Manual - Section 1: perlguts

NAME

perlguts - Introduction to the Perl API

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 questions or comments to the author below.

Variables

Datatypes

Perl has three typedefs that handle Perl’s three main data types:


    SV  Scalar Value
    AV  Array Value
    HV  Hash Value

Each typedef has specific routines that manipulate the various data types.

What is an ``\s-1IV\s0''?

Perl uses a special typedef IV which is a simple signed integer type that is guaranteed to be large enough to hold a pointer (as well as an integer). 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, as well.) They will usually be exactly 32 and 16 bits long, but on Crays they will both be 64 bits.

Working with SVs

An SV can be created and loaded with one command. There are five types of values that can be loaded: an integer value (IV), an unsigned integer value (UV), a double (NV), a string (PV), and another scalar (SV).

The seven routines are:


    SV*  newSViv(IV);
    SV*  newSVuv(UV);
    SV*  newSVnv(double);
    SV*  newSVpv(const char*, STRLEN);
    SV*  newSVpvn(const char*, STRLEN);
    SV*  newSVpvf(const char*, ...);
    SV*  newSVsv(SV*);

STRLEN

is an integer type (Size_t, usually defined as size_t in config.h) guaranteed to be large enough to represent the size of any string that perl can handle.

In the unlikely case of a SV requiring more complex initialisation, you can create an empty SV with newSV(len). If

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 SV has value undef.


    SV *sv = newSV(0);   /* no storage allocated  */
    SV *sv = newSV(10);  /* 10 (+1) bytes of uninitialised storage allocated  */

To change the value of an already-existing SV, there are eight routines:


    void  sv_setiv(SV*, IV);
    void  sv_setuv(SV*, UV);
    void  sv_setnv(SV*, double);
    void  sv_setpv(SV*, const char*);
    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 *);
    void  sv_setsv(SV*, SV*);

Notice that you can choose to specify the length of the string to be assigned by using

sv_setpvn

newSVpvn

, 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.

The arguments of

sv_setpvf

are processed like

sprintf

, and the formatted output becomes the value.

sv_vsetpvfn

is an analogue of

vsprintf

, but it allows you to specify either a pointer to a variable argument list or the address and length of an array of SVs. The last argument points to a boolean; on return, if that boolean is true, then locale-specific information has been used to format the string, and the string’s contents are therefore untrustworthy (see perlsec). This pointer may be NULL if that information is not important. Note that this function requires you to specify the length of the format.

The

sv_set*()

functions are not generic enough to operate on values that have magic. See 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 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*)
    SvUV(SV*)
    SvNV(SV*)
    SvPV(SV*, STRLEN len)
    SvPV_nolen(SV*)

which will automatically coerce the actual scalar type into an IV, UV, 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

SvPV_nolen

macro. Historically the

SvPV

macro with the global variable

PL_na

has been used in this case. But that can be quite inefficient because

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.

Also remember that C doesn’t allow you to safely say

foo(SvPV(s, len),
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;
    STRLEN len;
    char * ptr;
    ptr = SvPV(s, len);
    foo(ptr, len);

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(sv, len + 1)

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*, const char*);
    void  sv_catpvn(SV*, const char*, STRLEN);
    void  sv_catpvf(SV*, const char*, ...);
    void  sv_vcatpvfn(SV*, const char*, STRLEN, va_list *, SV **, I32, bool);
    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 processes its arguments like

sprintf

and appends the formatted output. The fourth function works like

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 SV with the string stored in the second SV. It also forces the second SV to be interpreted as a string.

The

sv_cat*()

functions are not generic enough to operate on values that have magic. See 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:


    SV*  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

PL_sv_undef

Its address can be used whenever an

SV*

is needed. Make sure that you don’t try to compare a random sv with

&PL_sv_undef

. For example when interfacing Perl code, it’ll work correctly for:


  foo(undef);

But won’t work when called as:


  $x = undef;
  foo($x);

So to repeat always use SvOK() to check whether an sv is defined.

Also you have to be careful when using

&PL_sv_undef

as a value in AVs or HVs (see AVs, HVs and undefined values).

There are also the two values

PL_sv_yes

and

PL_sv_no

, which contain boolean TRUE and FALSE values, respectively. Like

PL_sv_undef

, their addresses can be used whenever an

SV*

is needed.

Do not be fooled into thinking that

(SV *) 0

is the same as

&PL_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

&PL_sv_undef

in the first line and all will be well.

To free an SV that you’ve created, call

SvREFCNT_dec(SV*)

. Normally this call is not necessary (see Reference Counts and Mortality).

Offsets

Perl provides the function

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 actually removing the characters,

sv_chop

sets the flag

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

SvPVX

) forward that many bytes, and adjusts

SvCUR

and

SvLEN

Hence, at this point, the start of the buffer that we allocated lives at

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:


  % ./perl -Ilib -MDevel::Peek -le $a="12345"; $a=~s/.//; Dump($a)
  SV = PVIV(0x8128450) at 0x81340f0
    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

Devel::Peek::Dump

helpfully reminds us that this is an offset. The portion of the string between the real and the fake beginnings is shown in parentheses, and the values of

SvCUR

and

SvLEN

reflect the fake beginning, not the real one.

Something similar to the offset hack is performed on AVs to enable efficient shifting and splicing off the beginning of the array; while

AvARRAY

points to the first element in the array that is visible from Perl,

AvALLOC

points to the real start of the C array. These are usually the same, but a

shift

operation can be carried out by increasing

AvARRAY

by one and decreasing

AvFILL

and

AvLEN

. Again, the location of the real start of the C array only comes into play when freeing the array. See

av_shift

in av.c.

What's Really Stored in an \s-1SV\s0?

Recall that the usual method of determining the type of scalar you have is to use

Sv*OK

macros. Because a scalar can be both a number and a string, usually these macros will always return TRUE and calling the

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.

The are various ways in which the private and public flags may differ. For example, a tied SV may have a valid underlying value in the IV slot (so SvIOKp is true), but the data should be accessed via the FETCH routine rather than directly, so SvIOK is false. Another is when numeric conversion has occurred and precision has been lost: only the private flag is set on ’lossy’ values. So when an NV is converted to an IV with loss, SvIOKp, SvNOKp and SvNOK will be set, while SvIOK wont be.

In general, though, it’s best to use the

Sv*V

macros.

Working with AVs

There are two ways to create and load an AV. The first method creates an empty AV:


    AV*  newAV();

The second method both creates the AV and initially populates it with SVs:


    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 SVs can be destroyed, if so desired.

Once the AV has been created, the following operations are possible on AVs:


    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

, and does not increment the reference count of

val

. Thus the caller is responsible for taking care of that, and if

av_store

returns NULL, the caller will have to decrement the reference count to avoid a memory leak. 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 at least

key+1

elements. If

key+1

is less than the currently allocated 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*  get_av("package::varname", FALSE);

This returns NULL if the variable does not exist.

See Understanding the Magic of Tied Hashes and Arrays for more information on how to use the array access functions on tied arrays.

Working with HVs

To create an HV, you use the following routine:


    HV*  newHV();

Once the HV has been created, the following operations are possible on HVs:


    SV**  hv_store(HV*, const char* key, U32 klen, SV* val, U32 hash);
    SV**  hv_fetch(HV*, const 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 precomputed 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*, const char* key, U32 klen);
    SV*   hv_delete(HV*, const char* key, U32 klen, I32 flags);

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 an 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*  get_hv("package::varname", FALSE);

This returns NULL if the variable does not exist.

The hash algorithm is defined in the

PERL_HASH(hash, key, klen)

macro:


    hash = 0;
    while (klen--)
        hash = (hash * 33) + *key++;
    hash = hash + (hash >> 5);                  /* after 5.6 */

The last step was added in version 5.6 to improve distribution of lower bits in the resulting hash value.

