SV Scalar Value
AV Array Value
HV Hash Value
Each typedef has specific routines that manipulate the various data types.
Perl also uses two special typedefs, I32 and I16, which will always be at least 32-bits and 16-bits long, respectively.
The four routines are:
SV* newSViv(IV);
SV* newSVnv(double);
SV* newSVpv(char*, int);
SV* newSVsv(SV*);
To change the value of an *already-existing* SV, there are five routines:
void sv_setiv(SV*, IV);
void sv_setnv(SV*, double);
void sv_setpvn(SV*, char*, int)
void sv_setpv(SV*, char*);
void sv_setsv(SV*, SV*);
Notice that you can choose to specify the length of the string to be
assigned by using sv_setpvn or newSVpv, or you may allow Perl to calculate the length by using sv_setpv or by specifying 0 as the second argument to newSVpv. Be warned, though, that Perl will determine the string's length by using strlen, which depends on the string terminating with a
NUL character.
All SV's that will contain strings should, but need not, be terminated with a NUL character. If it is not 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. Nevertheless, you should be very careful when you pass a string stored in an SV to a C function or system call.
To access the actual value that an SV points to, you can use the macros:
SvIV(SV*)
SvNV(SV*)
SvPV(SV*, STRLEN len)
which will automatically coerce the actual scalar type into an IV, double, or string.
In the SvPV macro, the length of the string returned is placed into the variable len (this is a macro, so you do not use &len). If you do not care what the length of the data is, use the global
variable na. Remember, however, that Perl allows arbitrary strings of data that may both contain NUL's and might not be terminated by a
NUL.
If you want to know if the scalar value is TRUE, you can use:
SvTRUE(SV*)
Although Perl will automatically grow strings for you, if you need to force Perl to allocate more memory for your SV, you can use the macro
SvGROW(SV*, STRLEN newlen)
which will determine if more memory needs to be allocated. If so, it will call the function sv_grow. Note that SvGROW can only increase, not decrease, the allocated memory of an SV and that it does not automatically add a byte for the a trailing NUL (perl's own string functions typically do SvGROW).
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.
SvIOK(SV*)
SvNOK(SV*)
SvPOK(SV*)
You can get and set the current length of the string stored in an SV with the following macros:
SvCUR(SV*)
SvCUR_set(SV*, I32 val)
You can also get a pointer to the end of the string stored in the SV with the macro:
SvEND(SV*)
But note that these last three macros are valid only if SvPOK is true.
If you want to append something to the end of string stored in an SV*, you can use the following functions:
void sv_catpv(SV*, char*);
void sv_catpvn(SV*, char*, int);
void sv_catsv(SV*, SV*);
The first function calculates the length of the string to be appended by
using strlen. In the second, you specify the length of the string yourself. The third 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.
If you know the name of a scalar variable, you can get a pointer to its SV by using the following:
SV* perl_get_sv("package::varname", FALSE);
This returns NULL if the variable does not exist.
If you want to know if this variable (or any other SV) is actually defined, you can call:
SvOK(SV*)
The scalar undef value is stored in an
SV instance called sv_undef. Its address can be used whenever an SV* is needed.
There are also the two values sv_yes and sv_no, which contain Boolean
TRUE and
FALSE values, respectively. Like
sv_undef, their addresses can be used whenever an SV* is needed.
Do not be fooled into thinking that 0 is the same as &sv_undef. Take this code:
SV* sv = (SV*) 0;
if (I-am-to-return-a-real-value) {
sv = sv_2mortal(newSViv(42));
}
sv_setsv(ST(0), sv);
This code tries to return a new SV (which contains the value 42) if it should return a real value, or undef otherwise. Instead it has returned a null pointer which, somewhere down the line, will cause a segmentation violation, bus error, or just weird results. Change the zero to &sv_undef in the first line and all will be well.
To free an SV that you've created, call SvREFCNT_dec. Normally this call is not necessary (see the section on Reference Counts and Mortality).
Sv*V
macros will do the appropriate conversion of string to integer/double or
integer/double to string.
If you really need to know if you have an integer, double, or string pointer in an SV, you can use the following three macros instead:
SvIOKp(SV*)
SvNOKp(SV*)
SvPOKp(SV*)
These will tell you if you truly have an integer, double, or string pointer stored in your SV. The ``p'' stands for private.
In general, though, it's best to use the Sv*V macros.
AV* newAV();
The second method both creates the AV and initially populates it with SV's:
AV* av_make(I32 num, SV **ptr);
The second argument points to an array containing num SV*'s. Once the
AV has been created, the SV's can be destroyed, if so
desired.
Once the AV has been created, the following operations are possible on AV's:
void av_push(AV*, SV*);
SV* av_pop(AV*);
SV* av_shift(AV*);
void av_unshift(AV*, I32 num);
These should be familiar operations, with the exception of av_unshift. This routine adds num elements at the front of the array with the undef
value. You must then use av_store (described below) to assign values to these new elements.
Here are some other functions:
I32 av_len(AV*);
SV** av_fetch(AV*, I32 key, I32 lval);
SV** av_store(AV*, I32 key, SV* val);
The av_len function returns the highest index value in array (just like $#array in
Perl). If the array is empty, -1 is returned. The
av_fetch function returns the value at index key, but if lval
is non-zero, then av_fetch will store an undef value at that index. The av_store function stores the value val at index key. note that av_fetch and av_store both return SV**'s, not SV*'s as their return value.
void av_clear(AV*);
void av_undef(AV*);
void av_extend(AV*, I32 key);
The av_clear function deletes all the elements in the
AV* array, but does not actually delete the array
itself. The av_undef function will delete all the elements in the array plus the array itself.
The
av_extend function extends the array so that it contains key
elements. If key is less than the current length of the array, then nothing is done.
If you know the name of an array variable, you can get a pointer to its AV by using the following:
AV* perl_get_av("package::varname", FALSE);
This returns NULL if the variable does not exist.
