About Us Documentation

Contact Site Map



Man Page for PERLGUTS

     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

         SV  Scalar Value
         AV  Array Value
         HV  Hash Value

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

     What is an

     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.

     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

     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 allow Perl to
     calculate the length by using sv_setpv.  Be warned, though,
     that sv_setpv determines the string's length by using
     strlen, which depends on the string terminating with a NUL

     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 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:


     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

     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)

     But note that these are valid only if 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:


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

     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

     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

     Private and Public Values

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

     In general, though, it's best to just use the Sv[INP]V

     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 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 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.

     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
         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

     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);

     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
                    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++;


     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):


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

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


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

     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:

         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

     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).

     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

     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.

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.

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

     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

         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 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, it is legal to
     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.

     [This section under construction]

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

     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 sv_setiv and sv_setpv had been reversed,
     then the macro SvPOK_on would need to be called instead of

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 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 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:


     For more information, consult the perlcall manpage.

Memory Allocation
     [This section under construction]

     Jeff Okamoto <>

     With lots of help and suggestions from Dean Roehrich,
     Malcolm Beattie, Andreas Koenig, Paul Hudson, Ilya
     Zakharevich, Paul Marquess, and Neil Bowers.

     Version 12: 1994/10/16


Email addresses listed on this site may  NOT be used for unsolicited commercial email.

Ready-to-Run Software, Inc Privacy Statement

Portions (c)Copyright, 1996-2005 by Ready-to-Run Software, Inc
(All rights reserved.)
212 Cedar Cove
Lansing, NY 14882
Phone: 607 533 UNIX (8649)
Fax: 607 533 4002