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Perl: the Markov chain saw


by gods
on Aug 25, 1999 at 07:29 UTC ( #424=perlman: print w/replies, xml ) Need Help??


Current Perl documentation can be found at

Here is our local, out-dated (pre-5.6) version:


perlguts - Perl's Internal Functions


This document attempts to describe some of the internal functions of the Perl executable. 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.

What is an "IV"?

Perl uses a special typedef IV which is a simple integer type that is guaranteed to be large enough to hold a pointer (as well as an integer).

Perl also uses two special typedefs, I32 and I16, which will always be at least 32-bits and 16-bits long, respectively.

Working with SVs

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

The six routines are:

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

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

    void  sv_setiv(SV*, IV);
    void  sv_setuv(SV*, UV);
    void  sv_setnv(SV*, double);
    void  sv_setpv(SV*, char*);
    void  sv_setpvn(SV*, char*, int)
    void  sv_setpvf(SV*, const char*, ...);
    void  sv_setpvfn(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 perlman:perlguts, perlman:perlguts, or perlman:perlguts, or you may allow Perl to calculate the length by using perlman:perlguts or by specifying 0 as the second argument to perlman:perlguts. Be warned, though, that Perl will determine the string's length by using strlen, which depends on the string terminating with a NUL character.

The arguments of perlman:perlguts are processed like sprintf, and the formatted output becomes the value.

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

The sv_set*() functions are not generic enough to operate on values that have ``magic''. See Magic Virtual Tables later in this document.

All SVs that contain strings should be terminated with a NUL character. If it is not NUL-terminated there is a risk of core dumps and corruptions from code which passes the string to C functions or system calls which expect a NUL-terminated string. Perl's own functions typically add a trailing NUL for this reason. Nevertheless, you should be very careful when you pass a string stored in an SV to a C function or system call.

To access the actual value that an SV points to, you can use the macros:

    SvPV(SV*, STRLEN len)

which will automatically coerce the actual scalar type into an IV, double, or string.

In the perlman:perlguts macro, the length of the string returned is placed into the variable len (this is a macro, so you do not use &len). If you do not care what the length of the data is, use the global variable perlman:perlguts. Remember, however, that Perl allows arbitrary strings of data that may both contain NULs and might not be terminated by a NUL.

If you want to know if the scalar value is TRUE, you can use:


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 perlman:perlguts. Note that perlman:perlguts 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 perlman:perlguts).

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.


You can get and set the current length of the string stored in an SV with the following macros:

    SvCUR_set(SV*, I32 val)

You can also get a pointer to the end of the string stored in the SV with the macro:


But note that these last three macros are valid only if perlman:perlguts is true.

If you want to append something to the end of string stored in an perlman:perlguts, you can use the following functions:

    void  sv_catpv(SV*, char*);
    void  sv_catpvn(SV*, char*, int);
    void  sv_catpvf(SV*, const char*, ...);
    void  sv_catpvfn(SV*, const char*, STRLEN, va_list *, SV **, I32, bool);
    void  sv_catsv(SV*, SV*);

The first function calculates the length of the string to be appended by using strlen. In the second, you specify the length of the string yourself. The third function processes its arguments like sprintf and appends the formatted output. The fourth function works like vsprintf. You can specify the address and length of an array of SVs instead of the va_list argument. The fifth function extends the string stored in the first SV with the string stored in the second SV. It also forces the second SV to be interpreted as a string.

The sv_cat*() functions are not generic enough to operate on values that have ``magic''. See Magic Virtual Tables later in this document.

If you know the name of a scalar variable, you can get a pointer to its SV by using the following:

    SV*  perl_get_sv("package::varname", FALSE);

This returns NULL if the variable does not exist.

If you want to know if this variable (or any other SV) is actually defined, you can call:


The scalar undef value is stored in an SV instance called perlman:perlguts. Its address can be used whenever an perlman:perlguts is needed.

There are also the two values perlman:perlguts and perlman:perlguts, which contain Boolean TRUE and FALSE values, respectively. Like perlman:perlguts, their addresses can be used whenever an perlman:perlguts is needed.

