scalars can also be read-only, so the above
+check is incorrect. You just want:
+
+ SvREADONLY(sv)
+
+If you need to do this check often, define your own macro like this:
+
+ #if PERL_VERSION >= 18
+ # define SvTRULYREADONLY(sv) SvREADONLY(sv)
+ #else
+ # define SvTRULYREADONLY(sv) (SvREADONLY(sv) && !SvIsCOW(sv))
+ #endif
+
+=head2 Copy on Write
+
+Perl implements a copy-on-write (COW) mechanism for scalars, in which
+string copies are not immediately made when requested, but are deferred
+until made necessary by one or the other scalar changing. This is mostly
+transparent, but one must take care not to modify string buffers that are
+shared by multiple SVs.
+
+You can test whether an SV is using copy-on-write with C<SvIsCOW(sv)>.
+
+You can force an SV to make its own copy of its string buffer by calling C<sv_force_normal(sv)> or SvPV_force_nolen(sv).
+
+If you want to make the SV drop its string buffer, use
+C<sv_force_normal_flags(sv, SV_COW_DROP_PV)> or simply
+C<sv_setsv(sv, NULL)>.
+
+All of these functions will croak on read-only scalars (see the previous
+section for more on those).
+
+To test that your code is behaving correctly and not modifying COW buffers,
+on systems that support L<mmap(2)> (i.e., Unix) you can configure perl with
+C<-Accflags=-DPERL_DEBUG_READONLY_COW> and it will turn buffer violations
+into crashes. You will find it to be marvellously slow, so you may want to
+skip perl's own tests.
+
=head2 Magic Variables
[This section still under construction. Ignore everything here. Post no
feature.
If C<sv> is not already magical, Perl uses the C<SvUPGRADE> macro to
-convert C<sv> to type C<SVt_PVMG>. Perl then continues by adding new magic
+convert C<sv> to type C<SVt_PVMG>.
+Perl then continues by adding new magic
to the beginning of the linked list of magical features. Any prior entry
of the same type of magic is deleted. Note that this can be overridden,
and multiple instances of the same type of magic can be associated with an
SV.
The C<name> and C<namlen> arguments are used to associate a string with
-the magic, typically the name of a variable. C<namlen> is stored in the
+the magic, typically the name of a variable. C<namlen> is stored in the
C<mg_len> field and if C<name> is non-null then either a C<savepvn> copy of
C<name> or C<name> itself is stored in the C<mg_ptr> field, depending on
whether C<namlen> is greater than zero or equal to zero respectively. As a
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 L<Magic Virtual Tables> section below. The C<how> argument is also
-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 in F<perl.h>. Note that before
+See the L</Magic Virtual Tables> section below. The C<how> argument is also
+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 in F<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
referring 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 C<PERL_MAGIC_arylen>, 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>, C<PERL_MAGIC_regdatum>,
+C<PERL_MAGIC_regdata>, or if it is a NULL pointer, then C<obj> is merely
+stored, without the reference count being incremented.
See also C<sv_magicext> in L<perlapi> for a more flexible way to add magic
to an SV.
was initially made magical.
However, note that C<sv_unmagic> removes all magic of a certain C<type> from the
-C<SV>. If you want to remove only certain magic of a C<type> based on the magic
+C<SV>. If you want to remove only certain
+magic of a C<type> based on the magic
virtual table, use C<sv_unmagicext> instead:
int sv_unmagicext(SV *sv, int type, MGVTBL *vtbl);
The C<MGVTBL> has five (or sometimes eight) pointers to the following
routine types:
- int (*svt_get)(SV* sv, MAGIC* mg);
- int (*svt_set)(SV* sv, MAGIC* mg);
- U32 (*svt_len)(SV* sv, MAGIC* mg);
- int (*svt_clear)(SV* sv, MAGIC* mg);
- int (*svt_free)(SV* sv, MAGIC* mg);
+ int (*svt_get) (pTHX_ SV* sv, MAGIC* mg);
+ int (*svt_set) (pTHX_ SV* sv, MAGIC* mg);
+ U32 (*svt_len) (pTHX_ SV* sv, MAGIC* mg);
+ int (*svt_clear)(pTHX_ SV* sv, MAGIC* mg);
+ int (*svt_free) (pTHX_ SV* sv, MAGIC* mg);
- int (*svt_copy)(SV *sv, MAGIC* mg, SV *nsv,
+ int (*svt_copy) (pTHX_ SV *sv, MAGIC* mg, SV *nsv,
const char *name, I32 namlen);
- int (*svt_dup)(MAGIC *mg, CLONE_PARAMS *param);
- int (*svt_local)(SV *nsv, MAGIC *mg);
+ int (*svt_dup) (pTHX_ MAGIC *mg, CLONE_PARAMS *param);
+ int (*svt_local)(pTHX_ SV *nsv, MAGIC *mg);
This MGVTBL structure is set at compile-time in F<perl.h> and there are
The last three slots are a recent addition, and for source code
compatibility they are only checked for if one of the three flags
-MGf_COPY, MGf_DUP or MGf_LOCAL is set in mg_flags. This means that most
-code can continue declaring a vtable as a 5-element value. These three are
+MGf_COPY, MGf_DUP or MGf_LOCAL is set in mg_flags.
+This means that most code can continue declaring
+a vtable as a 5-element value. These three are
currently used exclusively by the threading code, and are highly subject
to change.
-------------------------- ------ -------------
\0 PERL_MAGIC_sv vtbl_sv Special scalar variable
# PERL_MAGIC_arylen vtbl_arylen Array length ($#ary)
- % PERL_MAGIC_rhash (none) extra data for restricted
+ % PERL_MAGIC_rhash (none) Extra data for restricted
hashes
- & PERL_MAGIC_proto (none) my sub prototype CV
+ * PERL_MAGIC_debugvar vtbl_debugvar $DB::single, signal, trace
+ vars
. PERL_MAGIC_pos vtbl_pos pos() lvalue
- : PERL_MAGIC_symtab (none) extra data for symbol
+ : PERL_MAGIC_symtab (none) Extra data for symbol
tables
- < PERL_MAGIC_backref vtbl_backref for weak ref data
- @ PERL_MAGIC_arylen_p (none) to move arylen out of XPVAV
+ < PERL_MAGIC_backref vtbl_backref For weak ref data
+ @ PERL_MAGIC_arylen_p (none) To move arylen out of XPVAV
B PERL_MAGIC_bm vtbl_regexp Boyer-Moore
(fast string search)
c PERL_MAGIC_overload_table vtbl_ovrld Holds overload table
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_regexp precompiled qr// regex
+ r PERL_MAGIC_qr vtbl_regexp Precompiled qr// regex
S PERL_MAGIC_sig (none) %SIG hash
s PERL_MAGIC_sigelem vtbl_sigelem %SIG hash element
t PERL_MAGIC_taint vtbl_taint Taintedness
v PERL_MAGIC_vec vtbl_vec vec() lvalue
w PERL_MAGIC_utf8 vtbl_utf8 Cached UTF-8 information
x PERL_MAGIC_substr vtbl_substr substr() lvalue
+ Y PERL_MAGIC_nonelem vtbl_nonelem Array element that does not
+ exist
y PERL_MAGIC_defelem vtbl_defelem Shadow "foreach" iterator
variable / smart parameter
vivification
- ] PERL_MAGIC_checkcall vtbl_checkcall inlining/mutation of call
+ \ PERL_MAGIC_lvref vtbl_lvref Lvalue reference
+ constructor
+ ] PERL_MAGIC_checkcall vtbl_checkcall Inlining/mutation of call
to this CV
~ PERL_MAGIC_ext (none) Available for use by
extensions
+
+=for apidoc Amnh||PERL_MAGIC_sv
+=for apidoc Amnh||PERL_MAGIC_arylen
+=for apidoc Amnh||PERL_MAGIC_rhash
+=for apidoc Amnh||PERL_MAGIC_debugvar
+=for apidoc Amnh||PERL_MAGIC_pos
+=for apidoc Amnh||PERL_MAGIC_symtab
+=for apidoc Amnh||PERL_MAGIC_backref
+=for apidoc Amnh||PERL_MAGIC_arylen_p
+=for apidoc Amnh||PERL_MAGIC_bm
+=for apidoc Amnh||PERL_MAGIC_overload_table
+=for apidoc Amnh||PERL_MAGIC_regdata
+=for apidoc Amnh||PERL_MAGIC_regdatum
+=for apidoc Amnh||PERL_MAGIC_env
+=for apidoc Amnh||PERL_MAGIC_envelem
+=for apidoc Amnh||PERL_MAGIC_fm
+=for apidoc Amnh||PERL_MAGIC_regex_global
+=for apidoc Amnh||PERL_MAGIC_hints
+=for apidoc Amnh||PERL_MAGIC_hintselem
+=for apidoc Amnh||PERL_MAGIC_isa
+=for apidoc Amnh||PERL_MAGIC_isaelem
+=for apidoc Amnh||PERL_MAGIC_nkeys
+=for apidoc Amnh||PERL_MAGIC_dbfile
+=for apidoc Amnh||PERL_MAGIC_dbline
+=for apidoc Amnh||PERL_MAGIC_shared
+=for apidoc Amnh||PERL_MAGIC_shared_scalar
+=for apidoc Amnh||PERL_MAGIC_collxfrm
+=for apidoc Amnh||PERL_MAGIC_tied
+=for apidoc Amnh||PERL_MAGIC_tiedelem
+=for apidoc Amnh||PERL_MAGIC_tiedscalar
+=for apidoc Amnh||PERL_MAGIC_qr
+=for apidoc Amnh||PERL_MAGIC_sig
+=for apidoc Amnh||PERL_MAGIC_sigelem
+=for apidoc Amnh||PERL_MAGIC_taint
+=for apidoc Amnh||PERL_MAGIC_uvar
+=for apidoc Amnh||PERL_MAGIC_uvar_elem
+=for apidoc Amnh||PERL_MAGIC_vstring
+=for apidoc Amnh||PERL_MAGIC_vec
+=for apidoc Amnh||PERL_MAGIC_utf8
+=for apidoc Amnh||PERL_MAGIC_substr
+=for apidoc Amnh||PERL_MAGIC_nonelem
+=for apidoc Amnh||PERL_MAGIC_defelem
+=for apidoc Amnh||PERL_MAGIC_lvref
+=for apidoc Amnh||PERL_MAGIC_checkcall
+=for apidoc Amnh||PERL_MAGIC_ext
+
=for mg_vtable.pl end
When an uppercase and lowercase letter both exist in the table, then the
uppercase letter is typically 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. Some internals code makes use of this case
+of that composite type. Some internals code makes use of this case
relationship. However, 'v' and 'V' (vec and v-string) are in no way related.
The C<PERL_MAGIC_ext> and C<PERL_MAGIC_uvar> magic types are defined
For C<PERL_MAGIC_ext> magic, it is usually a good idea to define an
C<MGVTBL>, even if all its fields will be C<0>, so that individual
C<MAGIC> pointers can be identified as a particular kind of magic
-using their magic virtual table. C<mg_findext> provides an easy way
+using their magic virtual table. C<mg_findext> provides an easy way
to do that:
STATIC MGVTBL my_vtbl = { 0, 0, 0, 0, 0, 0, 0, 0 };
* type */
This routine returns a pointer to a C<MAGIC> structure stored in the SV.
