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 type.

=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 a special typedef I32 which will always be a 32-bit integer.

=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 allow Perl to calculate the length by
using C<sv_setpv>.  Be warned, though, that C<sv_setpv> determines 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 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 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 Private and Public Values

Recall that the usual method of determining the type of scalar you have is
to use C<Sv[INP]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[INP]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.

In general, though, it's best to just use the C<Sv[INP]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 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 length of array */

    SV**  av_fetch(AV*, I32 key, I32 lval);
            /* Fetches value at key offset, but it seems to
	       set the value to lval if lval is non-zero */
    SV**  av_store(AV*, I32 key, SV* val);
            /* Stores val at offset key */

    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);

And more miscellaneous functions:

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

    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 */

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++;

=head2 References

References are a special type of scalar that point to other scalar types
(including references).  To treat an AV or HV as a scalar, it is simply
a matter of casting an AV or HV to an SV.

To create a reference, use the following command:

    SV*  newRV((SV*) pointer);

Once you have a reference, you can use the following macro with a cast to
the appropriate typedef (SV, AV, HV):

    SvRV(SV*)

then call the appropriate routines, casting the returned C<SV*> to either an
C<AV*> or C<HV*>.

To determine, after dereferencing a reference, if you still have a reference,
you can use the following macro:

    SvROK(SV*)

=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.

=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).

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.

=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.

=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
hash table 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).  The 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 GV structure (for global variable) but
represents them with an HV structure.

To get the HV 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.

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, it is legal to 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.

=head1 Magic

[This section under construction]

=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 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 should be 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 tells the appropriate routine what,
if any, variables are being returned by the Perl subroutine.

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

When using these four routines, 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

[This section under construction]

=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, and Neil
Bowers.

=head1 DATE

Version 12: 1994/10/16