perlguts - Introduction to the Perl API

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.

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. 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.

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*);

CWSTRLEN 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 CWlen 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(1);  /* 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 CWsv_setpvn, CWnewSVpvn, or CWnewSVpv, or you may allow Perl to calculate the length by using CWsv_setpv or by specifying 0 as the second argument to CWnewSVpv. Be warned, though, that Perl will determine the string's length by using CWstrlen, which depends on the string terminating with a NUL character.

The arguments of CWsv_setpvf are processed like CWsprintf, and the formatted output becomes the value.

CWsv_vsetpvfn is an analogue of CWvsprintf, 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 CWsv_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 CWSvPV macro, the length of the string returned is placed into the variable CWlen (this is a macro, so you do not use CW&len). If you do not care what the length of the data is, use the CWSvPV_nolen macro. Historically the CWSvPV macro with the global variable CWPL_na has been used in this case. But that can be quite inefficient because CWPL_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 CWfoo(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 CWsv_grow. Note that CWSvGROW 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 CWSvGROW(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 CWSvPOK() is true.

If you want to append something to the end of string stored in an CWSV*, 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 CWstrlen. In the second, you specify the length of the string yourself. The third function processes its arguments like CWsprintf and appends the formatted output. The fourth function works like CWvsprintf. 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 CWsv_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 CWdefined, you can call:

    SvOK(SV*)

The scalar CWundef value is stored in an SV instance called CWPL_sv_undef.

Its address can be used whenever an CWSV* is needed. Make sure that you don't try to compare a random sv with CW&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 CW&PL_sv_undef as a value in AVs or HVs (see AVs, HVs and undefined values).

There are also the two values CWPL_sv_yes and CWPL_sv_no, which contain boolean TRUE and FALSE values, respectively. Like CWPL_sv_undef, their addresses can be used whenever an CWSV* is needed.

Do not be fooled into thinking that CW(SV *) 0 is the same as CW&PL_sv_undef. Take this code:

    SV* sv = (SV*) 0;
    if (I-am-to-return-a-real-value) {
            sv = sv_2mortal(newSViv(4));
    }
    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 CW&PL_sv_undef in the first line and all will be well.

To free an SV that you've created, call CWSvREFCNT_dec(SV*). Normally this call is not necessary (see Reference Counts and Mortality).

Perl provides the function CWsv_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, CWsv_chop sets the flag CWOOK (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 CWSvPVX) forward that many bytes, and adjusts CWSvCUR and CWSvLEN.

Hence, at this point, the start of the buffer that we allocated lives at CWSvPVX(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(0) 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 CWDevel::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 CWSvCUR and CWSvLEN 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 CWAvARRAY points to the first element in the array that is visible from Perl, CWAvALLOC points to the real start of the C array. These are usually the same, but a CWshift operation can be carried out by increasing CWAvARRAY by one and decreasing CWAvFILL and CWAvLEN. Again, the location of the real start of the C array only comes into play when freeing the array. See CWav_shift in av.c.

Recall that the usual method of determining the type of scalar you have is to use CWSv*OK macros. Because a scalar can be both a number and a string, usually these macros will always return TRUE and calling the CWSv*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 occured and precision has been lost: only the private flag is set on 'lossy' values. So when an NV is converted to an IV with loss, SvIOKp, SvNOKp and SvNOK will be set, while SvIOK wont be.

In general, though, it's best to use the CWSv*V macros.