See Understanding the Magic of Tied Hashes and Arrays for more information on how to use the hash access functions on tied hashes.

Hash \s-1API\s0 Extensions

Beginning with version 5.004, the following functions are also supported:


    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 perlapi 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 perlapi 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*


    HeKEY(HE* he)
    HeKLEN(HE* he)

Note that both

hv_store

and

hv_store_ent

do not increment the reference count of the stored

val

, which is the caller’s responsibility. If these functions return a NULL value, the caller will usually have to decrement the reference count of

val

to avoid a memory leak.

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 used to using

&PL_sv_undef

if you need an undefined SV.

For example, intuition tells you that this XS code:


    AV *av = newAV();
    av_store( av, 0, &PL_sv_undef );

is equivalent to this Perl code:


    my @av;
    $av[0] = undef;

Unfortunately, this isn’t true. AVs use

&PL_sv_undef

as a marker for indicating that an array element has not yet been initialized. Thus,

exists $av[0]

would be true for the above Perl code, but false for the array generated by the XS code.

References

References are a special type of scalar that point to other data types (including references).

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*

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.

Blessed References and Class Objects

References are also used to support object-oriented programming. In perl’s OO lexicon, an object is simply a reference that has been blessed into a package (or class). Once blessed, the programmer may now use the reference to access the various methods in the class.

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 value. The

stash

argument specifies which class the reference will belong to. See 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, const char* classname);

Copies integer, unsigned integer or double into an SV whose reference is

rv

. SV is blessed if

classname

is non-null.


        SV* sv_setref_iv(SV* rv, const char* classname, IV iv);
        SV* sv_setref_uv(SV* rv, const char* classname, UV uv);
        SV* sv_setref_nv(SV* rv, const 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, const 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, const char* classname, PV iv, STRLEN length);

Tests whether the SV is blessed into the specified class. It does not check inheritance relationships.


        int  sv_isa(SV* sv, const char* name);

Tests whether the SV is a reference to a blessed object.


        int  sv_isobject(SV* sv);

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

UNIVERSAL::isa

functionality.


        bool sv_derived_from(SV* sv, const char* name);

To check if you’ve got an object derived from a specific class you have to write:


        if (sv_isobject(sv) && sv_derived_from(sv, class)) { ... }

Creating New Variables

To create a new Perl variable with an undef value which can be accessed from your Perl script, use the following routines, depending on the variable type.


    SV*  get_sv("package::varname", TRUE);
    AV*  get_av("package::varname", TRUE);
    HV*  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:


  Name <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.

Reference Counts and Mortality

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:


    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 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 perlcall and perlxs 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.

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:


    SV*  sv_newmortal()
    SV*  sv_2mortal(SV*)
    SV*  sv_mortalcopy(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

SvREFCNT_dec

), and the third creates a mortal copy of an existing SV. Because

sv_newmortal

gives the new SV no value,it must normally be given one via

sv_setpv

sv_setiv

, etc. :


    SV *tmp = sv_newmortal();
    sv_setiv(tmp, an_integer);

As that is multiple C statements it is quite common so see this idiom instead:


    SV *tmp = sv_2mortal(newSViv(an_integer));

You should be careful about creating mortal variables. Strange things can happen if you make the same value mortal within multiple contexts, or if you make a variable mortal multiple times. Thinking of Mortalization as deferred

SvREFCNT_dec

should help to minimize such problems. For example if you are passing an SV which you know has high enough REFCNT to survive its use on the stack you need not do any mortalization. If you are not sure then doing an

SvREFCNT_inc

and

sv_2mortal

, or making a

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

SV*

) to the

sv_2mortal

sv_mortalcopy

routines.

Stashes and Globs

A stash is a hash that contains all variables that are defined within a package. Each key of the stash is a symbol name (shared by all the different types of objects that have the same name), and each value in the hash table is a GV (Glob Value). This GV in turn contains references to the various objects of that name, including (but not limited to) the following:


    Scalar Value
    Array Value
    Hash Value
    I/O Handle
    Format
    Subroutine

There is a single stash called

PL_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 PL_defstash. The items in the

Bar::Baz

package are in the stash

Baz::

Bar::

’s stash.

To get the stash pointer for a particular package, use the function:


    HV*  gv_stashpv(const char* name, I32 flags)
    HV*  gv_stashsv(SV*, I32 flags)

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

flags

flag will create a new package if it is set to GV_ADD.

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 perlref.

Double-Typed SVs

Scalar variables normally contain only one type of value, an integer, double, pointer, or reference. Perl will automatically convert the actual scalar data from the stored type into the requested type.

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

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 = 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

Magic Variables

[This section still under construction. Ignore everything here. Post no bills. Everything not permitted is forbidden.]

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;
        I32         mg_len;
        SV*         mg_obj;
        char*       mg_ptr;
    };

Note this is current as of patchlevel 0, and could change at any time.

Assigning Magic

Perl adds magic to an SV using the sv_magic function:


    void sv_magic(SV* sv, SV* obj, int how, const char* name, I32 namlen);

The

sv

argument is a pointer to the SV that is to acquire a new magical feature.

sv

is not already magical, Perl uses the

SvUPGRADE

macro to convert

sv

to type

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

name

and

namlen

arguments are used to associate a string with the magic, typically the name of a variable.

namlen

is stored in the

mg_len

field and if

name

is non-null then either a

savepvn

copy of

name

name

itself is stored in the

mg_ptr

field, depending on whether

namlen

is greater than zero or equal to zero respectively. As a special case, if

(name && namlen == HEf_SVKEY)

then

name

is assumed to contain an

SV*

and is stored as-is with its REFCNT incremented.

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 Tables section below. The

how

argument is also stored in the

mg_type

field. The value of

how

should be chosen from the set of macros

PERL_MAGIC_foo

found in 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

PERL_MAGIC_uvar

for example.

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

PERL_MAGIC_arylen

, or if it is a NULL pointer, then

obj

is merely stored, without the reference count being incremented.

Magic Virtual Tables

The

mg_virtual

field in the

MAGIC

structure is a pointer to an

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 (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_copy)(SV *sv, MAGIC* mg, SV *nsv, const char *name, int namlen);
    int  (*svt_dup)(MAGIC *mg, CLONE_PARAMS *param);
    int  (*svt_local)(SV *nsv, 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 before the value of the SV is retrieved.
    svt_set             Do something after the SV is assigned a value.
    svt_len             Report on the SVs length.
    svt_clear           Clear something the SV represents.
    svt_free            Free any extra storage associated with the SV.

    svt_copy            copy tied variable magic to a tied element
    svt_dup             duplicate a magic structure during thread cloning
    svt_local           copy magic to local value during local

For instance, the MGVTBL structure called

vtbl_sv

(which corresponds to an

mg_type

PERL_MAGIC_sv

) contains:


    { magic_get, magic_set, magic_len, 0, 0 }

Thus, when an SV is determined to be magical and of type

PERL_MAGIC_sv

, 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_

. NOTE: the magic routines are not considered part of the Perl API, and may not be exported by the Perl library.

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 currently used exclusively by the threading code, and are highly subject to change.

