# NAME

perlguts - Perl's Internal Functions

# DESCRIPTION

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.

# Datatypes

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

        SV  Scalar Value
        AV  Array Value
        HV  Hash Value

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

## What is an "IV"?

Perl uses a special typedef IV which is large enough to hold either an integer or a pointer.

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

## Working with SV's

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 four routines are:

        SV*  newSViv(IV);
        SV*  newSVnv(double);
        SV*  newSVpv(char*, int);
        SV*  newSVsv(SV*);

To change the value of an *already-existing* scalar, there are five routines:

        void  sv_setiv(SV*, IV);
        void  sv_setnv(SV*, double);
        void  sv_setpvn(SV*, char*, int)
        void  sv_setpv(SV*, char*);
        void  sv_setsv(SV*, SV*);

Notice that you can choose to specify the length of the string to be assigned by using `sv_setpvn` or `newSVpv`, or you may allow Perl to calculate the length by using `sv_setpv` or specifying 0 as the second argument to `newSVpv`. Be warned, though, that Perl will determine the string's length by using `strlen`, which depends on the string terminating with a NUL character.

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

        SvIV(SV*)
        SvNV(SV*)
        SvPV(SV*, STRLEN len)

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

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

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

        SvTRUE(SV*)

Although Perl will automatically grow strings for you, if you need to force Perl to allocate more memory for your SV, you can use the macro

        SvGROW(SV*, STRLEN newlen)

which will determine if more memory needs to be allocated. If so, it will call the function `sv_grow`. Note that `SvGROW` can only increase, not decrease, the allocated memory of an SV.

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)

But note that these are valid only if `SvPOK()` is true.

If you want to append something to the end of string stored in an `SV*`, you can use the following functions:

        void  sv_catpv(SV*, char*);
        void  sv_catpvn(SV*, char*, int);
        void  sv_catsv(SV*, SV*);

The first function calculates the length of the string to be appended by using `strlen`. In the second, you specify the length of the string yourself. The third function extends the string stored in the first SV with the string stored in the second SV. It also forces the second SV to be interpreted as a string.

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

        SV*  perl_get_sv("varname", FALSE);

This returns NULL if the variable does not exist.

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

        SvOK(SV*)

The scalar undef value is stored in an SV instance called `sv_undef`. Its address can be used whenever an `SV*` is needed.

There are also the two values `sv_yes` and `sv_no`, which contain Boolean TRUE and FALSE values, respectively. Like `sv_undef`, their addresses can be used whenever an `SV*` is needed.

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

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

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

To free an SV that you've created, call `SvREFCNT_dec(SV*)`. Normally this call is not necessary. See the section on MORTALITY.

## What's Really Stored in an SV?

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

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

        SvIOKp(SV*)
        SvNOKp(SV*)
        SvPOKp(SV*)

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

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

## Working with AV's

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

        AV*  newAV();

The second method both creates the AV and initially populates it with SV's:

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

The second argument points to an array containing `num` `SV*`'s. Once the AV has been created, the SV's can be destroyed, if so desired.

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

        void  av_push(AV*, SV*);
        SV*   av_pop(AV*);
        SV*   av_shift(AV*);
        void  av_unshift(AV*, I32 num);

These should be familiar operations, with the exception of `av_unshift`. This routine adds `num` elements at the front of the array with the undef value. You must then use `av_store` (described below) to assign values to these new elements.

Here are some other functions:

        I32   av_len(AV*); /* Returns highest index value in array */
        SV**  av_fetch(AV*, I32 key, I32 lval);
                /* Fetches value at key offset, but it stores an undef value
                   at the offset if lval is non-zero */
        SV**  av_store(AV*, I32 key, SV* val);
                /* Stores val at offset key */

Take note that these two functions return `SV**`'s, not `SV*`'s.

        void  av_clear(AV*);
                /* Clear out all elements, but leave the array */
        void  av_undef(AV*);
                /* Undefines the array, removing all elements */

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("varname", FALSE);

This returns NULL if the variable does not exist.

## Working with HV's

To create an HV, you use the following routine:

        HV*  newHV();

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

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

The `klen` parameter is the length of the key being passed in. The `val` argument contains the SV pointer to the scalar being stored, and `hash` is the pre-computed hash value (zero if you want `hv_store` to calculate it for you). The `lval` parameter indicates whether this fetch is actually a part of a store operation.