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 CWnum CWSV*'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 CWav_unshift. This routine adds CWnum elements at the front of the array with the CWundef value. You must then use CWav_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 CWav_len function returns the highest index value in array (just like $#array in Perl). If the array is empty, -1 is returned. The CWav_fetch function returns the value at index CWkey, but if CWlval is non-zero, then CWav_fetch will store an undef value at that index. The CWav_store function stores the value CWval at index CWkey, and does not increment the reference count of CWval. Thus the caller is responsible for taking care of that, and if CWav_store returns NULL, the caller will have to decrement the reference count to avoid a memory leak. Note that CWav_fetch and CWav_store both return CWSV**'s, not CWSV*'s as their return value.

    void  av_clear(AV*);
    void  av_undef(AV*);
    void  av_extend(AV*, I32 key);

The CWav_clear function deletes all the elements in the AV* array, but does not actually delete the array itself. The CWav_undef function will delete all the elements in the array plus the array itself. The CWav_extend function extends the array so that it contains at least CWkey+1 elements. If CWkey+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.

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 CWklen parameter is the length of the key being passed in (Note that you cannot pass 0 in as a value of CWklen to tell Perl to measure the length of the key). The CWval argument contains the SV pointer to the scalar being stored, and CWhash is the precomputed hash value (zero if you want CWhv_store to calculate it for you). The CWlval 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 CWhv_fetch will return as if the value had already existed.

Remember that CWhv_store and CWhv_fetch return CWSV**'s and not just CWSV*. 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);

If CWflags does not include the CWG_DISCARD flag then CWhv_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, CWhv_clear deletes all the entries in the hash table but does not actually delete the hash table. The CWhv_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 CWSV*. However, once you have an CWHE*, 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 CWPERL_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.

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 CWSV* keys, which simplifies writing of extension code that deals with hash structures. These functions also allow passing of CWSV* keys to CWtie functions without forcing you to stringify the keys (unlike the previous set of functions).

They also return and accept whole hash entries (CWHE*), 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 CWSV*s:

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

Note that both CWhv_store and CWhv_store_ent do not increment the reference count of the stored CWval, which is the caller's responsibility. If these functions return a NULL value, the caller will usually have to decrement the reference count of CWval to avoid a memory leak.

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 CW&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 CW&PL_sv_undef as a marker for indicating that an array element has not yet been initialized. Thus, CWexists $av[0] would be true for the above Perl code, but false for the array generated by the XS code.

Other problems can occur when storing CW&PL_sv_undef in HVs:

    hv_store( hv, "key", 3, &PL_sv_undef, 0 );

This will indeed make the value CWundef, but if you try to modify the value of CWkey, you'll get the following error:

    Modification of non-creatable hash value attempted

In perl 5.8.0, CW&PL_sv_undef was also used to mark placeholders in restricted hashes. This caused such hash entries not to appear when iterating over the hash or when checking for the keys with the CWhv_exists function.

You can run into similar problems when you store CW&PL_sv_true or CW&PL_sv_false into AVs or HVs. Trying to modify such elements will give you the following error:

    Modification of a read-only value attempted

To make a long story short, you can use the special variables CW&PL_sv_undef, CW&PL_sv_true and CW&PL_sv_false with AVs and HVs, but you have to make sure you know what you're doing.

Generally, if you want to store an undefined value in an AV or HV, you should not use CW&PL_sv_undef, but rather create a new undefined value using the CWnewSV function, for example:

    av_store( av, 42, newSV(0) );
    hv_store( hv, "foo", 3, newSV(0), 0 );

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 CWthing argument can be any of an CWSV*, CWAV*, or CWHV*. The functions are identical except that CWnewRV_inc increments the reference count of the CWthing, while CWnewRV_noinc does not. For historical reasons, CWnewRV is a synonym for CWnewRV_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 CWSV* to either an CWAV* or CWHV*, 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.

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 CWsv argument must be a reference value. The CWstash 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 CWclassname 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 CWrv. SV is blessed if CWclassname 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 CWclassname is non-null.

        SV* sv_setref_pv(SV* rv, const char* classname, PV iv);

Copies string into an SV whose reference is CWrv. Set length to 0 to let Perl calculate the string length. SV is blessed if CWclassname 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 CWUNIVERSAL::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)) { ... }

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 CWTRUE 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.