HV* newHV();
Once the HV has been created, the following operations are possible on HV's:
SV** hv_store(HV*, char* key, U32 klen, SV* val, U32 hash);
SV** hv_fetch(HV*, char* key, U32 klen, I32 lval);
The klen parameter is the length of the key being passed in (Note that you cannot
pass 0 in as a value of klen to tell Perl to measure the length of the key). The val argument contains the
SV pointer to the scalar being stored, and hash is the pre-computed hash value (zero if you want hv_store to calculate it for you). The lval parameter indicates whether this fetch is actually a part of a store operation, in which case a new undefined value will be added to the
HV with the supplied key and
hv_fetch will return as if the value had already existed.
Remember that hv_store and hv_fetch return SV**'s and not just
SV*. To access the scalar value, you must first dereference the return value. However, you should check to make sure that the return value is not
NULL before dereferencing it.
These two functions check if a hash table entry exists, and deletes it.
bool hv_exists(HV*, char* key, U32 klen);
SV* hv_delete(HV*, char* key, U32 klen, I32 flags);
If flags does not include the G_DISCARD flag then hv_delete will create and return a mortal copy of the deleted value.
And more miscellaneous functions:
void hv_clear(HV*);
void hv_undef(HV*);
Like their AV counterparts, hv_clear deletes all the entries in the hash table but does not actually delete the hash table. The hv_undef deletes both the entries and the hash table itself.
Perl keeps the actual data in linked list of structures with a typedef of
HE. These contain the actual key and value pointers
(plus extra administrative overhead). The key is a string pointer; the
value is an SV*. However, once you have an HE*, to get the actual key and value, use the routines specified below.
I32 hv_iterinit(HV*);
/* Prepares starting point to traverse hash table */
HE* hv_iternext(HV*);
/* Get the next entry, and return a pointer to a
structure that has both the key and value */
char* hv_iterkey(HE* entry, I32* retlen);
/* Get the key from an HE structure and also return
the length of the key string */
SV* hv_iterval(HV*, HE* entry);
/* Return a SV pointer to the value of the HE
structure */
SV* hv_iternextsv(HV*, char** key, I32* retlen);
/* This convenience routine combines hv_iternext,
hv_iterkey, and hv_iterval. The key and retlen
arguments are return values for the key and its
length. The value is returned in the SV* argument */
If you know the name of a hash variable, you can get a pointer to its HV by using the following:
HV* perl_get_hv("package::varname", FALSE);
This returns NULL if the variable does not exist.
The hash algorithm is defined in the PERL_HASH macro:
i = klen;
hash = 0;
s = key;
while (i--)
hash = hash * 33 + *s++;
HE* hv_fetch_ent (HV* tb, SV* key, I32 lval, U32 hash);
HE* hv_store_ent (HV* tb, SV* key, SV* val, U32 hash);
bool hv_exists_ent (HV* tb, SV* key, U32 hash);
SV* hv_delete_ent (HV* tb, SV* key, I32 flags, U32 hash);
SV* hv_iterkeysv (HE* entry);
Note that these functions take SV* keys, which simplifies writing of extension code that deals with hash
structures. These functions also allow passing of SV* keys to tie functions without forcing you to stringify the keys (unlike the previous
set of functions).
They also return and accept whole hash entries (HE*), making their use more efficient (since the hash number for a particular
string doesn't have to be recomputed every time). See API LISTING later in this document for detailed descriptions.
The following macros must always be used to access the contents of hash entries. Note that the arguments to these macros must be simple variables, since they may get evaluated more than once. See API LISTING later in this document for detailed descriptions of these macros.
HePV(HE* he, STRLEN len)
HeVAL(HE* he)
HeHASH(HE* he)
HeSVKEY(HE* he)
HeSVKEY_force(HE* he)
HeSVKEY_set(HE* he, SV* sv)
These two lower level macros are defined, but must only be used when
dealing with keys that are not SV*s:
HeKEY(HE* he)
HeKLEN(HE* he)
To create a reference, use either of the following functions:
SV* newRV_inc((SV*) thing);
SV* newRV_noinc((SV*) thing);
The thing argument can be any of an SV*, AV*, or HV*. The functions are identical except that newRV_inc increments the reference count of the thing, while newRV_noinc does not. For historical reasons, newRV is a synonym for newRV_inc.
Once you have a reference, you can use the following macro to dereference the reference:
SvRV(SV*)
then call the appropriate routines, casting the returned SV* to either an
AV* or HV*, if required.
To determine if an SV is a reference, you can use the following macro:
SvROK(SV*)
To discover what type of value the reference refers to, use the following macro and then check the return value.
SvTYPE(SvRV(SV*))
The most useful types that will be returned are:
SVt_IV Scalar
SVt_NV Scalar
SVt_PV Scalar
SVt_RV Scalar
SVt_PVAV Array
SVt_PVHV Hash
SVt_PVCV Code
SVt_PVGV Glob (possible a file handle)
SVt_PVMG Blessed or Magical Scalar
See the sv.h header file for more details.
A reference can be blessed into a package with the following function:
SV* sv_bless(SV* sv, HV* stash);
The sv argument must be a reference. The stash argument specifies which class the reference will belong to. See the
section on
Stashes and Globs for information on converting class names into stashes.
/* Still under construction */
Upgrades rv to reference if not already one. Creates new
SV for rv to point to. If classname is non-null, the
SV is blessed into the specified class.
SV is returned.
SV* newSVrv(SV* rv, char* classname);
Copies integer or double into an
SV whose reference is rv.
SV is blessed if classname is non-null.
SV* sv_setref_iv(SV* rv, char* classname, IV iv); SV* sv_setref_nv(SV* rv, char* classname, NV iv);
Copies the pointer value (the address, not the string!) into an
SV whose reference is rv.
SV is blessed if
classname is non-null.
SV* sv_setref_pv(SV* rv, char* classname, PV iv);
Copies string into an
SV whose reference is rv. Set length to 0 to let Perl calculate the string length.