The current kinds of Magic Virtual Tables are:


    mg_type
    (old-style char and macro)   MGVTBL          Type of magic
    --------------------------   ------          -------------
    \0 PERL_MAGIC_sv             vtbl_sv         Special scalar variable
    A  PERL_MAGIC_overload       vtbl_amagic     %OVERLOAD hash
    a  PERL_MAGIC_overload_elem  vtbl_amagicelem %OVERLOAD hash element
    c  PERL_MAGIC_overload_table (none)          Holds overload table (AMT)
                                                 on stash
    B  PERL_MAGIC_bm             vtbl_bm         Boyer-Moore (fast string search)
    D  PERL_MAGIC_regdata        vtbl_regdata    Regex match position data
                                                 (@+ and @- vars)
    d  PERL_MAGIC_regdatum       vtbl_regdatum   Regex match position data
                                                 element
    E  PERL_MAGIC_env            vtbl_env        %ENV hash
    e  PERL_MAGIC_envelem        vtbl_envelem    %ENV hash element
    f  PERL_MAGIC_fm             vtbl_fm         Formline (compiled format)
    g  PERL_MAGIC_regex_global   vtbl_mglob      m//g target / study()ed string
    H  PERL_MAGIC_hints          vtbl_sig        %^H hash
    h  PERL_MAGIC_hintselem      vtbl_hintselem  %^H hash element
    I  PERL_MAGIC_isa            vtbl_isa        @ISA array
    i  PERL_MAGIC_isaelem        vtbl_isaelem    @ISA array element
    k  PERL_MAGIC_nkeys          vtbl_nkeys      scalar(keys()) lvalue
    L  PERL_MAGIC_dbfile         (none)          Debugger %_<filename
    l  PERL_MAGIC_dbline         vtbl_dbline     Debugger %_<filename element
    o  PERL_MAGIC_collxfrm       vtbl_collxfrm   Locale collate transformation
    P  PERL_MAGIC_tied           vtbl_pack       Tied array or hash
    p  PERL_MAGIC_tiedelem       vtbl_packelem   Tied array or hash element
    q  PERL_MAGIC_tiedscalar     vtbl_packelem   Tied scalar or handle
    r  PERL_MAGIC_qr             vtbl_qr         precompiled qr// regex
    S  PERL_MAGIC_sig            vtbl_sig        %SIG hash
    s  PERL_MAGIC_sigelem        vtbl_sigelem    %SIG hash element
    t  PERL_MAGIC_taint          vtbl_taint      Taintedness
    U  PERL_MAGIC_uvar           vtbl_uvar       Available for use by extensions
    v  PERL_MAGIC_vec            vtbl_vec        vec() lvalue
    V  PERL_MAGIC_vstring        (none)          v-string scalars
    w  PERL_MAGIC_utf8           vtbl_utf8       UTF-8 length+offset cache
    x  PERL_MAGIC_substr         vtbl_substr     substr() lvalue
    y  PERL_MAGIC_defelem        vtbl_defelem    Shadow "foreach" iterator
                                                 variable / smart parameter
                                                 vivification
    #  PERL_MAGIC_arylen         vtbl_arylen     Array length ($#ary)
    .  PERL_MAGIC_pos            vtbl_pos        pos() lvalue
    <  PERL_MAGIC_backref        vtbl_backref    back pointer to a weak ref
    ~  PERL_MAGIC_ext            (none)          Available for use by extensions
    :  PERL_MAGIC_symtab         (none)          hash used as symbol table
    %  PERL_MAGIC_rhash          (none)          hash used as restricted hash
    @  PERL_MAGIC_arylen_p       vtbl_arylen_p   pointer to $#a from @a

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 relationship. However, ’v’ and ’V’ (vec and v-string) are in no way related.

The

PERL_MAGIC_ext

and

PERL_MAGIC_uvar

magic types are defined specifically for use by extensions and will not be used by perl itself. Extensions can use

PERL_MAGIC_ext

magic to ’attach’ private information to variables (typically objects). This is especially useful because there is no way for normal perl code to corrupt this private information (unlike using extra elements of a hash object).

Similarly,

PERL_MAGIC_uvar

magic can be used much like tie() to call a C function any time a scalar’s value is used or changed. The

MAGIC

’s

mg_ptr

field points to a

ufuncs

structure:


    struct ufuncs {
        I32 (*uf_val)(pTHX_ IV, SV*);
        I32 (*uf_set)(pTHX_ IV, SV*);
        IV uf_index;
    };

When the SV is read from or written to, the

uf_val

uf_set

function will be called with

uf_index

as the first arg and a pointer to the SV as the second. A simple example of how to add

PERL_MAGIC_uvar

magic is shown below. Note that the ufuncs structure is copied by sv_magic, so you can safely allocate it on the stack.


    void
    Umagic(sv)
        SV *sv;
    PREINIT:
        struct ufuncs uf;
    CODE:
        uf.uf_val   = &my_get_fn;
        uf.uf_set   = &my_set_fn;
        uf.uf_index = 0;
        sv_magic(sv, 0, PERL_MAGIC_uvar, (char*)&uf, sizeof(uf));

Attaching

PERL_MAGIC_uvar

to arrays is permissible but has no effect.

For hashes there is a specialized hook that gives control over hash keys (but not values). This hook calls

PERL_MAGIC_uvar

’get’ magic if the set function in the

ufuncs

structure is NULL. The hook is activated whenever the hash is accessed with a key specified as an

SV

through the functions

hv_store_ent

hv_fetch_ent

hv_delete_ent

, and

hv_exists_ent

. Accessing the key as a string through the functions without the

..._ent

suffix circumvents the hook. See Guts in Hash::Util::Fieldhash for a detailed description.

Note that because multiple extensions may be using

PERL_MAGIC_ext

PERL_MAGIC_uvar

magic, it is important for extensions to take extra care to avoid conflict. Typically only using the magic on objects blessed into the same class as the extension is sufficient. For

PERL_MAGIC_ext

magic, it may also be appropriate to add an I32 ’signature’ at the top of the private data area and check that.

Also note that the

sv_set*()

and

sv_cat*()

functions described earlier do not invoke ’set’ magic on their targets. This must be done by the user either by calling the

SvSETMAGIC()

macro after calling these functions, or by using one of the

sv_set*_mg()

sv_cat*_mg()

functions. Similarly, generic C code must call the

SvGETMAGIC()

macro to invoke any ’get’ magic if they use an SV obtained from external sources in functions that don’t handle magic. See perlapi for a description of these functions. For example, calls to the

sv_cat*()

functions typically need to be followed by

SvSETMAGIC()

, but they don’t need a prior

SvGETMAGIC()

since their implementation handles ’get’ magic.

Finding Magic


    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, const char* key, STRLEN klen);

This routine checks to see what types of magic

sv

has. If the mg_type field is an uppercase letter, then the mg_obj is copied to

nsv

, but the mg_type field is changed to be the lowercase letter.

Understanding the Magic of Tied Hashes and Arrays

Tied hashes and arrays are magical beasts of the

PERL_MAGIC_tied

magic type.

WARNING: As of the 5.004 release, proper usage of the array and hash access functions requires understanding a few caveats. Some of these caveats are actually considered bugs in the API, to be fixed 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 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 reference to the new tied hash. Note that the code below does NOT call the TIEHASH method in the MyTie class - see Calling Perl Routines from within C Programs for details on how to do this.


    SV*
    mytie()
    PREINIT:
        HV *hash;
        HV *stash;
        SV *tie;
    CODE:
        hash = newHV();
        tie = newRV_noinc((SV*)newHV());
        stash = gv_stashpv("MyTie", GV_ADD);
        sv_bless(tie, stash);
        hv_magic(hash, (GV*)tie, PERL_MAGIC_tied);
        RETVAL = newRV_noinc(hash);
    OUTPUT:
        RETVAL

The

av_store

function, when given a tied array argument, merely copies the magic of the array onto the value to be stored, using

mg_copy

. It may also return NULL, indicating that the value did not actually need to be stored in the array. [MAYCHANGE] After a call to

av_store

on a tied array, the caller will usually need to call

mg_set(val)

to actually invoke the perl level STORE method on the TIEARRAY object. If

av_store

did return NULL, a call to

SvREFCNT_dec(val)

will also be usually necessary to avoid a memory leak. [/MAYCHANGE]

The previous paragraph is applicable verbatim to tied hash access using the

hv_store

and

hv_store_ent

functions as well.

av_fetch

and the corresponding hash functions

hv_fetch

and

hv_fetch_ent

actually return an undefined mortal value whose magic has been initialized using

mg_copy

. Note the value so returned does not need to be deallocated, as it is already mortal. [MAYCHANGE] But you will need to call

mg_get()

on the returned value in order to actually invoke the perl level FETCH method on the underlying TIE object. Similarly, you may also call

mg_set()

on the return value after possibly assigning a suitable value to it using

sv_setsv

, which will invoke the STORE method on the TIE object. [/MAYCHANGE]

[MAYCHANGE] In other words, the array or hash fetch/store functions don’t really fetch and store actual values in the case of tied arrays and hashes. They merely call

mg_copy

to attach magic to the values that were meant to be stored or fetched. Later calls to

mg_get

and

mg_set

actually do the job of invoking the TIE methods on the underlying objects. Thus the magic mechanism currently implements a kind of lazy access to arrays and hashes.