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 CWnewRV_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 CWnewRV_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 CWnewRV_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 CWnewRV_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 CWnewRV_noinc instead of CWnewRV_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 CWSvREFCNT_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 CWSvREFCNT_dec), and the third creates a mortal copy of an existing SV. Because CWsv_newmortal gives the new SV no value,it must normally be given one via CWsv_setpv, CWsv_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 CWSvREFCNT_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 CWSvREFCNT_inc and CWsv_2mortal, or making a CWsv_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 CWSV*) to the CWsv_2mortal or CWsv_mortalcopy routines.

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 CWPL_defstash that holds the items that exist in the CWmain package. To get at the items in other packages, append the string :: to the package name. The items in the CWFoo package are in the stash CWFoo:: in PL_defstash. The items in the CWBar::Baz package are in the stash CWBaz:: in CWBar::'s stash.

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

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

The first function takes a literal string, the second uses the string stored in the SV. Remember that a stash is just a hash table, so you get back an CWHV*. The CWcreate flag will create a new package if it is set.

The name that CWgv_stash*v wants is the name of the package whose symbol table you want. The default package is called CWmain. If you have multiply nested packages, pass their names to CWgv_stash*v, separated by CW:: 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 CWSV*, must be a reference, and the second argument is a stash. The returned CWSV* can now be used in the same way as any other SV.

For more information on references and blessings, consult perlref.

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 CW$! contains either the numeric value of CWerrno or its string equivalent from either CWstrerror or CWsys_errlist[].

To force multiple data values into an SV, you must do two things: use the CWsv_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 CWsv_set*v routine you called first. This is because every CWsv_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 CWsv_setiv and CWsv_setpv had been reversed, then the macro CWSvPOK_on would need to be called instead of CWSvIOK_on.

[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 CWstruct magic's, typedef'ed to CWMAGIC.

    struct magic {
        MAGIC*      mg_moremagic;
        MGVTBL*     mg_virtual;
        U16         mg_private;
        char        mg_type;
        U8          mg_flags;
        SV*         mg_obj;
        char*       mg_ptr;
        I32         mg_len;
    };

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

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

If CWsv is not already magical, Perl uses the CWSvUPGRADE macro to convert CWsv to type CWSVt_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 CWname and CWnamlen arguments are used to associate a string with the magic, typically the name of a variable. CWnamlen is stored in the CWmg_len field and if CWname is non-null then either a CWsavepvn copy of CWname or CWname itself is stored in the CWmg_ptr field, depending on whether CWnamlen is greater than zero or equal to zero respectively. As a special case, if CW(name && namlen == HEf_SVKEY) then CWname is assumed to contain an CWSV* and is stored as-is with its REFCNT incremented.

The sv_magic function uses CWhow to determine which, if any, predefined Magic Virtual Table should be assigned to the CWmg_virtual field. See the Magic Virtual Tables section below. The CWhow argument is also stored in the CWmg_type field. The value of CWhow should be chosen from the set of macros CWPERL_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 CWPERL_MAGIC_uvar for example.

The CWobj argument is stored in the CWmg_obj field of the CWMAGIC structure. If it is not the same as the CWsv argument, the reference count of the CWobj object is incremented. If it is the same, or if the CWhow argument is CWPERL_MAGIC_arylen, or if it is a NULL pointer, then CWobj is merely stored, without the reference count being incremented.

See also CWsv_magicext in perlapi for a more flexible way to add magic to an SV.

There is also a function to add magic to an CWHV:

    void hv_magic(HV *hv, GV *gv, int how);

This simply calls CWsv_magic and coerces the CWgv argument into an CWSV.

To remove the magic from an SV, call the function sv_unmagic:

    void sv_unmagic(SV *sv, int type);

The CWtype argument should be equal to the CWhow value when the CWSV was initially made magical.