-If the SV does not have that magical feature, C<NULL> is returned. If the
+If the SV does not have that magical
+feature, C<NULL> is returned. If the
SV has multiple instances of that magical feature, the first one will be
-returned. C<mg_findext> can be used to find a C<MAGIC> structure of an SV
+returned. C<mg_findext> can be used
+to find a C<MAGIC> structure of an SV
based on both its magic type and its magic virtual table:
MAGIC *mg_findext(SV *sv, int type, MGVTBL *vtbl);
WARNING: As of the 5.004 release, proper usage of the array and hash
access functions requires understanding a few caveats. Some
of these caveats are actually considered bugs in the API, to be fixed
-in later releases, and are bracketed with [MAYCHANGE] below. If
+in later releases, and are bracketed with [MAYCHANGE] below. If
you find yourself actually applying such information in this section, be
aware that the behavior may change in the future, umm, without warning.
The perl tie function associates a variable with an object that implements
the various GET, SET, etc methods. To perform the equivalent of the perl
tie function from an XSUB, you must mimic this behaviour. The code below
-carries out the necessary steps - firstly it creates a new hash, and then
+carries out the necessary steps -- firstly it creates a new hash, and then
creates a second hash which it blesses into the class which will implement
-the tie methods. Lastly it ties the two hashes together, and returns a
+the tie methods. Lastly it ties the two hashes together, and returns a
reference to the new tied hash. Note that the code below does NOT call the
TIEHASH method in the MyTie class -
-see L<Calling Perl Routines from within C Programs> for details on how
+see L</Calling Perl Routines from within C Programs> for details on how
to do this.
SV*
The biggest difference is that the first construction would
reinstate the initial value of $var, irrespective of how control exits
-the block: C<goto>, C<return>, C<die>/C<eval>, etc. It is a little bit
+the block: C<goto>, C<return>, C<die>/C<eval>, etc. It is a little bit
more efficient as well.
There is a way to achieve a similar task from C via Perl API: create a
I<pseudo-block>, and arrange for some changes to be automatically
undone at the end of it, either explicit, or via a non-local exit (via
-die()). A I<block>-like construct is created by a pair of
+die()). A I<block>-like construct is created by a pair of
C<ENTER>/C<LEAVE> macros (see L<perlcall/"Returning a Scalar">).
Such a construct may be created specially for some important localized
task, or an existing one (like boundaries of enclosing Perl
subroutine/block, or an existing pair for freeing TMPs) may be
-used. (In the second case the overhead of additional localization must
-be almost negligible.) Note that any XSUB is automatically enclosed in
+used. (In the second case the overhead of additional localization must
+be almost negligible.) Note that any XSUB is automatically enclosed in
an C<ENTER>/C<LEAVE> pair.
Inside such a I<pseudo-block> the following service is available:
=item C<SAVEPPTR(p)>
These macros arrange things to restore the value of pointers C<s> and
-C<p>. C<s> must be a pointer of a type which survives conversion to
+C<p>. C<s> must be a pointer of a type which survives conversion to
C<SV*> and back, C<p> should be able to survive conversion to C<char*>
and back.
=item C<SAVEFREESV(SV *sv)>
-The refcount of C<sv> would be decremented at the end of
+The refcount of C<sv> will be decremented at the end of
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,
=item C<SAVEDELETE(HV *hv, char *key, I32 length)>
-The key C<key> of C<hv> is deleted at the end of I<pseudo-block>. The
+The key C<key> of C<hv> is deleted at the end of I<pseudo-block>. The
string pointed to by C<key> is Safefree()ed. If one has a I<key> in
short-lived storage, the corresponding string may be reallocated like
this:
=item C<SV* save_scalar(GV *gv)>
+=for apidoc save_scalar
+
Equivalent to Perl code C<local $gv>.
=item C<AV* save_ary(GV *gv)>
+=for apidoc save_ary
+
=item C<HV* save_hash(GV *gv)>
+=for apidoc save_hash
+
Similar to C<save_scalar>, but localize C<@gv> and C<%gv>.
=item C<void save_item(SV *item)>
-Duplicates the current value of C<SV>, on the exit from the current
-C<ENTER>/C<LEAVE> I<pseudo-block> will restore the value of C<SV>
-using the stored value. It doesn't handle magic. Use C<save_scalar> if
+=for apidoc save_item
+
+Duplicates the current value of C<SV>. On the exit from the current
+C<ENTER>/C<LEAVE> I<pseudo-block> the value of C<SV> will be restored
+using the stored value. It doesn't handle magic. Use C<save_scalar> if
magic is affected.
=item C<void save_list(SV **sarg, I32 maxsarg)>
+=for apidoc save_list
+
A variant of C<save_item> which takes multiple arguments via an array
C<sarg> of C<SV*> of length C<maxsarg>.
=item C<SV* save_svref(SV **sptr)>
+=for apidoc save_svref
+
Similar to C<save_scalar>, but will reinstate an C<SV *>.
=item C<void save_aptr(AV **aptr)>
=item C<void save_hptr(HV **hptr)>
+=for apidoc save_aptr
+=for apidoc save_hptr
+
Similar to C<save_svref>, but localize C<AV *> and C<HV *>.
=back
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 C<PUSHs>
-macro. The pushed values will often need to be "mortal" (See
+macro. The pushed values will often need to be "mortal" (See
L</Reference Counts and Mortality>):
PUSHs(sv_2mortal(newSViv(an_integer)))
For a detailed description of calling conventions from C to Perl,
consult L<perlcall>.
-=head2 Memory Allocation
-
-=head3 Allocation
-
-All memory meant to be used with the Perl API functions should be manipulated
-using the macros described in this section. The macros provide the necessary
-transparency between differences in the actual malloc implementation that is
-used within perl.
-
-It is suggested that you enable the version of malloc that is distributed
-with Perl. It keeps pools of various sizes of unallocated memory in
-order to satisfy allocation requests more quickly. However, on some
-platforms, it may cause spurious malloc or free errors.
-
-The following three macros are used to initially allocate memory :
-
- Newx(pointer, number, type);
- Newxc(pointer, number, type, cast);
- Newxz(pointer, number, type);
-
-The first argument C<pointer> should be the name of a variable that will
-point to the newly allocated memory.
-
-The second and third arguments C<number> and C<type> specify how many of
-the specified type of data structure should be allocated. The argument
-C<type> is passed to C<sizeof>. The final argument to C<Newxc>, C<cast>,
-should be used if the C<pointer> argument is different from the C<type>
-argument.
-
-Unlike the C<Newx> and C<Newxc> macros, the C<Newxz> macro calls C<memzero>
-to zero out all the newly allocated memory.
-
-=head3 Reallocation
-
- Renew(pointer, number, type);
- Renewc(pointer, number, type, cast);
- Safefree(pointer)
-
-These three macros are used to change a memory buffer size or to free a
-piece of memory no longer needed. The arguments to C<Renew> and C<Renewc>
-match those of C<New> and C<Newc> with the exception of not needing the
-"magic cookie" argument.
-
-=head3 Moving
-
- Move(source, dest, number, type);
- Copy(source, dest, number, type);
- Zero(dest, number, type);
-
-These three macros are used to move, copy, or zero out previously allocated
-memory. The C<source> and C<dest> arguments point to the source and
-destination starting points. Perl will move, copy, or zero out C<number>
-instances of the size of the C<type> data structure (using the C<sizeof>
-function).
-
-=head2 PerlIO
-
-The most recent development releases of Perl have been experimenting with
-removing Perl's dependency on the "normal" standard I/O suite and allowing
-other stdio implementations to be used. This involves creating a new
-abstraction layer that then calls whichever implementation of stdio Perl
-was compiled with. All XSUBs should now use the functions in the PerlIO
-abstraction layer and not make any assumptions about what kind of stdio
-is being used.
-
-For a complete description of the PerlIO abstraction, consult L<perlapio>.
-
=head2 Putting a C value on Perl stack
A lot of opcodes (this is an elementary operation in the internal perl
-stack machine) put an SV* on the stack. However, as an optimization
-the corresponding SV is (usually) not recreated each time. The opcodes
+stack machine) put an SV* on the stack. However, as an optimization
+the corresponding SV is (usually) not recreated each time. The opcodes
reuse specially assigned SVs (I<target>s) which are (as a corollary)
not constantly freed/created.
Each of the targets is created only once (but see
-L<Scratchpads and recursion> below), and when an opcode needs to put
+L</Scratchpads and recursion> below), and when an opcode needs to put
an integer, a double, or a string on stack, it just sets the
corresponding parts of its I<target> and puts the I<target> on stack.
others, which use it via C<(X)PUSH[iunp]>.
Because the target is reused, you must be careful when pushing multiple
-values on the stack. The following code will not do what you think:
+values on the stack. The following code will not do what you think:
XPUSHi(10);
XPUSHi(20);
=head2 Scratchpads
The question remains on when the SVs which are I<target>s for opcodes
-are created. The answer is that they are created when the current
+are created. The answer is that they are created when the current
unit--a subroutine or a file (for opcodes for statements outside of
-subroutines)--is compiled. During this time a special anonymous Perl
+subroutines)--is compiled. During this time a special anonymous Perl
array is created, which is called a scratchpad for the current unit.
A scratchpad keeps SVs which are lexicals for the current unit and are
-targets for opcodes. One can deduce that an SV lives on a scratchpad
+targets for opcodes. A previous version of this document
+stated that one can deduce that an SV lives on a scratchpad
by looking on its flags: lexicals have C<SVs_PADMY> set, and
-I<target>s have C<SVs_PADTMP> set.
-
-The correspondence between OPs and I<target>s is not 1-to-1. Different
+I<target>s have C<SVs_PADTMP> set. But this has never been fully true.
+C<SVs_PADMY> could be set on a variable that no longer resides in any pad.
+While I<target>s do have C<SVs_PADTMP> set, it can also be set on variables
+that have never resided in a pad, but nonetheless act like I<target>s. As
+of perl 5.21.5, the C<SVs_PADMY> flag is no longer used and is defined as
+0. C<SvPADMY()> now returns true for anything without C<SVs_PADTMP>.
+
+The correspondence between OPs and I<target>s is not 1-to-1. Different
OPs in the compile tree of the unit can use the same target, if this
would not conflict with the expected life of the temporary.
=head2 Scratchpads and recursion
In fact it is not 100% true that a compiled unit contains a pointer to
-the scratchpad AV. In fact it contains a pointer to an AV of
-(initially) one element, and this element is the scratchpad AV. Why do
+the scratchpad AV. In fact it contains a pointer to an AV of
+(initially) one element, and this element is the scratchpad AV. Why do
we need an extra level of indirection?
-The answer is B<recursion>, and maybe B<threads>. Both
+The answer is B<recursion>, and maybe B<threads>. Both
these can create several execution pointers going into the same
-subroutine. For the subroutine-child not write over the temporaries
+subroutine. For the subroutine-child not write over the temporaries
for the subroutine-parent (lifespan of which covers the call to the
child), the parent and the child should have different
-scratchpads. (I<And> the lexicals should be separate anyway!)
+scratchpads.
(I<And> the lexicals should be separate anyway!)
So each subroutine is born with an array of scratchpads (of length 1).
On each entry to the subroutine it is checked that the current
The I<target>s on this scratchpad are C<undef>s, but they are already
marked with correct flags.
+=head1 Memory Allocation
+
+=head2 Allocation
+
+All memory meant to be used with the Perl API functions should be manipulated
+using the macros described in this section. The macros provide the necessary
+transparency between differences in the actual malloc implementation that is
+used within perl.