Currently (as of perl version 5.004), use of the hash and array access functions requires the user to be aware of whether they are operating on normal hashes and arrays, or on their tied variants. The API may be changed to provide more transparent access to both tied and normal data types in future versions. [/MAYCHANGE]

You would do well to understand that the TIEARRAY and TIEHASH interfaces are mere sugar to invoke some perl method calls while using the uniform hash and array syntax. The use of this sugar imposes some overhead (typically about two to four extra opcodes per FETCH/STORE operation, in addition to the creation of all the mortal variables required to invoke the methods). This overhead will be comparatively small if the TIE methods are themselves substantial, but if they are only a few statements long, the overhead will not be insignificant.

Localizing changes

Perl has a very handy construction


  {
    local $var = 2;
    ...
  }

This construction is approximately equivalent to


  {
    my $oldvar = $var;
    $var = 2;
    ...
    $var = $oldvar;
  }

The biggest difference is that the first construction would reinstate the initial value of

$var

, irrespective of how control exits the block:

goto

return

die

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 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 block-like construct is created by a pair of

ENTER

LEAVE

macros (see Returning a Scalar in perlcall). 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 an

ENTER

LEAVE

pair.

Inside such a pseudo-block the following service is available:

SAVEINT(int i)

SAVEIV(IV i)

SAVEI32(I32 i)

SAVELONG(long i)
These macros arrange things to restore the value of integer variable
i
at the end of enclosing pseudo-block.

SAVESPTR(s)

SAVEPPTR(p)
These macros arrange things to restore the value of pointers
s
and
p
.
s
must be a pointer of a type which survives conversion to
SV*
and back,
p
should be able to survive conversion to
char*
and back.

SAVEFREESV(SV *sv)
The refcount of
sv
would be decremented at the end of pseudo-block. This is similar to
sv_2mortal
in that it is also a mechanism for doing a delayed
SvREFCNT_dec
. However, while
sv_2mortal
extends the lifetime of
sv
until the beginning of the next statement,
SAVEFREESV
extends it until the end of the enclosing scope. These lifetimes can be wildly different.
Also compare
SAVEMORTALIZESV
.

SAVEMORTALIZESV(SV *sv)
Just like
SAVEFREESV
, but mortalizes
sv
at the end of the current scope instead of decrementing its reference count. This usually has the effect of keeping
sv
alive until the statement that called the currently live scope has finished executing.

SAVEFREEOP(OP *op)
The
OP *
is op_free()ed at the end of pseudo-block.

SAVEFREEPV(p)
The chunk of memory which is pointed to by
p
is Safefree()ed at the end of pseudo-block.

SAVECLEARSV(SV *sv)
Clears a slot in the current scratchpad which corresponds to
sv
at the end of pseudo-block.

SAVEDELETE(HV *hv, char *key, I32 length)
The key
key
of
hv
is deleted at the end of pseudo-block. The string pointed to by
key
is Safefree()ed. If one has a key in short-lived storage, the corresponding string may be reallocated like this:

SAVEDELETE(PL_defstash, savepv(tmpbuf), strlen(tmpbuf));

SAVEDESTRUCTOR(DESTRUCTORFUNC_NOCONTEXT_t f, void *p)
At the end of pseudo-block the function
f
is called with the only argument
p
.

SAVEDESTRUCTOR_X(DESTRUCTORFUNC_t f, void *p)
At the end of pseudo-block the function
f
is called with the implicit context argument (if any), and
p
.

SAVESTACK_POS()
The current offset on the Perl internal stack (cf.
SP
) is restored at the end of pseudo-block.

The following API list contains functions, thus one needs to provide pointers to the modifiable data explicitly (either C pointers, or Perlish

GV *

s). Where the above macros take

int

, a similar function takes

int *

SV* save_scalar(GV *gv)	Equivalent to Perl code local $gv .
AV* save_ary(GV *gv)
HV* save_hash(GV *gv)	Similar to save_scalar , but localize @gv and %gv .
void save_item(SV *item)	Duplicates the current value of SV , on the exit from the current ENTER / LEAVE pseudo-block will restore the value of SV using the stored value. It doesn’t handle magic. Use save_scalar if magic is affected.
void save_list(SV **sarg, I32 maxsarg)	A variant of save_item which takes multiple arguments via an array sarg of SV* of length maxsarg .
SV* save_svref(SV **sptr)	Similar to save_scalar , but will reinstate an SV * .
void save_aptr(AV **aptr)
void save_hptr(HV **hptr)	Similar to save_svref , but localize AV * and HV * .

The

Alias

module implements localization of the basic types within the caller’s scope. People who are interested in how to localize things in the containing scope should take a look there too.

Subroutines

XSUBs and the Argument Stack

The XSUB mechanism is a simple way for Perl programs to access C subroutines. An XSUB routine will have a stack that contains the arguments from the Perl program, and a way to map from the Perl data structures to a C equivalent.

The stack arguments are accessible through the

ST(n)

macro, which returns the

’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 macro that represents the local copy of 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

PUSHs

macro. The pushed values will often need to be mortal (See Reference Counts and Mortality):


    PUSHs(sv_2mortal(newSViv(an_integer)))
    PUSHs(sv_2mortal(newSVuv(an_unsigned_integer)))
    PUSHs(sv_2mortal(newSVnv(a_double)))
    PUSHs(sv_2mortal(newSVpv("Some String",0)))

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 macro:


    XPUSHs(SV*)

This macro automatically adjust the stack for you, if needed. Thus, you do not need to call

EXTEND

to extend the stack.

Despite their suggestions in earlier versions of this document the macros

(X)PUSH[iunp]

are not suited to XSUBs which return multiple results. For that, either stick to the

(X)PUSHs

macros shown above, or use the new

m(X)PUSH[iunp]

macros instead; see Putting a C value on Perl stack.

For more information, consult perlxs and perlxstut.

Calling Perl Routines from within C Programs

There are four routines that can be used to call a Perl subroutine from within a C program. These four are:


    I32  call_sv(SV*, I32);
    I32  call_pv(const char*, I32);
    I32  call_method(const char*, I32);
    I32  call_argv(const char*, I32, register char**);

The routine most often used is

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.

These routines used to be called

perl_call_sv

, etc., before Perl v5.6.0, but those names are now deprecated; macros of the same name are provided for compatibility.

When using any of these routines (except

call_argv

), the programmer must manipulate the Perl stack. These include the following macros and functions:


    dSP
    SP
    PUSHMARK()
    PUTBACK
    SPAGAIN
    ENTER
    SAVETMPS
    FREETMPS
    LEAVE
    XPUSH*()
    POP*()

For a detailed description of calling conventions from C to Perl, consult perlcall.

Memory Allocation

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

pointer

should be the name of a variable that will point to the newly allocated memory.

The second and third 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

Newxc

cast

, should be used if the

pointer

argument is different from the

type

argument.

Unlike the

Newx

and

Newxc

macros, the

Newxz

macro calls

memzero

to zero out all the newly allocated memory.

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

Renew

and

Renewc

match those of

New

and

Newc

with the exception of not needing the magic cookie argument.

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

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).