The CWmg_virtual field in the CWMAGIC structure is a pointer to an CWMGVTBL, 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 CWMGVTBL has five pointers to the following routine types:

    int  (*svt_get)(SV* sv, MAGIC* mg);
    int  (*svt_set)(SV* sv, MAGIC* mg);
    U32  (*svt_len)(SV* sv, MAGIC* mg);
    int  (*svt_clear)(SV* sv, MAGIC* mg);
    int  (*svt_free)(SV* sv, MAGIC* mg);

This MGVTBL structure is set at compile-time in perl.h and there are currently 19 types (or 21 with overloading turned on). These different structures contain pointers to various routines that perform additional actions depending on which function is being called.

    Function pointer    Action taken
    ----------------    ------------
    svt_get             Do something before the value of the SV is retrieved.
    svt_set             Do something after the SV is assigned a value.
    svt_len             Report on the SV's length.
    svt_clear           Clear something the SV represents.
    svt_free            Free any extra storage associated with the SV.

For instance, the MGVTBL structure called CWvtbl_sv (which corresponds to an CWmg_type of CWPERL_MAGIC_sv) contains:

    { magic_get, magic_set, magic_len, 0, 0 }

Thus, when an SV is determined to be magical and of type CWPERL_MAGIC_sv, if a get operation is being performed, the routine CWmagic_get is called. All the various routines for the various magical types begin with CWmagic_. NOTE: the magic routines are not considered part of the Perl API, and may not be exported by the Perl library.

The current kinds of Magic Virtual Tables are:

    mg_type
    (old-style char and macro)   MGVTBL         Type of magic
    --------------------------   ------         ----------------------------
    \0 PERL_MAGIC_sv             vtbl_sv        Special scalar variable
    A  PERL_MAGIC_overload       vtbl_amagic    %OVERLOAD hash
    a  PERL_MAGIC_overload_elem  vtbl_amagicelem %OVERLOAD hash element
    c  PERL_MAGIC_overload_table (none)         Holds overload table (AMT)
                                                on stash
    B  PERL_MAGIC_bm             vtbl_bm        Boyer-Moore (fast string search)
    D  PERL_MAGIC_regdata        vtbl_regdata   Regex match position data
                                                (@+ and @- vars)
    d  PERL_MAGIC_regdatum       vtbl_regdatum  Regex match position data
                                                element
    E  PERL_MAGIC_env            vtbl_env       %ENV hash
    e  PERL_MAGIC_envelem        vtbl_envelem   %ENV hash element
    f  PERL_MAGIC_fm             vtbl_fm        Formline ('compiled' format)
    g  PERL_MAGIC_regex_global   vtbl_mglob     m//g target / study()ed string
    I  PERL_MAGIC_isa            vtbl_isa       @ISA array
    i  PERL_MAGIC_isaelem        vtbl_isaelem   @ISA array element
    k  PERL_MAGIC_nkeys          vtbl_nkeys     scalar(keys()) lvalue
    L  PERL_MAGIC_dbfile         (none)         Debugger %_<filename
    l  PERL_MAGIC_dbline         vtbl_dbline    Debugger %_<filename element
    m  PERL_MAGIC_mutex          vtbl_mutex     ???
    o  PERL_MAGIC_collxfrm       vtbl_collxfrm  Locale 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_glob           vtbl_glob      GV (typeglob)
    #  PERL_MAGIC_arylen         vtbl_arylen    Array length ($#ary)
    .  PERL_MAGIC_pos            vtbl_pos       pos() lvalue
    <  PERL_MAGIC_backref        vtbl_backref   ???
    ~  PERL_MAGIC_ext            (none)         Available for use by extensions

When an uppercase and lowercase letter both exist in the table, then the uppercase letter is 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 CWPERL_MAGIC_ext and CWPERL_MAGIC_uvar magic types are defined specifically for use by extensions and will not be used by perl itself. Extensions can use CWPERL_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, CWPERL_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 CWMAGIC's CWmg_ptr field points to a CWufuncs 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 CWuf_val or CWuf_set function will be called with CWuf_index as the first arg and a pointer to the SV as the second. A simple example of how to add CWPERL_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));