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

    SV* sv = (SV*) 0;
    if (I-am-to-return-a-real-value) {
            sv = sv_2mortal(newSViv(42));
    sv_setsv(ST(0), sv);

This code tries to return a new SV (which contains the value 42) if it should return a real value, or undef otherwise. Instead it has returned a NULL pointer which, somewhere down the line, will cause a segmentation violation, bus error, or just weird results. Change the zero to perlman:perlguts in the first line and all will be well.

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

What's Really Stored in an SV?

Recall that the usual method of determining the type of scalar you have is to use perlman:perlguts macros. Because a scalar can be both a number and a string, usually these macros will always return TRUE and calling the Sv*V macros will do the appropriate conversion of string to integer/double or integer/double to string.

If you really need to know if you have an integer, double, or string pointer in an SV, you can use the following three macros instead:


These will tell you if you truly have an integer, double, or string pointer stored in your SV. The ``p'' stands for private.

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

Working with AVs

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

    AV*  newAV();

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

    AV*  av_make(I32 num, SV **ptr);

The second argument points to an array containing num perlman:perlguts'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 perlman:perlguts. This routine adds num elements at the front of the array with the undef value. You must then use perlman:perlguts (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 perlman:perlguts function returns the highest index value in array (just like $#array in Perl). If the array is empty, -1 is returned. The perlman:perlguts function returns the value at index key, but if lval is non-zero, then perlman:perlguts will store an undef value at that index. The perlman:perlguts function stores the value val at index key, and does not increment the reference count of val. Thus the caller is responsible for taking care of that, and if perlman:perlguts returns NULL, the caller will have to decrement the reference count to avoid a memory leak. Note that perlman:perlguts and perlman:perlguts both return perlman:perlguts's, not perlman:perlguts's as their return value.

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

The perlman:perlguts function deletes all the elements in the AV* array, but does not actually delete the array itself. The perlman:perlguts function will delete all the elements in the array plus the array itself. The perlman:perlguts function extends the array so that it contains key elements. If key is less than the current length of the array, then nothing is done.

If you know the name of an array variable, you can get a pointer to its AV by using the following:

    AV*  perl_get_av("package::varname", FALSE);

This returns NULL if the variable does not exist.

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

Working with HVs

To create an HV, you use the following routine:

    HV*  newHV();

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

    SV**  hv_store(HV*, char* key, U32 klen, SV* val, U32 hash);
    SV**  hv_fetch(HV*, char* key, U32 klen, I32 lval);

The klen parameter is the length of the key being passed in (Note that you cannot pass 0 in as a value of klen to tell Perl to measure the length of the key). The val argument contains the SV pointer to the scalar being stored, and hash is the precomputed hash value (zero if you want perlman:perlguts to calculate it for you). The lval parameter indicates whether this fetch is actually a part of a store operation, in which case a new undefined value will be added to the HV with the supplied key and perlman:perlguts will return as if the value had already existed.

Remember that perlman:perlguts and perlman:perlguts return perlman:perlguts's and not just perlman:perlguts. To access the scalar value, you must first dereference the return value. However, you should check to make sure that the return value is not NULL before dereferencing it.

These two functions check if a hash table entry exists, and deletes it.

    bool  hv_exists(HV*, char* key, U32 klen);
    SV*   hv_delete(HV*, char* key, U32 klen, I32 flags);

If flags does not include the perlman:perlguts flag then perlman:perlguts 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, perlman:perlguts deletes all the entries in the hash table but does not actually delete the hash table. The perlman:perlguts 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 perlman:perlguts. However, once you have an HE*, to get the actual key and value, use the routines specified below.

    I32    hv_iterinit(HV*);
            /* Prepares starting point to traverse hash table */
    HE*    hv_iternext(HV*);
            /* Get the next entry, and return a pointer to a
               structure that has both the key and value */
    char*  hv_iterkey(HE* entry, I32* retlen);
            /* Get the key from an HE structure and also return
               the length of the key string */
    SV*    hv_iterval(HV*, HE* entry);
            /* Return a SV pointer to the value of the HE
               structure */
    SV*    hv_iternextsv(HV*, char** key, I32* retlen);
            /* This convenience routine combines hv_iternext,
               hv_iterkey, and hv_iterval.  The key and retlen
               arguments are return values for the key and its
               length.  The value is returned in the SV* argument */

If you know the name of a hash variable, you can get a pointer to its HV by using the following:

    HV*  perl_get_hv("package::varname", FALSE);

This returns NULL if the variable does not exist.