SV is blessed if classname is non-null.
SV* sv_setref_pvn(SV* rv, char* classname, PV iv, int length);
int sv_isa(SV* sv, char* name); int sv_isobject(SV* sv);
SV* perl_get_sv("package::varname", TRUE);
AV* perl_get_av("package::varname", TRUE);
HV* perl_get_hv("package::varname", TRUE);
Notice the use of TRUE as the second parameter. The new variable can now be set, using the routines appropriate to the data type.
There are additional macros whose values may be bitwise OR'ed with the
TRUE argument to enable certain extra features. Those bits are:
GV_ADDMULTI Marks the variable as multiply defined, thus preventing the
"Indentifier <varname> used only once: possible typo" warning.
GV_ADDWARN Issues the warning "Had to create <varname> unexpectedly" if
the variable did not exist before the function was called.
If you do not specify a package name, the variable is created in the current package.
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:
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 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 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 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 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 newRV_noinc instead of 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 some convenience functions available that can help with the destruction of xV's. 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 xV's have their reference count decremented depends on two macros, SAVETMPS and FREETMPS. See the perlcall manpage and the perlxs manpage for more details on these macros.
``Mortalization'' then is at its simplest a deferred SvREFCNT_dec. However, if you mortalize a variable twice, the reference count will later be decremented twice.
You should be careful about creating mortal variables. Strange things can happen if you make the same value mortal within multiple contexts, or if you make a variable mortal multiple times.
To create a mortal variable, use the functions:
SV* sv_newmortal()
SV* sv_2mortal(SV*)
SV* sv_mortalcopy(SV*)
The first call creates a mortal SV, the second converts an existing SV to a mortal SV (and thus defers a call to SvREFCNT_dec), and the third creates a mortal copy of an existing SV.
The mortal routines are not just for SV's -- AV's and HV's can be made
mortal by passing their address (type-casted to SV*) to the
sv_2mortal or sv_mortalcopy routines.
Scalar Value
Array Value
Hash Value
File Handle
Directory Handle
Format
Subroutine
There is a single stash called ``defstash'' that holds the items that exist in the ``main'' package. To get at the items in other packages, append the string ``::'' to the package name. The items in the ``Foo'' package are in the stash ``Foo::'' in defstash. The items in the ``Bar::Baz'' package are in the stash ``Baz::'' in ``Bar::'''s stash.
To get the stash pointer for a particular package, use the function:
HV* gv_stashpv(char* name, I32 create)
HV* gv_stashsv(SV*, I32 create)
The first function takes a literal string, the second uses the string stored in the
SV. Remember that a stash is just a hash table, so you get back an
HV*. The create flag will create a new package if it is set.
The name that gv_stash*v wants is the name of the package whose symbol table you want. The default
package is called main. If you have multiply nested packages, pass their names to gv_stash*v, separated by :: as in the Perl language itself.
Alternately, if you have an SV that is a blessed reference, you can find out the stash pointer by using:
HV* SvSTASH(SvRV(SV*));
then use the following to get the package name itself:
char* HvNAME(HV* stash);
If you need to bless or re-bless an object you can use the following function:
SV* sv_bless(SV*, HV* stash)
where the first argument, an SV*, must be a reference, and the second argument is a stash. The returned SV* can now be used in the same way as any other
SV.
For more information on references and blessings, consult the perlref manpage.
Some scalar variables contain more than one type of scalar data. For
example, the variable $! contains either the numeric value of errno
or its string equivalent from either strerror or sys_errlist[].
To force multiple data values into an
SV, you must do two things: use the
sv_set*v routines to add the additional scalar type, then set a flag so that Perl
will believe it contains more than one type of data. The four macros to set
the flags are:
SvIOK_on SvNOK_on SvPOK_on SvROK_on
The particular macro you must use depends on which sv_set*v routine you called first. This is because every sv_set*v routine turns on only the bit for the particular type of data being set,
and turns off all the rest.
For example, to create a new Perl variable called ``dberror'' that contains both the numeric and descriptive string error values, you could use the following code:
extern int dberror;
extern char *dberror_list;
SV* sv = perl_get_sv("dberror", TRUE);
sv_setiv(sv, (IV) dberror);
sv_setpv(sv, dberror_list[dberror]);
SvIOK_on(sv);
If the order of sv_setiv and sv_setpv had been reversed, then the macro SvPOK_on would need to be called instead of SvIOK_on.
Any
SV may be magical, that is, it has special features that a normal
SV does not have. These features are stored in the
SV structure in a linked list of
struct magic's, typedef'ed to MAGIC.
struct magic {
MAGIC* mg_moremagic;
MGVTBL* mg_virtual;
U16 mg_private;
char mg_type;
U8 mg_flags;
SV* mg_obj;
char* mg_ptr;
I32 mg_len;
};
Note this is current as of patchlevel 0, and could change at any time.
void sv_magic(SV* sv, SV* obj, int how, char* name, I32 namlen);
The sv argument is a pointer to the
SV that is to acquire a new magical feature.
If sv is not already magical, Perl uses the SvUPGRADE macro to set the SVt_PVMG flag for the sv. Perl then continues by adding it 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 name and namlem arguments are used to associate a string with the magic, typically the name
of a variable. namlem is stored in the
mg_len field and if name is non-null and namlem >= 0 a malloc'd copy of the name is stored in mg_ptr field.
The sv_magic function uses how to determine which, if any, predefined ``Magic Virtual Table'' should be
assigned to the mg_virtual field. See the ``Magic Virtual Table'' section below. The how argument is also stored in the mg_type field.
The obj argument is stored in the mg_obj field of the MAGIC
structure. If it is not the same as the sv argument, the reference count of the obj object is incremented. If it is the same, or if the how argument is ``#'', or if it is a null pointer, then obj is merely stored, without the reference count being incremented.