Note that because multiple extensions may be using CWPERL_MAGIC_ext or CWPERL_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 CWPERL_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 CWsv_set*() and CWsv_cat*() functions described earlier do not invoke 'set' magic on their targets. This must be done by the user either by calling the CWSvSETMAGIC() macro after calling these functions, or by using one of the CWsv_set*_mg() or CWsv_cat*_mg() functions. Similarly, generic C code must call the CWSvGETMAGIC() 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 CWsv_cat*() functions typically need to be followed by CWSvSETMAGIC(), but they don't need a prior CWSvGETMAGIC() since their implementation handles 'get' magic.

    MAGIC* mg_find(SV*, int type); /* Finds the magic pointer of that type */

This routine returns a pointer to the CWMAGIC structure stored in the SV. If the SV does not have that magical feature, CWNULL 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 CWsv has. If the mg_type field is an uppercase letter, then the mg_obj is copied to CWnsv, but the mg_type field is changed to be the lowercase letter.

Tied hashes and arrays are magical beasts of the CWPERL_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", TRUE);
        sv_bless(tie, stash);
        hv_magic(hash, (GV*)tie, PERL_MAGIC_tied);
        RETVAL = newRV_noinc(hash);
    OUTPUT:
        RETVAL

The CWav_store function, when given a tied array argument, merely copies the magic of the array onto the value to be stored, using CWmg_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 CWav_store on a tied array, the caller will usually need to call CWmg_set(val) to actually invoke the perl level STORE method on the TIEARRAY object. If CWav_store did return NULL, a call to CWSvREFCNT_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 CWhv_store and CWhv_store_ent functions as well.

CWav_fetch and the corresponding hash functions CWhv_fetch and CWhv_fetch_ent actually return an undefined mortal value whose magic has been initialized using CWmg_copy. Note the value so returned does not need to be deallocated, as it is already mortal. [MAYCHANGE] But you will need to call CWmg_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 CWmg_set() on the return value after possibly assigning a suitable value to it using CWsv_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 CWmg_copy to attach magic to the values that were meant to be stored or fetched. Later calls to CWmg_get and CWmg_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.

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 CW$var, irrespective of how control exits the block: CWgoto, CWreturn, CWdie/CWeval, 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 CWENTER/CWLEAVE 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 CWENTER/CWLEAVE pair.

Inside such a pseudo-block the following service is available: These macros arrange things to restore the value of integer variable CWi at the end of enclosing pseudo-block. These macros arrange things to restore the value of pointers CWs and CWp. CWs must be a pointer of a type which survives conversion to CWSV* and back, CWp should be able to survive conversion to CWchar* and back. The refcount of CWsv would be decremented at the end of pseudo-block. This is similar to CWsv_2mortal in that it is also a mechanism for doing a delayed CWSvREFCNT_dec. However, while CWsv_2mortal extends the lifetime of CWsv until the beginning of the next statement, CWSAVEFREESV extends it until the end of the enclosing scope. These lifetimes can be wildly different. Also compare CWSAVEMORTALIZESV. Just like CWSAVEFREESV, but mortalizes CWsv at the end of the current scope instead of decrementing its reference count. This usually has the effect of keeping CWsv alive until the statement that called the currently live scope has finished executing. The CWOP * is op_free()ed at the end of pseudo-block. The chunk of memory which is pointed to by CWp is Safefree()ed at the end of pseudo-block. Clears a slot in the current scratchpad which corresponds to CWsv at the end of pseudo-block. The key CWkey of CWhv is deleted at the end of pseudo-block. The string pointed to by CWkey 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));

The following API list contains functions, thus one needs to provide pointers to the modifiable data explicitly (either C pointers, or Perlish CWGV *s). Where the above macros take CWint, a similar function takes CWint *. Equivalent to Perl code CWlocal $gv. Similar to CWsave_scalar, but localize CW@gv and CW%gv. Duplicates the current value of CWSV, on the exit from the current CWENTER/CWLEAVE pseudo-block will restore the value of CWSV using the stored value. A variant of CWsave_item which takes multiple arguments via an array CWsarg of CWSV* of length CWmaxsarg. Similar to CWsave_scalar, but will reinstate an CWSV *. Similar to CWsave_svref, but localize CWAV * and CWHV *.