PerlIO

The most recent development releases of Perl has 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 perlapio.

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 reuse specially assigned SVs (targets) which are (as a corollary) not constantly freed/created.

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

(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:


    XPUSHi(10);
    XPUSHi(20);

This translates as "set

TARG

to 10, push a pointer to

TARG

onto the stack; set

TARG

to 20, push a pointer to

TARG

onto the stack". At the end of the operation, the stack does not contain the values 10 and 20, but actually contains two pointers to

TARG

, which we have set to 20.

If you need to push multiple different values then you should either use the

(X)PUSHs

macros, or else use the new

m(X)PUSH[iunp]

macros, none of which make use of

TARG

. The

(X)PUSHs

macros simply push an SV* on the stack, which, as noted under XSUBs and the Argument Stack, will often need to be mortal. The new

m(X)PUSH[iunp]

macros make this a little easier to achieve by creating a new mortal for you (via

(X)PUSHmortal

), pushing that onto the stack (extending it if necessary in the case of the

mXPUSH[iunp]

macros), and then setting its value. Thus, instead of writing this to fix the example above:


    XPUSHs(sv_2mortal(newSViv(10)))
    XPUSHs(sv_2mortal(newSViv(20)))

you can simply write:


    mXPUSHi(10)
    mXPUSHi(20)

On a related note, if you do use

(X)PUSH[iunp]

, then you’re going to need a

dTARG

in your variable declarations so that the

*PUSH*

macros can make use of the local variable

TARG

. See also

dTARGET

and

dXSTARG

Scratchpads

The question remains on when the SVs which are targets for opcodes 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 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 by looking on its flags: lexicals have

SVs_PADMY

set, and targets have

SVs_PADTMP

set.

The correspondence between OPs and targets 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.

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 we need an extra level of indirection?

The answer is recursion, and maybe 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

undef

s, but they are already marked with correct flags.

Compiled code

Code tree

Here we describe the internal form your code is converted to by Perl. Start with a simple example:


  $a = $b + $c;

This is converted to a tree similar to this one:


             assign-to
           /           \
          +             $a
        /   \
      $b     $c

(but slightly more complicated). This tree reflects 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.

Examining the tree

If you have your perl compiled for debugging (usually done with

-DDEBUGGING

on the

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

is not included into above listing), i.e.,

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 pp*.c files; the function which implements the op with type

gvsv

pp_gvsv

, and so on. As the tree above shows, different ops have different numbers of children:

add

is a binary operator, as one would 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

OP

: this has no children. Unary operators,

UNOP

s, have one child, and this is pointed to by the

op_first

field. Binary operators (

BINOP

s) have not only an

op_first

field but also an

op_last

field. The most complex type of op is a

LISTOP

, which has any number of children. In this case, the first child is pointed to by

op_first

and the last child by

op_last

. The children in between can be found by iteratively following the

op_sibling

pointer from the first child to the last.

There are also two other op types: a

PMOP

holds a regular expression, and has no children, and a

LOOP

may or may not have children. If the

op_children

field is non-zero, it behaves like a

LISTOP

. To complicate matters, if a

UNOP

is actually a

null

op after optimization (see Compile pass 2: context propagation) it will still have children in accordance with its former type.

Another way to examine the tree is to use a compiler back-end module, such as B::Concise.

Compile pass 1: check routines

The tree is created by the compiler while yacc code feeds it the constructions it recognizes. Since yacc works bottom-up, so does the first pass of perl compilation.

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 are 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).

Compile pass 1a: constant folding

Immediately after the check routine is called the returned node is checked for being compile-time executable. If it is (the value is judged to be constant) it is immediately executed, and a constant node with the return value of the corresponding subtree is substituted instead. The subtree is deleted.

If constant folding was not performed, the execution-order thread is created.

Compile pass 2: context propagation

When a context for a part of compile tree is known, it is propagated down through the tree. At this time the context can have 5 values (instead of 2 for runtime context): void, boolean, scalar, list, and lvalue. In contrast with the pass 1 this pass is processed from top to bottom: a node’s context determines the context for its children.

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 free()d now. To allow optimized-away nodes at this stage, such nodes are null()ified instead of free()ing (i.e. their type is changed to OP_NULL).

Compile pass 3: peephole optimization

After the compile tree for a subroutine (or for an

eval

or a file) 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). These optimizations are done in the subroutine peep(). Optimizations performed at this stage are subject to the same restrictions as in the pass 2.

Pluggable runops

The compile tree is executed in a runops function. There are two runops functions, in run.c and in dump.c.

Perl_runops_debug

is used with DEBUGGING and

Perl_runops_standard

is used otherwise. For fine control over the execution of the compile tree it is possible to provide your own runops function.

It’s probably best to copy one of the existing runops functions and change it to suit your needs. Then, in the BOOT section of your XS file, add the line:


  PL_runops = my_runops;

This function should be as efficient as possible to keep your programs running as fast as possible.

Examining internal data structures with the \f(CWdump\fP functions

To aid debugging, the source file dump.c contains a number of functions which produce formatted output of internal data structures.

The most commonly used of these functions is

Perl_sv_dump

; it’s used for dumping SVs, AVs, HVs, and CVs. The

Devel::Peek

module calls

sv_dump

to produce debugging output from Perl-space, so users of that module should already be familiar with its format.

Perl_op_dump

can be used to dump an

OP

structure or any of its derivatives, and produces output similar to

perl -Dx

; in fact,

Perl_dump_eval

will dump the main root of the code being evaluated, exactly like

-Dx

Other useful functions are

Perl_dump_sub

, which turns a

GV

into an op tree,

Perl_dump_packsubs

which calls

Perl_dump_sub

on all the subroutines in a package like so: (Thankfully, these are all xsubs, so there is no op tree)


    (gdb) print Perl_dump_packsubs(PL_defstash)

    SUB attributes::bootstrap = (xsub 0x811fedc 0)

    SUB UNIVERSAL::can = (xsub 0x811f50c 0)

    SUB UNIVERSAL::isa = (xsub 0x811f304 0)

    SUB UNIVERSAL::VERSION = (xsub 0x811f7ac 0)

    SUB DynaLoader::boot_DynaLoader = (xsub 0x805b188 0)

and

Perl_dump_all

, which dumps all the subroutines in the stash and the op tree of the main root.

How multiple interpreters and concurrency are supported

Background and \s-1PERL_IMPLICIT_CONTEXT\s0

The Perl interpreter can be regarded as a closed box: it has an API for feeding it code or otherwise making it do things, but it also has functions for its own use. This smells a lot like an object, and there are ways for you to build Perl so that you can have multiple interpreters, with one interpreter represented either as a C structure, 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 MULTIPLICITY build has a C structure that packages all the interpreter 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 mutli-threaded perls possible (with the ithreads threading model, related to the macro USE_ITHREADS.)

Two other encapsulation macros are the PERL_GLOBAL_STRUCT and PERL_GLOBAL_STRUCT_PRIVATE (the latter turns on the former, and the former turns on MULTIPLICITY.) The PERL_GLOBAL_STRUCT causes all the internal variables of Perl to be wrapped inside a single global struct, struct perl_vars, accessible as (globals) &PL_Vars or PL_VarsPtr or the function Perl_GetVars(). The PERL_GLOBAL_STRUCT_PRIVATE goes one step further, there is still a single struct (allocated in main() either from heap or from stack) but there are no global data symbols pointing to it. In either case the global struct should be initialised as the very first thing in main() using Perl_init_global_struct() and correspondingly tear it down after perl_free() using Perl_free_global_struct(), please see miniperlmain.c for usage details. You may also need to use

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

nm


  nm libperl.a | grep -v  [TURtr]

If this displays any

symbols, you have non-const data.

For backward compatibility reasons defining just PERL_GLOBAL_STRUCT doesn’t actually hide all symbols inside a big global struct: some PerlIO_xxx vtables are left visible. The PERL_GLOBAL_STRUCT_PRIVATE then hides everything (see how the PERLIO_FUNCS_DECL is used).