There is also a function to add magic to an HV:
void hv_magic(HV *hv, GV *gv, int how);
This simply calls sv_magic and coerces the gv argument into an SV.
To remove the magic from an SV, call the function sv_unmagic:
void sv_unmagic(SV *sv, int type);
The type argument should be equal to the how value when the SV
was initially made magical.
mg_virtual field in the MAGIC structure is a pointer to a
MGVTBL, which is a structure of function pointers and stands for ``Magic Virtual
Table'' to handle the various operations that might be applied to that
variable.
The MGVTBL has five 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);
This
MGVTBL structure is set at compile-time in perl.h and there are currently 19 types (or 21 with overloading turned on). These
different structures contain pointers to various routines that perform
additional actions depending on which function is being called.
Function pointer Action taken
---------------- ------------
svt_get Do something after the value of the SV is retrieved.
svt_set Do something after the SV is assigned a value.
svt_len Report on the SV's length.
svt_clear Clear something the SV represents.
svt_free Free any extra storage associated with the SV.
For instance, the
MGVTBL structure called vtbl_sv (which corresponds to an mg_type of '\0') contains:
{ magic_get, magic_set, magic_len, 0, 0 }
Thus, when an
SV is determined to be magical and of type '\0', if a
get operation is being performed, the routine magic_get is called. All the various routines for the various magical types begin
with magic_.
The current kinds of Magic Virtual Tables are:
mg_type MGVTBL Type of magical
------- ------ ----------------------------
\0 vtbl_sv Regexp???
A vtbl_amagic Operator Overloading
a vtbl_amagicelem Operator Overloading
c 0 Used in Operator Overloading
B vtbl_bm Boyer-Moore???
E vtbl_env %ENV hash
e vtbl_envelem %ENV hash element
g vtbl_mglob Regexp /g flag???
I vtbl_isa @ISA array
i vtbl_isaelem @ISA array element
L 0 (but sets RMAGICAL) Perl Module/Debugger???
l vtbl_dbline Debugger?
o vtbl_collxfrm Locale transformation
P vtbl_pack Tied Array or Hash
p vtbl_packelem Tied Array or Hash element
q vtbl_packelem Tied Scalar or Handle
S vtbl_sig Signal Hash
s vtbl_sigelem Signal Hash element
t vtbl_taint Taintedness
U vtbl_uvar ???
v vtbl_vec Vector
x vtbl_substr Substring???
y vtbl_itervar Shadow "foreach" iterator variable
* vtbl_glob GV???
# vtbl_arylen Array Length
. vtbl_pos $. scalar variable
~ None Used by certain extensions
When an upper-case and lower-case letter both exist in the table, then the upper-case letter is used to represent some kind of composite type (a list or a hash), and the lower-case letter is used to represent an element of that composite type.
The '~' magic type is defined specifically for use by extensions and will not be used by perl itself. Extensions can use ~ magic to 'attach' private information to variables (typically objects). This is especially useful because there is no way for normal perl code to corrupt this private information (unlike using extra elements of a hash object).
Note that because multiple extensions may be using ~ magic it is important for extensions to take extra care with it. Typically only using it on objects blessed into the same class as the extension is sufficient. It may also be appropriate to add an I32 'signature' at the top of the private data area and check that.
MAGIC* mg_find(SV*, int type); /* Finds the magic pointer of that type */
This routine returns a pointer to the MAGIC structure stored in the
SV. If the
SV does not have that magical feature,
NULL is returned. Also, if the
SV is not of type SVt_PVMG, Perl may core-dump.
int mg_copy(SV* sv, SV* nsv, char* key, STRLEN klen);
This routine checks to see what types of magic sv has. If the mg_type field is an upper-case letter, then the mg_obj is
copied to nsv, but the mg_type field is changed to be the lower-case letter.
The stack arguments are accessible through the ST macro, which returns the n'th stack argument. Argument 0 is the first argument passed in the Perl
subroutine call. These arguments are SV*, and can be used anywhere an SV* is used.
Most of the time, output from the
C routine can be handled through use of the
RETVAL and
OUTPUT directives. However, there are some cases where the argument stack is not already long enough to handle all the return values. An example is the
POSIX tzname call, which takes no arguments, but returns two, the local time zone's standard and summer time abbreviations.
To handle this situation, the PPCODE directive is used and the stack is extended using the macro:
EXTEND(sp, num);
where sp is the stack pointer, and num is the number of elements the stack should be extended by.
Now that there is room on the stack, values can be pushed on it using the macros to push IV's, doubles, strings, and SV pointers respectively:
PUSHi(IV)
PUSHn(double)
PUSHp(char*, I32)
PUSHs(SV*)
And now the Perl program calling tzname, the two values will be assigned as in:
($standard_abbrev, $summer_abbrev) = POSIX::tzname;
An alternate (and possibly simpler) method to pushing values on the stack is to use the macros:
XPUSHi(IV)
XPUSHn(double)
XPUSHp(char*, I32)
XPUSHs(SV*)
These macros automatically adjust the stack for you, if needed. Thus, you do not need to call EXTEND to extend the stack.
For more information, consult the perlxs manpage and the perlxstut manpage.
I32 perl_call_sv(SV*, I32);
I32 perl_call_pv(char*, I32);
I32 perl_call_method(char*, I32);
I32 perl_call_argv(char*, I32, register char**);
The routine most often used is perl_call_sv. The SV* argument contains either the name of the Perl subroutine to be called, or a
reference to the subroutine. The second argument consists of flags that
control the context in which the subroutine is called, whether or not the
subroutine is being passed arguments, how errors should be trapped, and how
to treat return values.
All four routines return the number of arguments that the subroutine returned on the Perl stack.
When using any of these routines (except perl_call_argv), the programmer must manipulate the Perl stack. These include the following macros and functions:
dSP
PUSHMARK()
PUTBACK
SPAGAIN
ENTER
SAVETMPS
FREETMPS
LEAVE
XPUSH*()
POP*()
For a detailed description of calling conventions from C to Perl, consult the perlcall manpage.