The CWAlias 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.

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 CWST(n) macro, which returns the CWn'th stack argument. Argument 0 is the first argument passed in the Perl subroutine call. These arguments are CWSV*, and can be used anywhere an CWSV* 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 CWSP is the macro that represents the local copy of the stack pointer, and CWnum 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 CWPUSHs 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 CWtzname, 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 CWEXTEND to extend the stack.

Despite their suggestions in earlier versions of this document the macros CW(X)PUSH[iunp] are not suited to XSUBs which return multiple results. For that, either stick to the CW(X)PUSHs macros shown above, or use the new CWm(X)PUSH[iunp] macros instead; see Putting a C value on Perl stack.

For more information, consult perlxs and perlxstut.

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 CWcall_sv. The CWSV* 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 CWperl_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 CWcall_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.

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 :

    New(x, pointer, number, type);
    Newc(x, pointer, number, type, cast);
    Newz(x, pointer, number, type);

The first argument CWx was a magic cookie that was used to keep track of who called the macro, to help when debugging memory problems. However, the current code makes no use of this feature (most Perl developers now use run-time memory checkers), so this argument can be any number.

The second argument CWpointer should be the name of a variable that will point to the newly allocated memory.

The third and fourth arguments CWnumber and CWtype specify how many of the specified type of data structure should be allocated. The argument CWtype is passed to CWsizeof. The final argument to CWNewc, CWcast, should be used if the CWpointer argument is different from the CWtype argument.

Unlike the CWNew and CWNewc macros, the CWNewz macro calls CWmemzero 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 CWRenew and CWRenewc match those of CWNew and CWNewc 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 CWsource and CWdest arguments point to the source and destination starting points. Perl will move, copy, or zero out CWnumber instances of the size of the CWtype data structure (using the CWsizeof function).

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.

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 CWPUSHTARG, and it is directly used in some opcodes, as well as indirectly in zillions of others, which use it via CW(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(1);
    XPUSHi(2);

This translates as "set CWTARG to 10, push a pointer to CWTARG onto the stack; set CWTARG to 20, push a pointer to CWTARG 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 CWTARG, which we have set to 20.

If you need to push multiple different values then you should either use the CW(X)PUSHs macros, or else use the new CWm(X)PUSH[iunp] macros, none of which make use of CWTARG. The CW(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 CWm(X)PUSH[iunp] macros make this a little easier to achieve by creating a new mortal for you (via CW(X)PUSHmortal), pushing that onto the stack (extending it if necessary in the case of the CWmXPUSH[iunp] macros), and then setting its value. Thus, instead of writing this to fix the example above:

    XPUSHs(sv_2mortal(newSViv(1)))
    XPUSHs(sv_2mortal(newSViv(2)))

you can simply write:

    mXPUSHi(1)
    mXPUSHi(2)

On a related note, if you do use CW(X)PUSH[iunp], then you're going to need a CWdTARG in your variable declarations so that the CW*PUSH* macros can make use of the local variable CWTARG. See also CWdTARGET and CWdXSTARG.

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 CWSVs_PADMY set, and targets have CWSVs_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.

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 CWundefs, but they are already marked with correct flags.

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 CW$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.