The hash algorithm is defined in the PERL_HASH(hash, key, klen) macro:

    i = klen;
    hash = 0;
    s = key;
    while (i--)
        hash = hash * 33 + *s++;

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

Hash API Extensions

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

    HE*     hv_fetch_ent  (HV* tb, SV* key, I32 lval, U32 hash);
    HE*     hv_store_ent  (HV* tb, SV* key, SV* val, U32 hash);
    bool    hv_exists_ent (HV* tb, SV* key, U32 hash);
    SV*     hv_delete_ent (HV* tb, SV* key, I32 flags, U32 hash);
    SV*     hv_iterkeysv  (HE* entry);

Note that these functions take perlman:perlguts keys, which simplifies writing of extension code that deals with hash structures. These functions also allow passing of perlman:perlguts keys to tie functions without forcing you to stringify the keys (unlike the previous set of functions).

They also return and accept whole hash entries (HE*), making their use more efficient (since the hash number for a particular string doesn't have to be recomputed every time). See API LISTING later in this document for detailed descriptions.

The following macros must always be used to access the contents of hash entries. Note that the arguments to these macros must be simple variables, since they may get evaluated more than once. See API LISTING later in this document for detailed descriptions of these macros.

    HePV(HE* he, STRLEN len)
    HeVAL(HE* he)
    HeHASH(HE* he)
    HeSVKEY(HE* he)
    HeSVKEY_force(HE* he)
    HeSVKEY_set(HE* he, SV* sv)

These two lower level macros are defined, but must only be used when dealing with keys that are not perlman:perlgutss:

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

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


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

To create a reference, use either of the following functions:

    SV* newRV_inc((SV*) thing);
    SV* newRV_noinc((SV*) thing);

The thing argument can be any of an perlman:perlguts, perlman:perlguts, or perlman:perlguts. The functions are identical except that perlman:perlguts increments the reference count of the thing, while perlman:perlguts does not. For historical reasons, newRV is a synonym for perlman:perlguts.

Once you have a reference, you can use the following macro to dereference the reference:


then call the appropriate routines, casting the returned perlman:perlguts to either an perlman:perlguts or perlman:perlguts, if required.

To determine if an SV is a reference, you can use the following macro:


To discover what type of value the reference refers to, use the following macro and then check the return value.


The most useful types that will be returned are:

    SVt_IV    Scalar
    SVt_NV    Scalar
    SVt_PV    Scalar
    SVt_RV    Scalar
    SVt_PVAV  Array
    SVt_PVHV  Hash
    SVt_PVCV  Code
    SVt_PVGV  Glob (possible a file handle)
    SVt_PVMG  Blessed or Magical Scalar

    See the sv.h header file for more details.

Blessed References and Class Objects

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

A reference can be blessed into a package with the following function:

    SV* sv_bless(SV* sv, HV* stash);

The sv argument must be a reference. The stash argument specifies which class the reference will belong to. See Stashes and Globs for information on converting class names into stashes.

/* Still under construction */

Upgrades rv to reference if not already one. Creates new SV for rv to point to. If classname is non-null, the SV is blessed into the specified class. SV is returned.

        SV* newSVrv(SV* rv, char* classname);

Copies integer or double into an SV whose reference is rv. SV is blessed if classname is non-null.

        SV* sv_setref_iv(SV* rv, char* classname, IV iv);
        SV* sv_setref_nv(SV* rv, char* classname, NV iv);

Copies the pointer value (the address, not the string!) into an SV whose reference is rv. SV is blessed if classname is non-null.

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

Copies string into an SV whose reference is rv. Set length to 0 to let Perl calculate the string length. SV is blessed if classname is non-null.

        SV* sv_setref_pvn(SV* rv, char* classname, PV iv, int length);

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

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

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

        int  sv_isobject(SV* sv);

Tests whether the SV is derived from the specified class. SV can be either a reference to a blessed object or a string containing a class name. This is the function implementing the UNIVERSAL::isa functionality.

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

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

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

Creating New Variables

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

    SV*  perl_get_sv("package::varname", TRUE);
    AV*  perl_get_av("package::varname", TRUE);
    HV*  perl_get_hv("package::varname", TRUE);

Notice the use of TRUE as the second parameter. The new variable can now be set, using the routines appropriate to the data type.