New(x, pointer, number, type);
Newc(x, pointer, number, type, cast);
Newz(x, pointer, number, type);
These three macros are used to initially allocate memory.
The first argument x was a ``magic cookie'' that was used to keep track of who called the macro,
to help when debugging memory problems. However, the current code makes no
use of this feature (most Perl developers now use run-time memory
checkers), so this argument can be any number.
The second argument pointer should be the name of a variable that will point to the newly allocated
memory.
The third and fourth arguments number and type specify how many of the specified type of data structure should be
allocated. The argument
type is passed to sizeof. The final argument to Newc, cast, should be used if the pointer argument is different from the type
argument.
Unlike the New and Newc macros, the Newz macro calls memzero
to zero out all the newly allocated memory.
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 Renew and Renewc match those of New and Newc with the exception of not needing the ``magic cookie'' argument.
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 source and dest arguments point to the source and destination starting points. Perl will
move, copy, or zero out number
instances of the size of the type data structure (using the sizeof
function).
For a complete description of the PerlIO abstraction, consult the perlapio manpage.
Each of the targets is created only once (but see 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 target and puts the target on stack.
The macro to put this target on stack is PUSHTARG, and it is directly used in some opcodes, as well as indirectly in
zillions of others, which use it via PUSH[pni].
A scratchpad keeps SV's which are lexicals for the current unit and are targets for opcodes. One can deduce that an
SV lives on a scratchpad by looking on its flags: lexicals have
SVs_PADMY set, and
targets have SVs_PADTMP set.
The correspondence between OP's and targets is not 1-to-1. Different OP's in the compile tree of the unit can use the same target, if this would not conflict with the expected life of the temporary.
The answer is recursion, and maybe (sometime soon) threads. Both these can create several execution pointers going into the same 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. (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 depth of the recursion is not more than the length of this array, and if it is, new scratchpad is created and pushed into the array.
The targets on this scratchpad are undefs, but they are already marked with correct flags.
$a = $b + $c;
This is converted to a tree similar to this one:
assign-to
/ \
+ $a
/ \
$b $c
(but slightly more complicated). This tree reflect the way Perl parsed your code, but has nothing to do with the execution order. There is an additional ``thread'' going through the nodes of the tree which shows the order of execution of the nodes. In our simplified example above it looks like:
$b ---> $c ---> + ---> $a ---> assign-to
But with the actual compile tree for $a = $b + $c it is different: some nodes optimized away. As a corollary, though the actual tree contains more nodes than our
simplified example, the execution order is the same as in our example.
-D
optimize=-g on Configure command line), you may examine the compiled tree by specifying -Dx on the Perl command line. The output takes several lines per node, and for $b+$c it looks like this:
5 TYPE = add ===> 6
TARG = 1
FLAGS = (SCALAR,KIDS)
{
TYPE = null ===> (4)
(was rv2sv)
FLAGS = (SCALAR,KIDS)
{
3 TYPE = gvsv ===> 4
FLAGS = (SCALAR)
GV = main::b
}
}
{
TYPE = null ===> (5)
(was rv2sv)
FLAGS = (SCALAR,KIDS)
{
4 TYPE = gvsv ===> 5
FLAGS = (SCALAR)
GV = main::c
}
}
This tree has 5 nodes (one per TYPE specifier), only 3 of them are not optimized away (one per number in the
left column). The immediate children of the given node correspond to {} pairs on the same level of indentation, thus this listing corresponds to
the tree:
add
/ \
null null
| |
gvsv gvsv
The execution order is indicated by ===> marks, thus it is 3
4 5 6 (node 6 is not included into above listing), i.e.,
gvsv gvsv add whatever.
What makes this pass interesting for perl developers is that some
optimization may be performed on this pass. This is optimization by
so-called check routines. The correspondence between node names and corresponding check routines is
described in opcode.pl (do not forget to run make regen_headers if you modify this file).
A check routine is called when the node is fully constructed except for the execution-order thread. Since at this time there is no back-links to the currently constructed node, one can do most any operation to the top-level node, including freeing it and/or creating new nodes above/below it.
The check routine returns the node which should be inserted into the tree (if the top-level node was not modified, check routine returns its argument).
By convention, check routines have names ck_*. They are usually called from new*OP subroutines (or convert) (which in turn are called from perly.y).
If constant folding was not performed, the execution-order thread is created.
Additional context-dependent optimizations are performed at this time. Since at this moment the compile tree contains back-references (via ``thread'' pointers), nodes cannot be freed now. To allow optimized-away nodes at this stage, such nodes are nullified instead of freeing (i.e. their type is changed to
OP_NULL).
peep. Optimizations performed at this stage
are subject to the same restrictions as in the pass 2.
void av_clear _((AV* ar));
key is the index to which the array should be extended.
void av_extend _((AV* ar, I32 key));
key is the index. If lval is set then the fetch will be part of a store. Check that the return value
is non-null before dereferencing it to a SV*.
SV** av_fetch _((AV* ar, I32 key, I32 lval));
I32 av_len _((AV* ar));
AV* av_make _((I32 size, SV** svp));
SV* av_pop _((AV* ar));
void av_push _((AV* ar, SV* val));
SV* av_shift _((AV* ar));
key. The return value will be null if the operation failed, otherwise it can
be dereferenced to get the original SV*.
SV** av_store _((AV* ar, I32 key, SV* val));
void av_undef _((AV* ar));
void av_unshift _((AV* ar, I32 num));
xsubpp to indicate the class name for a
C++
XS constructor. This is always a
char*. See THIS and
Using XS With C++.
memcpy function. The s is the source, d is the destination, n is the number of items, and t is the type.
(void) Copy( s, d, n, t );
HV * CvSTASH( SV* sv )
SvPV( GvSV( DBsub ), na )
mark, for the
XSUB. See MARK and
dORIGMARK.
sp, for the
XSUB. See SP.
xsubpp. Declares the items variable to indicate the number of items on the stack.
xsubpp.
xsubpp.