+
+It is suggested that you enable the version of malloc that is distributed
+with Perl. It keeps pools of various sizes of unallocated memory in
+order to satisfy allocation requests more quickly. However, on some
+platforms, it may cause spurious malloc or free errors.
+
+The following three macros are used to initially allocate memory :
+
+ Newx(pointer, number, type);
+ Newxc(pointer, number, type, cast);
+ Newxz(pointer, number, type);
+
+The first argument C<pointer> should be the name of a variable that will
+point to the newly allocated memory.
+
+The second and third arguments C<number> and C<type> specify how many of
+the specified type of data structure should be allocated. The argument
+C<type> is passed to C<sizeof>. The final argument to C<Newxc>, C<cast>,
+should be used if the C<pointer> argument is different from the C<type>
+argument.
+
+Unlike the C<Newx> and C<Newxc> macros, the C<Newxz> macro calls C<memzero>
+to zero out all the newly allocated memory.
+
+=head2 Reallocation
+
+ Renew(pointer, number, type);
+ Renewc(pointer, number, type, cast);
+ Safefree(pointer)
+
+These three macros are used to change a memory buffer size or to free a
+piece of memory no longer needed. The arguments to C<Renew> and C<Renewc>
+match those of C<New> and C<Newc> with the exception of not needing the
+"magic cookie" argument.
+
+=head2 Moving
+
+ Move(source, dest, number, type);
+ Copy(source, dest, number, type);
+ Zero(dest, number, type);
+
+These three macros are used to move, copy, or zero out previously allocated
+memory. The C<source> and C<dest> arguments point to the source and
+destination starting points. Perl will move, copy, or zero out C<number>
+instances of the size of the C<type> data structure (using the C<sizeof>
+function).
+
+=head1 PerlIO
+
+The most recent development releases of Perl have been experimenting with
+removing Perl's dependency on the "normal" standard I/O suite and allowing
+other stdio implementations to be used. This involves creating a new
+abstraction layer that then calls whichever implementation of stdio Perl
+was compiled with. All XSUBs should now use the functions in the PerlIO
+abstraction layer and not make any assumptions about what kind of stdio
+is being used.
+
+For a complete description of the PerlIO abstraction, consult L<perlapio>.
+
=head1 Compiled code
=head2 Code tree
Here we describe the internal form your code is converted to by
-Perl. Start with a simple example:
+Perl. Start with a simple example:
$a = $b + $c;
C<gvsv gvsv add whatever>.
Each of these nodes represents an op, a fundamental operation inside the
-Perl core. The code which implements each operation can be found in the
+Perl core. The code which implements each operation can be found in the
F<pp*.c> files; the function which implements the op with type C<gvsv>
-is C<pp_gvsv>, and so on. As the tree above shows, different ops have
+is C<pp_gvsv>, and so on. As the tree above shows, different ops have
different numbers of children: C<add> is a binary operator, as one would
-expect, and so has two children. To accommodate the various different
+expect, and so has two children. To accommodate the various different
numbers of children, there are various types of op data structure, and
they link together in different ways.
-The simplest type of op structure is C<OP>: this has no children. Unary
+The simplest type of op structure is C<OP>: this has no children. Unary
operators, C<UNOP>s, have one child, and this is pointed to by the
-C<op_first> field. Binary operators (C<BINOP>s) have not only an
-C<op_first> field but also an C<op_last> field. The most complex type of
-op is a C<LISTOP>, which has any number of children. In this case, the
+C<op_first> field. Binary operators (C<BINOP>s) have not only an
+C<op_first> field but also an C<op_last> field. The most complex type of
+op is a C<LISTOP>, which has any number of children. In this case, the
first child is pointed to by C<op_first> and the last child by
-C<op_last>. The children in between can be found by iteratively
-following the C<op_sibling> pointer from the first child to the last.
+C<op_last>. The children in between can be found by iteratively
+following the C<OpSIBLING> pointer from the first child to the last (but
+see below).
-There are also
two other op types: a C<PMOP> holds a regular expression,
-and has no children, and a C<LOOP> may or may not have children. If the
-C<op_children> field is non-zero, it behaves like a C<LISTOP>. To
+There are also
some other op types: a C<PMOP> holds a regular expression,
+and has no children, and a C<LOOP> may or may not have children. If the
+C<op_children> field is non-zero, it behaves like a C<LISTOP>. To
complicate matters, if a C<UNOP> is actually a C<null> op after
optimization (see L</Compile pass 2: context propagation>) it will still
have children in accordance with its former type.
+Finally, there is a C<LOGOP>, or logic op. Like a C<LISTOP>, this has one
+or more children, but it doesn't have an C<op_last> field: so you have to
+follow C<op_first> and then the C<OpSIBLING> chain itself to find the
+last child. Instead it has an C<op_other> field, which is comparable to
+the C<op_next> field described below, and represents an alternate
+execution path. Operators like C<and>, C<or> and C<?> are C<LOGOP>s. Note
+that in general, C<op_other> may not point to any of the direct children
+of the C<LOGOP>.
+
+Starting in version 5.21.2, perls built with the experimental
+define C<-DPERL_OP_PARENT> add an extra boolean flag for each op,
+C<op_moresib>. When not set, this indicates that this is the last op in an
+C<OpSIBLING> chain. This frees up the C<op_sibling> field on the last
+sibling to point back to the parent op. Under this build, that field is
+also renamed C<op_sibparent> to reflect its joint role. The macro
+C<OpSIBLING(o)> wraps this special behaviour, and always returns NULL on
+the last sibling. With this build the C<op_parent(o)> function can be
+used to find the parent of any op. Thus for forward compatibility, you
+should always use the C<OpSIBLING(o)> macro rather than accessing
+C<op_sibling> directly.
+
Another way to examine the tree is to use a compiler back-end module, such
as L<B::Concise>.
=head2 Compile pass 1: check routines
The tree is created by the compiler while I<yacc> code feeds it
-the constructions it recognizes. Since I<yacc> works bottom-up, so does
+the constructions it recognizes. Since I<yacc> works bottom-up, so does
the first pass of perl compilation.
What makes this pass interesting for perl developers is that some
tree (if the top-level node was not modified, check routine returns
its argument).
-By convention, check routines have names C<ck_*>. They are usually
+By convention, check routines have names C<ck_*>. They are usually
called from C<new*OP> subroutines (or C<convert>) (which in turn are
called from F<perly.y>).
=head2 Compile pass 3: peephole optimization
After the compile tree for a subroutine (or for an C<eval> or a file)
-is created, an additional pass over the code is performed. This pass
+is created, an additional pass over the code is performed. This pass
is neither top-down or bottom-up, but in the execution order (with
additional complications for conditionals). Optimizations performed
at this stage are subject to the same restrictions as in the pass 2.
=head2 Compile-time scope hooks
As of perl 5.14 it is possible to hook into the compile-time lexical
-scope mechanism using C<Perl_blockhook_register>. This is used like
+scope mechanism using C<Perl_blockhook_register>. This is used like
this:
STATIC void my_start_hook(pTHX_ int full);
Perl_blockhook_register(aTHX_ &my_hooks);
This will arrange to have C<my_start_hook> called at the start of
-compiling every lexical scope. The available hooks are:
+compiling every lexical scope. The available hooks are:
=over 4
=item C<void bhk_start(pTHX_ int full)>
-This is called just after starting a new lexical scope. Note that Perl
+This is called just after starting a new lexical scope. Note that Perl
code like
if ($x) { ... }
creates two scopes: the first starts at the C<(> and has C<full == 1>,
-the second starts at the C<{> and has C<full == 0>. Both end at the
-C<}>, so calls to C<start> and C<pre/post_end> will match. Anything
+the second starts at the C<{> and has C<full == 0>. Both end at the
+C<}>, so calls to C<start> and C<pre>/C<post_end> will match. Anything
pushed onto the save stack by this hook will be popped just before the
scope ends (between the C<pre_> and C<post_end> hooks, in fact).
=item C<void bhk_pre_end(pTHX_ OP **o)>
This is called at the end of a lexical scope, just before unwinding the
-stack. I<o> is the root of the optree representing the scope; it is a
+stack. I<o> is the root of the optree representing the scope; it is a
double pointer so you can replace the OP if you need to.
=item C<void bhk_post_end(pTHX_ OP **o)>
This is called at the end of a lexical scope, just after unwinding the
-stack.
I<o> is as above. Note that it is possible for calls to C<pre_>
+stack.
I<o> is as above. Note that it is possible for calls to C<pre_>
and C<post_end> to nest, if there is something on the save stack that
calls string eval.
=item C<void bhk_eval(pTHX_ OP *const o)>
This is called just before starting to compile an C<eval STRING>, C<do
-FILE>, C<require> or C<use>, after the eval has been set up. I<o> is the
+FILE>, C<require> or C<use>, after the eval has been set up. I<o> is the
OP that requested the eval, and will normally be an C<OP_ENTEREVAL>,
C<OP_DOFILE> or C<OP_REQUIRE>.
=back
Once you have your hook functions, you need a C<BHK> structure to put
-them in. It's best to allocate it statically, since there is no way to
-free it once it's registered. The function pointers should be inserted
+them in. It's best to allocate it statically, since there is no way to
+free it once it's registered. The function pointers should be inserted
into this structure using the C<BhkENTRY_set> macro, which will also set
-flags indicating which entries are valid. If you do need to allocate
+flags indicating which entries are valid. If you do need to allocate
your C<BHK> dynamically for some reason, be sure to zero it before you
start.
Once registered, there is no mechanism to switch these hooks off, so if
-that is necessary you will need to do this yourself. An entry in C<%^H>
+that is necessary you will need to do this yourself. An entry in C<%^H>
is probably the best way, so the effect is lexically scoped; however it
is also possible to use the C<BhkDISABLE> and C<BhkENABLE> macros to
-temporarily switch entries on and off. You should also be aware that
+temporarily switch entries on and off. You should also be aware that
generally speaking at least one scope will have opened before your
-extension is loaded, so you will see some C<pre/post_end> pairs that
+extension is loaded, so you will see some C<pre>/C<post_end> pairs that
didn't have a matching C<start>.
=head1 Examining internal data structures with the C<dump> functions
functions which produce formatted output of internal data structures.
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
+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.
or inside a thread-specific structure. These structures contain all
the context, the state of that interpreter.
-One macro controls the major Perl build flavor: MULTIPLICITY. The
+One macro controls the major Perl build flavor: MULTIPLICITY. The
MULTIPLICITY build has a C structure that packages all the interpreter
-state. With multiplicity-enabled perls, PERL_IMPLICIT_CONTEXT is also
+state. With multiplicity-enabled perls, PERL_IMPLICIT_CONTEXT is also
normally defined, and enables the support for passing in a "hidden" first
-argument that represents all three data structures. MULTIPLICITY makes
+argument that represents all three data structures. MULTIPLICITY makes
multi-threaded perls possible (with the ithreads threading model, related
to the macro USE_ITHREADS.)
to use C<dVAR> in your coding to "declare the global variables"
when you are using them. dTHX does this for you automatically.
-To see whether you have non-const data you can use a BSD-compatible C<nm>:
+=for apidoc Amnh||dVAR
+
+To see whether you have non-const data you can use a BSD (or GNU)
+compatible C<nm>:
nm libperl.a | grep -v ' [TURtr] '
-If this displays any C<D> or C<d> symbols, you have non-const data.