If you have your perl compiled for debugging (usually done with CW-DDEBUGGING on the CWConfigure command line), you may examine the compiled tree by specifying CW-Dx on the Perl command line. The output takes several lines per node, and for CW$b+$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 CWTYPE 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 CW{} 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 CW===> marks, thus it is CW3 4 5 6 (node CW6 is not included into above listing), i.e., CWgvsv 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 CWgvsv is CWpp_gvsv, and so on. As the tree above shows, different ops have different numbers of children: CWadd 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 CWOP: this has no children. Unary operators, CWUNOPs, have one child, and this is pointed to by the CWop_first field. Binary operators (CWBINOPs) have not only an CWop_first field but also an CWop_last field. The most complex type of op is a CWLISTOP, which has any number of children. In this case, the first child is pointed to by CWop_first and the last child by CWop_last. The children in between can be found by iteratively following the CWop_sibling pointer from the first child to the last.

There are also two other op types: a CWPMOP holds a regular expression, and has no children, and a CWLOOP may or may not have children. If the CWop_children field is non-zero, it behaves like a CWLISTOP. To complicate matters, if a CWUNOP is actually a CWnull 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.

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 CWmake 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 CWck_*. They are usually called from CWnew*OP subroutines (or CWconvert) (which in turn are called from perly.y).

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.

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).

After the compile tree for a subroutine (or for an CWeval 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.

The compile tree is executed in a runops function. There are two runops functions, in run.c and in dump.c. CWPerl_runops_debug is used with DEBUGGING and CWPerl_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. 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 CWPerl_sv_dump; it's used for dumping SVs, AVs, HVs, and CVs. The CWDevel::Peek module calls CWsv_dump to produce debugging output from Perl-space, so users of that module should already be familiar with its format.

CWPerl_op_dump can be used to dump an CWOP structure or any of its derivatives, and produces output similar to CWperl -Dx; in fact, CWPerl_dump_eval will dump the main root of the code being evaluated, exactly like CW-Dx.

Other useful functions are CWPerl_dump_sub, which turns a CWGV into an op tree, CWPerl_dump_packsubs which calls CWPerl_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 CWPerl_dump_all, which dumps all the subroutines in the stash and the op tree of the main root.

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.

Two macros control the major Perl build flavors: MULTIPLICITY and USE_5005THREADS. The MULTIPLICITY build has a C structure that packages all the interpreter state, and there is a similar thread-specific data structure under USE_5005THREADS. In both cases, PERL_IMPLICIT_CONTEXT is also normally defined, and enables the support for passing in a hidden first argument that represents all three data structures.

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 CWS_ 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)

CWpTHX_ 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 CWpTHX, CWaTHX and CWdTHX, 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 CWsv_setiv. It expands into something like this:

    #ifdef PERL_IMPLICIT_CONTEXT
      #define sv_setiv(a,b)      Perl_sv_setiv(aTHX_ a, b)
      /* can't 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 CWaTHX_ 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 CW#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.

CWdTHR was introduced in perl 5.005 to support the older thread model. The older thread model now uses the CWTHX mechanism to pass context pointers around, so CWdTHR 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.

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 my_private_function(int arg1, int arg2);

        static SV *
        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 CW#define PERL_NO_GET_CONTEXT before including the Perl headers, followed by a CWdTHX; 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 my_private_function(pTHX_ int arg1, int arg2);

        static SV *
        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 CWpTHX yourselfalways 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 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 CWperl_alloc and CWperl_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 CWPERL_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 ...

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 and USE_5005THREADS on Windows (see inside iperlsys.h).

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.

All of Perl's internal functions which will be exposed to the outside world are prefixed by CWPerl_ 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 CWPL_. (By convention, static functions start with CWS_.)

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

p

This function has a CWPerl_ prefix; ie, it is defined as CWPerl_av_fetch

d

This function has documentation using the CWapidoc feature which we'll look at in a second.

Other available flags are:

s

This is a static function and is defined as CWS_whatever, and usually called within the sources as CWwhatever(...).

n

This does not use CWaTHX_ and CWpTHX to pass interpreter context. (See Background and PERL_IMPLICIT_CONTEXT in perlguts.)

r

This function never returns; CWcroak, CWexit and friends.

f

This function takes a variable number of arguments, CWprintf style. The argument list should end with CW..., 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, CWPerl_parse to CWparse. It must be called as CWPerl_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 CWPerl_ implementation (which is exported).