There are additional macros whose values may be bitwise OR'ed with the TRUE argument to enable certain extra features. Those bits are:

    GV_ADDMULTI Marks the variable as multiply defined, thus preventing the
                "Name <varname> used only once: possible typo" warning.
    GV_ADDWARN  Issues the warning "Had to create <varname> unexpectedly" if
                the variable did not exist before the function was called.

If you do not specify a package name, the variable is created in the current package.

Reference Counts and Mortality

Perl uses an 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 perlman:perlguts 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 perlman:perlguts 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 perlman:perlguts, passing it the just-created SV. This returns the reference as a new SV, but the reference count of the SV you passed to perlman:perlguts 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 perlman:perlguts instead of perlman:perlguts. 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 the perlcall manpage and the perlxs manpage for more details on these macros.

``Mortalization'' then is at its simplest a deferred perlman:perlguts. However, if you mortalize a variable twice, the reference count will later be decremented twice.

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

To create a mortal variable, use the functions:

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

The first call creates a mortal SV, the second converts an existing SV to a mortal SV (and thus defers a call to perlman:perlguts), and the third creates a mortal copy of an existing SV.

The mortal routines are not just for SVs -- AVs and HVs can be made mortal by passing their address (type-casted to perlman:perlguts) to the perlman:perlguts or perlman:perlguts routines.

Stashes and Globs

A ``stash'' is a hash that contains all of the different objects that are contained 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

There is a single stash called ``PL_defstash'' that holds the items that exist in the ``main'' package. To get at the items in other packages, append the string ``::'' to the package name. The items in the ``Foo'' package are in the stash ``Foo::'' in PL_defstash. The items in the ``Bar::Baz'' package are in the stash ``Baz::'' in ``Bar::'''s stash.

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

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

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

The name that gv_stash*v wants is the name of the package whose symbol table you want. The default package is called main. If you have multiply nested packages, pass their names to gv_stash*v, separated by :: as in the Perl language itself.

Alternately, if you have an SV that is a blessed reference, you can find out the stash pointer by using:

    HV*  SvSTASH(SvRV(SV*));

then use the following to get the package name itself:

    char*  HvNAME(HV* stash);

If you need to bless or re-bless an object you can use the following function:

    SV*  sv_bless(SV*, HV* stash)

where the first argument, an perlman:perlguts, must be a reference, and the second argument is a stash. The returned perlman:perlguts can now be used in the same way as any other SV.

For more information on references and blessings, consult the perlref manpage.

Double-Typed SVs

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

Some scalar variables contain more than one type of scalar data. For example, the variable $! contains either the numeric value of errno or its string equivalent from either strerror or sys_errlist[].

To force multiple data values into an SV, you must do two things: use the sv_set*v routines to add the additional scalar type, then set a flag so that Perl will believe it contains more than one type of data. The four macros to set the flags are:


The particular macro you must use depends on which sv_set*v routine you called first. This is because every sv_set*v routine turns on only the bit for the particular type of data being set, and turns off all the rest.

For example, to create a new Perl variable called ``dberror'' that contains both the numeric and descriptive string error values, you could use the following code:

    extern int  dberror;
    extern char *dberror_list;

    SV* sv = perl_get_sv("dberror", TRUE);
    sv_setiv(sv, (IV) dberror);
    sv_setpv(sv, dberror_list[dberror]);

If the order of perlman:perlguts and perlman:perlguts had been reversed, then the macro perlman:perlguts would need to be called instead of perlman:perlguts.

Magic Variables

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

Any SV may be magical, that is, it has special features that a normal SV does not have. These features are stored in the SV structure in a linked list of struct magic's, typedef'ed to MAGIC.

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

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

Assigning Magic

Perl adds magic to an SV using the sv_magic function:

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

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

If sv is not already magical, Perl uses the perlman:perlguts macro to set the perlman:perlguts flag for the sv. Perl then continues by adding it to the beginning of the linked list of magical features. Any prior entry of the same type of magic is deleted. Note that this can be overridden, and multiple instances of the same type of magic can be associated with an SV.