+If this displays any C<D> or C<d> symbols (or possibly C<C> or C<c>),
+you have non-const data. The symbols the C<grep> removed are as follows:
+C<Tt> are I<text>, or code, the C<Rr> are I<read-only> (const) data,
+and the C<U> is <undefined>, external symbols referred to.
+
+The test F<t/porting/libperl.t> does this kind of symbol sanity
+checking on C<libperl.a>.
For backward compatibility reasons defining just PERL_GLOBAL_STRUCT
doesn't actually hide all symbols inside a big global struct: some
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
+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
or 'd' for B<d>eclaration, so we have C<pTHX>, C<aTHX> and C<dTHX>, and
their variants.
+=for apidoc Amnh||aTHX
+=for apidoc Amnh||aTHX_
+=for apidoc Amnh||dTHX
+=for apidoc Amnh||pTHX
+=for apidoc Amnh||pTHX_
+
When Perl is built without options that set PERL_IMPLICIT_CONTEXT, there is no
first argument containing the interpreter's context. The trailing underscore
in the pTHX_ macro indicates that the macro expansion needs a comma
=head2 So what happened to dTHR?
+=for apidoc Amnh||dTHR
+
C<dTHR> was introduced in perl 5.005 to support the older thread model.
The older thread model now uses the C<THX> mechanism to pass context
pointers around, so C<dTHR> is not useful any more. Perl 5.6.0 and
All of Perl's internal functions which will be exposed to the outside
world are prefixed by C<Perl_> so that they will not conflict with XS
functions or functions used in a program in which Perl is embedded.
-Similarly, all global variables begin with C<PL_>. (By convention,
+Similarly, all global variables begin with C<PL_>. (By convention,
static functions start with C<S_>.)
Inside the Perl core (C<PERL_CORE> defined), you can get at the functions
either with or without the C<Perl_> prefix, thanks to a bunch of defines
-that live in F<embed.h>. Note that extension code should I<not> set
+that live in F<embed.h>. Note that extension code should I<not> set
C<PERL_CORE>; this exposes the full perl internals, and is likely to cause
breakage of the XS in each new perl release.
The file F<embed.h> is generated automatically from
-F<embed.pl> and F<embed.fnc>. F<embed.pl> also creates the prototyping
+F<embed.pl> and F<embed.fnc>. F<embed.pl> also creates the prototyping
header files for the internal functions, generates the documentation
-and a lot of other bits and pieces. It's important that when you add
+and a lot of other bits and pieces. It's important that when you add
a new function to the core or change an existing one, you change the
-data in the table in F<embed.fnc> as well. Here's a sample entry from
+data in the table in F<embed.fnc> as well. Here's a sample entry from
that table:
Apd |SV** |av_fetch |AV* ar|I32 key|I32 lval
-The second column is the return type, the third column the name. Columns
-after that are the arguments. The first column is a set of flags:
-
-=over 3
-
-=item A
-
-This function is a part of the public API. All such functions should also
-have 'd', very few do not.
-
-=item p
-
-This function has a C<Perl_> prefix; i.e. it is defined as
-C<Perl_av_fetch>.
-
-=item d
-
-This function has documentation using the C<apidoc> feature which we'll
-look at in a second. Some functions have 'd' but not 'A'; docs are good.
-
-=back
-
-Other available flags are:
-
-=over 3
-
-=item s
-
-This is a static function and is defined as C<STATIC S_whatever>, and
-usually called within the sources as C<whatever(...)>.
-
-=item n
-
-This does not need an interpreter context, so the definition has no
-C<pTHX>, and it follows that callers don't use C<aTHX>. (See
-L</Background and PERL_IMPLICIT_CONTEXT>.)
-
-=item r
-
-This function never returns; C<croak>, C<exit> and friends.
-
-=item f
-
-This function takes a variable number of arguments, C<printf> style.
-The argument list should end with C<...>, like this:
-
- Afprd |void |croak |const char* pat|...
-
-=item M
-
-This function is part of the experimental development API, and may change
-or disappear without notice.
-
-=item o
-
-This function should not have a compatibility macro to define, say,
-C<Perl_parse> to C<parse>. It must be called as C<Perl_parse>.
-
-=item x
-
-This function isn't exported out of the Perl core.
-
-=item m
-
-This is implemented as a macro.
-
-=item X
-
-This function is explicitly exported.
-
-=item E
-
-This function is visible to extensions included in the Perl core.
-
-=item b
-
-Binary backward compatibility; this function is a macro but also has
-a C<Perl_> implementation (which is exported).
-
-=item others
-
-See the comments at the top of C<embed.fnc> for others.
-
-=back
+The first column is a set of flags, the second column the return type,
+the third column the name. Columns after that are the arguments.
+The flags are documented at the top of F<embed.fnc>.
If you edit F<embed.pl> or F<embed.fnc>, you will need to run
C<make regen_headers> to force a rebuild of F<embed.h> and other
NVff NV %f-like
NVgf NV %g-like
+=for apidoc Amnh||IVdf
+=for apidoc Amnh||UVuf
+=for apidoc Amnh||UVof
+=for apidoc Amnh||UVxf
+=for apidoc Amnh||NVef
+=for apidoc Amnh||NVff
+=for apidoc Amnh||NVgf
+
These will take care of 64-bit integers and long doubles.
For example:
- printf("IV is %"IVdf"\n", iv);
+ printf("IV is %" IVdf "\n", iv);
-The IVdf will expand to whatever is the correct format for the IVs.
+The C<IVdf> will expand to whatever is the correct format for the IVs.
+Note that the spaces are required around the format in case the code is
+compiled with C++, to maintain compliance with its standard.
+
+Note that there are different "long doubles": Perl will use
+whatever the compiler has.
If you are printing addresses of pointers, use UVxf combined
with PTR2UV(), do not use %lx or %p.
+=head2 Formatted Printing of SvPVs
+
+If you just want the bytes printed in a NUL-terminated string, you can
+just use C<%s> (assuming they are all printables). But if there is a
+possibility the value will be encoded as UTF-8, you should instead use
+the C<UTF8f> format. And as its parameter, use the C<UTF8fARG()> macro.
+Below is a general example using the SV C<err_msg> which is known to
+contain a string and not need magic handling:
+
+ Perl_croak(aTHX_ "This croaked because: " UTF8f "\n",
+ UTF8fARG(SvUTF8(err_msg),
+ SvCUR(err_msg),
+ SvPVX(err_msg)));
+
+The first parameter to C<UTF8fARG> is a boolean: 1 if the string is in
+UTF-8; 0 if bytes.
+The second parameter is the number of bytes in the string to print.
+And the third and final parameter is a pointer to the first byte in the
+string.
+
+=head2 Formatted Printing of C<Size_t> and C<SSize_t>
+
+The most general way to do this is to cast them to a UV or IV, and
+print as in the
+L<previous section|/Formatted Printing of IVs, UVs, and NVs>.
+
+But if you're using C<PerlIO_printf()>, it's less typing and visual
+clutter to use the C<"%z"> length modifier (for I<siZe>):
+
+ PerlIO_printf("STRLEN is %zu\n", len);
+
+This modifier is not portable, so its use should be restricted to
+C<PerlIO_printf()>.
+
=head2 Pointer-To-Integer and Integer-To-Pointer
Because pointer size does not necessarily equal integer size,
PTR2NV(pointer)
INT2PTR(pointertotype, integer)
+=for apidoc Amh|void *|INT2PTR|type|int value
+=for apidoc Amh|UV|PTR2UV|void *
+=for apidoc Amh|IV|PTR2IV|void *
+=for apidoc Amh|NV|PTR2NV|void *
+
For example:
IV iv = ...;
=head2 Exception Handling
There are a couple of macros to do very basic exception handling in XS
-modules. You have to define C<NO_XSLOCKS> before including F<XSUB.h> to
+modules. You have to define C<NO_XSLOCKS> before including F<XSUB.h> to
be able to use these macros:
#define NO_XSLOCKS
#include "XSUB.h"
You can use these macros if you call code that may croak, but you need
-to do some cleanup before giving control back to Perl. For example:
+to do some cleanup before giving control back to Perl. For example:
dXCPT; /* set up necessary variables */
}
Note that you always have to rethrow an exception that has been
-caught. Using these macros, it is not possible to just catch the
-exception and ignore it. If you have to ignore the exception, you
+caught. Using these macros, it is not possible to just catch the
+exception and ignore it. If you have to ignore the exception, you
have to use the C<call_*> function.
The advantage of using the above macros is that you don't have
=head2 Source Documentation
There's an effort going on to document the internal functions and
-automatically produce reference manuals from them - L<perlapi> is one
+automatically produce reference manuals from them -
- L<perlapi> is one
such manual which details all the functions which are available to XS
-writers. L<perlintern> is the autogenerated manual for the functions
+writers.
L<perlintern> is the autogenerated manual for the functions
which are not part of the API and are supposedly for internal use only.
Source documentation is created by putting POD comments into the C
=for apidoc sv_setiv
Copies an integer into the given SV. Does not handle 'set' magic. See
- C<sv_setiv_mg>.
+ L<perlapi/sv_setiv_mg>.
=cut
*/
=head2 Backwards compatibility
-The Perl API changes over time. New functions are added or the interfaces
-of existing functions are changed. The C<Devel::PPPort> module tries to
+The Perl API changes over time. New functions are
+added or the interfaces of existing functions are
+changed. The C<Devel::PPPort> module tries to
provide compatibility code for some of these changes, so XS writers don't
have to code it themselves when supporting multiple versions of Perl.
C<Devel::PPPort> generates a C header file F<ppport.h> that can also
-be run as a Perl script. To generate F<ppport.h>, run:
+be run as a Perl script.
To generate F<ppport.h>, run:
perl -MDevel::PPPort -eDevel::PPPort::WriteFile
Besides checking existing XS code, the script can also be used to retrieve
compatibility information for various API calls using the C<--api-info>
-command line switch. For example:
+command line switch. For example:
% perl ppport.h --api-info=sv_magicext
=head1 Unicode Support
-Perl 5.6.0 introduced Unicode support. It's important for porters and XS
+Perl 5.6.0 introduced Unicode support. It's important for porters and XS
writers to understand this support and make sure that the code they
write does not corrupt Unicode data.
=head2 What B<is> Unicode, anyway?
-In the olden, less enlightened times, we all used to use ASCII. Most of
-us did, anyway. The big problem with ASCII is that it's American. Well,
+In the olden, less enlightened times, we all used to use ASCII. Most of
+us did, anyway. The big problem with ASCII is that it's American. Well,
no, that's not actually the problem; the problem is that it's not
-particularly useful for people who don't use the Roman alphabet. What
+particularly useful for people who don't use the Roman alphabet. What
used to happen was that particular languages would stick their own
-alphabet in the upper range of the sequence, between 128 and 255. Of
+alphabet in the upper range of the sequence, between 128 and 255. Of
course, we then ended up with plenty of variants that weren't quite
ASCII, and the whole point of it being a standard was lost.
To fix this, some people formed Unicode, Inc. and
produced a new character set containing all the characters you can
-possibly think of and more. There are several ways of representing these
-characters, and the one Perl uses is called UTF-8. UTF-8 uses
-a variable number of bytes to represent a character. You can learn more
+possibly think of and more. There are several ways of representing these
+characters, and the one Perl uses is called UTF-8. UTF-8 uses
+a variable number of bytes to represent a character. You can learn more
about Unicode and Perl's Unicode model in L<perlunicode>.
