Programming Perl in C

variables without worrying about when exactly the current scope will end; the 

answer is usually  soon enough,  which is usually good enough!

The Perl Environment

Often when you're writing a C module for Perl you'll need to interact with the Perl 

environment. The most basic means for this interaction providing functions 

written in C that can be called from Perl will be described later in Chapters 9 and 

10. This section is about going the other way calling back into Perl from C.

Accessing Variables

The simplest way to get information from Perl is to access global and package vari 

ables. The Perl API supports this with the get_ family of calls:

SV *sv = get_sv("Data::Dumper::Purity", 0); // access $Data::Dumper::Purity

AV *av = get_av("main::DATA", 1);           // create/access the global @DATA

HV *av = get_hv("main::VALUES", 1);         // create/access the global %VALUES

Each of these calls take two parameters the fully qualified name of the variable 

and a Boolean indicating whether to create the variable if it doesn't yet exist. By 

using a get_ function with the second argument set to true, you can create new 

variable in Perl space from C. If you set the second parameter to false, the calls will 

return NULL if the variable cannot be found.

Calling Perl Subroutines from C

The subroutine calling convention is probably the Perl API's most complicated 

feature. Fortunately for you, it's also its best documented one. Perl comes with an 

excellent manual page on the subject perlcall. I'll demonstrate some simple 

examples here; you can find all the gritty details in perlcall.

Example 1: No Parameters, No Return Value

The simplest type of call is one that passes no parameters and accepts no return 

values. Let's say I've defined a subroutine in the package Hello called say_hello:

sub say_hello {

   print "Hello, world.\n";

}

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