All this obviously requires a way for the Perl internal functions to be either subroutines taking some kind of structure as the first argument, or subroutines taking nothing as the first argument. To enable these two very different ways of building the interpreter, the Perl source (as it does in so many other situations) makes heavy use of macros and subroutine naming conventions.

First problem: deciding which functions will be public API functions and which will be private. All functions whose names begin

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 Internal Functions.) The easiest way to be sure a function is part of the API is to find its entry in perlapi. If it exists in 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 perlbug explaining why you think it should be.

Second problem: there must be a syntax so that the same subroutine declarations and calls can pass a structure as their first argument, or pass nothing. To solve this, the subroutines are named and declared in a particular way. Here’s a typical start of a static function used within the Perl guts:


  STATIC void
  S_incline(pTHX_ char *s)

STATIC becomes static in C, and may be #define’d to nothing in some configurations in future.

A public function (i.e. part of the internal API, but not necessarily sanctioned for use in extensions) begins like this:


  void
  Perl_sv_setiv(pTHX_ SV* dsv, IV num)

pTHX_

is one of a number of macros (in perl.h) that hide the details of the interpreter’s context. THX stands for thread, this, or thingy, as the case may be. (And no, George Lucas is not involved. :-) The first character could be ’p’ for a prototype, ’a’ for argument, or ’d’ for declaration, so we have

pTHX

aTHX

and

dTHX

, and their variants.

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 after the context argument because other arguments follow it. If PERL_IMPLICIT_CONTEXT is not defined, pTHX_ will be ignored, and the subroutine is not prototyped to take the extra argument. The form of the macro without the trailing underscore is used when there are no additional explicit arguments.

When a core function calls another, it must pass the context. This is normally hidden via macros. Consider

sv_setiv

. It expands into something like this:


    #ifdef PERL_IMPLICIT_CONTEXT
      #define sv_setiv(a,b)      Perl_sv_setiv(aTHX_ a, b)
      /* cant do this for vararg functions, see below */
    #else
      #define sv_setiv           Perl_sv_setiv
    #endif

This works well, and means that XS authors can gleefully write:


    sv_setiv(foo, bar);

and still have it work under all the modes Perl could have been compiled with.

This doesn’t work so cleanly for varargs functions, though, as macros imply that the number of arguments is known in advance. Instead we either need to spell them out fully, passing

aTHX_

as the first argument (the Perl core tends to do this with functions like Perl_warner), or use a context-free version.

The context-free version of Perl_warner is called Perl_warner_nocontext, and does not take the extra argument. Instead it does dTHX; to get the context from thread-local storage. We

#define warner Perl_warner_nocontext

so that extensions get source compatibility at the expense of performance. (Passing an arg is cheaper than grabbing it from thread-local storage.)

You can ignore [pad]THXx when browsing the Perl headers/sources. Those are strictly for use within the core. Extensions and embedders need only be aware of [pad]THX.

So what happened to dTHR?

dTHR

was introduced in perl 5.005 to support the older thread model. The older thread model now uses the

THX

mechanism to pass context pointers around, so

dTHR

is not useful any more. Perl 5.6.0 and later still have it for backward source compatibility, but it is defined to be a no-op.

How do I use all this in extensions?

When Perl is built with PERL_IMPLICIT_CONTEXT, extensions that call any functions in the Perl API will need to pass the initial context argument somehow. The kicker is that you will need to write it in such a way that the extension still compiles when Perl hasn’t been built with PERL_IMPLICIT_CONTEXT enabled.

There are three ways to do this. First, the easy but inefficient way, which is also the default, in order to maintain source compatibility with extensions: whenever XSUB.h is #included, it redefines the aTHX and aTHX_ macros to call a function that will return the context. Thus, something like:


        sv_setiv(sv, num);

in your extension will translate to this when PERL_IMPLICIT_CONTEXT is in effect:


        Perl_sv_setiv(Perl_get_context(), sv, num);

or to this otherwise:


        Perl_sv_setiv(sv, num);

You have to do nothing new in your extension to get this; since the Perl library provides Perl_get_context(), it will all just work.

The second, more efficient way is to use the following template for your Foo.xs:


        #define PERL_NO_GET_CONTEXT     /* we want efficiency */
        #include "EXTERN.h"
        #include "perl.h"
        #include "XSUB.h"

        STATIC void my_private_function(int arg1, int arg2);

        STATIC void
        my_private_function(int arg1, int arg2)
        {
            dTHX;       /* fetch context */
            ... call many Perl API functions ...
        }

        [... etc ...]

        MODULE = Foo            PACKAGE = Foo

        /* typical XSUB */

        void
        my_xsub(arg)
                int arg
            CODE:
                my_private_function(arg, 10);

Note that the only two changes from the normal way of writing an extension is the addition of a

#define PERL_NO_GET_CONTEXT

before including the Perl headers, followed by a

dTHX;

declaration at the start of every function that will call the Perl API. (You’ll know which functions need this, because the C compiler will complain that there’s an undeclared identifier in those functions.) No changes are needed for the XSUBs themselves, because the XS() macro is correctly defined to pass in the implicit context if needed.

The third, even more efficient way is to ape how it is done within the Perl guts:


        #define PERL_NO_GET_CONTEXT     /* we want efficiency */
        #include "EXTERN.h"
        #include "perl.h"
        #include "XSUB.h"

        /* pTHX_ only needed for functions that call Perl API */
        STATIC void my_private_function(pTHX_ int arg1, int arg2);

        STATIC void
        my_private_function(pTHX_ int arg1, int arg2)
        {
            /* dTHX; not needed here, because THX is an argument */
            ... call Perl API functions ...
        }

        [... etc ...]

        MODULE = Foo            PACKAGE = Foo

        /* typical XSUB */

        void
        my_xsub(arg)
                int arg
            CODE:
                my_private_function(aTHX_ arg, 10);

This implementation never has to fetch the context using a function call, since it is always passed as an extra argument. Depending on your needs for simplicity or efficiency, you may mix the previous two approaches freely.

Never add a comma after

pTHX

yourself—always use the form of the macro with the underscore for functions that take explicit arguments, or the form without the argument for functions with no explicit arguments.

If one is compiling Perl with the

-DPERL_GLOBAL_STRUCT

the

dVAR

definition is needed if the Perl global variables (see perlvars.h or globvar.sym) are accessed in the function and

dTHX

is not used (the

dTHX

includes the

dVAR

if necessary). One notices the need for

dVAR

only with the said compile-time define, because otherwise the Perl global variables are visible as-is.

Should I do anything special if I call perl from multiple threads?

If you create interpreters in one thread and then proceed to call them in another, you need to make sure perl’s own Thread Local Storage (TLS) slot is initialized correctly in each of those threads.

The

perl_alloc

and

perl_clone

API functions will automatically set the TLS slot to the interpreter they created, so that there is no need to do anything special if the interpreter is always accessed in the same thread that created it, and that thread did not create or call any other interpreters afterwards. If that is not the case, you have to set the TLS slot of the thread before calling any functions in the Perl API on that particular interpreter. This is done by calling the

PERL_SET_CONTEXT

macro in that thread as the first thing you do:


        /* do this before doing anything else with some_perl */
        PERL_SET_CONTEXT(some_perl);

        ... other Perl API calls on some_perl go here ...

Future Plans and \s-1PERL_IMPLICIT_SYS\s0

Just as PERL_IMPLICIT_CONTEXT provides a way to bundle up everything that the interpreter knows about itself and pass it around, so too are there plans to allow the interpreter to bundle up everything it knows about the environment it’s running on. This is enabled with the PERL_IMPLICIT_SYS macro. Currently it only works with USE_ITHREADS on Windows.