+(On EBCDIC platforms, Perl uses instead UTF-EBCDIC, which is a form of
+UTF-8 adapted for EBCDIC platforms. Below, we just talk about UTF-8.
+UTF-EBCDIC is like UTF-8, but the details are different. The macros
+hide the differences from you, just remember that the particular numbers
+and bit patterns presented below will differ in UTF-EBCDIC.)
+
=head2 How can I recognise a UTF-8 string?
-You can't. This is because UTF-8 data is stored in bytes just like
-non-UTF-8 data. The Unicode character 200, (C<0xC8> for you hex types)
+You can't. This is because UTF-8 data is stored in bytes just like
+non-UTF-8 data. The Unicode character 200, (C<0xC8> for you hex types)
capital E with a grave accent, is represented by the two bytes
-C<v196.172>. Unfortunately, the non-Unicode string C<chr(196).chr(172)>
-has that byte sequence as well. So you can't tell just by looking - this
+C<v196.172>. Unfortunately, the non-Unicode string C<chr(196).chr(172)>
+has that byte sequence as well. So you can't tell just by looking -- this
is what makes Unicode input an interesting problem.
In general, you either have to know what you're dealing with, or you
have to guess. The API function C<is_utf8_string> can help; it'll tell
-you if a string contains only valid UTF-8 characters. However, it can't
-do the work for you. On a character-by-character basis,
-C<is_utf8_char_buf>
+you if a string contains only valid UTF-8 characters, and the chances
+of a non-UTF-8 string looking like valid UTF-8 become very small very
+quickly with increasing string length. On a character-by-character
+basis, C<isUTF8_CHAR>
will tell you whether the current character in a string is valid UTF-8.
=head2 How does UTF-8 represent Unicode characters?
As mentioned above, UTF-8 uses a variable number of bytes to store a
-character. Characters with values 0...127 are stored in one byte, just
-like good ol' ASCII. Character 128 is stored as C<v194.128>; this
-continues up to character 191, which is C<v194.191>. Now we've run out of
-bits (191 is binary C<10111111>) so we move on; 192 is C<v195.128>. And
+character. Characters with values 0...127 are stored in one
+byte, just like good ol' ASCII. Character 128 is stored as
+C<v194.128>; this continues up to character 191, which is
+C<v194.191>. Now we've run out of bits (191 is binary
+C<10111111>) so we move on; character 192 is C<v195.128>. And
so it goes on, moving to three bytes at character 2048.
+L<perlunicode/Unicode Encodings> has pictures of how this works.
Assuming you know you're dealing with a UTF-8 string, you can find out
how long the first character in it is with the C<UTF8SKIP> macro:
Another way to skip over characters in a UTF-8 string is to use
C<utf8_hop>, which takes a string and a number of characters to skip
-over. You're on your own about bounds checking, though, so don't use it
+over. You're on your own about bounds checking, though, so don't use it
lightly.
All bytes in a multi-byte UTF-8 character will have the high bit set,
so you can test if you need to do something special with this
-character like this (the UTF8_IS_INVARIANT() is a macro that tests
-whether the byte can be encoded as a single byte even in UTF-8):
+character like this (the C<UTF8_IS_INVARIANT()> is a macro that tests
+whether the byte is encoded as a single byte even in UTF-8):
- U8 *utf;
- U8 *utf_end; /* 1 beyond buffer pointed to by utf */
- UV uv; /* Note: a UV, not a U8, not a char */
- STRLEN len; /* length of character in bytes */
+ U8 *utf; /* Initialize this to point to the beginning of the
+ sequence to convert */
+ U8 *utf_end; /* Initialize this to 1 beyond the end of the sequence
+ pointed to by 'utf' */
+ UV uv; /* Returned code point; note: a UV, not a U8, not a
+ char */
+ STRLEN len; /* Returned length of character in bytes */
if (!UTF8_IS_INVARIANT(*utf))
/* Must treat this as UTF-8 */
value of the character; the inverse function C<uvchr_to_utf8> is available
for putting a UV into UTF-8:
- if (!UTF8_IS_INVARIANT(uv))
+ if (!UVCHR_IS_INVARIANT(uv))
/* Must treat this as UTF8 */
utf8 = uvchr_to_utf8(utf8, uv);
else
You B<must> convert characters to UVs using the above functions if
you're ever in a situation where you have to match UTF-8 and non-UTF-8
-characters. You may not skip over UTF-8 characters in this case. If you
+characters. You may not skip over UTF-8 characters in this case. If you
do this, you'll lose the ability to match hi-bit non-UTF-8 characters;
for instance, if your UTF-8 string contains C<v196.172>, and you skip
that character, you can never match a C<chr(200)> in a non-UTF-8 string.
So don't do that!
+(Note that we don't have to test for invariant characters in the
+examples above. The functions work on any well-formed UTF-8 input.
+It's just that its faster to avoid the function overhead when it's not
+needed.)
+
=head2 How does Perl store UTF-8 strings?
-Currently, Perl deals with Unicode strings and non-Unicode strings
-slightly differently. A flag in the SV, C<SVf_UTF8>, indicates that the
-string is internally encoded as UTF-8. Without it, the byte value is the
-codepoint number and vice versa (in other words, the string is encoded
-as iso-8859-1, but C<use feature 'unicode_strings'> is needed to get iso-8859-1
-semantics). You can check and manipulate this flag with the
+Currently, Perl deals with UTF-8 strings and non-UTF-8 strings
+slightly differently. A flag in the SV, C<SVf_UTF8>, indicates that the
+string is internally encoded as UTF-8. Without it, the byte value is the
+codepoint number and vice versa. This flag is only meaningful if the SV
+is C<SvPOK> or immediately after stringification via C<SvPV> or a
+similar macro. You can check and manipulate this flag with the
following macros:
SvUTF8(sv)
SvUTF8_off(sv)
This flag has an important effect on Perl's treatment of the string: if
-Unicode data is not properly distinguished, regular expressions,
+UTF-8 data is not properly distinguished, regular expressions,
C<length>, C<substr> and other string handling operations will have
-undesirable results.
+undesirable (wrong) results.
The problem comes when you have, for instance, a string that isn't
-flagged as UTF-8, and contains a byte sequence that could be UTF-8 -
+flagged as UTF-8, and contains a byte sequence that could be UTF-8 --
especially when combining non-UTF-8 and UTF-8 strings.
-Never forget that the C<SVf_UTF8> flag is separate to the PV value; you
-need be sure you don't accidentally knock it off while you're
-manipulating SVs. More specifically, you cannot expect to do this:
+Never forget that the C<SVf_UTF8> flag is separate from the PV value; you
+need to be sure you don't accidentally knock it off while you're
+manipulating SVs. More specifically, you cannot expect to do this:
SV *sv;
SV *nsv;
nsv = newSVpvn(p, len);
The C<char*> string does not tell you the whole story, and you can't
-copy or reconstruct an SV just by copying the string value. Check if the
-old SV has the UTF8 flag set, and act accordingly:
+copy or reconstruct an SV just by copying the string value. Check if the
+old SV has the UTF8 flag set (I<after> the C<SvPV> call), and act
+accordingly:
p = SvPV(sv, len);
- frobnicate(p);
+ is_utf8 = SvUTF8(sv);
+ frobnicate(p, is_utf8);
nsv = newSVpvn(p, len);
- if (SvUTF8(sv))
+ if (is_utf8)
SvUTF8_on(nsv);
-In fact, your C<frobnicate> function should be made aware of whether or
-not it's dealing with UTF-8 data, so that it can handle the string
-appropriately.
+In the above, your C<frobnicate> function has been changed to be made
+aware of whether or not it's dealing with UTF-8 data, so that it can
+handle the string appropriately.
Since just passing an SV to an XS function and copying the data of
the SV is not enough to copy the UTF8 flags, even less right is just
-passing a C<char *> to an XS function.
+passing a S<C<char *>> to an XS function.
+
+For full generality, use the L<C<DO_UTF8>|perlapi/DO_UTF8> macro to see if the
+string in an SV is to be I<treated> as UTF-8. This takes into account
+if the call to the XS function is being made from within the scope of
+L<S<C<use bytes>>|bytes>. If so, the underlying bytes that comprise the
+UTF-8 string are to be exposed, rather than the character they
+represent. But this pragma should only really be used for debugging and
+perhaps low-level testing at the byte level. Hence most XS code need
+not concern itself with this, but various areas of the perl core do need
+to support it.
+
+And this isn't the whole story. Starting in Perl v5.12, strings that
+aren't encoded in UTF-8 may also be treated as Unicode under various
+conditions (see L<perlunicode/ASCII Rules versus Unicode Rules>).
+This is only really a problem for characters whose ordinals are between
+128 and 255, and their behavior varies under ASCII versus Unicode rules
+in ways that your code cares about (see L<perlunicode/The "Unicode Bug">).
+There is no published API for dealing with this, as it is subject to
+change, but you can look at the code for C<pp_lc> in F<pp.c> for an
+example as to how it's currently done.
=head2 How do I convert a string to UTF-8?
If you're mixing UTF-8 and non-UTF-8 strings, it is necessary to upgrade
-one of the strings to UTF-8. If you've got an SV, the easiest way to do
+the non-UTF-8 strings to UTF-8. If you've got an SV, the easiest way to do
this is:
sv_utf8_upgrade(sv);
by the end user, it can cause problems in deficient code.
Instead, C<bytes_to_utf8> will give you a UTF-8-encoded B<copy> of its
-string argument. This is useful for having the data available for
-comparisons and so on, without harming the original SV. There's also
+string argument. This is useful for having the data available for
+comparisons and so on, without harming the original SV. There's also
C<utf8_to_bytes> to go the other way, but naturally, this will fail if
the string contains any characters above 255 that can't be represented
in a single byte.
+=head2 How do I compare strings?
+
+L<perlapi/sv_cmp> and L<perlapi/sv_cmp_flags> do a lexigraphic
+comparison of two SV's, and handle UTF-8ness properly. Note, however,
+that Unicode specifies a much fancier mechanism for collation, available
+via the L<Unicode::Collate> module.
+
+To just compare two strings for equality/non-equality, you can just use
+L<C<memEQ()>|perlapi/memEQ> and L<C<memNE()>|perlapi/memEQ> as usual,
+except the strings must be both UTF-8 or not UTF-8 encoded.
+
+To compare two strings case-insensitively, use
+L<C<foldEQ_utf8()>|perlapi/foldEQ_utf8> (the strings don't have to have
+the same UTF-8ness).
+
=head2 Is there anything else I need to know?
-Not really. Just remember these things:
+Not really. Just remember these things:
=over 3
=item *
-There's no way to tell if a string is UTF-8 or not. You can tell if an SV
-is UTF-8 by looking at its C<SvUTF8> flag. Don't forget to set the flag if
-something should be UTF-8. Treat the flag as part of the PV, even though
-it's not - if you pass on the PV to somewhere, pass on the flag too.
+There's no way to tell if a S<C<char *>> or S<C<U8 *>> string is UTF-8
+or not. But you can tell if an SV is to be treated as UTF-8 by calling
+C<DO_UTF8> on it, after stringifying it with C<SvPV> or a similar
+macro. And, you can tell if SV is actually UTF-8 (even if it is not to
+be treated as such) by looking at its C<SvUTF8> flag (again after
+stringifying it). Don't forget to set the flag if something should be
+UTF-8.
