Programming Perl in C

Perl's data types are like objects in another sense they support a lightweight 

form of polymorphism. Polymorphism is the property of an object in a derived 

class to behave like an object of a parent class. In this case the inheritance tree is 

simple SV behaves as the parent class, and all the other types derive from SV. The 

Perl internals exploit this by using function parameter and return types of SV* to 

actually contain any of the available data types.

Scalar Values (SV)

The SV type represents a Perl scalar (that is, $scalar). A scalar may contain a signed 

integer (IV), an unsigned integer (UV), a floating point number (NV), or a string 

value (PV). These types are typedef aliases for fundamental C types. For example, IV

may be a typedef for int or long. NV is usually a typedef for double. You can find out 

what the underlying types are on your system using the perl  V switch. For example, 

here's the output on my system:

$ perl  V:ivtype  V:uvtype  V:nvtype

ivtype= long ;

uvtype= unsigned long ;

nvtype= double ;

Notice that PV wasn't included here that's because PV is always char * regardless 

of the platform.

Perl uses aliases for C's fundamental types to improve portability; C's types 

can differ wildly between platforms, but Perl's aliases maintain a modicum of con 

sistency. Perl's IV and UV types are guaranteed to be at least 32 bits wide and large 

enough to safely hold a pointer. For exact bit widths, Perl contains typedefs called 

I8, U8, I16, U16, I32, and U32 that are guaranteed to be at least the size specified and 

as small as possible.

Like all Perl data types, SV is an opaque type. An opaque type is one that you are 

not intended to interact with directly. Instead, you call functions that operate on 

the data type. This means that you should never dereference a pointer to a Perl 

data type directly doing so exposes the underlying implementation of the data 

type, which may change wildly between Perl versions.

Creation

The simplest way to create a new SV is with the NEWSV macro:

SV *sv = NEWSV(0,0);

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