This allows the ability to provide an extra pointer (called the host environment) for all the system calls. This makes it possible for all the system stuff to maintain their own state, broken down into seven C structures. These are thin wrappers around the usual system calls (see win32/perllib.c) for the default perl executable, but for a more ambitious host (like the one that would do fork() emulation) all the extra work needed to pretend that different interpreters are actually different processes, would be done here.

The Perl engine/interpreter and the host are orthogonal entities. There could be one or more interpreters in a process, and one or more hosts, with free association between them.

Internal Functions

All of Perl’s internal functions which will be exposed to the outside world are prefixed by

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

PL_

. (By convention, static functions start with

S_

Inside the Perl core, you can get at the functions either with or without the

Perl_

prefix, thanks to a bunch of defines that live in embed.h. This header file is generated automatically from embed.pl and embed.fnc. 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 a new function to the core or change an existing one, you change the data in the table in 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:

A This function is a part of the public API. All such functions should also have ’d’, very few do not.

p This function has a
Perl_
prefix; i.e. it is defined as
Perl_av_fetch
.

d This function has documentation using the
apidoc
feature which we’ll look at in a second. Some functions have ’d’ but not ’A’; docs are good.

Other available flags are:

s This is a static function and is defined as
STATIC S_whatever
, and usually called within the sources as
whatever(...)
.

n This does not need a interpreter context, so the definition has no
pTHX
, and it follows that callers don’t use
aTHX
. (See Background and PERL_IMPLICIT_CONTEXT in perlguts.)

r This function never returns;
croak
,
exit
and friends.

f This function takes a variable number of arguments,
printf
style. The argument list should end with
...
, like this:

Afprd |void |croak |const char* pat|...

M This function is part of the experimental development API, and may change or disappear without notice.

o This function should not have a compatibility macro to define, say,
Perl_parse
to
parse
. It must be called as
Perl_parse
.

x This function isn’t exported out of the Perl core.

m This is implemented as a macro.

X This function is explicitly exported.

E This function is visible to extensions included in the Perl core.

b Binary backward compatibility; this function is a macro but also has a
Perl_
implementation (which is exported).

others See the comments at the top of
embed.fnc
for others.

If you edit embed.pl or embed.fnc, you will need to run

make regen_headers

to force a rebuild of embed.h and other auto-generated files.

Formatted Printing of IVs, UVs, and NVs

If you are printing IVs, UVs, or NVS instead of the stdio(3) style formatting codes like

%d

%ld

%f

, you should use the following macros for portability


        IVdf            IV in decimal
        UVuf            UV in decimal
        UVof            UV in octal
        UVxf            UV in hexadecimal
        NVef            NV %e-like
        NVff            NV %f-like
        NVgf            NV %g-like

These will take care of 64-bit integers and long doubles. For example:


        printf("IV is %"IVdf"\n", iv);

The IVdf will expand to whatever is the correct format for the IVs.

If you are printing addresses of pointers, use UVxf combined with PTR2UV(), do not use

%lx

%p

Pointer-To-Integer and Integer-To-Pointer

Because pointer size does not necessarily equal integer size, use the follow macros to do it right.


        PTR2UV(pointer)
        PTR2IV(pointer)
        PTR2NV(pointer)
        INT2PTR(pointertotype, integer)

For example:


        IV  iv = ...;
        SV *sv = INT2PTR(SV*, iv);

and


        AV *av = ...;
        UV  uv = PTR2UV(av);

Exception Handling

There are a couple of macros to do very basic exception handling in XS modules. You have to define

NO_XSLOCKS

before including 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:


        dXCPT;    /* set up necessary variables */

        XCPT_TRY_START {
          code_that_may_croak();
        } XCPT_TRY_END

        XCPT_CATCH
        {
          /* do cleanup here */
          XCPT_RETHROW;
        }

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 have to use the

call_*

function.

The advantage of using the above macros is that you don’t have to setup an extra function for

call_*

, and that using these macros is faster than using

call_*

Source Documentation

There’s an effort going on to document the internal functions and automatically produce reference manuals from them - perlapi is one such manual which details all the functions which are available to XS writers. 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 source, like this:


 /*
 =for apidoc sv_setiv

 Copies an integer into the given SV.  Does not handle set magic.  See
 C<sv_setiv_mg>.

 =cut
 */

Please try and supply some documentation if you add functions to the Perl core.

Backwards compatibility

The Perl API changes over time. New functions are added or the interfaces of existing functions are changed. The

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.

Devel::PPPort

generates a C header file ppport.h that can also be run as a Perl script. To generate 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

--api-info

command line switch. For example:


  % perl ppport.h --api-info=sv_magicext

For details, see

perldoc ppport.h

Unicode Support

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.

What \fBis\fP 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, 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 used to happen was that particular languages would stick their own 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.

Worse still, if you’ve got a language like Chinese or Japanese that has hundreds or thousands of characters, then you really can’t fit them into a mere 256, so they had to forget about ASCII altogether, and build their own systems using pairs of numbers to refer to one character.

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 about Unicode and Perl’s Unicode model in perlunicode.

How can I recognise a \s-1UTF\-8\s0 string?

You can’t. This is because UTF-8 data is stored in bytes just like non-UTF-8 data. The Unicode character 200, (

0xC8

for you hex types) capital E with a grave accent, is represented by the two bytes

v196.172

. Unfortunately, the non-Unicode string

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

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,

is_utf8_char

will tell you whether the current character in a string is valid UTF-8.

How does \s-1UTF\-8\s0 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

v194.128

; this continues up to character 191, which is

v194.191

. Now we’ve run out of bits (191 is binary

10111111

) so we move on; 192 is

v195.128

. And so it goes on, moving to three bytes at character 2048.

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

UTF8SKIP

macro:


    char *utf = "\305\233\340\240\201";
    I32 len;

    len = UTF8SKIP(utf); /* len is 2 here */
    utf += len;
    len = UTF8SKIP(utf); /* len is 3 here */

Another way to skip over characters in a UTF-8 string is to use

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 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):


    U8 *utf;
    UV uv;      /* Note: a UV, not a U8, not a char */

    if (!UTF8_IS_INVARIANT(*utf))
        /* Must treat this as UTF-8 */
        uv = utf8_to_uv(utf);
    else
        /* OK to treat this character as a byte */
        uv = *utf;

You can also see in that example that we use

utf8_to_uv

to get the value of the character; the inverse function

uv_to_utf8

is available for putting a UV into UTF-8:


    if (!UTF8_IS_INVARIANT(uv))
        /* Must treat this as UTF8 */
        utf8 = uv_to_utf8(utf8, uv);
    else
        /* OK to treat this character as a byte */
        *utf8++ = uv;

You 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 do this, you’ll lose the ability to match hi-bit non-UTF-8 characters; for instance, if your UTF-8 string contains

v196.172

, and you skip that character, you can never match a

chr(200)

in a non-UTF-8 string. So don’t do that!

How does Perl store \s-1UTF\-8\s0 strings?

Currently, Perl deals with Unicode strings and non-Unicode strings slightly differently. A flag in the SV,

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). You can check and manipulate this flag with the following macros:


    SvUTF8(sv)
    SvUTF8_on(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,

length

substr

and other string handling operations will have undesirable 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 - especially when combining non-UTF-8 and UTF-8 strings.

Never forget that the

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:


    SV *sv;
    SV *nsv;
    STRLEN len;
    char *p;

    p = SvPV(sv, len);
    frobnicate(p);
    nsv = newSVpvn(p, len);

The

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:


    p = SvPV(sv, len);
    frobnicate(p);
    nsv = newSVpvn(p, len);
    if (SvUTF8(sv))
        SvUTF8_on(nsv);

In fact, your

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.

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

char *

to an XS function.

How do I convert a string to \s-1UTF\-8\s0?