ENTER;
EXTEND( sp, int x );
FREETMPS;
name and a defined subroutine or
NULL. The glob lives in the given stash, or in the stashes accessable via @ISA and
@<UNIVERSAL>.
The argument level should be either 0 or -1. If level==0, as a side-effect creates a glob with the given name in the given
stash which in the case of success contains an alias for the subroutine, and sets
up caching info for this glob. Similarly for all the searched stashes.
This function grants "SUPER" token as a postfix of the stash name.
The
GV returned from gv_fetchmeth may be a method cache entry, which is not visible to Perl code. So when
calling perl_call_sv, you should not use the
GV directly; instead, you should use the method's
CV, which can be obtained from the
GV with the
GvCV macro.
GV* gv_fetchmeth _((HV* stash, char* name, STRLEN len, I32 level));
stash. In fact in the presense of autoloading this may be the glob for
``AUTOLOAD''. In this case the corresponing variable
$AUTOLOAD is already setup.
Note that if you want to keep this glob for a long time, you need to check for it being
``AUTOLOAD'', since at the later time the the call may load a different subroutine due to $AUTOLOAD changing its value. Use the glob created via a side effect to do this.
This function grants "SUPER" token as a prefix of the method name.
Has the same side-effects and as gv_fetchmeth with level==0.
name should be writable if contains ':' or '\''. The warning against passing the
GV returned by gv_fetchmeth to
perl_call_sv apply equally to gv_fetchmethod.
GV* gv_fetchmethod _((HV* stash, char* name));
create is set then the package will be created if it does not already exist. If create
is not set and the package does not exist then
NULL is returned.
HV* gv_stashpv _((char* name, I32 create));
HV* gv_stashsv _((SV* sv, I32 create));
void he_delayfree _((HV* hv, HE* hent));
SV* pointer where a
char* pointer is to be expected. (For information only--not to be used).
void he_free _((HV* hv, HE* hent));
U32) stored in the hash entry.
HeHASH(HE* he)
char* or SV*, depending on the value of
HeKLEN. Can be assigned to. The HePV or HeSVKEY macros are usually preferable for finding the value of a key.
HeKEY(HE* he)
SV* key. Otherwise, holds the actual length of the key. Can be assigned to. The HePV macro is usually preferable for finding key lengths.
HeKLEN(HE* he)
char* value, doing any necessary dereferencing of possibly SV* keys. The length of the string is placed in len (this is a macro, so do not use
&len). If you do not care about what the length of the key is, you may use the
global variable na. Remember though, that hash keys in perl are free to contain embedded
nulls, so using strlen
or similar is not a good way to find the length of hash keys. This is very
similar to the SvPV macro described elsewhere in this document.
HePV(HE* he, STRLEN len)
SV*, or Nullsv if the hash entry does not contain an SV* key.
HeSVKEY(HE* he)
SV*. Will create and return a temporary mortal SV* if the hash entry contains only a char* key.
HeSVKEY_force(HE* he)
SV*, taking care to set the appropriate flags to indicate the presence of an SV* key, and returns the same SV*.
HeSVKEY_set(HE* he, SV* sv)
SV*) stored in the hash entry.
HeVAL(HE* he)
void hv_clear _((HV* tb));
klen is the length of the key. The
flags value will normally be zero; if set to
G_DISCARD then null will be returned.
SV* hv_delete _((HV* tb, char* key, U32 klen, I32 flags));
flags value will normally be zero; if set to
G_DISCARD then null will be returned. hash can be a valid pre-computed hash value, or 0 to ask for it to be computed.
SV* hv_delete_ent _((HV* tb, SV* key, I32 flags, U32 hash));
klen is the length of the key.
bool hv_exists _((HV* tb, char* key, U32 klen));
hash
can be a valid pre-computed hash value, or 0 to ask for it to be computed.
bool hv_exists_ent _((HV* tb, SV* key, U32 hash));
klen is the length of the key. If lval is set then the fetch will be part of a store. Check that the return value
is non-null before dereferencing it to a SV*.
SV** hv_fetch _((HV* tb, char* key, U32 klen, I32 lval));
hash must be a valid pre-computed hash number for the given key, or 0 if you want the function to compute it.
IF lval is set then the fetch will be part of a store. Make sure the return value
is non-null before accessing it. The return value when tb is a tied hash is a pointer to a static location, so be sure to make a copy
of the structure if you need to store it somewhere.
HE* hv_fetch_ent _((HV* tb, SV* key, I32 lval, U32 hash));
I32 hv_iterinit _((HV* tb));
char* hv_iterkey _((HE* entry, I32* retlen));
SV* hv_iterkeysv _((HE* entry));
HE* hv_iternext _((HV* tb));
SV * hv_iternextsv _((HV* hv, char** key, I32* retlen));
SV* hv_iterval _((HV* tb, HE* entry));
void hv_magic _((HV* hv, GV* gv, int how));
char *HvNAME (HV* stash)
key and klen is the length of the key. The hash parameter is the pre-computed hash value; if it is zero then Perl will
compute it. The return value will be null if the operation failed,
otherwise it can be dereferenced to get the original SV*.
SV** hv_store _((HV* tb, char* key, U32 klen, SV* val, U32 hash));
val in a hash. The hash key is specified as key. The hash
parameter is the pre-computed hash value; if it is zero then Perl will
compute it. The return value is the new hash entry so created. It will be
null if the operation failed or if the entry was stored in a tied hash.
Otherwise the contents of the return value can be accessed using the
He??? macros described here.