If you edit embed.pl or embed.fnc, you will need to run CWmake regen_headers to force a rebuild of embed.h and other auto-generated files.

If you are printing IVs, UVs, or NVS instead of the stdio(3) style formatting codes like CW%d, CW%ld, CW%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 CW%lx or CW%p.

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);

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.

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

CWDevel::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 CW--api-info command line switch. For example:

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

For details, see CWperldoc ppport.h.

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.

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, instead of just one. You can learn more about Unicode at http://www.unicode.org/

You can't. This is because UTF-8 data is stored in bytes just like non-UTF-8 data. The Unicode character 200, (CW0xC8 for you hex types) capital E with a grave accent, is represented by the two bytes CWv196.172. Unfortunately, the non-Unicode string CWchr(1).chr(1) has that byte sequence as well. So you can't tell just by looking - this is what makes Unicode input an interesting problem.

The API function CWis_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, CWis_utf8_char will tell you whether the current character in a string is valid UTF-8.

As mentioned above, UTF-8 uses a variable number of bytes to store a character. Characters with values 1...128 are stored in one byte, just like good ol' ASCII. Character 129 is stored as CWv194.129; this continues up to character 191, which is CWv194.191. Now we've run out of bits (191 is binary CW10111111) so we move on; 192 is CWv195.128. 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 CWUTF8SKIP 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 CWutf8_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 CWutf8_to_uv to get the value of the character; the inverse function CWuv_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 CWv196.172, and you skip that character, you can never match a CWchr(2) in a non-UTF-8 string. So don't do that!

Currently, Perl deals with Unicode strings and non-Unicode strings slightly differently. If a string has been identified as being UTF-8 encoded, Perl will set a flag in the SV, CWSVf_UTF8. 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, CWlength, CWsubstr and other string handling operations will have undesirable results.

The problem comes when you have, for instance, a string that isn't flagged is 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 CWSVf_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 CWchar* 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 UTF-8 flag set, and act accordingly:

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

In fact, your CWfrobnicate 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 UTF-8 flags, even less right is just passing a CWchar * to an XS function.

If you're mixing UTF-8 and non-UTF-8 strings, you might find it 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.

Instead, CWbytes_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 CWutf8_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.

Not really. Just remember these things:

•

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 CWSvUTF8 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.

•

If a string is UTF-8, always use CWutf8_to_uv to get at the value, unless CWUTF8_IS_INVARIANT(*s) in which case you can use CW*s.

•

When writing a character CWuv to a UTF-8 string, always use CWuv_to_utf8, unless CWUTF8_IS_INVARIANT(uv)) in which case you can use CW*s = uv.

•

Mixing UTF-8 and non-UTF-8 strings is tricky. Use CWbytes_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 - CWHALF_UPGRADE is one of those.

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 CWgvsv, gvsv, add.)

This feature is implemented as a new op type, CWOP_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 CWCHECK block and the CWB::Generate module, or by adding a custom peephole optimizer with the CWoptimize module.

When you do this, you replace ordinary Perl ops with custom ops by creating ops with the type CWOP_CUSTOM and the CWpp_addr of your own PP function. This should be defined in XS code, and should look like the PP ops in CWpp_*.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 CWOP_CUSTOM, Perl uses the value of CWo->op_ppaddr as a key into the CWPL_custom_op_descs and CWPL_custom_op_names hashes. This means you need to enter a name and description for your op at the appropriate place in the CWPL_custom_op_names and CWPL_custom_op_descs hashes.

Forthcoming versions of CWB::Generate (version 1.0 and above) should directly support the creation of custom ops by name; CWOpcodes::Custom will provide functions which make it trivial to register custom ops to the Perl interpreter.

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.

perlapi(1), perlintern(1), perlxs(1), perlembed(1)