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 this is:


    sv_utf8_upgrade(sv);

However, you must not do this, for example:


    if (!SvUTF8(left))
        sv_utf8_upgrade(left);

If you do this in a binary operator, you will actually change one of the strings that came into the operator, and, while it shouldn’t be noticeable by the end user, it can cause problems in deficient code.

Instead,

bytes_to_utf8

will give you a UTF-8-encoded 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

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.

Is there anything else I need to know?

Not really. Just remember these things:

o 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 is
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.

o If a string is UTF-8, always use
utf8_to_uv
to get at the value, unless
UTF8_IS_INVARIANT(*s)
in which case you can use
*s
.

o When writing a character
uv
to a UTF-8 string, always use
uv_to_utf8
, unless
UTF8_IS_INVARIANT(uv))
in which case you can use
*s = uv
.

o Mixing UTF-8 and non-UTF-8 strings is tricky. Use
bytes_to_utf8
to get a new string which is UTF-8 encoded. There are tricks you can use to delay deciding whether you need to use a UTF-8 string until you get to a high character -
HALF_UPGRADE
is one of those.

Custom Operators

Custom operator support is a new experimental feature that allows you to 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

gvsv, gvsv, add

This feature is implemented as a new op type,

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 tree using a

CHECK

block and the

B::Generate

module, or by adding a custom peephole optimizer with the

optimize

module.

When you do this, you replace ordinary Perl ops with custom ops by creating ops with the type

OP_CUSTOM

and the

pp_addr

of your own PP function. This should be defined in XS code, and should look like the PP ops in

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 have multiple custom ops within the one logical op type

OP_CUSTOM

, Perl uses the value of

o->op_ppaddr

as a key into the

PL_custom_op_descs

and

PL_custom_op_names

hashes. This means you need to enter a name and description for your op at the appropriate place in the

PL_custom_op_names

and

PL_custom_op_descs

hashes.

Forthcoming versions of

B::Generate

(version 1.0 and above) should directly support the creation of custom ops by name.

AUTHORS

Until May 1997, this document was maintained by Jeff Okamoto <okamoto@corp.hp.com>. It is now maintained as part of Perl itself by the Perl 5 Porters <perl5-porters@perl.org>.

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, Ulrich Pfeifer, Stephen McCamant, and Gurusamy Sarathy.

SAVEINT(int i)
SAVEIV(IV i)
SAVEI32(I32 i)
SAVELONG(long i)	These macros arrange things to restore the value of integer variable i at the end of enclosing pseudo-block.
SAVESPTR(s)
SAVEPPTR(p)	These macros arrange things to restore the value of pointers s and p . s must be a pointer of a type which survives conversion to SV* and back, p should be able to survive conversion to char* and back.
SAVEFREESV(SV *sv)	The refcount of sv would be decremented at the end of pseudo-block. This is similar to sv_2mortal in that it is also a mechanism for doing a delayed SvREFCNT_dec . However, while sv_2mortal extends the lifetime of sv until the beginning of the next statement, SAVEFREESV extends it until the end of the enclosing scope. These lifetimes can be wildly different. Also compare SAVEMORTALIZESV .
SAVEMORTALIZESV(SV *sv)	Just like SAVEFREESV , but mortalizes sv at the end of the current scope instead of decrementing its reference count. This usually has the effect of keeping sv alive until the statement that called the currently live scope has finished executing.
SAVEFREEOP(OP *op)	The OP * is op_free()ed at the end of pseudo-block.
SAVEFREEPV(p)	The chunk of memory which is pointed to by p is Safefree()ed at the end of pseudo-block.
SAVECLEARSV(SV *sv)	Clears a slot in the current scratchpad which corresponds to sv at the end of pseudo-block.
SAVEDELETE(HV hv, char key, I32 length)	The key key of hv is deleted at the end of pseudo-block. The string pointed to by key is Safefree()ed. If one has a key in short-lived storage, the corresponding string may be reallocated like this: SAVEDELETE(PL_defstash, savepv(tmpbuf), strlen(tmpbuf));
SAVEDESTRUCTOR(DESTRUCTORFUNC_NOCONTEXT_t f, void *p)	At the end of pseudo-block the function f is called with the only argument p .
SAVEDESTRUCTOR_X(DESTRUCTORFUNC_t f, void *p)	At the end of pseudo-block the function f is called with the implicit context argument (if any), and p .
SAVESTACK_POS()	The current offset on the Perl internal stack (cf. SP ) is restored at the end of pseudo-block.

s	This is a static function and is defined as STATIC S_whatever , and usually called within the sources as whatever(...) .
n	This does not need a interpreter context, so the definition has no pTHX , and it follows that callers don’t use aTHX . (See Background and PERL_IMPLICIT_CONTEXT in perlguts.)
r	This function never returns; croak , exit and friends.
f	This function takes a variable number of arguments, printf style. The argument list should end with ... , like this: Afprd \|void \|croak \|const char* pat\|...
M	This function is part of the experimental development API, and may change or disappear without notice.
o	This function should not have a compatibility macro to define, say, Perl_parse to parse . It must be called as Perl_parse .
x	This function isn’t exported out of the Perl core.
m	This is implemented as a macro.
X	This function is explicitly exported.
E	This function is visible to extensions included in the Perl core.
b	Binary backward compatibility; this function is a macro but also has a Perl_ implementation (which is exported).
others	See the comments at the top of embed.fnc for others.

o	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 is 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.
o	If a string is UTF-8, always use utf8_to_uv to get at the value, unless UTF8_IS_INVARIANT(s) in which case you can use s .
o	When writing a character uv to a UTF-8 string, always use uv_to_utf8 , unless UTF8_IS_INVARIANT(uv)) in which case you can use *s = uv .
o	Mixing UTF-8 and non-UTF-8 strings is tricky. Use bytes_to_utf8 to get a new string which is UTF-8 encoded. There are tricks you can use to delay deciding whether you need to use a UTF-8 string until you get to a high character - HALF_UPGRADE is one of those.

Manual - Section 1: perlguts

Linux Online Man Pages for openSUSE 11.2 "Emerald"

NAME

DESCRIPTION

Variables

Datatypes

What is an ``\s-1IV\s0''?

Working with SVs

Offsets

What's Really Stored in an \s-1SV\s0?

Working with AVs

Working with HVs

Hash \s-1API\s0 Extensions

AVs, HVs and undefined values

References

Blessed References and Class Objects

Creating New Variables

Reference Counts and Mortality

Stashes and Globs

Double-Typed SVs

Magic Variables

Assigning Magic

Magic Virtual Tables

Finding Magic

Understanding the Magic of Tied Hashes and Arrays

Localizing changes

Subroutines

XSUBs and the Argument Stack

Calling Perl Routines from within C Programs

Memory Allocation

PerlIO

Putting a C value on Perl stack

Scratchpads

Scratchpads and recursion

Compiled code

Code tree

Examining the tree

Compile pass 1: check routines

Compile pass 1a: constant folding

Compile pass 2: context propagation

Compile pass 3: peephole optimization

Pluggable runops

Examining internal data structures with the \f(CWdump\fP functions

How multiple interpreters and concurrency are supported

Background and \s-1PERL_IMPLICIT_CONTEXT\s0

So what happened to dTHR?

How do I use all this in extensions?

Should I do anything special if I call perl from multiple threads?

Future Plans and \s-1PERL_IMPLICIT_SYS\s0

Internal Functions

Formatted Printing of IVs, UVs, and NVs

Pointer-To-Integer and Integer-To-Pointer

Exception Handling

Source Documentation

Backwards compatibility

Unicode Support

What \fBis\fP Unicode, anyway?

How can I recognise a \s-1UTF\-8\s0 string?

How does \s-1UTF\-8\s0 represent Unicode characters?

How does Perl store \s-1UTF\-8\s0 strings?

How do I convert a string to \s-1UTF\-8\s0?

Is there anything else I need to know?

Custom Operators

AUTHORS

SEE ALSO

Contents