The name and namlen arguments are used to associate a string with the magic, typically the name of a variable. namlen is stored in the perlman:perlguts field and if name is non-null and namlen >= 0 a malloc'd copy of the name is stored in mg_ptr field.

The sv_magic function uses how to determine which, if any, predefined ``Magic Virtual Table'' should be assigned to the mg_virtual field. See the ``Magic Virtual Table'' section below. The how argument is also stored in the mg_type field.

The obj argument is stored in the mg_obj field of the MAGIC structure. If it is not the same as the sv argument, the reference count of the obj object is incremented. If it is the same, or if the how argument is ``#'', or if it is a NULL pointer, then obj is merely stored, without the reference count being incremented.

There is also a function to add magic to an perlman:perlguts:

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

This simply calls perlman:perlguts and coerces the gv argument into an perlman:perlguts.

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

    void sv_unmagic(SV *sv, int type);

The type argument should be equal to the how value when the perlman:perlguts was initially made magical.

Magic Virtual Tables

The mg_virtual field in the MAGIC structure is a pointer to a MGVTBL, which is a structure of function pointers and stands for ``Magic Virtual Table'' to handle the various operations that might be applied to that variable.

The MGVTBL has five pointers to the following routine types:

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

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

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

For instance, the MGVTBL structure called vtbl_sv (which corresponds to an mg_type of '\0') contains:

    { magic_get, magic_set, magic_len, 0, 0 }

Thus, when an SV is determined to be magical and of type '\0', if a get operation is being performed, the routine magic_get is called. All the various routines for the various magical types begin with magic_.

The current kinds of Magic Virtual Tables are:

    mg_type  MGVTBL              Type of magic
    -------  ------              ----------------------------
    \0       vtbl_sv             Special scalar variable
    A        vtbl_amagic         %OVERLOAD hash
    a        vtbl_amagicelem     %OVERLOAD hash element
    c        (none)              Holds overload table (AMT) on stash
    B        vtbl_bm             Boyer-Moore (fast string search)
    E        vtbl_env            %ENV hash
    e        vtbl_envelem        %ENV hash element
    f        vtbl_fm             Formline ('compiled' format)
    g        vtbl_mglob          m//g target / study()ed string
    I        vtbl_isa            @ISA array
    i        vtbl_isaelem        @ISA array element
    k        vtbl_nkeys          scalar(keys()) lvalue
    L        (none)              Debugger %_<filename 
    l        vtbl_dbline         Debugger %_<filename element
    o        vtbl_collxfrm       Locale transformation
    P        vtbl_pack           Tied array or hash
    p        vtbl_packelem       Tied array or hash element
    q        vtbl_packelem       Tied scalar or handle
    S        vtbl_sig            %SIG hash
    s        vtbl_sigelem        %SIG hash element
    t        vtbl_taint          Taintedness
    U        vtbl_uvar           Available for use by extensions
    v        vtbl_vec            vec() lvalue
    x        vtbl_substr         substr() lvalue
    y        vtbl_defelem        Shadow "foreach" iterator variable /
                                  smart parameter vivification
    *        vtbl_glob           GV (typeglob)
    #        vtbl_arylen         Array length ($#ary)
    .        vtbl_pos            pos() lvalue
    ~        (none)              Available for use by extensions

When an uppercase and lowercase letter both exist in the table, then the uppercase letter is used to represent some kind of composite type (a list or a hash), and the lowercase letter is used to represent an element of that composite type.

The '~' and 'U' magic types are defined specifically for use by extensions and will not be used by perl itself. Extensions can use '~' magic to 'attach' private information to variables (typically objects). This is especially useful because there is no way for normal perl code to corrupt this private information (unlike using extra elements of a hash object).

Similarly, 'U' magic can be used much like tie() to call a C function any time a scalar's value is used or changed. The MAGIC's mg_ptr field points to a ufuncs structure:

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

When the SV is read from or written to, the uf_val or uf_set function will be called with uf_index as the first arg and a pointer to the SV as the second.

Note that because multiple extensions may be using '~' or 'U' magic, it is important for extensions to take extra care to avoid conflict. Typically only using the magic on objects blessed into the same class as the extension is sufficient. For '~' magic, it may also be appropriate to add an I32 'signature' at the top of the private data area and check that.