+Treat the flag as part of the PV, even though it's not -- if you pass on
+the PV to somewhere, pass on the flag too.
=item *
=item *
-When writing a character
C<uv> to a UTF-8 string, B<always> use
-C<uvchr_to_utf8>, unless C<UTF8_IS_INVARIANT(uv))> in which case
+When writing a character
UV to a UTF-8 string, B<always> use
+C<uvchr_to_utf8>, unless C<UVCHR_IS_INVARIANT(uv))> in which case
you can use C<*s = uv>.
=item *
-Mixing UTF-8 and non-UTF-8 strings is tricky. Use C<bytes_to_utf8> to get
+Mixing UTF-8 and non-UTF-8 strings is
+tricky. Use C<bytes_to_utf8> to get
a new string which is UTF-8 encoded, and then combine them.
=back
=head1 Custom Operators
Custom operator support is an experimental feature that allows you to
-define your own ops. This is primarily to allow the building of
+define your own ops. This is primarily to allow the building of
interpreters for other languages in the Perl core, but it also allows
optimizations through the creation of "macro-ops" (ops which perform the
functions of multiple ops which are usually executed together, such as
C<gvsv, gvsv, add>.)
-This feature is implemented as a new op type, C<OP_CUSTOM>. The Perl
+This feature is implemented as a new op type, C<OP_CUSTOM>. The Perl
core does not "know" anything special about this op type, and so it will
-not be involved in any optimizations. This also means that you can
-define your custom ops to be any op structure - unary, binary, list and
-so on - you like.
-
-It's important to know what custom operators won't do for you. They
-won't let you add new syntax to Perl, directly. They won't even let you
-add new keywords, directly. In fact, they won't change the way Perl
-compiles a program at all. You have to do those changes yourself, after
-Perl has compiled the program. You do this either by manipulating the op
+not be involved in any optimizations. This also means that you can
+define your custom ops to be any op structure -- unary, binary, list and
+so on -- you like.
+
+It's important to know what custom operators won't do for you. They
+won't let you add new syntax to Perl, directly. They won't even let you
+add new keywords, directly. In fact, they won't change the way Perl
+compiles a program at all. You have to do those changes yourself, after
+Perl has compiled the program. You do this either by manipulating the op
tree using a C<CHECK> block and the C<B::Generate> module, or by adding
a custom peephole optimizer with the C<optimize> module.
When you do this, you replace ordinary Perl ops with custom ops by
creating ops with the type C<OP_CUSTOM> and the C<op_ppaddr> of your own
-PP function. This should be defined in XS code, and should look like
-the PP ops in C<pp_*.c>. You are responsible for ensuring that your op
+PP function. This should be defined in XS code, and should look like
+the PP ops in C<pp_*.c>. You are responsible for ensuring that your op
takes the appropriate number of values from the stack, and you are
responsible for adding stack marks if necessary.
You should also "register" your op with the Perl interpreter so that it
-can produce sensible error and warning messages. Since it is possible to
+can produce sensible error and warning messages. Since it is possible to
have multiple custom ops within the one "logical" op type C<OP_CUSTOM>,
Perl uses the value of C<< o->op_ppaddr >> to determine which custom op
-it is dealing with. You should create an C<XOP> structure for each
+it is dealing with. You should create an C<XOP> structure for each
ppaddr you use, set the properties of the custom op with
C<XopENTRY_set>, and register the structure against the ppaddr using
-C<Perl_custom_op_register>. A trivial example might look like:
+C<Perl_custom_op_register>. A trivial example might look like:
static XOP my_xop;
static OP *my_pp(pTHX);
=item xop_name
-A short name for your op. This will be included in some error messages,
+A short name for your op. This will be included in some error messages,
and will also be returned as C<< $op->name >> by the L<B|B> module, so
it will appear in the output of module like L<B::Concise|B::Concise>.
=item xop_class
-Which of the various C<*OP> structures this op uses. This should be one of
+Which of the various C<*OP> structures this op uses. This should be one of
the C<OA_*> constants from F<op.h>, namely
=over 4
=item OA_PVOP_OR_SVOP
-This should be interpreted as 'C<PVOP>' only. The C<_OR_SVOP> is because
+This should be interpreted as 'C<PVOP>' only. The C<_OR_SVOP> is because
the only core C<PVOP>, C<OP_TRANS>, can sometimes be a C<SVOP> instead.
=item OA_LOOP
=item xop_peep
This member is of type C<Perl_cpeep_t>, which expands to C<void
-(*Perl_cpeep_t)(aTHX_ OP *o, OP *oldop)>. If it is set, this function
+(*Perl_cpeep_t)(aTHX_ OP *o, OP *oldop)>. If it is set, this function
will be called from C<Perl_rpeep> when ops of this type are encountered
-by the peephole optimizer. I<o> is the OP that needs optimizing;
+by the peephole optimizer. I<o> is the OP that needs optimizing;
I<oldop> is the previous OP optimized, whose C<op_next> points to I<o>.
=back
C<B::Generate> directly supports the creation of custom ops by name.
+=head1 Stacks
+
+Descriptions above occasionally refer to "the stack", but there are in fact
+many stack-like data structures within the perl interpreter. When otherwise
+unqualified, "the stack" usually refers to the value stack.
+
+The various stacks have different purposes, and operate in slightly different
+ways. Their differences are noted below.
+
+=head2 Value Stack
+
+This stack stores the values that regular perl code is operating on, usually
+intermediate values of expressions within a statement. The stack itself is
+formed of an array of SV pointers.
+
+The base of this stack is pointed to by the interpreter variable
+C<PL_stack_base>, of type C<SV **>.
+
+The head of the stack is C<PL_stack_sp>, and points to the most
+recently-pushed item.
+
+Items are pushed to the stack by using the C<PUSHs()> macro or its variants
+described above; C<XPUSHs()>, C<mPUSHs()>, C<mXPUSHs()> and the typed
+versions. Note carefully that the non-C<X> versions of these macros do not
+check the size of the stack and assume it to be big enough. These must be
+paired with a suitable check of the stack's size, such as the C<EXTEND> macro
+to ensure it is large enough. For example
+
+ EXTEND(SP, 4);
+ mPUSHi(10);
+ mPUSHi(20);
+ mPUSHi(30);
+ mPUSHi(40);
+
+This is slightly more performant than making four separate checks in four
+separate C<mXPUSHi()> calls.
+
+As a further performance optimisation, the various C<PUSH> macros all operate
+using a local variable C<SP>, rather than the interpreter-global variable
+C<PL_stack_sp>. This variable is declared by the C<dSP> macro - though it is
+normally implied by XSUBs and similar so it is rare you have to consider it
+directly. Once declared, the C<PUSH> macros will operate only on this local
+variable, so before invoking any other perl core functions you must use the
+C<PUTBACK> macro to return the value from the local C<SP> variable back to
+the interpreter variable. Similarly, after calling a perl core function which
+may have had reason to move the stack or push/pop values to it, you must use
+the C<SPAGAIN> macro which refreshes the local C<SP> value back from the
+interpreter one.
+
+Items are popped from the stack by using the C<POPs> macro or its typed
+versions, There is also a macro C<TOPs> that inspects the topmost item without
+removing it.
+
+Note specifically that SV pointers on the value stack do not contribute to the
+overall reference count of the xVs being referred to. If newly-created xVs are
+being pushed to the stack you must arrange for them to be destroyed at a
+suitable time; usually by using one of the C<mPUSH*> macros or C<sv_2mortal()>
+to mortalise the xV.
+
+=head2 Mark Stack
+
+The value stack stores individual perl scalar values as temporaries between
+expressions. Some perl expressions operate on entire lists; for that purpose
+we need to know where on the stack each list begins. This is the purpose of the
+mark stack.
+
+The mark stack stores integers as I32 values, which are the height of the
+value stack at the time before the list began; thus the mark itself actually
+points to the value stack entry one before the list. The list itself starts at
+C<mark + 1>.
+
+The base of this stack is pointed to by the interpreter variable
+C<PL_markstack>, of type C<I32 *>.
+
+The head of the stack is C<PL_markstack_ptr>, and points to the most
+recently-pushed item.
+
+Items are pushed to the stack by using the C<PUSHMARK()> macro. Even though
+the stack itself stores (value) stack indices as integers, the C<PUSHMARK>
+macro should be given a stack pointer directly; it will calculate the index
+offset by comparing to the C<PL_stack_sp> variable. Thus almost always the
+code to perform this is
+
+ PUSHMARK(SP);
+
+Items are popped from the stack by the C<POPMARK> macro. There is also a macro
+C<TOPMARK> that inspects the topmost item without removing it. These macros
+return I32 index values directly. There is also the C<dMARK> macro which
+declares a new SV double-pointer variable, called C<mark>, which points at the
+marked stack slot; this is the usual macro that C code will use when operating
+on lists given on the stack.
+
+As noted above, the C<mark> variable itself will point at the most recently
+pushed value on the value stack before the list begins, and so the list itself
+starts at C<mark + 1>. The values of the list may be iterated by code such as
+
+ for(SV **svp = mark + 1; svp <= PL_stack_sp; svp++) {
+ SV *item = *svp;
+ ...
+ }
+
+Note specifically in the case that the list is already empty, C<mark> will
+equal C<PL_stack_sp>.
+
+Because the C<mark> variable is converted to a pointer on the value stack,
+extra care must be taken if C<EXTEND> or any of the C<XPUSH> macros are
+invoked within the function, because the stack may need to be moved to
+extend it and so the existing pointer will now be invalid. If this may be a
+problem, a possible solution is to track the mark offset as an integer and
+track the mark itself later on after the stack had been moved.
+
+ I32 markoff = POPMARK;
+
+ ...
+
+ SP **mark = PL_stack_base + markoff;
+
+=head2 Temporaries Stack
+
+As noted above, xV references on the main value stack do not contribute to the
+reference count of an xV, and so another mechanism is used to track when
+temporary values which live on the stack must be released. This is the job of
+the temporaries stack.
+
+The temporaries stack stores pointers to xVs whose reference counts will be
+decremented soon.
+
+The base of this stack is pointed to by the interpreter variable
+C<PL_tmps_stack>, of type C<SV **>.
+
+The head of the stack is indexed by C<PL_tmps_ix>, an integer which stores the
+index in the array of the most recently-pushed item.
+
+There is no public API to directly push items to the temporaries stack. Instead,
+the API function C<sv_2mortal()> is used to mortalize an xV, adding its
+address to the temporaries stack.
+
+Likewise, there is no public API to read values from the temporaries stack.
+Instead. the macros C<SAVETMPS> and C<FREETPMS> are used. The C<SAVETMPS>
+macro establishes the base levels of the temporaries stack, by capturing the
+current value of C<PL_tmps_ix> into C<PL_tmps_floor> and saving the previous
+value to the save stack. Thereafter, whenever C<FREETMPS> is invoked all of
+the temporaries that have been pushed since that level are reclaimed.
+
+While it is common to see these two macros in pairs within an C<ENTER>/
+C<LEAVE> pair, it is not necessary to match them. It is permitted to invoke
+C<FREETMPS> multiple times since the most recent C<SAVETMPS>; for example in a
+loop iterating over elements of a list. While you can invoke C<SAVETMPS>
+multiple times within a scope pair, it is unlikely to be useful. Subsequent
+invocations will move the temporaries floor further up, thus effectively
+trapping the existing temporaries to only be released at the end of the scope.