Remember that `hv_store` and `hv_fetch` return `SV**`'s and not just `SV*`. In order 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);

And more miscellaneous functions:

        void   hv_clear(HV*);
                /* Clears all entries in hash table */
        void   hv_undef(HV*);
                /* Undefines the hash table */

Perl keeps the actual data in linked list of structures with a typedef of HE. These contain the actual key and value pointers (plus extra administrative overhead). The key is a string pointer; the value is an `SV*`. However, once you have an `HE*`, to get the actual key and value, use the routines specified below.

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

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

        HV*  perl_get_hv("varname", FALSE);

This returns NULL if the variable does not exist.

The hash algorithm, for those who are interested, is:

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

# Creating New Variables

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

        SV*  perl_get_sv("varname", TRUE);
        AV*  perl_get_av("varname", TRUE);
        HV*  perl_get_hv("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 bits that may be OR'ed with the TRUE argument to enable certain extra features. Those bits are:

        0x02  Marks the variable as multiply defined, thus preventing the
              "Indentifier <varname> used only once: possible typo" warning.
        0x04  Issues a "Had to create <varname> unexpectedly" warning if
              the variable didn't actually exist.  This is useful if
              you expected the variable to already exist and want to propagate
              this warning back to the user.

    If the C<varname> argument does not contain a package specifier, it is
    created in the current package.

## References

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

To create a reference, use the following command:

        SV*  newRV((SV*) thing);

The `thing` argument can be any of an `SV*`, `AV*`, or `HV*`. Once you have a reference, you can use the following macro to dereference the reference:

        SvRV(SV*)

then call the appropriate routines, casting the returned `SV*` to either an `AV*` or `HV*`, if required.

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

        SvROK(SV*)

To actually discover what the reference refers to, you must use the following macro and then check the value returned.

        SvTYPE(SvRV(SV*))

The most useful types that will be returned are:

        SVt_IV    Scalar
        SVt_NV    Scalar
        SVt_PV    Scalar
        SVt_PVAV  Array
        SVt_PVHV  Hash
        SVt_PVCV  Code
        SVt_PVMG  Blessed Scalar

# XSUB's and the Argument Stack

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

The stack arguments are accessible through the `ST(n)` macro, which returns the `n`'th stack argument. Argument 0 is the first argument passed in the Perl subroutine call. These arguments are `SV*`, and can be used anywhere an `SV*` is used.

Most of the time, output from the C routine can be handled through use of the RETVAL and OUTPUT directives. However, there are some cases where the argument stack is not already long enough to handle all the return values. An example is the POSIX tzname() call, which takes no arguments, but returns two, the local timezone's standard and summer time abbreviations.

To handle this situation, the PPCODE directive is used and the stack is extended using the macro:

        EXTEND(sp, num);

where `sp` is the stack pointer, and `num` is the number of elements the stack should be extended by.

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

        PUSHi(IV)
        PUSHn(double)
        PUSHp(char*, I32)
        PUSHs(SV*)

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

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

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

        XPUSHi(IV)
        XPUSHn(double)
        XPUSHp(char*, I32)
        XPUSHs(SV*)

These macros automatically adjust the stack for you, if needed.

For more information, consult the perlapi manpage .

# Mortality

In Perl, values are normally "immortal" -- that is, they are not freed unless explicitly done so (via the Perl undef call or other routines in Perl itself).

Add cruft about reference counts.

In the above example with `tzname`, we needed to create two new SV's to push onto the argument stack, that being the two strings. However, we don't want these new SV's to stick around forever because they will eventually be copied into the SV's that hold the two scalar variables.

An SV (or AV or HV) that is "mortal" acts in all ways as a normal "immortal" SV, AV, or HV, but is only valid in the "current context". When the Perl interpreter leaves the current context, the mortal SV, AV, or HV is automatically freed. Generally the "current context" means a single Perl statement.

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

The mortal routines are not just for SV's -- AV's and HV's can be made mortal by passing their address (and casting them to `SV*`) to the `sv_2mortal` or `sv_mortalcopy` routines.

From Ilya: Beware that the sv_2mortal() call is eventually equivalent to svREFCNT_dec(). A value can happily be mortal in two different contexts, and it will be svREFCNT_dec()ed twice, once on exit from these contexts. It can also be mortal twice in the same context. This means that you should be very careful to make a value mortal exactly as many times as it is needed. The value that go to the Perl stack should be mortal.

