path: root/pod/perlguts.pod

=head1 NAME

perlguts - Perl's Internal Functions

=head1 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.

=head1 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.

=head2 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.

=head2 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 C<sv_setpvn> or C<newSVpv>, or you may allow Perl to
calculate the length by using C<sv_setpv> or specifying 0 as the second
argument to C<newSVpv>.  Be warned, though, that Perl will determine the
string's length by using C<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 C<SvPV> macro, the length of the string returned is placed into the
variable C<len> (this is a macro, so you do I<not> use C<&len>).  If you do not
care what the length of the data is, use the global variable C<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 C<sv_grow>.  Note that C<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 C<SvPOK()> is true.

If you want to append something to the end of string stored in an C<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 C<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 C<defined>,
you can call:

    SvOK(SV*)

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

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

Do not be fooled into thinking that C<(SV *) 0> is the same as C<&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 C<&sv_undef> in the first line and
all will be well.

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

=head2 What's Really Stored in an SV?

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

If you I<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 C<Sv*V> macros.

=head2 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 C<num> C<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 C<av_unshift>.
This routine adds C<num> elements at the front of the array with the C<undef>
value.  You must then use C<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 C<SV**>'s, not C<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.

=head2 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 C<klen> parameter is the length of the key being passed in.  The C<val>
argument contains the SV pointer to the scalar being stored, and C<hash> is
the pre-computed hash value (zero if you want C<hv_store> to calculate it
for you).  The C<lval> parameter indicates whether this fetch is actually a
part of a store operation.

Remember that C<hv_store> and C<hv_fetch> return C<SV**>'s and not just
C<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 C<SV*>.  However,
once you have an C<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++;

=head1 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.

=head2 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 C<thing> argument can be any of an C<SV*>, C<AV*>, or C<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 C<SV*> to either an
C<AV*> or C<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

=head1 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 C<ST(n)> macro, which returns
the C<n>'th stack argument.  Argument 0 is the first argument passed in the
Perl subroutine call.  These arguments are C<SV*>, and can be used anywhere
an C<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 C<sp> is the stack pointer, and C<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 C<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 L<perlxs>.

=head1 Mortality

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

Add cruft about reference counts.

In the above example with C<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 C<SV*>) to the C<sv_2mortal> or
C<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 I<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.

=head1 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 C<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
C<HV*>.

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

For more information on references and blessings, consult L<perlref>.

=head1 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 C<struct magic>'s, typedef'ed to C<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.

=head2 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 C<sv> argument is a pointer to the SV that is to acquire a new magical
feature.

If C<sv> is not already magical, Perl uses the C<SvUPGRADE> macro to
set the C<SVt_PVMG> flag for the C<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 C<name> and C<namlem> arguments are used to associate a string with
the magic, typically the name of a variable. C<namlem> is stored in the
C<mg_len> field and if C<name> is non-null and C<namlem> >= 0 a malloc'd
copy of the name is stored in C<mg_ptr> field.

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

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

=head2 Magic Virtual Tables

The C<mg_virtual> field in the C<MAGIC> structure is a pointer to a
C<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 C<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 C<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 C<vtbl_sv> (which corresponds
to an C<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 C<magic_get> is called.  All
the various routines for the various magical types begin with C<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.

=head2 Finding Magic

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

This routine returns a pointer to the C<MAGIC> structure stored in the SV.
If the SV does not have that magical feature, C<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 C<sv> has.  If the mg_type
field is an upper-case letter, then the mg_obj is copied to C<nsv>, but
the mg_type field is changed to be the lower-case letter.

=head1 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 C<$!> contains either the numeric value of C<errno>
or its string equivalent from either C<strerror> or C<sys_errlist[]>.

To force multiple data values into an SV, you must do two things: use the
C<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 C<sv_set*v> routine
you called first.  This is because every C<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 C<sv_setiv> and C<sv_setpv> had been reversed, then the
macro C<SvPOK_on> would need to be called instead of C<SvIOK_on>.

=head1 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 C<perl_call_sv>.  The C<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 C<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 L<perlcall>.

=head1 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
C<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 C<pointer> will point to the newly allocated memory.
The third and fourth arguments C<number> and C<type> specify how many of
the specified type of data structure should be allocated.  The argument
C<type> is passed to C<sizeof>.  The final argument to C<Newc>, C<cast>,
should be used if the C<pointer> argument is different from the C<type>
argument.

Unlike the C<New> and C<Newc> macros, the C<Newz> macro calls C<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 C<Renew> and C<Renewc>
match those of C<New> and C<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 C<source> and C<dest> arguments point to the source and
destination starting points.  Perl will move, copy, or zero out C<number>
instances of the size of the C<type> data structure (using the C<sizeof>
function).

=head1 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.

=head1 DATE

Version 19: 1995/4/26