+
+=head2 Save Stack
+
+The save stack is used by perl to implement the C<local> keyword and other
+similar behaviours; any cleanup operations that need to be performed when
+leaving the current scope. Items pushed to this stack generally capture the
+current value of some internal variable or state, which will be restored when
+the scope is unwound due to leaving, C<return>, C<die>, C<goto> or other
+reasons.
+
+Whereas other perl internal stacks store individual items all of the same type
+(usually SV pointers or integers), the items pushed to the save stack are
+formed of many different types, having multiple fields to them. For example,
+the C<SAVEt_INT> type needs to store both the address of the C<int> variable
+to restore, and the value to restore it to. This information could have been
+stored using fields of a C<struct>, but would have to be large enough to store
+three pointers in the largest case, which would waste a lot of space in most
+of the smaller cases.
+
+Instead, the stack stores information in a variable-length encoding of C<ANY>
+structures. The final value pushed is stored in the C<UV> field which encodes
+the kind of item held by the preceeding items; the count and types of which
+will depend on what kind of item is being stored. The kind field is pushed
+last because that will be the first field to be popped when unwinding items
+from the stack.
+
+The base of this stack is pointed to by the interpreter variable
+C<PL_savestack>, of type C<ANY *>.
+
+The head of the stack is indexed by C<PL_savestack_ix>, an integer which
+stores the index in the array at which the next item should be pushed. (Note
+that this is different to most other stacks, which reference the most
+recently-pushed item).
+
+Items are pushed to the save stack by using the various C<SAVE...()> macros.
+Many of these macros take a variable and store both its address and current
+value on the save stack, ensuring that value gets restored on scope exit.
+
+ SAVEI8(i8)
+ SAVEI16(i16)
+ SAVEI32(i32)
+ SAVEINT(i)
+ ...
+
+There are also a variety of other special-purpose macros which save particular
+types or values of interest. C<SAVETMPS> has already been mentioned above.
+Others include C<SAVEFREEPV> which arranges for a PV (i.e. a string buffer) to
+be freed, or C<SAVEDESTRUCTOR> which arranges for a given function pointer to
+be invoked on scope exit. A full list of such macros can be found in
+F<scope.h>.
+
+There is no public API for popping individual values or items from the save
+stack. Instead, via the scope stack, the C<ENTER> and C<LEAVE> pair form a way
+to start and stop nested scopes. Leaving a nested scope via C<LEAVE> will
+restore all of the saved values that had been pushed since the most recent
+C<ENTER>.
+
+=head2 Scope Stack
+
+As with the mark stack to the value stack, the scope stack forms a pair with
+the save stack. The scope stack stores the height of the scope stack at which
+nested scopes begin, and allows the save stack to be unwound back to that
+point when the scope is left.
+
+When perl is built with debugging enabled, there is a second part to this
+stack storing human-readable string names describing the type of stack
+context. Each push operation saves the name as well as the height of the save
+stack, and each pop operation checks the topmost name with what is expected,
+causing an assertion failure if the name does not match.
+
+The base of this stack is pointed to by the interpreter variable
+C<PL_scopestack>, of type C<I32 *>. If enabled, the scope stack names are
+stored in a separate array pointed to by C<PL_scopestack_name>, of type
+C<const char **>.
+
+The head of the stack is indexed by C<PL_scopestack_ix>, an integer which
+stores the index of the array or arrays at which the next item should be
+pushed. (Note that this is different to most other stacks, which reference the
+most recently-pushed item).
+
+Values are pushed to the scope stack using the C<ENTER> macro, which begins a
+new nested scope. Any items pushed to the save stack are then restored at the
+next nested invocation of the C<LEAVE> macro.
+
+=head1 Dynamic Scope and the Context Stack
+
+B<Note:> this section describes a non-public internal API that is subject
+to change without notice.
+
+=head2 Introduction to the context stack
+
+In Perl, dynamic scoping refers to the runtime nesting of things like
+subroutine calls, evals etc, as well as the entering and exiting of block
+scopes. For example, the restoring of a C<local>ised variable is
+determined by the dynamic scope.
+
+Perl tracks the dynamic scope by a data structure called the context
+stack, which is an array of C<PERL_CONTEXT> structures, and which is
+itself a big union for all the types of context. Whenever a new scope is
+entered (such as a block, a C<for> loop, or a subroutine call), a new
+context entry is pushed onto the stack. Similarly when leaving a block or
+returning from a subroutine call etc. a context is popped. Since the
+context stack represents the current dynamic scope, it can be searched.
+For example, C<next LABEL> searches back through the stack looking for a
+loop context that matches the label; C<return> pops contexts until it
+finds a sub or eval context or similar; C<caller> examines sub contexts on
+the stack.
+
+Each context entry is labelled with a context type, C<cx_type>. Typical
+context types are C<CXt_SUB>, C<CXt_EVAL> etc., as well as C<CXt_BLOCK>
+and C<CXt_NULL> which represent a basic scope (as pushed by C<pp_enter>)
+and a sort block. The type determines which part of the context union are
+valid.
+
+The main division in the context struct is between a substitution scope
+(C<CXt_SUBST>) and block scopes, which are everything else. The former is
+just used while executing C<s///e>, and won't be discussed further
+here.
+
+All the block scope types share a common base, which corresponds to
+C<CXt_BLOCK>. This stores the old values of various scope-related
+variables like C<PL_curpm>, as well as information about the current
+scope, such as C<gimme>. On scope exit, the old variables are restored.
+
+Particular block scope types store extra per-type information. For
+example, C<CXt_SUB> stores the currently executing CV, while the various
+for loop types might hold the original loop variable SV. On scope exit,
+the per-type data is processed; for example the CV has its reference count
+decremented, and the original loop variable is restored.
+
+The macro C<cxstack> returns the base of the current context stack, while
+C<cxstack_ix> is the index of the current frame within that stack.
+
+In fact, the context stack is actually part of a stack-of-stacks system;
+whenever something unusual is done such as calling a C<DESTROY> or tie
+handler, a new stack is pushed, then popped at the end.
+
+Note that the API described here changed considerably in perl 5.24; prior
+to that, big macros like C<PUSHBLOCK> and C<POPSUB> were used; in 5.24
+they were replaced by the inline static functions described below. In
+addition, the ordering and detail of how these macros/function work
+changed in many ways, often subtly. In particular they didn't handle
+saving the savestack and temps stack positions, and required additional
+C<ENTER>, C<SAVETMPS> and C<LEAVE> compared to the new functions. The
+old-style macros will not be described further.
+
+
+=head2 Pushing contexts
+
+For pushing a new context, the two basic functions are
+C<cx = cx_pushblock()>, which pushes a new basic context block and returns
+its address, and a family of similar functions with names like
+C<cx_pushsub(cx)> which populate the additional type-dependent fields in
+the C<cx> struct. Note that C<CXt_NULL> and C<CXt_BLOCK> don't have their
+own push functions, as they don't store any data beyond that pushed by
+C<cx_pushblock>.
+
+The fields of the context struct and the arguments to the C<cx_*>
+functions are subject to change between perl releases, representing
+whatever is convenient or efficient for that release.
+
+A typical context stack pushing can be found in C<pp_entersub>; the
+following shows a simplified and stripped-down example of a non-XS call,
+along with comments showing roughly what each function does.
+
+ dMARK;
+ U8 gimme = GIMME_V;
+ bool hasargs = cBOOL(PL_op->op_flags & OPf_STACKED);
+ OP *retop = PL_op->op_next;
+ I32 old_ss_ix = PL_savestack_ix;
+ CV *cv = ....;
+
+ /* ... make mortal copies of stack args which are PADTMPs here ... */
+
+ /* ... do any additional savestack pushes here ... */
+
+ /* Now push a new context entry of type 'CXt_SUB'; initially just
+ * doing the actions common to all block types: */
+
+ cx = cx_pushblock(CXt_SUB, gimme, MARK, old_ss_ix);
+
+ /* this does (approximately):
+ CXINC; /* cxstack_ix++ (grow if necessary) */
+ cx = CX_CUR(); /* and get the address of new frame */
+ cx->cx_type = CXt_SUB;
+ cx->blk_gimme = gimme;
+ cx->blk_oldsp = MARK - PL_stack_base;
+ cx->blk_oldsaveix = old_ss_ix;
+ cx->blk_oldcop = PL_curcop;
+ cx->blk_oldmarksp = PL_markstack_ptr - PL_markstack;
+ cx->blk_oldscopesp = PL_scopestack_ix;
+ cx->blk_oldpm = PL_curpm;
+ cx->blk_old_tmpsfloor = PL_tmps_floor;
+
+ PL_tmps_floor = PL_tmps_ix;
+ */
+
+
+ /* then update the new context frame with subroutine-specific info,
+ * such as the CV about to be executed: */
+
+ cx_pushsub(cx, cv, retop, hasargs);
+
+ /* this does (approximately):
+ cx->blk_sub.cv = cv;
+ cx->blk_sub.olddepth = CvDEPTH(cv);
+ cx->blk_sub.prevcomppad = PL_comppad;
+ cx->cx_type |= (hasargs) ? CXp_HASARGS : 0;
+ cx->blk_sub.retop = retop;
+ SvREFCNT_inc_simple_void_NN(cv);
+ */
+
+Note that C<cx_pushblock()> sets two new floors: for the args stack (to
+C<MARK>) and the temps stack (to C<PL_tmps_ix>). While executing at this
+scope level, every C<nextstate> (amongst others) will reset the args and
+tmps stack levels to these floors. Note that since C<cx_pushblock> uses
+the current value of C<PL_tmps_ix> rather than it being passed as an arg,
+this dictates at what point C<cx_pushblock> should be called. In
+particular, any new mortals which should be freed only on scope exit
+(rather than at the next C<nextstate>) should be created first.
+
+Most callers of C<cx_pushblock> simply set the new args stack floor to the
+top of the previous stack frame, but for C<CXt_LOOP_LIST> it stores the
+items being iterated over on the stack, and so sets C<blk_oldsp> to the
+top of these items instead. Note that, contrary to its name, C<blk_oldsp>
+doesn't always represent the value to restore C<PL_stack_sp> to on scope
+exit.
+
+Note the early capture of C<PL_savestack_ix> to C<old_ss_ix>, which is
+later passed as an arg to C<cx_pushblock>. In the case of C<pp_entersub>,
+this is because, although most values needing saving are stored in fields
+of the context struct, an extra value needs saving only when the debugger
+is running, and it doesn't make sense to bloat the struct for this rare
+case. So instead it is saved on the savestack. Since this value gets
+calculated and saved before the context is pushed, it is necessary to pass
+the old value of C<PL_savestack_ix> to C<cx_pushblock>, to ensure that the
+saved value gets freed during scope exit. For most users of
+C<cx_pushblock>, where nothing needs pushing on the save stack,
+C<PL_savestack_ix> is just passed directly as an arg to C<cx_pushblock>.
+
+Note that where possible, values should be saved in the context struct
+rather than on the save stack; it's much faster that way.
+
+Normally C<cx_pushblock> should be immediately followed by the appropriate
+C<cx_pushfoo>, with nothing between them; this is because if code
+in-between could die (e.g. a warning upgraded to fatal), then the context
+stack unwinding code in C<dounwind> would see (in the example above) a
+C<CXt_SUB> context frame, but without all the subroutine-specific fields
+set, and crashes would soon ensue.