You should be careful about creating mortal variables. It is possible for strange things to happen should you make the same value mortal within multiple contexts.

# Stashes and Objects

A stash is a hash table (associative array) 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 called a GV (for 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 File Handle Directory Handle Format Subroutine

Perl stores various stashes in a separate GV structure (for global variable) but represents them with an HV structure. The keys in this larger GV are the various package names; the values are the `GV*`'s which are stashes. It may help to think of a stash purely as an HV, and that the term "GV" means the global variable hash.

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

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

The first function takes a literal string, the second uses the string stored in the SV. Remember that a stash is just a hash table, so you get back an `HV*`.

The 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 return a blessed value to your Perl script, you can use the following function:

        SV*  sv_bless(SV*, HV* stash)

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

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

# Magic

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

# Version 6, 1995/1/27

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 `SvUPGRADE` macro to set the `SVt_PVMG` flag for the `sv`. Perl then continues by adding it to the beginning of the linked list of magical features. Any prior entry of the same type of magic is deleted. Note that this can be overriden, and multiple instances of the same type of magic can be associated with an SV.

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

The sv_magic function uses `how` to determine which, if any, predefined "Magic Virtual Table" should be assigned to the `mg_virtual` field. See the "Magic Virtual Table" section below.

The `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.

## 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 magicalness
        -------  ------              -------------------
        \0       vtbl_sv             Regexp???
        A        vtbl_amagic         Operator Overloading
        a        vtbl_amagicelem     Operator Overloading
        c        0                   Used in Operator Overloading
        B        vtbl_bm             Boyer-Moore???
        E        vtbl_env            %ENV hash
        e        vtbl_envelem        %ENV hash element
        g        vtbl_mglob          Regexp /g flag???
        I        vtbl_isa            @ISA array
        i        vtbl_isaelem        @ISA array element
        L        0 (but sets RMAGICAL)     Perl Module/Debugger???
        l        vtbl_dbline         Debugger?
        P        vtbl_pack           Tied Array or Hash
        p        vtbl_packelem       Tied Array or Hash element
        q        vtbl_packelem       Tied Scalar or Handle
        S        vtbl_sig            Signal Hash
        s        vtbl_sigelem        Signal Hash element
        t        vtbl_taint          Taintedness
        U        vtbl_uvar           ???
        v        vtbl_vec            Vector
        x        vtbl_substr         Substring???
        *        vtbl_glob           GV???
        #        vtbl_arylen         Array Length
        .        vtbl_pos            $. scalar variable
        ~        Reserved for extensions, but multiple extensions may clash

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

## 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 upper-case letter, then the mg_obj is copied to `nsv`, but the mg_type field is changed to be the lower-case letter.

# Double-Typed SV's

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:

            SvIOK_on
            SvNOK_on
            SvPOK_on
            SvROK_on

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

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

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

If the order of `sv_setiv` and `sv_setpv` had been reversed, then the macro `SvPOK_on` would need to be called instead of `SvIOK_on`.

# 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 `perl_call_sv`. The `SV*` argument contains either the name of the Perl subroutine to be called, or a reference to the subroutine. The second argument consists of flags that control the context in which the subroutine is called, whether or not the subroutine is being passed arguments, how errors should be trapped, and how to treat return values.

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

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

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

For more information, consult the perlcall manpage .

# Memory Allocation

It is strongly 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 more quickly satisfy allocation requests. 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 (Larry has switched to using a run-time memory checker), so this argument can be any number.

The second argument `pointer` will point to the newly allocated memory. The third and fourth arguments `number` and `type` specify how many of the specified type of data structure should be allocated. The argument `type` is passed to `sizeof`. The final argument to `Newc`, `cast`, should be used if the `pointer` argument is different from the `type` argument.

Unlike the `New` and `Newc` macros, the `Newz` macro calls `memzero` to zero out all the newly allocated memory.

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

These three macros are used to change a memory buffer size or to free a piece of memory no longer needed. The arguments to `Renew` and `Renewc` match those of `New` and `Newc` with the exception of not needing the "magic cookie" argument.

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

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

# AUTHOR

Jeff Okamoto<okamoto@corp.hp.com>

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, and Spider Boardman.

# DATE

Version 19: 1995/4/26