HE* hv_store_ent _((HV* tb, SV* key, SV* val, U32 hash));
void hv_undef _((HV* tb));
char is an ascii alphanumeric character or digit.
int isALNUM (char c)
char is an ascii alphabetic character.
int isALPHA (char c)
char is an ascii digit.
int isDIGIT (char c)
char is a lowercase character.
int isLOWER (char c)
char is whitespace.
int isSPACE (char c)
char is an uppercase character.
int isUPPER (char c)
xsubpp to indicate the number of items on the stack. See Variable-length Parameter Lists.
xsubpp to indicate which of an XSUB's aliases was used to invoke it. See The ALIAS: Keyword.
LEAVE;
int mg_clear _((SV* sv));
int mg_copy _((SV *, SV *, char *, STRLEN));
MAGIC* mg_find _((SV* sv, int type));
int mg_free _((SV* sv));
int mg_get _((SV* sv));
U32 mg_len _((SV* sv));
void mg_magical _((SV* sv));
int mg_set _((SV* sv));
memmove function. The s is the source, d is the destination, n is the number of items, and t is the type.
(void) Move( s, d, n, t );
malloc function.
void * New( x, void *ptr, int size, type )
malloc function, with cast.
void * Newc( x, void *ptr, int size, type, cast )
malloc function. The allocated memory is zeroed with memzero.
void * Newz( x, void *ptr, int size, type )
AV* newAV _((void));
HV* newHV _((void));
SV* newRV_inc _((SV* ref));
For historical reasons, ``newRV'' is a synonym for ``newRV_inc''.
SV* newRV_noinc _((SV* ref));
len parameter indicates the number of bytes of pre-allocated string space the
SV should have. The reference count for the new
SV is set to 1.
SV* newSV _((STRLEN len));
SV* newSViv _((IV i));
SV* newSVnv _((NV i));
len is zero then Perl will compute the length.
SV* newSVpv _((char* s, STRLEN len));
rv, to point to. If rv is not an
RV then it will be upgraded to one. If classname is non-null then the new
SV will be blessed in the specified package. The new
SV is returned and its reference count is 1.
SV* newSVrv _((SV* rv, char* classname));
SV* newSVsv _((SV* old));
xsubpp to hook up XSUBs as Perl subs.
xsubpp to hook up XSUBs as Perl subs. Adds Perl prototypes to the subs.
I32 perl_call_argv _((char* subname, I32 flags, char** argv));
I32 perl_call_method _((char* methname, I32 flags));
I32 perl_call_pv _((char* subname, I32 flags));
I32 perl_call_sv _((SV* sv, I32 flags));
I32 perl_eval_sv _((SV* sv, I32 flags));
create is set and the Perl variable does not exist then it will be created. If create is not set and the variable does not exist then null is returned.
AV* perl_get_av _((char* name, I32 create));
create is set and the Perl variable does not exist then it will be created. If create is not set and the variable does not exist then null is returned.
CV* perl_get_cv _((char* name, I32 create));
create is set and the Perl variable does not exist then it will be created. If create is not set and the variable does not exist then null is returned.
HV* perl_get_hv _((char* name, I32 create));
create is set and the Perl variable does not exist then it will be created. If create is not set and the variable does not exist then null is returned.
SV* perl_get_sv _((char* name, I32 create));
void perl_require_pv _((char* pv));
int POPi();
long POPl();
char * POPp();
double POPn();
SV* POPs();
PUSHMARK(p)
PUSHi(int d)
PUSHn(double d)
len indicates the length of the string. See XPUSHp.
PUSHp(char *c, int len )
PUSHs(sv)
xsubpp. See PUSHMARK and the perlcall manpage for other uses.
PUTBACK;
realloc function.
void * Renew( void *ptr, int size, type )
realloc function, with cast.
void * Renewc( void *ptr, int size, type, cast )
xsubpp to hold the return value for an
XSUB. This is always the proper type for the
XSUB. See
The RETVAL Variable.
free function.
malloc function.
realloc function.
char* savepv _((char* sv));
len indicates number of bytes to copy. This does not use an
SV.
char* savepvn _((char* sv, I32 len));
SAVETMPS;
xsubpp. See dSP and
SPAGAIN.
SPAGAIN;
SV* ST(int x)
int strEQ( char *s1, char *s2 )
s1, is greater than or equal to the second, s2. Returns true or false.
int strGE( char *s1, char *s2 )
s1, is greater than the second,
s2. Returns true or false.
int strGT( char *s1, char *s2 )
s1, is less than or equal to the second, s2. Returns true or false.
int strLE( char *s1, char *s2 )
s1, is less than the second,
s2. Returns true or false.
int strLT( char *s1, char *s2 )
int strNE( char *s1, char *s2 )
len parameter indicates the number of bytes to compare. Returns true or false.
int strnEQ( char *s1, char *s2 )
len parameter indicates the number of bytes to compare. Returns true or false.
int strnNE( char *s1, char *s2, int len )
SV* sv_2mortal _((SV* sv));
SV* sv_bless _((SV* sv, HV* stash));
void sv_catpv _((SV* sv, char* ptr));
len indicates number of bytes to copy.
void sv_catpvn _((SV* sv, char* ptr, STRLEN len));
ssv onto the end of the string in
SV
dsv.
void sv_catsv _((SV* dsv, SV* ssv));
sv1 is less than, equal to, or greater than the string in
sv2.
I32 sv_cmp _((SV* sv1, SV* sv2));
sv1 is less than, equal to, or greater than the string in
sv2.