+
+Where the two must be separate, initially set the type to C<CXt_NULL> or
+C<CXt_BLOCK>, and later change it to C<CXt_foo> when doing the
+C<cx_pushfoo>. This is exactly what C<pp_enteriter> does, once it's
+determined which type of loop it's pushing.
+
+=head2 Popping contexts
+
+Contexts are popped using C<cx_popsub()> etc. and C<cx_popblock()>. Note
+however, that unlike C<cx_pushblock>, neither of these functions actually
+decrement the current context stack index; this is done separately using
+C<CX_POP()>.
+
+There are two main ways that contexts are popped. During normal execution
+as scopes are exited, functions like C<pp_leave>, C<pp_leaveloop> and
+C<pp_leavesub> process and pop just one context using C<cx_popfoo> and
+C<cx_popblock>. On the other hand, things like C<pp_return> and C<next>
+may have to pop back several scopes until a sub or loop context is found,
+and exceptions (such as C<die>) need to pop back contexts until an eval
+context is found. Both of these are accomplished by C<dounwind()>, which
+is capable of processing and popping all contexts above the target one.
+
+Here is a typical example of context popping, as found in C<pp_leavesub>
+(simplified slightly):
+
+ U8 gimme;
+ PERL_CONTEXT *cx;
+ SV **oldsp;
+ OP *retop;
+
+ cx = CX_CUR();
+
+ gimme = cx->blk_gimme;
+ oldsp = PL_stack_base + cx->blk_oldsp; /* last arg of previous frame */
+
+ if (gimme == G_VOID)
+ PL_stack_sp = oldsp;
+ else
+ leave_adjust_stacks(oldsp, oldsp, gimme, 0);
+
+ CX_LEAVE_SCOPE(cx);
+ cx_popsub(cx);
+ cx_popblock(cx);
+ retop = cx->blk_sub.retop;
+ CX_POP(cx);
+
+ return retop;
+
+The steps above are in a very specific order, designed to be the reverse
+order of when the context was pushed. The first thing to do is to copy
+and/or protect any any return arguments and free any temps in the current
+scope. Scope exits like an rvalue sub normally return a mortal copy of
+their return args (as opposed to lvalue subs). It is important to make
+this copy before the save stack is popped or variables are restored, or
+bad things like the following can happen:
+
+ sub f { my $x =...; $x } # $x freed before we get to copy it
+ sub f { /(...)/; $1 } # PL_curpm restored before $1 copied
+
+Although we wish to free any temps at the same time, we have to be careful
+not to free any temps which are keeping return args alive; nor to free the
+temps we have just created while mortal copying return args. Fortunately,
+C<leave_adjust_stacks()> is capable of making mortal copies of return args,
+shifting args down the stack, and only processing those entries on the
+temps stack that are safe to do so.
+
+In void context no args are returned, so it's more efficient to skip
+calling C<leave_adjust_stacks()>. Also in void context, a C<nextstate> op
+is likely to be imminently called which will do a C<FREETMPS>, so there's
+no need to do that either.
+
+The next step is to pop savestack entries: C<CX_LEAVE_SCOPE(cx)> is just
+defined as C<< LEAVE_SCOPE(cx->blk_oldsaveix) >>. Note that during the
+popping, it's possible for perl to call destructors, call C<STORE> to undo
+localisations of tied vars, and so on. Any of these can die or call
+C<exit()>. In this case, C<dounwind()> will be called, and the current
+context stack frame will be re-processed. Thus it is vital that all steps
+in popping a context are done in such a way to support reentrancy. The
+other alternative, of decrementing C<cxstack_ix> I<before> processing the
+frame, would lead to leaks and the like if something died halfway through,
+or overwriting of the current frame.
+
+C<CX_LEAVE_SCOPE> itself is safely re-entrant: if only half the savestack
+items have been popped before dying and getting trapped by eval, then the
+C<CX_LEAVE_SCOPE>s in C<dounwind> or C<pp_leaveeval> will continue where
+the first one left off.
+
+The next step is the type-specific context processing; in this case
+C<cx_popsub>. In part, this looks like:
+
+ cv = cx->blk_sub.cv;
+ CvDEPTH(cv) = cx->blk_sub.olddepth;
+ cx->blk_sub.cv = NULL;
+ SvREFCNT_dec(cv);
+
+where its processing the just-executed CV. Note that before it decrements
+the CV's reference count, it nulls the C<blk_sub.cv>. This means that if
+it re-enters, the CV won't be freed twice. It also means that you can't
+rely on such type-specific fields having useful values after the return
+from C<cx_popfoo>.
+
+Next, C<cx_popblock> restores all the various interpreter vars to their
+previous values or previous high water marks; it expands to:
+
+ PL_markstack_ptr = PL_markstack + cx->blk_oldmarksp;
+ PL_scopestack_ix = cx->blk_oldscopesp;
+ PL_curpm = cx->blk_oldpm;
+ PL_curcop = cx->blk_oldcop;
+ PL_tmps_floor = cx->blk_old_tmpsfloor;
+
+Note that it I<doesn't> restore C<PL_stack_sp>; as mentioned earlier,
+which value to restore it to depends on the context type (specifically
+C<for (list) {}>), and what args (if any) it returns; and that will
+already have been sorted out earlier by C<leave_adjust_stacks()>.
+
+Finally, the context stack pointer is actually decremented by C<CX_POP(cx)>.
+After this point, it's possible that that the current context frame could
+be overwritten by other contexts being pushed. Although things like ties
+and C<DESTROY> are supposed to work within a new context stack, it's best
+not to assume this. Indeed on debugging builds, C<CX_POP(cx)> deliberately
+sets C<cx> to null to detect code that is still relying on the field
+values in that context frame. Note in the C<pp_leavesub()> example above,
+we grab C<blk_sub.retop> I<before> calling C<CX_POP>.
+
+=head2 Redoing contexts
+
+Finally, there is C<cx_topblock(cx)>, which acts like a super-C<nextstate>
+as regards to resetting various vars to their base values. It is used in
+places like C<pp_next>, C<pp_redo> and C<pp_goto> where rather than
+exiting a scope, we want to re-initialise the scope. As well as resetting
+C<PL_stack_sp> like C<nextstate>, it also resets C<PL_markstack_ptr>,
+C<PL_scopestack_ix> and C<PL_curpm>. Note that it doesn't do a
+C<FREETMPS>.
+
+
+=head1 Slab-based operator allocation
+
+B<Note:> this section describes a non-public internal API that is subject
+to change without notice.
+
+Perl's internal error-handling mechanisms implement C<die> (and its internal
+equivalents) using longjmp. If this occurs during lexing, parsing or
+compilation, we must ensure that any ops allocated as part of the compilation
+process are freed. (Older Perl versions did not adequately handle this
+situation: when failing a parse, they would leak ops that were stored in
+C C<auto> variables and not linked anywhere else.)
+
+To handle this situation, Perl uses I<op slabs> that are attached to the
+currently-compiling CV. A slab is a chunk of allocated memory. New ops are
+allocated as regions of the slab. If the slab fills up, a new one is created
+(and linked from the previous one). When an error occurs and the CV is freed,
+any ops remaining are freed.
+
+Each op is preceded by two pointers: one points to the next op in the slab, and
+the other points to the slab that owns it. The next-op pointer is needed so
+that Perl can iterate over a slab and free all its ops. (Op structures are of
+different sizes, so the slab's ops can't merely be treated as a dense array.)
+The slab pointer is needed for accessing a reference count on the slab: when
+the last op on a slab is freed, the slab itself is freed.
+
+The slab allocator puts the ops at the end of the slab first. This will tend to
+allocate the leaves of the op tree first, and the layout will therefore
+hopefully be cache-friendly. In addition, this means that there's no need to
+store the size of the slab (see below on why slabs vary in size), because Perl
+can follow pointers to find the last op.
+
+It might seem possible eliminate slab reference counts altogether, by having
+all ops implicitly attached to C<PL_compcv> when allocated and freed when the
+CV is freed. That would also allow C<op_free> to skip C<FreeOp> altogether, and
+thus free ops faster. But that doesn't work in those cases where ops need to
+survive beyond their CVs, such as re-evals.
+
+The CV also has to have a reference count on the slab. Sometimes the first op
+created is immediately freed. If the reference count of the slab reaches 0,
+then it will be freed with the CV still pointing to it.
+
+CVs use the C<CVf_SLABBED> flag to indicate that the CV has a reference count
+on the slab. When this flag is set, the slab is accessible via C<CvSTART> when
+C<CvROOT> is not set, or by subtracting two pointers C<(2*sizeof(I32 *))> from
+C<CvROOT> when it is set. The alternative to this approach of sneaking the slab
+into C<CvSTART> during compilation would be to enlarge the C<xpvcv> struct by
+another pointer. But that would make all CVs larger, even though slab-based op
+freeing is typically of benefit only for programs that make significant use of
+string eval.
+
+When the C<CVf_SLABBED> flag is set, the CV takes responsibility for freeing
+the slab. If C<CvROOT> is not set when the CV is freed or undeffed, it is
+assumed that a compilation error has occurred, so the op slab is traversed and
+all the ops are freed.
+
+Under normal circumstances, the CV forgets about its slab (decrementing the
+reference count) when the root is attached. So the slab reference counting that
+happens when ops are freed takes care of freeing the slab. In some cases, the
+CV is told to forget about the slab (C<cv_forget_slab>) precisely so that the
+ops can survive after the CV is done away with.
+
+Forgetting the slab when the root is attached is not strictly necessary, but
+avoids potential problems with C<CvROOT> being written over. There is code all
+over the place, both in core and on CPAN, that does things with C<CvROOT>, so
+forgetting the slab makes things more robust and avoids potential problems.
+
+Since the CV takes ownership of its slab when flagged, that flag is never
+copied when a CV is cloned, as one CV could free a slab that another CV still
+points to, since forced freeing of ops ignores the reference count (but asserts
+that it looks right).
+
+To avoid slab fragmentation, freed ops are marked as freed and attached to the
+slab's freed chain (an idea stolen from DBM::Deep). Those freed ops are reused
+when possible. Not reusing freed ops would be simpler, but it would result in
+significantly higher memory usage for programs with large C<if (DEBUG) {...}>
+blocks.
+
+C<SAVEFREEOP> is slightly problematic under this scheme. Sometimes it can cause
+an op to be freed after its CV. If the CV has forcibly freed the ops on its
+slab and the slab itself, then we will be fiddling with a freed slab. Making
+C<SAVEFREEOP> a no-op doesn't help, as sometimes an op can be savefreed when
+there is no compilation error, so the op would never be freed. It holds
+a reference count on the slab, so the whole slab would leak. So C<SAVEFREEOP>
+now sets a special flag on the op (C<< ->op_savefree >>). The forced freeing of
+ops after a compilation error won't free any ops thus marked.
+
+Since many pieces of code create tiny subroutines consisting of only a few ops,
+and since a huge slab would be quite a bit of baggage for those to carry
+around, the first slab is always very small. To avoid allocating too many
+slabs for a single CV, each subsequent slab is twice the size of the previous.
+
+Smartmatch expects to be able to allocate an op at run time, run it, and then
+throw it away. For that to work the op is simply malloced when PL_compcv hasn't
+been set up. So all slab-allocated ops are marked as such (C<< ->op_slabbed >>),
+to distinguish them from malloced ops.
+
+
=head1 AUTHORS
Until May 1997, this document was maintained by Jeff Okamoto
https://perl5.git.perl.org / perl5.git / blobdiff