Also note that the sv_set*() and sv_cat*() functions described earlier do not invoke 'set' magic on their targets. This must be done by the user either by calling the perlman:perlguts macro after calling these functions, or by using one of the sv_set*_mg() or sv_cat*_mg() functions. Similarly, generic C code must call the perlman:perlguts macro to invoke any 'get' magic if they use an SV obtained from external sources in functions that don't handle magic. API LISTING later in this document identifies such functions. For example, calls to the sv_cat*() functions typically need to be followed by perlman:perlguts, but they don't need a prior perlman:perlguts since their implementation handles 'get' magic.

Finding Magic

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

This routine returns a pointer to the MAGIC structure stored in the SV. If the SV does not have that magical feature, NULL is returned. Also, if the SV is not of type SVt_PVMG, Perl may core dump.

    int mg_copy(SV* sv, SV* nsv, char* key, STRLEN klen);

This routine checks to see what types of magic sv has. If the mg_type field is an uppercase letter, then the mg_obj is copied to nsv, but the mg_type field is changed to be the lowercase letter.

Understanding the Magic of Tied Hashes and Arrays

Tied hashes and arrays are magical beasts of the 'P' 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 perlman:perlguts function, when given a tied array argument, merely copies the magic of the array onto the value to be ``stored'', using perlman:perlguts. It may also return NULL, indicating that the value did not actually need to be stored in the array. [MAYCHANGE] After a call to perlman:perlguts on a tied array, the caller will usually need to call perlman:perlguts to actually invoke the perl level ``STORE'' method on the TIEARRAY object. If perlman:perlguts did return NULL, a call to perlman:perlguts will also be usually necessary to avoid a memory leak. [/MAYCHANGE]

The previous paragraph is applicable verbatim to tied hash access using the perlman:perlguts and perlman:perlguts functions as well.

perlman:perlguts and the corresponding hash functions perlman:perlguts and perlman:perlguts actually return an undefined mortal value whose magic has been initialized using perlman:perlguts. Note the value so returned does not need to be deallocated, as it is already mortal. [MAYCHANGE] But you will need to call perlman:perlguts on the returned value in order to actually invoke the perl level ``FETCH'' method on the underlying TIE object. Similarly, you may also call perlman:perlguts on the return value after possibly assigning a suitable value to it using perlman:perlguts, 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 perlman:perlguts to attach magic to the values that were meant to be ``stored'' or ``fetched''. Later calls to perlman:perlguts and perlman:perlguts actually do the job of invoking the TIE methods on the underlying objects. Thus the magic mechanism currently implements a kind of lazy access to arrays and hashes.

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

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

Localizing changes

Perl has a very handy construction

    local $var = 2;

This construction is approximately equivalent to

    my $oldvar = $var;
    $var = 2;
    $var = $oldvar;

The biggest difference is that the first construction would reinstate the initial value of $var, irrespective of how control exits the block: goto, return, die/eval etc. It is a little bit more efficient as well.

There is a way to achieve a similar task from C via Perl API: create a pseudo-block, and arrange for some changes to be automatically undone at the end of it, either explicit, or via a non-local exit (via die()). A block-like construct is created by a pair of perlman:perlguts/perlman:perlguts macros (see perlman: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 perlman:perlguts/perlman:perlguts pair.

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

SAVEINT(int i)
SAVEI32(I32 i)
SAVELONG(long i)

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


These macros arrange things to restore the value of pointers perlman:perlop and p. perlman:perlop must be a pointer of a type which survives conversion to perlman:perlguts and back, p should be able to survive conversion to char* and back.


The refcount of sv would be decremented at the end of pseudo-block. This is similar to perlman:perlguts, which should (?) be used instead.


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


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


Clears a slot in the current scratchpad which corresponds to sv at the end of pseudo-block.

SAVEDELETE(HV *hv, char *key, I32 length)

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

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

At the end of pseudo-block the function f is called with the only argument (of type perlman:perlguts) p.


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

The following API list contains functions, thus one needs to provide pointers to the modifiable data explicitly (either C pointers, or Perlish GV *s). Where the above macros take int, a similar function takes perlfunc:int.

SV* save_scalar(GV *gv)

Equivalent to Perl code local $gv.