I32 sv_cmp _((SV* sv1, SV* sv2));
int SvCUR (SV* sv)
SvCUR_set (SV* sv, int val )
void sv_dec _((SV* sv));
void sv_dec _((SV* sv));
*SvEND(sv)
I32 sv_eq _((SV* sv1, SV* sv2));
char * SvGROW( SV* sv, int len )
void sv_inc _((SV* sv));
int SvIOK (SV* SV)
SvIOK_off (SV* sv)
SvIOK_on (SV* sv)
SvIOK_on (SV* sv)
SvIOK_on (SV* sv)
int SvIOKp (SV* SV)
int sv_isa _((SV* sv, char* name));
int SvIV (SV* sv)
int sv_isobject _((SV* sv));
int SvIVX (SV* sv);
int SvLEN (SV* sv)
STRLEN sv_len _((SV* sv));
STRLEN sv_len _((SV* sv));
void sv_magic _((SV* sv, SV* obj, int how, char* name, I32 namlen));
SV* sv_mortalcopy _((SV* oldsv));
int SvOK (SV* sv)
SV* sv_newmortal _((void));
false
SV. See sv_yes. Always refer to this as &sv_no.
int SvNIOK (SV* SV)
SvNIOK_off (SV* sv)
int SvNIOKp (SV* SV)
int SvNOK (SV* SV)
SvNOK_off (SV* sv)
SvNOK_on (SV* sv)
SvNOK_on (SV* sv)
SvNOK_on (SV* sv)
int SvNOKp (SV* SV)
double SvNV (SV* sv);
double SvNVX (SV* sv);
int SvPOK (SV* SV)
SvPOK_off (SV* sv)
SvPOK_on (SV* sv)
SvPOK_on (SV* sv)
SvPOK_on (SV* sv)
int SvPOKp (SV* SV)
len is na then Perl will handle the length on its own.
char * SvPV (SV* sv, int len )
char * SvPVX (SV* sv)
int SvREFCNT (SV* sv);
void SvREFCNT_dec (SV* sv)
void SvREFCNT_inc (SV* sv)
int SvROK (SV* sv)
SvROK_off (SV* sv)
SvROK_on (SV* sv)
SV* SvRV (SV* sv);
void sv_setiv _((SV* sv, IV num));
void sv_setnv _((SV* sv, double num));
void sv_setpv _((SV* sv, char* ptr));
len parameter indicates the number of bytes to be copied.
void sv_setpvn _((SV* sv, char* ptr, STRLEN len));
rv
argument will be upgraded to an
RV. That
RV will be modified to point to the new
SV. The
classname argument indicates the package for the blessing. Set classname to Nullch to avoid the blessing. The new
SV will be returned and will have a reference count of
1.
SV* sv_setref_iv _((SV *rv, char *classname, IV iv));
rv
argument will be upgraded to an
RV. That
RV will be modified to point to the new
SV. The
classname argument indicates the package for the blessing. Set classname to Nullch to avoid the blessing. The new
SV will be returned and will have a reference count of
1.
SV* sv_setref_nv _((SV *rv, char *classname, double nv));
rv
argument will be upgraded to an
RV. That
RV will be modified to point to the new
SV. If the
pv argument is
NULL then sv_undef will be placed into the
SV. The classname argument indicates the package for the blessing. Set classname to Nullch to avoid the blessing. The new
SV will be returned and will have a reference count of
1.
SV* sv_setref_pv _((SV *rv, char *classname, void* pv));
Do not use with integral Perl types such as HV, AV, SV, CV, because those objects will become corrupted by the pointer copy process.
Note that sv_setref_pvn copies the string while this copies the pointer.
n. The rv argument will be upgraded to an
RV. That
RV will be modified to point to the new
SV. The
classname
argument indicates the package for the blessing. Set classname to
Nullch to avoid the blessing. The new
SV will be returned and will have a reference count of
1.
SV* sv_setref_pvn _((SV *rv, char *classname, char* pv, I32 n));
Note that sv_setref_pv copies the pointer while this copies the string.
ssv into the destination
SV dsv. The source
SV may be destroyed if it is mortal.
void sv_setsv _((SV* dsv, SV* ssv));
HV * SvSTASH (SV* sv)
int SvTRUE (SV* sv)
svtype SvTYPE (SV* sv)
bool SvUPGRADE _((SV* sv, svtype mt));
void sv_unref _((SV* sv));
ptr to find its string value. Normally the string is stored inside the
SV but sv_usepvn allows the
SV to use an outside string. The
ptr should point to memory that was allocated by malloc. The string length, len, must be supplied. This function will realloc the memory pointed to by ptr, so that pointer should not be freed or used by the programmer after
giving it to sv_usepvn.
void sv_usepvn _((SV* sv, char* ptr, STRLEN len));
true
SV. See sv_no. Always refer to this as &sv_yes.
xsubpp to designate the object in a
C++
XSUB. This is always the proper type for the
C++ object. See
CLASS and
Using XS With C++.
int toLOWER (char c)
int toUPPER (char c)
XPUSHi(int d)
XPUSHn(double d)
len
indicates the length of the string. See PUSHp.
XPUSHp(char *c, int len)
XPUSHs(sv)
xsubpp.
xsubpp.
XSRETURN(int x);
XSRETURN_EMPTY;
XSRETURN_IV(IV v);
XSRETURN_NO;
XSRETURN_NV(NV v);
XSRETURN_PV(char *v);
XSRETURN_UNDEF;
XSRETURN_YES;
i on the stack. The value is stored in a new mortal
SV.
XST_mIV( int i, IV v );
i on the stack. The value is stored in a new mortal
SV.
XST_mNV( int i, NV v );
i on the stack.
XST_mNO( int i );
i on the stack. The value is stored in a new mortal
SV.
XST_mPV( int i, char *v );
i on the stack.
XST_mUNDEF( int i );
i on the stack.
XST_mYES( int i );
ExtUtils::MakeMaker. See XS_VERSION_BOOTCHECK.
$VERSION variable matches the
XS module's
XS_VERSION variable. This is usually handled automatically by
xsubpp. See The VERSIONCHECK: Keyword.
memzero function. The d is the destination, n is the number of items, and t is the type.
(void) Zero( d, n, t );
With lots of help and suggestions from Dean Roehrich, Malcolm Beattie, Andreas Koenig, Paul Hudson, Ilya Zakharevich, Paul Marquess, Neil Bowers, Matthew Green, Tim Bunce, Spider Boardman, and Ulrich Pfeifer.
API Listing by Dean Roehrich <roehrich@cray.com>.