perlguts - Perl's Internal Functions


This document attempts to describe some of the internal functions of the Perl executable. It is far from complete and probably contains many errors. Please refer any questions or comments to the author below.


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

SV Scalar Value AV Array Value HV Hash Value

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

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.

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 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 character.

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


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 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.


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 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 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 .

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 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 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 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);

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++;


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 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.


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 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.

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 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 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:


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 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 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<okamoto @corp>

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