AV* save_ary(GV *gv)
HV* save_hash(GV *gv)

Similar to save_scalar, but localize @gv and %gv.

void save_item(SV *item)

Duplicates the current value of perlman:perlguts, on the exit from the current perlman:perlguts/perlman:perlguts pseudo-block will restore the value of perlman:perlguts using the stored value.

void save_list(SV **sarg, I32 maxsarg)

A variant of save_item which takes multiple arguments via an array sarg of perlman:perlguts of length maxsarg.

SV* save_svref(SV **sptr)

Similar to save_scalar, but will reinstate a perlman:perlguts.

void save_aptr(AV **aptr)
void save_hptr(HV **hptr)

Similar to save_svref, but localize perlman:perlguts and perlman:perlguts.

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


XSUBs and the Argument Stack

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

The stack arguments are accessible through the perlman:perlguts macro, which returns the n'th stack argument. Argument 0 is the first argument passed in the Perl subroutine call. These arguments are perlman:perlguts, and can be used anywhere an perlman:perlguts 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 perlman:perlguts is the macro that represents the local copy of the stack pointer, and num is the number of elements the stack should be extended by.

Now that there is room on the stack, values can be pushed on it using the macros to push IVs, doubles, strings, and SV pointers respectively:

    PUSHp(char*, I32)

And now the Perl program calling tzname, the two values will be assigned as in:

    ($standard_abbrev, $summer_abbrev) = POSIX::tzname;

An alternate (and possibly simpler) method to pushing values on the stack is to use the macros:

    XPUSHp(char*, I32)

These macros automatically adjust the stack for you, if needed. Thus, you do not need to call perlman:perlguts to extend the stack.

For more information, consult the perlxs manpage and the perlxstut manpage.

Calling Perl Routines from within C Programs

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

    I32  perl_call_sv(SV*, I32);
    I32  perl_call_pv(char*, I32);
    I32  perl_call_method(char*, I32);
    I32  perl_call_argv(char*, I32, register char**);

The routine most often used is perlman:perlguts. The perlman:perlguts argument contains either the name of the Perl subroutine to be called, or a reference to the subroutine. The second argument consists of flags that control the context in which the subroutine is called, whether or not the subroutine is being passed arguments, how errors should be trapped, and how to treat return values.

All four routines return the number of arguments that the subroutine returned on the Perl stack.

When using any of these routines (except perlman:perlguts), the programmer must manipulate the Perl stack. These include the following macros and functions:


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

Memory Allocation

It is suggested that you use 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.

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

These three macros are used to initially allocate memory.

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

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

The third and fourth arguments number and type specify how many of the specified type of data structure should be allocated. The argument type is passed to sizeof. The final argument to perlman:perlguts, cast, should be used if the pointer argument is different from the type argument.

Unlike the perlman:perlguts and perlman:perlguts macros, the perlman:perlguts macro calls memzero to zero out all the newly allocated memory.

    Renew(pointer, number, type);
    Renewc(pointer, number, type, cast);

These three macros are used to change a memory buffer size or to free a piece of memory no longer needed. The arguments to perlman:perlguts and perlman:perlguts match those of perlman:perlguts and perlman:perlguts with the exception of not needing the ``magic cookie'' argument.

    Move(source, dest, number, type);
    Copy(source, dest, number, type);
    Zero(dest, number, type);

These three macros are used to move, copy, or zero out previously allocated memory. The source and dest arguments point to the source and destination starting points. Perl will move, copy, or zero out number instances of the size of the type data structure (using the sizeof function).


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[choroba]: Dangers of WFH: a mailman rings, brings a parcel for my wife, I need to go outside, take my son with me, sign a paper *and* into the postman's mobile app, get back. Guess what the soup was doing meanwhiles?
[Discipulus]: soup without heat protections as CPUs are doomed to burned..
[1nickt]: The dangers of milk-based soup, you mean?
[Corion]: choroba: Ouch! But you don't need that many external events for that. I have on several occasions set up milk to cook and then programmed a bit, only to find the milk burned and congealed over the stove :-/
[choroba]: it's still edible, just need to scour the pot
[1nickt]: Corion I have a large site I need to check for broken links and absolute links. Making a scraper is easy of course; a spider that crawls a whole site is a little more involved ... I was planning a queue-based tool. Intersted to see what you do...

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